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  • 251.
    Finnström, Marie
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jet formation in liquid impact1984Licentiatavhandling, monografi (Övrigt vetenskapligt)
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  • 252.
    Forslund, Tobias
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Non-stokesian flows in thin porous media2020Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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  • 253.
    Forslund, Tobias
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    The impact of periodicity assumptions in porous media modellingManuskript (preprint) (Övrigt vetenskapligt)
  • 254.
    Forslund, Tobias
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Tomographic-PIV imaging of non-stokesian flow through ordered thin porous mediaIngår i: Artikel i tidskrift (Övrigt vetenskapligt)
  • 255.
    Frenne, N.
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Sound source characterisation and transfer path analysis with time histories2003Ingår i: International Journal for Transformative Research, ISSN 1027-5851, E-ISSN 2360-7866, Vol. 8, nr 3, s. 167-175Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The article describes a method to separate time histories of partial sound sources. The goal is to develop a noise control engineering tool for use in sound quality improvement applications. Contributions from partial sound sources are identified. The partial sound sources may be ranked for the purpose of creating a better mixture of sound in selected listening positions. The strategy is to reproduce time histories of sources of importance. The method described includes experimental and calculation parts. The experimental part consists of the recording of sound pressure time signals, reciprocal measurement of frequency response functions, and source strength estimation of partial sound sources. The calculation part comprises calculation of the cross-spectral matrix of source strength, calculation of filters, and filtered sound pressure recording to obtain time signals of the individual sources. Usually the contribution from partial sources is impossible to record directly. In this laboratory experiment, such control was possible. The laboratory experiment shows that the method described makes it possible to produce informative separation of time histories of partial sound sources. The effects of the errors in the calculated time histories are audible but not pronounced

  • 256.
    Frenne, Nicklas
    Luleå tekniska universitet.
    Acoustic time histories of vibrating partial sources2004Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The research in this thesis evaluates a method designed for separation of individual or partial sounds from multiple sounds generated by a sound source (e.g. a diesel engine). The sound of sound sources can be recorded to reproduce a sound event in an environment, just like a photo or moving picture can be used to illustrate a visual impression. By modifying the recordings, desired sounds can be described as targets of an improved sound environment. Furthermore, the reproduced sounds of the recordings can form the basis for improvements on sources, sound paths and sound environments which affect the cause and the perception of the sound. This is the founding idea of sound design and work that strives to bring about improved product sound quality. In cases when the sound comes from a multipart product, questions may arise about which parts produce which sound. With knowledge about which sound is caused by which part, modification of the right part and hence the sound is achievable. After the different sound signals are separated from each other, the components of a multipart sound source can be identified and then appropriately tested. For example, the sound of the separate parts of a heavy vehicle engine may be assessed in listening positions around the vehicle or in its cabin. Time signals of the parts of the engine and the sound paths from the engine to the listening positions are calculated and measured separately. The sound of similar engines can therefore be determined in different cabins regardless as to whether an engine actually had been positioned there. The foundation for this research originates from signal processing and inverse problems, in the aspect of calculated time histories from sources, and methods for empirical measurements of transfer functions between sources and microphones. The method used consists of six steps: Recording of sound pressure signals, measurement of transfer functions, measurement of source strength, calculation of source strength matrix, calculation of filters and filtering of recorded sound pressure signals. The first four steps of the method can be used for automated sound source quantification of the sources' source strength or sound power; the subsequent steps are needed to determine the acoustic time histories of the sources as source strength at the source or as sound pressure in the listening positions. The results of the research demonstrated that this approach is a viable method for the detection and isolation of partial sound sources. As the research used two types sound source it is also apparent that additional research is needed before the model can be widely applied to other types of sources. In particular, the effect of the positioning of fictitious monopoles needs to be better understood and also the effect of distance on the positioning of microphones from a partial source.

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  • 257.
    Frenne, Nicklas
    Luleå tekniska universitet.
    Auralisation of partial sound sources2002Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This Licentiate thesis focuses on the development of a method for auralisation of partial sound sources. In many work environments, noise is a primary annoyance factor. A specific problem is the masking of important aural information either by high noise levels or by too efficient sound insulation. A typical sound environment is made up of sound from a large number of sources. In order to improve the sound quality in such environments the noise control engineer needs a tool to find a proper balance between contributing sources. The aim of the proposed method is to properly reproduce the contribution from sources of importance. The method includes: Recording of sound pressure time signals in dedicated positions, Reciprocal measurement of frequency response functions, Sound power measurement of partial sound sources, Calculation of Cross-spectral matrix Sqq, Calculation of filters, Filter sound pressure recording to obtain time signals of the individual sources. Paper A shows an initial approach to the problem. In this work the significance of matrix inversion was not fully understood, necessary procedural measures were not used, and therefore the result was not satisfying. Paper B begins to develop general considerations regarding the inverse problem. Aspects of filtering, input interference and the uniqueness of frequency response functions are, however, not properly solved. In Paper C the result yielded good agreement between calculated and measured time signals, especially in the important frequency range 100-3000 Hz. In this frequency range, the standard deviation of the found differences is 3 dB. The audible effects of the errors in the calculated time histories are detectable but not pronounced. Finally, goals for further research are proposed.

  • 258.
    Frenne, Nicklas
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Acoustic time histories from vibrating surfaces of a diesel engine2006Ingår i: Applied Acoustics, ISSN 0003-682X, E-ISSN 1872-910X, Vol. 67, nr 3, s. 230-248Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An experiment on a diesel engine provides for validation of a method that retrieves source strength spectra, source strength time histories and sound pressure time histories of the engine's complex partial sources. The method is based on empirical transfer function measurements and inverse matrix calculations briefly described in the article. Different simplifying source models were selected by comparison of calculated and measured auto spectra. The results show: (1) indication of time efficient measurements of source strength spectra, (2) the importance of correct source models in the case of separated source strength time histories, and (3) spectra of separated sound pressure time histories. Listening tests reported that it is possible to detect well differentiated sounds of the partial sources as a result of the method.

  • 259.
    Frenne, Nicklas
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Assessing the sound of different transfer paths from a subdivided sound source1999Ingår i: Sixth International Congress on Sound and Vibration: , Technical University of Denmark, Lyngby, Denmark, 5-8 July 1999 / [ed] Finn Jacobsen, Lyngby: Technical university of Denmark, Dep. of acoustic technology , 1999Konferensbidrag (Refereegranskat)
  • 260.
    Frenne, Nicklas
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Calculation of time histories from a subdivided sound2000Ingår i: Proceedings: Inter.Noise 2000 : August 27 - 30, 2000, Nice, France / [ed] Didier Cassereau, Paris: SFA , 2000, s. 2125-2130Konferensbidrag (Refereegranskat)
  • 261.
    Frenne, Nicklas
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Sound Source Characterisation and Transfer Path Analysis with Time Histories2003Ingår i: International Journal of Acoustics and Vibration, ISSN 1027-5851, Vol. 8, nr 3, s. 167-175Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The article describes a method to separate time histories of partial sound sources. The goal is to develop a noise control engineering tool for use in sound quality improvement applications. Contributions from partial sound sources are identified. The partial sound sources may be ranked for the purpose of creating a better mixture of sound in selected listening positions. The strategy is to reproduce time histories of sources of importance. The method described includes experimental and calculation parts. The experimental part consists of the recording of sound pressure time signals, reciprocal measurement of frequency response functions, and source strength esti- mation of partial sound sources. The calculation part comprises calculation of the cross-spectral matrix of source strength, calculation of filters, and filtered sound pressure recording to obtain time signals of the individual sources. Usually the contribution from partial sources is impossible to record directly. In this laboratory experi- ment, such control was possible. The laboratory experiment shows that the method described makes it possible to produce informative separation of time histories of partial sound sources. The effects of the errors in the cal- culated time histories are audible but not pronounced.

  • 262.
    Frenne, Nicklas
    et al.
    Luleå tekniska universitet.
    Johansson, Örjan
    Time histories from partial sound sources and generalised cross validation2003Ingår i: Proceedings of the 10th International Congress on Sound and Vibration (ICSV10): Stockholm, July 7-10, 2003 / [ed] A. Nilsson; H. Bodén, International Institute of Acoustics and Vibration , 2003, s. 3476-3486Konferensbidrag (Refereegranskat)
  • 263. Frishfelds, Vilnis
    et al.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Flow-induced deformation of non-crimp fabrics during composites manufacturing2009Ingår i: Proceedings of ITP2009: Interdisciplinary Transport Phenomena VI: Fluid, Thermal, Biological, Materials and Space Sciences, October 4-9, 2009, Volterra, Italy, 2009, artikel-id ITP-09-52Konferensbidrag (Refereegranskat)
    Abstract [en]

    Flow induced alteration in permeability of deformable systems of fibres is studied. Low Reynolds number transversal flow through random arrays of aligned cylinders is considered by using a combined methodology of directly solving the twodimensional Navier-Stokes equations for the flow in the vicinity of a single fibre and minimisation of the dissipation rate in a system of fibres. The permeability of large random arrays increases always which is most apparent for compact systems with equal sized fibres. The permeability can also decrease but then for structured or small systems. The elastic deformations of fibre bundles are calculated basing microscopic fibre structure.

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  • 264. Frishfelds, Vilnis
    et al.
    Hellström, J. Gunnar I.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mattsson, Hans
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Fluid flow induced deformation of porous medium: modeling of the no erosion filter test experiment2010Ingår i: Proceedings of the 3rd International Conference on Porous Media and its Applications in Science and Engineering: ICPM3, 2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    To study internal erosion in depth it is necessary to know either the detailed flow or how it varies in a statistical manner. It is also important to know how the internal erosion process initiates and progresses due to the fluid flow-induced forces. The underlying reason for this is that internal erosion will initiate exactly where the forces from the fluid are higher than the retaining forces that keep the particles together. Hence, a new model is here developed where fluid flow induced deformations of a large number of particles is studied. The model is applied to the No Erosion Filter test and simulated results resemble experimental results from the literature. The NEF test is used to investigate parameters such as the hydraulic conductivity and also in detail the process of internal erosion. The simulations are performed on different set-ups to exemplify successful and unsuccessful sealing. In the model, minimization of the dissipation rate of energy is accompanied with discretization of the system with modified Voronoi diagrams. Then Computational Fluid Dynamics is applied to solve the flow within each part of the Voronoi diagrams. Different parameters, such as the vorticity, calculated with the CFD-software are then used as input to the Monte Carlo-simulations. An overall good conformity between simulated results and experimental results from the literature is obtained.

  • 265.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Alteration of permeability caused by transversal flow-induced deformation of fibres during composites manufacturing2010Ingår i: Proceedings of The 10th International Conference on Flow Processes in Composite Materials (FPCM10): Monte Verità, Ascona, CH – July 11-15, 2010, 2010Konferensbidrag (Refereegranskat)
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  • 266.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Flow-induced deformations within random packed beds of spheres2014Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 104, nr 1, s. 43-56Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Low Reynolds number flow-induced alterations of permeability of random packing of mono-sized spheres is studied. The number of spheres is several thousands and the porosities ranges between 0.4 and 0.6. The change of permeability is obtained for elastic deformations of the positions of the spheres using either of two methods. Each sphere is elastically attached to single points or the spheres that are connected via an elastic porous network. The system of spheres is divided into smaller volumes with Voronoi diagrams and the flow is derived by usage of a dual stream function. The local saturated flow fields are approximated as for close packed spheres and the overall flow pattern is obtained by minimising the dissipation rate of energy. The results show that the permeability for large random systems increases as a function of velocity and thus the deformation. The alteration is, however, much less than for two-dimensional cases like parallel cylinders. The relative increase in permeability becomes larger as the porosity increases from 0.4 to 0.6.

  • 267.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mattsson, Hans
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Fluid flow induced internal erosion within porous media: modelling of the no erosion filter test experiment2011Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 89, nr 3, s. 441-457Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An investigation of the potential to numerically model the no erosion filter test is performed here, where the flow through a large ensemble of particles is considered by applying minimisation of dissipation rate of energy on the ensemble that is discretised with modified Voronoi diagrams and Delaunay triangulation. Low-Reynolds number simulations are applied to each part of the Voronoi diagram using computational fluid dynamics. The mechanical friction between particles is modelled by increasing the effective viscosity for closely spaced particles. Microscopic mechanisms for successful and unsuccessful sealing of filters are obtained. The numerical results agree with previously presented experimental observations by Sherard and Dunnigan. A conformity is that the sealing starts from the end of the channel and continues outwards in the radial direction. The sealing implies that the permeability can be reduced several orders of magnitude during a test.

  • 268.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Jakovics, Andris
    Faculty of Physics and Mathematics, University of Latvia.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Automatic recognition and analysis of scanned non-crimp fabrics for calculation of their fluid flow permeability2007Ingår i: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 26, nr 3, s. 285-296Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Automatic recognition of scanned images of distorted bi-axial fiber bundle arrangements is considered in order to obtain the overall permeability of the formed fiber network. Scanned images are pre-processed with color normalization followed by usage of a threshold to find the pixels belonging to the bundles, the threads keeping the bundles together, and the distinct gaps formed between the bundles. Since the scanned images virtually have a perfect grayscale, the intensity can be treated as a corresponding signal of the image. Next the regular character of the fiber network is investigated using Fourier analysis on the fiber bundles as well as on the threads. The direction, position, and spatial separation of the fiber bundle and the threads are obtained in this way. In order to recognize the bottom fiber bundle layer a fine structure technique is used. Small clusters falsely identified as belonging to the bottom bundle layer are removed by application of a threshold to the perimeter of the cluster. The gaps in the top bundle layer are identified more clearly in this way. Next, a local Fourier analysis is applied to obtain the local distortion of the bundle and the thread structure yielding the characteristic distribution of the gaps between the bundles. Finally the distribution of the width of the threads is obtained by simply identifying the minimal distance between the sides of the threads.

  • 269. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Formering av bubblor vid vätning av textil bestående av lager av raka fiberbuntar2009Ingår i: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, s. 90-Konferensbidrag (Övrigt vetenskapligt)
  • 270. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Influence of flow-induced deformations of fabrics on the formation and transport of bubbles during liquid moulding processes2011Ingår i: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 47, nr 2, s. 221-232Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The viscous flow-induced deformation of non-crimp fabrics during liquid moulding processes, such as resin transfer moulding, and its influence on the creation of bubbles behind the liquid front are studied. A transverse flow with a low Reynolds number through random arrays of aligned cylinders is considered with account of changes in the transverse permeability of fibre bundles. A combined methodology of directly solving the two-dimensional Navier-Stokes equations for the flow in the vicinity of a single fibre and minimization of the dissipation rate in a system of fibres is employed. Sethian's level set method is used for transient calculations of the motion of the liquid-gas front, at which the capillary pressure is accounted for. The continuity is maintained, and local correlations between the dimensions of neighbouring gaps formed between bundles are used. The elastic deformations of the fibre bundles are calculated based on micromechanical analyses. The void fractions of inter-and intrabundle bubbles obtained differ for deformable and non-deformable fabrics, but both the cases compare well with those from real mouldings

  • 271.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Influence of flow-induced forces on creation of bubbles during resin transfer moulding in non-crimp fabrics2010Konferensbidrag (Refereegranskat)
  • 272. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modelling of particle deposition during impregnation of dual-scale fabrics2011Ingår i: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 40, nr 2, s. 65-69Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The motion of particles through a system of permeable fibre bundles is considered. The system is discretisised with Voronoi diagrams and the dissipation rate of energy is minimised with respect to the stream function in a system with periodic boundary conditions. The flow of the particles is in the transversal direction to the fibre bundles and the particles are hindered to move out of the plane allowing for two-dimensional calculations. The motion of particles is assumed to be slow with respect to the flowrate so that particles are driven by the Stokesian force for stationary particles. In this case, the flow distribution is essentially dependent on the particle configuration and strictly follows the motion of particles. When testing different sizes of the particles, it is shown that there is a qualitative agreement between model and experiments previously performed. In particular, stationary flow leads to particle depositions in front of the fibre bundles and small particles move into the fibre bundles while large ones are stuck at the border

  • 273. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jacovics, Andris
    University of Latvia.
    Lattice gas analysis of liquid front in non-crimp fabrics2010Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 84, nr 1, s. 75-93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The liquid flow front during impregnation of non-crimp fabrics is considered. Irregularities in fibre bundle architecture lead to generation of bubbles at this front. The velocity of this interface is highly influenced by capillary forces mainly caused by the small fibres inside the bundles. In order to better understand which shapes the liquid front takes up at different conditions, a lattice gas model has been applied. First, the macroscopic properties of the solved gas in the liquid are discussed. Next, bubble inclusions are analyzed as to liquid-gas interface position and concentrations of minor component in each phase. The capillary effects at the fluid front are studied for systems both with and without gaps between the bundles. The flow in the interior of the fibre bundles is scrutinized, as well, by also considering the viscous stresses. The flow through unidirectional fabrics is considered by a one-dimensional model, which suggests that the liquid front inside bundles and gaps moves with the same speed when the liquid front inside the bundle has to catch up with the liquid front in the gap

  • 274. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jakovics, A.
    University of Latvia.
    Bubble motion through non-crimp fabrics during composites manufacturing2008Ingår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 39, nr 2, s. 243-251Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bubbles motion through inter bundle channels in bi-axial non crimp fabrics is modelled. The scenario is that formed bubbles move with the resin through these channels and are trapped if the channels become too narrow. By usage of a permeability network model, existing criteria on bubble deformation and a variety of analytical and probabilistic methods it is found that the paths of the bubbles depend significantly on the position of the threads keeping the fabric together and the number of fibres crossing the interbundle channels. Another result is that the pressure difference over a trapped bubble increases with 50 % in a 3D geometry possible helping the bubble to escape. A third result is that, on average, the bubbles move biased to the direction of the tows. Finally it is found that the predicted void distribution of bubbles after a major part of bubbles have moved through the system are in qualitative agreement with experimental data.

  • 275. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jakovics, A.
    Recognition technique for analysis of permeability of clustered fibre network2006Ingår i: Book of abstracts: Fourteenth International Conference Mechanics of Composite Materials, May 29 - June 2, 2006, Riga, Latvia / [ed] V. Tamuzs, Institute of Polymer Mechanics, University of Latvia , 2006Konferensbidrag (Övrigt vetenskapligt)
  • 276. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jakovics, Andris
    Lattice-gas analysis of fluid front in non-crimp fabrics2008Ingår i: Proceedings of the 19th International Symposium on Transport Phenomena (ISTP-19): Reykjavik, Iceland, August 17-21, 2008 / [ed] Sigurdur Brynjolfsson; Olafur Petur Palsson; Jong H. Kim, University of Iceland, Faculty of Industrial Engineering, Mechanical Engineering and Computer Science , 2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    The fluid flow front during impregnation of non-crimp fabrics is considered. Irregularities in fiber bundle architecture lead to generation of bubbles at the fluid front. The velocity of this interface is highly influenced by capillary forces mainly caused by the small fibers inside the bundles. In order to derive the shapes of the fluid front, a lattice-gas model has been applied. First, the macroscopic properties of the solved gas in the fluid are discussed. Next, the bubble inclusions are analyzed as to fluid-liquid interface position and concentrations of minor component in each phase. Finally, the flow in the interior of the fiber bundles is scrutinized, where the viscous stresses are considered, as well.

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  • 277. Frishfelds, Vilnis
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jakovics, Andris
    University of Latvia.
    Permeability of clustered fibre networks: modelling of unit cell2003Ingår i: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 39, nr 3, s. 265-272Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper is focused on estimating the permeability of a clustered fiber network by variational methods. First, a laminar flow in ducts is considered by using polynomial trial functions. Then, a longitudinal flow through a square array is described by expanding the flow-rate field in trigonometric and Laurent series. Finally, a formal scheme for estimating the longitudinal permeability in a cluster with an irregular distribution of fibers is given. The irregular distribution is modeled by setting an individual effective radius for each fiber and then letting this fiber reach its minimum gravitational energy. The results obtained here form a basis for future predictions of the permeability of fibrous reinforcements.

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  • 278.
    Frishfelds, Vilnis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Lundström, T. Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modelling particulate flow during impregnation of dual-scale fabrics2011Ingår i: 18th International Conference on Composites Materials, ICCM 2011: Jeju; South Korea; 21 August 2011 through 26 August 2011, 2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Filtration of particles during impregnation of dual-scale fabrics is studied numerically for a number of configurations with a previously derived model. The initial position and size of the particles are varied. The main result is that structural composites can be tailor-made as to additional properties by such an approach.

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  • 279. Fällström, Karl-Evert
    et al.
    Gustavsson, Håkan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Molin, Nils-Erik
    Wåhlin, Anders
    Transient bending waves in plates studied by hologram interferometry1989Ingår i: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 29, nr 4, s. 378-387Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.

  • 280.
    Gantasala, Sudhakar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Luneno, Jean-Claude
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Aidanpää, Jan-Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Aeroelastic simulations of wind turbine using 13 DOF rigid beam model2016Ingår i: Open archives of the 16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, Symposia on Rotating Machinery , 2016, artikel-id hal-01516690Konferensbidrag (Refereegranskat)
    Abstract [en]

    The vibration behavior of wind turbine substructures is mainly dominated by their first few vibration modes because wind turbines operate at low rotational speeds. In this study, 13 degrees of freedom (DOF) model of a wind turbine is derived considering fundamental vibration modes of the tower and blades which are modelled as rigid beams with torsional springs attached at their root. Linear equations of motion (EOM) governing the structural behavior of wind turbines are derived by assuming small amplitude vibrations. This model is used to study the coupling between the structural and aerodynamic behavior of NREL 5 MWmodel wind turbine. Aeroelastic natural frequencies of the current model are compared with the results obtained from the finite element model of this wind turbine. Quasi-steady aerodynamic loads are calculated considering wind velocity changes due to height and tower shadow effects. In this study, vibration responses are simulated at various wind velocities. The derived 13 DOF simplified model of the wind turbine enables to simulate the influence ofchange in parameters and operating conditions on vibration behavior with less computational effort. Besides that, the results of the simplified models can be interpreted with much ease.

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  • 281.
    Gantasala, Sudhakar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Tabatabaei, Narges
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Aidanpää, Jan-Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Numerical Investigation of the Aeroelastic Behavior of a Wind Turbine with Iced Blades2019Ingår i: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, nr 12, artikel-id 2422Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wind turbines installed in cold-climate regions are prone to the risks of ice accumulation which affects their aeroelastic behavior. The studies carried out on this topic so far considered icing in a few sections of the blade, mostly located in the outer part of the blade, and their influence on the loads and power production of the turbine are only analyzed. The knowledge about the influence of icing in different locations of the blade and asymmetrical icing of the blades on loads, power, and vibration behavior of the turbine is still not matured. To improve this knowledge, multiple simulation cases are needed to run with different ice accumulations on the blade considering structural and aerodynamic property changes due to ice. Such simulations can be easily run by automating the ice shape creation on aerofoil sections and two-dimensional (2-D) Computational Fluid Dynamics (CFD) analysis of those sections. The current work proposes such methodology and it is illustrated on the National Renewable Energy Laboratory (NREL) 5 MW baseline wind turbine model. The influence of symmetrical icing in different locations of the blade and asymmetrical icing of the blade assembly is analyzed on the turbine’s dynamic behavior using the aeroelastic computer-aided engineering tool FAST. The outer third of the blade produces about 50% of the turbine’s total power and severe icing in this part of the blade reduces power output and aeroelastic damping of the blade’s flapwise vibration modes. The increase in blade mass due to ice reduces its natural frequencies which can be extracted from the vibration responses of the turbine operating under turbulent wind conditions. Symmetrical icing of the blades reduces loads acting on the turbine components, whereas asymmetrical icing of the blades induces loads and vibrations in the tower, hub, and nacelle assembly at a frequency synchronous to rotational speed of the turbine.

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  • 282.
    Gebart, Rikard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Analysis of heat transfer and fluid flow in the resin transfer moulding process1992Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This thesis contains an analysis of fluid flow and heat transfer problems in the resin transfer moulding (RTM) process for manufacturing of polymer based fibre composites and it consists of five separate papers. The permeability of unidirectional fabrics, that are often used in advanced composites, is considered in Paper A and a theory for the permeability dependence on the micro geometry is developed. The theory is based on lubrication theory for narrow gaps which is motivated by the fact that most of the flow resistance comes from a small region where the fibres are closest to each other. Despite this limitation the results agree excellently with numerical results. 'Me best performance of the theory is expected at high fibre volume fractions (Vf) but the dependence on Vf is surprisingly good even at as low values as 0.3. Although the theory is formulated for an idealised geometry it can be used to predict the variation of the anisotropic permeability tensor with fibre volume fraction in real fabrics after fitting of three model parameters. Paper B is a study of the influence from different process parameters on the void content in the laminate. The void content is shown to be reduced strongly by an applied vacuum during mould filling. The main mechanism for void formation appears to be mechanical entrapment at the flow front. The voids are convected by the flow so that their concentration is highest close to the flow front. Microscopy investigation of the bubbles show that they are of two basic types, large spherical bubbles in the interstices between fibre bundles and smaller cylindrical bubbles inside the fibre bundles. The positive influence of vacuum compared to no vacuum can be explained as a combined effect of an increased mobility due to larger volume changes during mould filling and compression by the increased pressure during cure. In Paper C a comparison is made between the mould filling times for different injection strategies. The possible alternatives for a normal laminate are point injection, edge injection and peripheral injection. Theoretical results are derived that can be used to estimate the mould filling time with the different alternatives. In addition, fundamental theoretical results are derived from the governing equations showing the scaling of the mould filling time with the process parameters. This analysis also shows that the flow front motion during mould filling is only a function of the anisotropy of the reinforcement and the location of the gates. Paper D presents an analysis of the non-uniform flow at the flow front during impregnation of a stack of fabrics consisting of layers with different flow resistance. A detailed derivation of the theory and an analytical solution to the equations are presented in an addendum to Paper D. The theoretical model is compared with experimental results and is found to describe the experiment qualitatively well. The resulting permeability of a stack of different fabrics is derived from the basic equations and is found to be a weighted average of the permeability in the individual layers. This result is compared with experiments with different stacking sequences and it is found that the stacking sequence has no influence on the resulting permeability as expected from the theory. Experimental results in excellent agreement with Darcy's law are also presented for the case with radial flow and with unidirectional flow. Finally, Paper E is a theoretical study of the curing behaviour of thick laminates. A general solution independent of the cure kinetic model is derived. The solution is valid for low exothermal peak temperatures and it is characterised by two dimensionless numbers. The first parameter is the ratio between the time scales for the reaction and for heat conduction, the second parameter is the ratio between the processing temperature and the adiabatic temperature rise. The general solution is specialised to a second order autocatalytic cure model so that the results can be compared to numerical results. The agreement between the numerical and the analytical solution is excellent for small exothermal peak temperatures, as expected. The particular model used also serves as an example of the additional dimensionless parameters that are introduced by a specific kinetic model.

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  • 283.
    Gebart, Rikard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Gustavsson, HåkanLuleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.Karlsson, Rolf
    Proceedings of Turbine-99: workshop on draft tube flow2000Samlingsverk (redaktörskap) (Övrigt vetenskapligt)
    Abstract [en]

    The Turbine 99 workshop, held in Porjus, Sweden, 20-23 June 1999, was organized to determine the state-of-the-art of CFD simulations of draft tube flows. A total of 16 groups accepted the invitation to compute the draft tube flow. The following experimental data was available before the workshop: the axial and swirl velocity components at the inlet (with rms-values and one Reynolds' stress component) and the pressure distribution around the outlet cross section. Before the workshop, the groups submitted simulation results (using 12 different CFD codes) which were compiled by the organizers to an extensive set of data available at the workshop. The experimental data for the pressure recovery factor, pressure distributions along the draft tube walls and the detailed velocity field in one downstream cross section was presented during the workshop. In this report a summary of the main results and conclusions of the workshop is given, together with the written reports from the experiments and the simulations. In addition, the report contains all relevant background information for the workshop such as the draft tube geometry, provided data, requested information etc. and is thus the main document from the workshop. As the simulations provided more data than was available for comparison with the experiments, the simulation results will be available in a separate document provided by the organizers. One major conclusion of the workshop is that much attention must (still) be payed to the grid quality and the boundary conditions, factors that need to be strictly specified before a fully relevant comparison can be made between different flow models.

  • 284.
    Gebart, Rikard
    et al.
    Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Lundström, Staffan
    Influence on the permeability of unidirectional reinforcement from fibre geometry1991Ingår i: The polymer processing society: european regional meeting , final program and abstracts, Palermo-Italy September 15-18/1991, 1991Konferensbidrag (Övrigt vetenskapligt)
  • 285.
    Gebart, Rikard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sandlund, Erik
    In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique1999Ingår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 20, nr 1, s. 146-154Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.

  • 286.
    Gebart, Rikard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Vahlund, C. Fredrik
    Luleå tekniska universitet.
    Analysis of an image processing method for fiber orientation in polymer composites2001Ingår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 22, nr 3, s. 327-336Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The errors in fiber orientation distribution measurements of compression molding materials have been investigated, for generated as well as for real distributions. Because of the size of the sample, only a finite number of fibers are visible in a picture leading to a statistical error in the measurement. A method is proposed to express this error as function of the number of visible fibers and the number of subintervals the distribution is divided into. Studies of the error In a Fraunhofer/FT based fiber orientation distribution analysis have been performed, including effects of increasing number of fibers (fiber-fiber intersections) in generated pictures and investigation of the errors in real fiber images. All steps in a fiber orientation distribution analysis of SMC/GMT kind of material are described, including suitable equipment, image enhancement methods and investigation of the errors present in analysis of a real image as well as comparisons to hand-measured data.

  • 287.
    Gebart, Rikard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Vahlund, C.F.
    Squeeze flow rheology in large tools1999Ingår i: Fifth International Conference on Flow Processes in Composite Materials: Proceedings, University of Plymouth Press, 1999, s. 365-372Konferensbidrag (Refereegranskat)
  • 288. Gererkiden, Berhanu Mulu
    et al.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    LDA measurements in a Kaplan spiral casing model2010Ingår i: 13th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery 2010 (ISROMAC-13): Honolulu, Hawaii, USA, 4 - 7 April 2010, Red Hook, NY: Curran Associates, Inc., 2010, s. 85-92Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper presents an experimental investigation of a Kaplan spiral casing turbine model. A two-component laser Doppler anemometry (LDA) apparatus was used to measure the velocity profiles at different locations in the turbine. To improve the signal quality and measurement accuracy, a refractive index matching optical box was mounted on the circular pipe of the spiral casing inlet. The investigations were carried out with a constant runner- blade angle and at three different loads: the best operating point of the turbine and two off-design operating points (left and right side of the propeller curve) with the presence of a vortex breakdown. The mean velocity profiles and corresponding RMS at the spiral casing before the guide vanes and at inlet of the spiral casing are presented for the different loads investigated.

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  • 289.
    Giessman, Ray
    et al.
    Luleå tekniska universitet.
    Ågren, Anders
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Structural modifications of A 9 litre diesel engine to decrease noise radiation: A, Text B, Appendices1992Rapport (Övrigt vetenskapligt)
  • 290.
    Gonzalez, Sergio Galvan
    et al.
    Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo.
    Ibarra-Bracamontes, Laura
    Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo.
    Diaz, Gildardo Solorio
    Mechanical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Assement study of the RNG turbulence model for steady state swirling flow analysis in a draft tube2012Ingår i: Avances en Simulacion Computacional y Modelado Numerico: memorias del XI congreso internacional de métodos numéricos en ingeniería y ciencias aplicadas CIMENICS'2012, Isla de Margarita, Venezuela, 26 al 28 de marzo de 2012 / [ed] E. Dávila; G. Uzcátegui; M. Cerrolaza, Facultad de Ingeniería , Universidad Central de Venezuela , 2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    The flow in a hydraulic turbine draft tube is a challenging industrial application for CFD because is characterized by a turbulent flow with different flow phenomena, e.g. unsteadiness, flow separation, swirling flow and strong adverse pressure gradient. Thus, its simulation is complex and time-consuming high computational capacities are required Additionally, adequate turbulence modeling is needed to predict such flows accurately. The objective of the present work is to investigate the accuracy of the Renormalization Group Theory (RNG) k-c turbulence model on the draft tube flow close to the best efficiency of the turbine. The effect of the discerization schemes on these models has been studied As the flow is significantlyaffected by the walls, two different grid concentrations near to the wall (y + I and y + 50) were evaluated The numerical results are compared to detailed experimental results at different section and discussed.

  • 291.
    Goyal, Rahul
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    FLOW FIELD IN A HIGH HEAD FRANCIS TURBINE DRAFT TUBE DURING TRANSIENT OPERATIONS2017Doktorsavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    Hydroelectricity plays an important role to balance the stability of grid network.  In order to improve the stability of presently high loaded grids, hydropower plants are being operated over a wide range of operations and experiencing frequent start-stop, load rejection, and load acceptance. The turbines need to sustain sudden change in their operating condition to balance the grid frequency. Francis turbines have been widely used because of their wider operating range and higher stability in operation during rapid load variation. This has resulted in severe damage to the turbines as they are not normally designed to operate under such transient conditions.

    Several low and high frequency pressure fluctuations prevail during transients operating conditions. Generally, wall pressure measurements are performed which may not provide sufficient information to investigate the flow instabilities related to these fluctuations. Thus, the main objective of the present work is to simplify and perform optical measurements in a turbine during transient operating conditions to investigate the flow field. The measurements have been performed at the Water Power Laboratory using a high head model Francis turbine. The turbine is a 1:5.1 scale down model of a prototype operating at the Tokke Power Plant, Norway. The model runner diameter, net head, and discharge at the best efficiency point (BEP) were 0.349 m, 12 m, and 0.2 m3 s-1, respectively. A total ten pressure sensors were mounted at different locations namely, turbine inlet, vaneless space, and draft tube. The data were acquired at a sampling rate of 5 kHz. The instruments and sensors have been calibrated according to guidelines available in IEC standards. The determined total uncertainty in the measurement of hydraulic efficiency was ±0.15% at BEP condition. The velocity measurements in the draft tube cone were performed using a 2D PIV system and the images were sampled at a rate of 40 Hz.

         Steady state measurements were carried out considering the realistic design and off-design operating conditions of the prototype turbine. Therefore, the angular speed of the runner was maintained constant for all steady state conditions during the measurements. The maximum hydraulic efficiency (92.4%) was observed at nED = 0.18, QED = 0.15, and a = 9.8º, which is named BEP. It is observed that the turbine experiences significant pressure fluctuations at the vaneless space, runner, and the draft tube. The fluctuations due to rotor-stator interaction (RSI) were observed to be most dominating at high load condition, however, fluctuations due to the rotating vortex rope (RVR) at part load (PL) condition. Two different modes (synchronous and asynchronous) modes of vortex rope are observed at PL condition of the turbine. An asymmetry in the flow leaving the runner was detected at both design and off-design conditions, with a stronger effect during off-design operating condition. Numerical simulations of the model turbine were carried out at PL operating condition. The simulations were performed using two turbulence models, standard k-ε and SST k-ω, with high-resolution advection scheme. The numerical pressure values obtained with both standard k-ε model and SST k-ω showed a small difference with the experimental values. The amplitudes of numerical pressure values were higher (~2.8%) in the vaneless space and lower (~5.0%) in the draft tube than the experimental values. The frequencies of the RSI and RVR were well captured in the turbine but the amplitudes were overestimated.  

    During load rejection from BEP to PL, the plunging mode of the vortex rope was observed to appear first in the system than that of the rotating mode. Whereas during the load acceptance from PL to BEP, both the modes were observed to disappear simultaneously from the system. In the velocity data, the axial velocity only contributed to the development of the plunging mode and radial velocity to the rotating mode. The region of low velocity, stagnation point, flow separation, recirculation, oscillating flow and high axial velocity gradients were well captured in the system during the transients. The induced high-velocity gradients during the load acceptance from BEP to HL was observed to develop a vortex core in the draft tube.

    During startup and shutdown, the guide vanes angular position was moved from one to another steady state condition to achieve the minimum load condition of the turbine. At this condition, the generator of the turbine was magnetized at the synchronous speed during startup and shutdown, respectively. The frequency of wave propagation was observed to vary with the runner angular speed during startup and complete shutdown of the turbine. Comparatively high-pressure fluctuations in the draft tube were observed during the guide vane movement from the high discharge conditions. Some unsteady phenomena such as the formation of dead velocity zone, backward flow, and flow oscillations were observed during startup and shutdown of the turbine.  

    The current work has been also used to continue a series of workshops, i.e., Francis-99. The first workshop was held on December 2014 with the cooperation of LTU and NTNU. The measurements performed in this work were used for the second workshop which was held on December 2016. The investigations presented in this thesis will be further explored in the third workshop scheduled for December 2018. 

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  • 292.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Indian Institute of Technology Roorkee.
    Bergan, Carl
    Waterpower Laboratory, Norwegian University of Science and Technology.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Dalhaug, Ole Gunnar
    Waterpower Laboratory, Norwegian University of Science and Technology.
    Experimental investigation on a high head model Francis turbine during load rejection2016Ingår i: IOP Conference Series: Earth and Environment, ISSN 1755-1307, E-ISSN 1755-1315, Vol. 49, nr 8, artikel-id 082004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Francis-99 is a set of workshop aiming to determine the state of the art of high head model Francis turbine simulations (flow and structure) under steady and transient operating conditions as well as to promote their development and knowledge dissemination openly. The first workshop (Trondheim, 2014) was concerned with steady state operation. The second workshop will focus on transient operations such as load variation and start-stop. In the present work, 2-D particle image velocimetry (PIV) with synchronized pressure measurements performed in the draft tube cone of the Francis-99 test case during load rejection is presented. Pressure sensors were mounted in the vaneless space and draft tube cone to estimate the instantaneous pressure fluctuations while operating the turbine from the best efficiency point (9.8°) to part load (6.7°) with the presence of a rotating vortex rope (RVR). The time-resolved velocity and pressure data are presented in this paper showing the transition in the turbine from one state to another

  • 293.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Vortex Rope Formation in a High Head Model Francis Turbine2017Ingår i: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 139, nr 4, artikel-id 041102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the twodimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4±0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor-stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope

  • 294.
    Goyal, Rahul
    et al.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology.
    Characteristics of Synchronous and Asynchronous modes of fluctuations in Francis turbine draft tube during load variation2017Ingår i: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, E-ISSN 1882-9554, Vol. 10, nr 2, s. 164-175Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Francis turbines are often operated over a wide load range due to high flexibility in electricity demand and penetration of other renewable energies. This has raised significant concerns about the existing designing criteria. Hydraulic turbines are not designed to withstand large dynamic pressure loadings on the stationary and rotating parts during such conditions. Previous investigations on transient operating conditions of turbine were mainly focused on the pressure fluctuations due to the rotor-stator interaction. This study characterizes the synchronous and asynchronous pressure and velocity fluctuations due to rotor-stator interaction and rotating vortex rope during load variation, i.e. best efficiency point to part load and vice versa. The measurements were performed on the Francis-99 test case. The repeatability of the measurements was estimated by providing similar movement to guide vanes twenty times for both load rejection and load acceptance operations. Synchronized two dimensional particle image velocimetry and pressure measurements were performed to investigate the dominant frequencies of fluctuations, vortex rope formation, and modes (rotating and plunging) of the rotating vortex rope. The time of appearance and disappearance of rotating and plunging modes of vortex rope was investigated simultaneously in the pressure and velocity data. The asynchronous mode was observed to dominate over the synchronous mode in both velocity and pressure measurements.

  • 295.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, India.
    Cervantes, Michel J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, India.
    Synchronized PIV and pressure measurements on a model Francis turbine during start-up2020Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 58, nr 1, s. 70-86Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents the experiments performed on a high head model Francis turbine during start-up. Synchronized time dependent pressure and velocity measurements were performed to investigate the instabilities in the turbine. A total of four steady state operating points, namely synchronous load, part load, best efficiency point, and high load are considered to perform the turbine start-up. The runner angular speed was observed to increase almost exponentially during the guide vane positions from completely closed to no load condition. The frequency of wave propagation due to the interaction between runner blades and guide vanes was observed to follow the trend of increase of runner angular speed. A vortex rope frequency was captured in the draft tube during synchronous load to part load of the start-up. Two different mechanisms, namely, the development of stagnation point and the available recirculation regions were observed to cause the formation of vortex rope in the draft tube.

  • 296.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology.
    Experimental study of mitigation of a spiral vortex breakdown at high Reynolds number under an adverse pressure gradient2017Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 9, artikel-id 104104Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow in the off-design operation of a Francis turbine may lead to the formation of spiral vortex breakdowns in the draft tube, a diffuser installed after the runner. The spiral vortex breakdown, also named a vortex rope, may induce several low-frequency fluctuations leading to structural vibrations and a reduction in the overall efficiency of the turbine. In the present study, synchronized particle image velocimetry, pressure, and turbine flow parameter (Q, H, α, and T) measurements have been carried out in the draft tube cone of a high head model Francis turbine. The transient operating condition from the part load to the best efficiency point was selected to investigate the mitigation of the vortex rope in the draft tube cone. The experiments were performed 20 times to assess the significance of the results. A precession frequency of 1.61 Hz [i.e., 0.29 times the runner rotational frequency (Rheingans frequency)] is observed in the draft tube cone. The frequency is captured in both pressure and velocity data with its harmonics. The accelerating flow condition at the center of the cone with a guide vane opening is observed to diminish the spiral form of the vortex breakdown in the quasi-stagnant region. This further mitigates the stagnant part of the cone with a highly dominated axial flow condition of the turbine at the best efficiency point. The disappearance of the stagnant region is the most important state in the present case, which mitigates the spiral vortex breakdown of the cone at high Reynolds numbers. In contrast to a typical transition, a new type of transition from wake to jet is observed during the mitigation of the breakdown. The obtained 2D instantaneous velocity fields demonstrate the disappearance region of shear layers and stagnation in the cone. The results also demonstrate the existence of high axial velocity gradients in an elbow draft tube cone.

  • 297.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Indian Institute of Technology, Department of Mechanics & Industrial Engineering, Roorkee .
    Gandhi, Bhupendra K.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee .
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    PIV measurements in Francis turbine: A review and application to transient operations2018Ingår i: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 81, nr 2, s. 2976-2991Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Penetration of solar and wind energy into the grid network has raised the concern for grid stability which is generally balanced by operating the hydropower plants over a wide range. This results in several issues, such as rotor-stator interaction (RSI), vortex breakdown, rotating vortex rope (RVR), pressure shocks, vibration, and noise which may lead to failure. Particle Image Velocimetry (PIV) has been used to understand several physical mechanisms in the flow at various operating conditions. A non-negligible uncertainty may arise in the measurements due to calibration, abbreviation, and distortion of the light. Various parameters such as laser sheet thickness, particle type, particle size, particle density, camera resolution, image size and number of images may affect the quality of the measurements. In the present work, a review of PIV measurements performed in hydraulic turbines, mainly Francis, has been carried out. The objective is to develop an experimental set up to perform steady and transient measurements on a model Francis turbine. A maximum deviation of 1.8% in absolute velocity is estimated in the present study as compared to 2–3% reported in the previously performed measurements on Francis turbines. The repeatability of transient measurements is also investigated by extracting two velocity points on a PIV plane

  • 298.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Indian Institute of Technology Roorkee.
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Transient Pressure Measurements in the Vaneless Space of a Francis Turbine during Load Acceptances from Minimum Load2018Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 1042, artikel-id 012009Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Increased penetration of solar and the wind impels the designers of the hydroelectric power generation unit to provide more flexibility in operation for the stability of the grid. The power generating unit includes turbine which needs to sustain sudden change in its operating conditions. Thus, the hydraulic turbine experiences more transients per day which result in chronic problems such as fatigue to the runner, instrument malfunctioning, vibrations, wear and tear etc. This paper describes experiments performed on a high model (1.5:1) Francis turbine for load acceptances from the minimum load. The experiments presented in the paper are the part of Francis-99 workshop which aims to determine the performance of numerical models in simulations of model Francis turbine under steady and transient operating conditions. The aim of the paper is to present the transient pressure variation in the vaneless space of a Francis turbine where high-frequency pulsations are normally expected. For this, two pressure sensors, VL1 and VL2, are mounted at the vaneless space, one near the beginning of the spiral casing and the other before the end of the spiral casing. Both are used to capture the unsteady pressure field developed in the space between guide vanes and runner inlet. The time-resolved pressure signals are analyzed and presented during the transient to observe the pressure variation and dominant frequencies of pulsations.

  • 299.
    Goyal, Rahul
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Grenoble-INP/CNRS/UJF-Grenoble 1, Grenoble, France.
    Gandhi, BK
    Indian institute of Technology, Roorkee, India.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Experimental Investigation of a High Head Francis Turbine Model During Shutdown Operation2019Ingår i: IOP Conference Series: Earth and Environment, Institute of Physics (IOP), 2019, Vol. 240, artikel-id 022028Konferensbidrag (Refereegranskat)
    Abstract [en]

    Increased penetration of intermittent energy resources disturbs the power grid network. The frequency band of the power grid is normally controlled by automatic opening and closing of the guide vanes of hydraulic turbines. This has increased the number of shutdown cycles as compared to the defined ones for the normal operation of turbines. Turbine shutdown induced a significantly higher level of pressure fluctuations and unsteadiness in the flow field, decreasing its expected life. This paper presents experiments performed on a high head model Francis turbine during shutdown. The pressure and 2D Particle Image Velocimetry (PIV) measurements were performed to investigate the pressure fluctuations and flow instabilities in the turbine. The pressure sensors were mounted in the draft tube cone and vaneless space to measure the instantaneous pressure fluctuations. In the present study, the initial high load operating condition was considered to perform the turbine shutdown. The data were logged at the sampling frequency of 40 Hz and 5 kHz for PIV and pressure measurements, respectively. Time-resolved velocity and pressure data are presented in this paper to show the pressure fluctuations and causes of generation of unsteady flow in the draft tube.  

  • 300.
    Goyal, Rahul
    et al.
    Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee.
    Trivedi, Chirag
    Norwegian University of Science and Technology, Trondheim, .
    Gandhi, Bhupendra K.
    Indian Institute of Technology Roorkee.
    Cervantes, Michel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Numerical Simulation and Validation of a High Head Model Francis Turbine at Part Load Operating Condition2018Ingår i: Journal of The Institution of Engineers (India): Series C, ISSN 2250-0545, Vol. 99, nr 5, s. 557-570Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydraulic turbines are operated over an extended operating range to meet the real time electricity demand. Turbines operated at part load have flow parameters not matching the designed ones. This results in unstable flow conditions in the runner and draft tube developing low frequency and high amplitude pressure pulsations. The unsteady pressure pulsations affect the dynamic stability of the turbine and cause additional fatigue. The work presented in this paper discusses the flow field investigation of a high head model Francis turbine at part load: 50% of the rated load. Numerical simulation of the complete turbine has been performed. Unsteady pressure pulsations in the vaneless space, runner, and draft tube are investigated and validated with available experimental data. Detailed analysis of the rotor stator interaction and draft tube flow field are performed and discussed. The analysis shows the presence of a rotating vortex rope in the draft tube at the frequency of 0.3 times of the runner rotational frequency. The frequency of the vortex rope precession, which causes severe fluctuations and vibrations in the draft tube, is predicted within 3.9% of the experimental measured value. The vortex rope results pressure pulsations propagating in the system whose frequency is also perceive in the runner and upstream the runner.

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