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  • 51.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    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.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Representative volume size when modelling flow through porous structures2014Konferensbidrag (Refereegranskat)
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  • 52.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Lundström, Staffan
    Mechanisms of flow-induced deformation of porous media2010Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 664, s. 220-237Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The study investigates creeping flow-induced alteration in the permeability of deformable particle systems. Low-Reynolds-number transversal flow through random arrays of aligned cylinders is considered by means of a combined methodology of directly solving the two-dimensional (2D) Stokes equations for the flow in the vicinity of two particles and minimising the dissipation rate in a system comprising thousands of particles. The results demonstrate that the more compact the system, the greater the possible relative change of permeability when a high flow rate is applied. The permeability of large random arrays always increases when increasing the flow rate, which is most apparent in compact systems with equal-sized particles. The permeability can sometimes decrease but only in structured or small systems.

  • 53.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    Lundström, Staffan
    Mechanisms of fluid flow induced deformation of porous medium2009Ingår i: Proceedings of 4th ICAPM: August 10-12, 2009, Istanbul, Turkey, 2009Konferensbidrag (Refereegranskat)
  • 54.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    Lundström, Staffan
    Mekanismer för flödesinducerad deformation av ett poröst material2009Ingår i: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, s. 103-Konferensbidrag (Övrigt vetenskapligt)
  • 55.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsson, Jonas
    Epsilon HighTech AB, Göteborg.
    Lundström, Staffan
    Laminar and turbulent flow through an array of cylinders2010Ingår i: Journal of Porous Media, ISSN 1091-028X, E-ISSN 1934-0508, Vol. 13, nr 12, s. 1073-1085Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    When modeling fluid flow through porous media it is necessary to know when to take inertia effects into account, as well as when to switch to a turbulent description of the flow. From an engineering point of view, the problem is often solved with the empirically derived Ergun equation or a recently upgraded version by Nemec and Levec [Chem. Eng. Sci., vol. 60, pp. 6947−6957, 2005]. The drawback with this approach is, however, that the mechanisms for the transitions between the three states of flow are not revealed and time-consuming experiments have to be performed. In order to increase knowledge of the detailed flow, numerical studies of flow through arrays of quadratically packed cylinders at a variety of Re values were carried out. One result is that the laminar and turbulent approaches used both mimic experimental results for low Re, while for higher Re only the turbulent approach resembles the empirically derived equations. The deviation from Darcy’s law for different porosities of the array can be defined by usage of Re based on the hydraulic radius and the average interstitial velocity. However, to find a common Re when turbulence need to be accounted for, another Re based solely on the averaged interstitial velocity and the diameter of the cylinders was used. It was found that at low Re the laminar and turbulent setups give practically the same velocity fields, while the turbulent dissipation at higher Re results in larger circulation zones and weaker jets.

  • 56. Hellström, J. Gunnar I.
    et al.
    Ljung, Anna-Lena
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Flow through a hexagonal array of perturbed spheres at low to high Reynolds number2007Ingår i: Abstracts of the Second International Conference on Porous Media and its Applications in Science, Engineering and Industry, Engineering Conferences International , 2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    When performing numerical simulations of fluid flow through porous media it is necessary to know when to switch from a creeping flow formulation to a more elaborate laminar description. In the creeping flow regime the Darcy law is sufficient while when inertia-effects become significant it is necessarily to use the full Navier-Stokes equations or at least add a non-linear term to Darcy's law as done in the empirically derived Ergun equation. The latter equation has also turned out to be valid for turbulent flows. It is however not obvious which equation to use at a certain Reynolds number. In order to solve this problem Computational Fluid Dynamics is used to derive the apparent permeability of a hexagonal packed array of spheres. In addition the forces acting on the spheres are derived when a perturbation in the form of a spherically shaped particle is introduced in the pore space. Then simulations are performed at various Reynolds number ranging from the creeping flow region to moderate Reynolds number flows. The simulations are carried out with the commercially available software, ANSYS CFX 11.0, with a particular effort on grid refinement and numerical iteration in order to secure that the errors are sufficiently small. One result is that inertia effects become important already at Reynolds number about 5 for as well the array as the perturbed geometry. As the particle radius increases the shear and normal forces per unit area decreases. In general, these forces increase with Reynolds number. The simulations however show that for some cases the normal forces per unit area decreases and even change sign as Reynolds number increases.

  • 57.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ljung, Anna-Lena
    Lundström, Staffan
    Forces on grains located in model geometry with application to internal erosion in embankment dams2007Ingår i: International Symposium on Modern Technology of Dams: The 4th EADC Symposium, 2007, s. 375-386Konferensbidrag (Refereegranskat)
    Abstract [en]

    For a comprehensive understanding of internal erosion in embankment dams it is necessary to elucidate the detailed seepage flow. A neat tool that can be used for this purpose is Computational Fluid Dynamics. With such a tool forces on individual particles can be derived and means to decide when to switch from a creeping flow formulation to a more elaborate laminar description for macroscopic flow simulations can be derived. It can even be decided when the transition from a laminar formulation to a fully turbulent description should take place. In the creeping flow regime a Darcy law formulation is sufficient while when inertia-effects become significant it is necessary to use the Navier-Stokes equations or at least add a non-linear term to Darcy's law as done in the empirically derived Ergun equation. It is, however, not obvious which equation to use at a certain Reynolds number. Hence, Computational Fluid Dynamics is here used to derive the apparent permeability of a hexagonal packed array of spheres. Then, grains are introduced in the pore space between the spheres and forces acting on the grains are derived. It will then be possible to decide at what conditions such particles will start to move, due to flow induced forces, and thereby initiating internal erosion. The simulations are performed at various Reynolds number ranging from the creeping flow region to the transition regime. The software ANSYS CFX 11.0 is applied with particular effort on grid refinement and numerical iteration in order to secure that the numerical errors are sufficiently small. One result is that inertia-effects become important already at a Reynolds number of 10. Another is that the forces acting on the grains can decrease as a function of Reynolds number and can as well be dependent on the geometry of the grains, even though the force per unit area on the array of spheres increases. Interestingly, the direction of the forces on the grains can even be opposite to the main flow direction.

  • 58.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    CFD-simulations of fluid flow through an array of hexagonal or quadratic packed cylinders: laminar versus turbulent formulations2009Ingår i: Proceedings of 4th ICAPM: August 10-12, 2009, Istanbul, Turkey, 2009Konferensbidrag (Refereegranskat)
  • 59. Hellström, J. Gunnar I.
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Flow induced forces in porous media with application to internal erosion2008Ingår 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]

    For a comprehensive understanding of internal erosion in embankment dams it is necessary to elucidate the detailed seepage flow. A neat tool that can be used for this purpose is Computational Fluid Dynamics in which forces on individual particles in a porous media can be derived. The model geometry chosen to represent the porous media is a hexagonal array of spheres into which smaller particles are introduced which are supposed to move at a certain level of flow induced forces. It is shown that the mesh deformation method introduced has a potential to model internal erosion.

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  • 60. Hellström, J. Gunnar I.
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Flöde genom poröost material vid varierande Reynolds tal2007Ingår i: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, s. 63-Konferensbidrag (Övrigt vetenskapligt)
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  • 61. Hellström, J. Gunnar I.
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Numerical simulations of fluid flow through model geometries of porous media: at low to high Reynolds number2008Ingår i: Heat Transfer, Fluid Mechanics & Thermodynamics, HEFAT 2008: Proceedings of the Sixth International Conference, Pretoria, South Africa, 30 June-2 July 2008, Pretoria: University of Pretoria, South Africa , 2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    When modeling fluid flow through porous media it is necessary to know when to switch from a creeping flow formulation to a more elaborate laminar description or to a fully turbulent one. This is of importance in a large number of industrial processes such as flow through embankment dams, composites manufacturing, filtering and in the refinement of iron ore pellets. Regarding the creeping flow regime the Darcy law is sufficient while when inertia-effects become significant it is necessary to use the full Navier-Stokes equations or at least add a non-linear term to Darcy's law as done in the empirically derived Ergun equation, which has also turned out to be valid for some turbulent flows. It is however not obvious which equation to use at a certain Reynolds number and on what velocities and length scales Reynolds number should be based on. In order to shed some light on this Computational Fluid Dynamics is here applied to simple model geometries of porous media. In particular the flow through quadratic and hexagonal arrays of cylinders is studied. The main quantities of interest are the apparent permeability, the Blake-type friction factor as well as the forces acting on the cylinders. The simulations are carried out for a wide range of Reynolds number ranging from the creeping region to rather high Reynolds number flow, considering flow in porous media. The simulations are based on as well a laminar flow formulation as a turbulent one where the turbulence model chosen is the Shear Stress Transport model, and the CFD-software used is ANSYS CFX with extra care regarding grid resolution and numerical iteration in order to secure that the numerical errors are sufficiently small. One result is that inertia-effects become significant already at Reynolds number of about 10, for the quadratic packing, but around 50 for the hexagonal arrangement and the region where the laminar simulations differ considerably from the turbulent calculations is dependent on the different array arrangements.

  • 62.
    Hellström, J. Gunnar I.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Mattsson, Hans
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Fluid mechanics of internal erosion in embankment dams2007Ingår i: Parallel computing of fluid flow through porous media, Luleå: Luleå tekniska universitet, 2007Kapitel i bok, del av antologi (Övrigt vetenskapligt)
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  • 63.
    Hellström, J. Gunnar I.
    et al.
    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.
    A numerical study of the flow near smolt guidance devices for increased fish migration2012Konferensbidrag (Refereegranskat)
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  • 64.
    Hellström, J. Gunnar I.
    et al.
    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.
    Numeriska strömningsberäkningar av smoltavledare för förbättrad fiskvandring2011Konferensbidrag (Refereegranskat)
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  • 65.
    Höller, J. Bastian
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, Anders G.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Integrating Downstream Ecological, Social and Economic Effects of Hydropower to Hydraulic Modeling: A Review2023Ingår i: World Journal of Mechanics, ISSN 2160-049X, E-ISSN 2160-0503, Vol. 13, nr 8, s. 149-172Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Hydropower gains increasing importance as a steerable and controllable power source in a renewable energy mix and deregulated markets. Although hydropower produces fossil-free energy, it has a significant impact on the local environment. This review investigates the effects of flow alterations by hydropower on the downstream river system and the possibilities to integrate these effects into hydraulic modeling. The results show that various effects of flow regulation on the ecosystem, but also social and economic effects on related communities were observed in the last decades. The application of hydraulic models for investigations of ecological effects is common. Especially hydraulic effects and effects on fish were extensively modeled with the help of hydraulic 1D- and 2D-simulations. Current applications to investigate social and economic effects integrated into hydraulic modeling are meanwhile limited. Approaches to realizing this integration are presented. Further research on the economic valuation of ecosystems and integration of social and economic effects to hydraulic models is necessary to develop holistic tools to support decision-making on sustainable hydropower.

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  • 66.
    Jonsson, Patrick
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andreasson, Patrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Smoothed Particle Hydrodynamic Simulation of Hydraulic Jump using Periodic Open Boundaries2016Ingår i: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 40, nr 19-20, s. 8391-8405Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The natural phenomena hydraulic jump that is commonly used in spillways as an energy dissipater coupled to hydropower applications has been investigated with Smoothed Particle Hydrodynamics. A new approach was applied based on a periodic open boundary condition. The model consisted of a tank, a gate, a stilling basin and periodic open boundaries at each end of the computational domain. The tank provided a hydraulic head and in turn a specific flow through the gate, and a downstream condition in terms of a depth for the jump. The gate elevation had a major impact and was calibrated to ensure a correct and stable flow rate, when compared to experiments. With the proper flow rate, the position of the jump toe was significantly improved. The jump toe oscillated with a frequency in good agreement with experimental findings found in the literature and the oscillation amplitude increased with Froude number. However, for high Froude number cases the position was still too close to the gate but could be improved by including a correction based on the length of the jump. The depths in both the super- and subcritical zones was in good agreement with experiments and previous numerical studies. Furthermore, the Froude number was in-line with the definition of super- and subcritical flows.

  • 67.
    Jonsson, Patrick
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Andreasson, Patrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    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.
    Smoothed particle hydrodynamics modellering av hydrauliska språng2011Konferensbidrag (Övrigt vetenskapligt)
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  • 68.
    Jonsson, Patrick
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Andreasson, Patrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modelling Dam Break Evolution over a Wet Bed with Smoothed Particle Hydrodynamics: A Parameter Study2015Ingår i: Engineering, ISSN 1947-3931, E-ISSN 1947-394X, Vol. 7, nr 5, s. 248-260Artikel i tidskrift (Refereegranskat)
  • 69.
    Jonsson, Patrick
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Andreasson, Patrik
    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.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Smoothed Particle Hydrodynamics Modeling of a Hydraulic Jump2012Konferensbidrag (Refereegranskat)
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  • 70.
    Jonsson, Patrick
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Andreasson, Patrik
    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.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Smoothed particle hydrodynamics modeling of hydraulic jumps2011Ingår i: Particle-based Methods – Fundamentals and Applications / [ed] E. Oñate; D.R.J. Owen, Barcelona: International Center for Numerical Methods in Engineering (CIMNE), 2011, s. 490-501Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    This study focus on Smoothed Particle Hydrodynamics (SPH) modeling of twodimensional hydraulic jumps in horizontal open channel flows. Insights to the complex dynamics of hydraulic jumps in a generalized test case serves as a knowledgebase for real world applications such as spillway channel flows in hydropower systems. In spillways, the strong energy dissipative mechanism associated with hydraulic jumps is a utilized feature to reduce negative effects of erosion to spillway channel banks and in the old river bed. The SPH-method with its mesh-free Lagrangian formulation and adaptive nature results in a method that handles extremely large deformations and numerous publications using the SPH-method for free-surface flow computations can be found in the literature. Hence, the main objectives with this work are to explore the SPH-methods capabilities to accurately capture the main features of a hydraulic jump and to investigate the influence of the number of particles that represent the system. The geometrical setup consists of an inlet which discharges to a horizontal plane with an attached weir close to the outlet. To investigate the influence of the number of particles that represents the system, three initial interparticle distances were studied, coarse, mid and fine. For all cases it is shown that the SPH-method accurately captures the main features of a hydraulic jump such as the transition between supercritical- and subcritical flow and the dynamics of the highly turbulent roller and the air entrapment process. The latter was captured even though a single phase was modeled only. Comparison of theoretically derived values and numerical results show good agreement for the coarse and mid cases. However, the fine case show oscillating tendencies which might be due to inherent numerical instabilities of the SPH-method or it might show a more physically correct solution. Further validation with experimental results is needed to clarify these issues.

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  • 71.
    Jourak, Amir
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    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.
    Derivation of dispersion coefficients in large random arrays of spheres and cylinders2012Konferensbidrag (Refereegranskat)
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  • 72.
    Jourak, Amir
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    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.
    Herrmann, Inga
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Arkitektur och vatten.
    Hedström, Annelie
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Arkitektur och vatten.
    Longitudinal dispersion coefficient: effects of particle-size distribution2013Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 99, nr 1, s. 1-16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effects of particle-size distribution on the longitudinal dispersion coefficient (DL) in packed beds of spherical particles are studied by simulating a tracer column experiment. The packed-bed models consist of uniform and different-sized spherical particles with a ratio of maximum to minimum particle diameter in the range of 1–4. The modified version of Euclidian Voronoi diagrams is used to discretize the system of particles into cells that each contains one sphere. The local flow distribution is derived with the use of Laurent series. The flow pattern at low particle Reynolds number is then obtained by minimization of dissipation rate of energy for the dual stream function. The value of DL is obtained by comparing the effluent curve from large discrete systems of spherical particles to the solution of the one-dimensional advection-dispersion equation. Main results are that at Peclet numbers above 1, increasing the width of the particle-size distribution increases the values of DL in the packed bed. At Peclet numbers below 1, increasing the width of the particle-size distribution slightly lowers DL.

  • 73.
    Jourak, Amir
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    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.
    Frishfelds, Vilnis
    Faculty of Natural Sciences, Liepaja University, Liela iela 14, Liepaja, LV-3401 Latvia.
    Numerical derivation of dispersion coefficients for flow through three-dimensional randomly packed beds of monodisperse spheres2014Ingår i: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 60, nr 2, s. 749-761Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The longitudinal (DL) and transverse (DT) dispersion coefficients for flow through randomly packed beds of discrete monosized spherical particles are studied. The three-dimensional (3-D) porous-medium model consists of thousands of spherical particles that are divided into cells using Voronoi diagrams. The relationship between the variation of the dual stream function and the vorticity between neighboring particles is derived using Laurent series. The whole flow pattern at low particle Reynolds number is then obtained by minimization of the dissipation rate of energy with respect to the dual stream function. The DL is obtained by fitting the resulting effluent curve to a 1-D solution of a continuous model. The DT is obtained by fitting the numerical concentration profile to an approximate 2-D solution. The derived DL and DT values are in agreement with 3-D experimental data from the literature enabling a study of the effects of pore structure and porosity on DL and DT.

  • 74.
    Jouybari, Nima Fallah
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media2020Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 81, artikel-id 108523Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the present study, the importance of the thermal dispersion and the turbulent heat flux in porous media and their effects on the macroscopic distribution of thermal energy are investigated. To this end, turbulent flow and heat transfer within five unit-cells mimicking porous media are solved using large eddy simulation. It is shown that the thermal dispersion and the turbulent heat flux are negligible as compared to the convection term in the macroscopic energy equation. When further scrutinizing this equation, it is revealed that except for the longitudinal components of the thermal dispersion, the other components of thermal dispersion and turbulent heat flux may be neglected away from the boundaries as compared to the interfacial heat transfer. Visualizations of vortices show that the size of the turbulence structures within the cells is of the same order as the size of the pores; therefore, the turbulent heat flux is limited to the intra-pore level. Finally, a discussion is provided on the accuracy of the gradient type diffusion model commonly used for turbulent heat flux in porous media in the absence of macroscopic turbulence. It is shown that the intra-pore turbulence does not affect the macroscopic transport of thermal energy within the porous media studied.

  • 75.
    Jouybari, Nima Fallah
    et al.
    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.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media2020Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 131, nr 2, s. 739-765Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

  • 76.
    Khayamyan, Shervin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Transitional and Turbulent Flow in a Bed of Spheres as Measured with Stereoscopic Particle Image Velocimetry2017Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 117, nr 1, s. 45-67Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Stereoscopic particle image velocimetry has been used to investigate inertia dominated, transitional and turbulent flow in a randomly packed bed of monosized PMMA spheres. By using an index-matched fluid, the bed is optically transparent and measurements can be performed in an arbitrary position within the porous bed. The velocity field observations are carried out for particle Reynolds numbers, (Formula presented.), between 20 and 3220, and the sampling is done at a frequency of 75 Hz. Results show that, in porous media, the dynamics of the flow can vary significantly from pore to pore. At (Formula presented.) around 400 the spatially averaged time fluctuations of total velocity reach a maximum and the spatial variation of the time-averaged total velocity, (Formula presented.) increases up to about the same (Formula presented.) and then it decreases. Also in the studied planes, a considerable amount of the fluid moves in the perpendicular directions to the main flow direction and the time-averaged magnitude of the velocity in the main direction, (Formula presented.), has an averaged minimum of 40% of the magnitude of (Formula presented.) at (Formula presented.) about 400. For (Formula presented.), this ratio is nearly constant and (Formula presented.) is on average a little bit less than 50% of (Formula presented.). The importance of the results for longitudinal and transverse dispersion is discussed.

  • 77.
    Khayamyan, Shervin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Measurements of Transitional and Turbulent Flow in a Randomly Packed Bed of Spheres with Particle Image Velocimetry2017Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 116, nr 1, s. 413-431Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Particle image velocimetry (PIV) has been used to investigate transitional and turbulent flow in a randomly packed bed of mono-sized transparent spheres at particle Reynolds number, (Formula presented.). The refractive index of the liquid is matched with the spheres to provide optical access to the flow within the bed without distortions. Integrated pressure drop data yield that Darcy law is valid at (Formula presented.). The PIV measurements show that the velocity fluctuations increase and that the time-averaged velocity distribution start to change at lower (Formula presented.). The probability for relatively low and high velocities decreases with (Formula presented.) and recirculation zones that appear in inertia dominated flows are suppressed by the turbulent flow at higher (Formula presented.). Hence there is a maximum of recirculation at about (Formula presented.). Finally, statistical analysis of the spatial distribution of time-averaged velocities shows that the velocity distribution is clearly and weakly self-similar with respect to (Formula presented.) for turbulent and laminar flow, respectively

  • 78. Khayamyan, Shervin
    et al.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    PIV measurements within a randomly packed bed of spheres2016Konferensbidrag (Refereegranskat)
  • 79.
    Knutsson, Sven
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    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.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Project: Internal erosion in embankment dams2010Övrigt (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    Since older embankment dams have a tendency to gradually display an increased degree of leakage it is interesting to find out if this phenomenon can be captured by an inverse modelling approach. Numerical tests with a model system will hopefully lead to a better understanding of the decomposition of material in dams and enhance the knowledge about the long term effects in order to forsee potential problems in the future.These embankment dams typically consist of a central core of moraine that is surrounded by a filter, a transition layer and a fill material. Ideally the seepage through the dam is relatively small and the effect on the dam from the flow is negligible. However, in reality there are several situations that promote a fatal failure of the dam including overtopping at high flood discharges, internal erosion and seepage problems in the embankment and the foundation. The status of embankment dams are generally examined by in-situ measurement of one or several of the following quantities: pore pressure, inclination and settlement, temperature, resistivity, self-potential, seepage rate, turbidity of the seepage water, ground penetration radar and bore hole tomography. These indirect and/or discrete methods give indication of the condition of the dam leaving the real situation for the imagination. Hence, when there are alterations to measured values great efforts are often spent on reinforcing dams in order to keep a high safety level. An increased knowledge of the composition of material in dams would enable the usage of focused measures reducing the costs and increasing the safety.

  • 80.
    Lundqvist, Hans
    et al.
    Swedish University of Agricultural Sciences, Department of Wildlife, Fish and Environmental Studies.
    Leonardsson, Kjell
    Swedish University of Agricultural Sciences, Department of Wildlife, Fish and Environmental Studies.
    Lindberg, Dan-Erik
    Swedish University of Agricultural Sciences, Department of Wildlife, Fish and Environmental Studies.
    Westbergh, Stig
    Ume- Vindelälvens Fiskeråd.
    Forssén, Åke
    Vattenfall Vattenkraft.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Laxens nedströmsvandring mot fiskavledare till Stornorrfors fisktrappa i Umeälvens nedre del2014Rapport (Övrigt vetenskapligt)
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  • 81.
    Lundström, Staffan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Lindmark, Elianne
    Flow design of guiding device for downstream fish migration2010Ingår i: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 26, nr 2, s. 166-182Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Downstream migrating smolt must be guided around hydropower plants to avoid fish mortality due to the turbines. In Piteå River, which is already regulated, open spillways serve this purpose but few fish find this route. Hence, action must be taken to enhance downstream fish migration. One way to attract the fish to the spillways is to direct the surface flow towards them by means of a guiding device. The hydrodynamic design of one such device is outlined using numerical calculations of the flow upstream the spillways and by the assumption that the fish moves near the surface of the water. A number of geometries are evaluated by starting from a straight impermeable barrier that extends 2 m down from the water surface and stretches over a part of the river. A major result is that it is possible to redirect the surface water towards the spillways at very low spilling rates which means high energy efficiency. Another finding was that the device should stretch over a large part of the river. For optimal functionality, the spilling should match the guiding device geometry. High spilling implies that the guiding has a low impact while for low spilling the geometry is crucial for successful downstream migration.

  • 82.
    Lundström, T. Staffan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Frishfelds, Vilnis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Transversal flow-induced deformation of fibres during composites manufacturing and the effect on permeability2013Ingår i: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 32, nr 15, s. 1129-1135Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Low Reynolds number flow of a viscous fluid through bundles in non-crimp stitched fabrics is considered for different arrangements of the fibres within the bundles. Using a previous derived model, two-dimensional Navier–Stokes solutions are sampled for the flow between a few fibres with a subsequent minimisation of the dissipation rate in the total system of fibres. Based on the detailed geometry of the fabrics flow induced elastic deformations of the fibre bundles are then derived and the overall permeability is computed for different pressure gradients. The permeability of random arrays of a large number of as well mono-dispersed as poly-dispersed fibres increases as the flow-induced deformation increases and despite the relative shift of the fibres is small the overall change in permeability is essential. For a system with gaps between bundles the change in permeability depends on the orientation of the flow field with respect to the geometry studied while for a regular packing the alteration in permeability is negligible.

  • 83.
    Lundström, T. Staffan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Rahkola, Mattias Brynjell
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Ljung, Anna-Lena
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Green, Torbjörn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Evaluation of Guiding Device for Downstream Fish Migration with In-field Particle Tracking Velocimetry and CFD2015Ingår i: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 8, nr 3, s. 579-589Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The performance of a fish guiding device located just upstream a hydropower plant is scrutinized. The device is designed to redirect surface orientated down-stream migrating fish (smolts) away from the turbines towards a spillway that act as a relatively safe fishway. Particles are added up-stream the device and the fraction particles going to the spillway is measured. A two-frame Particle Tracking Velocimetry algorithm is used to derive the velocity field of the water. The experimental results are compared to simulations with CFD. If the smolts move passively as the particles used in the study the guiding device works very well and some modifications may optimize its performance. In-field Particle Tracking Velocimetry is a suitable technique for the current case and the results compare well with numerical simulations.

  • 84.
    Mattsson, Hans
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    On internal erosion in embankment dams: a literature survey of the phenomenon and the prospect to model it numerically2008Rapport (Övrigt vetenskapligt)
    Abstract [en]

    The main objective with this literature survey is to elucidate the state of the art of internal erosion in embankment dams in order to be able to formulate a research program for numerical modelling of internal erosion in a physically sound manner. Since these processes normally are localised to specific zones in a dam, the ordinary continuum approach frequently utilised in soil modelling will not, by itself, be successful. The plan of the research group is therefore to treat internal erosion numerically as a type of localisation and describe the constitutive behaviour with micromechanical models in localised zones. In the next step, the internal erosion model developed will be implemented in a mathematical consistent fashion in a continuum model, based on e.g. the finite element method. In such software, ordinary computations of stresses, strains, deformations and pore pressures in an embankment dam can be performed; results which possible lead to conclusions about the initiation of internal erosion processes. When internal erosion is initiated, the micromechanical models will describe these processes in localised zones. It was decided to restrict the literature survey to areas important for the research direction defined above. The chosen areas are thus: numerical modelling of embankment dams, internal erosion processes, embankment dam failures and accidents, filter design, micromechanical models of hydro dynamical loads on single particles and system of particles as well as micromechanical models for friction between single grains and system of grains. The most essential findings for each area, in the context of the research, will be described below. There exists much experience in the field numerical modelling of embankment dams which is exemplified by the large amount of published papers on the subject. However, the processes of internal erosion have not been modelled in a general manner, as outlined in the research idea above, in any of the papers cited in this literature survey. There are three main internal erosion processes that can initiate piping: backward erosion, concentrated leak and suffusion. Piping can occur in the embankment, through the foundation and from the embankment into the foundation. Embankment dams are normally constructed in zones of different materials. Different compressibilities of the various zones might lead to internal redistribution of stresses and uneven settlements, which might cause cracks or "soft zones" where internal erosion can be initiated. Cracks can also occur later on by hydraulic fracturing. All of that described above are examples of mechanical processes that are theoretically possible to model numerically. To receive information about potential internal erosion problems in a dam at an early stage, geophysical methods are a promising alternative or complement to numerical methods. However, much research and development remain before the results from geophysical methods utilised for permanent monitoring and surveillance of dam structures are completely trustful. It is often very difficult to determine the exact reason for a dam accident or a failure since the processes involved have a tendency to destroy evidence which might have existed. Statistical data, however, shows that failure by overtopping and piping are the most common modes of failure while failure by slides is less common. Many piping failures occur very fast, leaving a short time for proper actions. The safety of large embankment dams is strongly dependent on the reliability of the performance of their critical filters. Existing filter design criteria are in most cases empirically derived, implying that a lot of knowledge can be gained by taking a mechanical approach to the problem. More research would also be desirable in the field of ageing effects of dams and uncertainties of core/filter appearance during and after accidents, incidents etc. For this, numerical modelling also seems to be a promising approach. Continuum formulations of flow through porous media often results in equations consisting of a few unknown parameters such as the permeability. The physical background of such parameters can often be traced to the detailed flow in the pores and it is therefore in place to study the flow on this level, as well. The forces on individual particles have been exploited for certain geometries and for a number of flow conditions. We however need to investigate further higher Reynolds number flows, more complex geometries and instationary conditions. The forces from the micromechanical models must be balanced by gravity and forces that emanates from particle interactions for the dam to be stable. As a first, and most simple, criteria the size distribution of the particles being subjected to hydrodynamic pressure is compared to the pore size distribution of the porous medium. Hence the particles will move if they are small enough independent on magnitude of the force on it. In reality, however, the hydraulic forces must exceed particle interaction forces and or gravitational forces keeping the particles in place at normal conditions. The forces are strongly dependent on the size of the particles and therefore dependent on different phenomena such as cohesion, adhesion and static friction. In the final part of the literature survey a concept for numerical modelling of internal erosion is presented based on ideas that have emerged from this work. The concept involves as well mathematical developments in order to formulate a micromechanical model for internal erosion as laboratory tests. The theoretical work and the practical laboratory work should be performed simultaneously and in an interactive manner. A software containing a model that simulates internal erosion could be useful for: an increase of the knowledge about internal erosion processes, evaluating the risk for dam incidents caused by internal erosion, estimating the time for progression of internal erosion and piping, studying self-healing of leaks, changes in filter behaviour subsequent to particle accumulation and ageing effects in dams, analysing the amount of instrumentation needed in a dam and the proper location for monitoring and surveillance as well as designing dams to mention a few examples. It is therefore apparent that the route suggested has a high potential to become a tool for future improvements of dam safety.

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  • 85.
    Mattsson, Hans
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Knutsson, Sven
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    On numerical modelling of internal erosion in embankment dams2009Ingår i: Proceedings 1st International Symposium on Rockfill Dams: October 18-21, 2009, Chengdu, China, 2009Konferensbidrag (Refereegranskat)
  • 86.
    Pérez-Ràfols, Francesc
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    A Stochastic Two-Scale Model for Rarefied Gas Flow in Highly Heterogeneous Porous Media2020Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 135, nr 1, s. 219-242Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents the development of a model enabling the analysis of rarefied gas flow through highly heterogeneous porous media. To capture the characteristics associated with the global- and the local-scale topology of the permeable phase in a typical porous medium, the heterogeneous multi-scale method, which is a flexible framework for constructing two-scale models, was employed. The rapid spatial variations associated with the local-scale topology are accounted for stochastically, by treating the permeability of different local-scale domains as a random variable. The results obtained with the present model show that an increase in the spatial variability in the heterogeneous topology of the porous medium significantly reduces the relevance of rarefaction effects. This clearly shows the necessity of considering a realistic description of the pore topology and questions the applicability of the results obtained for topologies exhibiting regular pore patterns. Although the present model is developed to study low Knudsen number flows, i.e. the slip-flow regime, the same development procedure could be readily adapted for other regimes as well.

  • 87.
    Risberg, Daniel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Westerlund, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Computational fluid dynamics simulation of indoor climate in low energy buildings computational set up2017Ingår i: Thermal Science, ISSN 0354-9836, E-ISSN 2334-7163, Vol. 21, nr 5, s. 1985-1998Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper CFD was used for simulation of the indoor climate in a part of a low energy building. The focus of the work was on investigating the computational set up, such as grid size and boundary conditions in order to solve the indoor climate problems in an accurate way. Future work is to model a complete building, with reasonable calculation time and accuracy. A limited number of grid elements and knowledge of boundary settings are therefore essential. An accurate grid edge size of around 0.1 m was enough to predict the climate according to a grid independency study. Different turbulence models were compared with only small differences in the indoor air velocities and temperatures. The models show that radiation between building surfaces has a large impact on the temperature field inside the building, with the largest differences at the floor level. Simpling the simulations by modelling the radiator as a surface in the outer wall of the room is appropriate for the calculations. The overall indoor climate is finally compared between three different cases for the outdoor air temperature. The results show a good indoor climate for a low energy building all around the year.

  • 88.
    Saber, Ammar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Mechanical Engineering Department, University of Mosul, Mosul 41001, Iraq.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Influence of Inertial Particles on Turbulence Characteristics in Outer and Near Wall Flow as Revealed With High Resolution Particle Image Velocimetry2016Ingår i: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 138, nr 9, artikel-id FE-15-1428Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A fully developed turbulent particle-gas flow in a rectangular horizontalchannel 100 × 10 × 4000 mm3 is disclosed with high spatial resolution2D Particle Image Velocimetry. The objective is to increase the knowledge ofthe mechanisms behind alterations in turbulent characteristics when adding twosets of relatively large solid spherical particles with mean diameters of 525and 755 μm and particle size distributions of 450 - 600 and 710 - 800 μm,respectively. Reynolds numbers are 4000 and 5600 and relatively high volumefraction of 5.4 × 10-4 and 8.0 × 10-4 are tested. Both thenear wall turbulent boundary layer flow and outer core flow are considered. Resultsshow that the carrier phase turbulent intensities increase with volume fractionof the inertial particles. The overall mean flowvelocity is affected when adding the particles but only to a minor extent. Nearthe wall, averaged velocity decreases while fluctuating velocity componentsincrease when particles are added to the flow. Quadrant analysis shows theimportance of sweep near the wall and ejection events in the region defined byy+ > 20. In conclusion, high inertia particles can enhanceturbulence even at relatively low Particle Reynolds number < 90. In the nearbottom wall region particles tend to be a source of instability reflected asenhancement in rms values of the normal velocity component.

  • 89.
    Sam, Ali al
    et al.
    Lund University, Division of Fluid Mechanics.
    Andersson, Anders
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Szász, Robert-Zoltán
    Lund University, Division of Fluid Mechanics.
    Hellström, Gunnar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Revstedt, Johan
    Lund University, Division of Fluid Mechanics.
    Measurement and simulation of turbulent flow over rough surface2014Konferensbidrag (Refereegranskat)
  • 90.
    Sjöstedt, Lovisa M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andersson, Anders G.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    CFD Modelling as a Tool to Better Understand Fish Behavior2023Ingår i: Proceedings of the 40th IAHR World Congress - 2023: Rivers - Connecting Mountains and Coasts / [ed] Helmut Habersack; Michael Tritthart; Lisa Waldenberger, International Association for Hydro-Environment Engineering and Research (IAHR) , 2023, s. 2665-2672Konferensbidrag (Refereegranskat)
  • 91.
    Teng, Penghua
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Johansson, Fredrik
    KTH Royal Institute of Technology, Stockholm, Sweden.
    Nilsson, Carl-Oscar
    Uniper – Sydkraft Hydropower AB, Östersund, Sweden.
    Modelling of erosion in rock spillway channels2023Konferensbidrag (Refereegranskat)
    Abstract [en]

    Erosion of rock channels downstream spillways may cause the significant deterioration of the spillways, raisingconcern for the safety of the dam. To evaluate the occurrence of rock erosion, this paper aims to employ a resolvedComputational Fluid Dynamics (CFD)-Discrete Element Method (DEM) approach to model the erosion process of asingle rock block and to investigate the effects of different parameters. The approach models the interaction behaviorsbetween flowing water and rock blocks. Simulation results visualize the erosion process of the rock block, whichprovides a reference to determine the threshold of the initiation of rock erosion. The results show that the incipientmotion of the rock block is significantly affected by the joint aperture. With the increase of the joint aperture size, thepressure distribution on the block surface is altered. Simulation results of the rough channel model are compared witha smooth model, which points out the influence of channel roughness on the rock erosion process.

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    fulltext
  • 92.
    Teng, Penghua
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Johansson, Fredrik
    Division of Soil and Rock Mechanics, KTH Royal Institute of Technology, Stockholm, SE-100 44, Sweden.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Modelling erosion of a single rock block using a coupled CFD-DEM approach2023Ingår i: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 15, nr 9, s. 2375-2387Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Rock block removal is the prevalent physical mechanism for rock erosion and could affect the stability of dam foundations and spillways. Despite this, understanding of block removal is still inadequate because of the complex interactions among block characteristics, hydraulic forces, and erosive processes acting on the block. Herein, based on a previously conducted physical experiment of erosion of a single rock block, the removal processes of two different protruding blocks are represented by a coupled computational fluid dynamics-discrete element model (CFD-DEM) approach under varied flow conditions. Additionally, the blocks could be rotated with respect to the flow direction to consider the effect of the discontinuity orientation on the block removal process. Simulation results visualize the entire block removal process. The simulations reproduce the effects of the discontinuity orientation on the critical flow velocity inducing block incipient motion and the trajectory of the block motion observed in the physical experiments. The numerical results present a similar tendency of the critical velocities at different discontinuity orientations but have slightly lower values. The trajectory of the block in the simulations fits well with the experimental measurements. The relationship between the dimensionless critical shear stress and discontinuity orientation observed from the simulations shows that the effect of block protrusion becomes more dominant on the block incipient motion with the increase of relative protrusion height. To our knowledge, this present study is the first attempt to use the coupled finite volume method (FVM)-DEM approach for modelling the interaction behavior between the block and the flowing water so that the block removal process can be reproduced and analyzed.

    Ladda ner fulltext (pdf)
    fulltext
  • 93.
    Trieu, Hang
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hellström, J. Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Andreasson, Patrik
    Wildlife, fish and environmental studies, Swedish University of Agricultural Sciences.
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Natural surface floaters in image-based river surface velocimetry: Insights from a case study2024Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Flow Measurement and Instrumentation, ISSN 0955-5986, Vol. 96, nr 102557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study focuses on utilizing image techniques for river velocity measurement, with a specific emphasis on natural surface floating patterns. Employing a multi-camera system, we conducted 3D measurements on river surfaces, including surface velocity and water surface reconstruction. A pattern-based tracking approach has been adopted to improve the performance of image measurements on different types of natural floating tracers. The study employs the following approaches: 3D Lagrangian Pattern Tracking Velocimetry (3D-LPTV), 2D Lagrangian Pattern Velocimetry (2D- LPTV), and Large-scale Particle Image Velocimetry (LSPIV), for surface velocity estimation. The outcomes revealed that all three approaches yielded consistent results in terms of averaged velocity. However, the LSPIV method produced about two times higher uncertainty in measured velocities compared to the other methods. A strategy to assess the quality of river surface patterns in velocity estimation is presented. Specifically, the sum of squared interrogation area intensity gradient (SSIAIG) was found to be strongly correlated with measurement uncertainty. Additionally, a term related to the peak sidelobe ratio (PSR) of the cross-correlation map was found as an effective constraint, ensuring the image-tracking process achieves high reliability. The precision of measurements increases corresponding to the increase of image intensity gradient and PSR.

  • 94.
    Trieu, Hang
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Hellström, J.Gunnar I.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Andreasson, Patrik
    Lycksam, Henrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Natural surface floaters in image-based river surface velocimetry: Insights from a case study2024Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 96, s. 102557-102557, artikel-id 102557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study focuses on utilizing image techniques for river velocity measurement, with a specific emphasis onnatural surface floating patterns. Employing a multi-camera system, we conducted 3D measurements on riversurfaces, including surface velocity and water surface reconstruction. A pattern-based tracking approach hasbeen adopted to improve the performance of image measurements on different types of natural floating tracers.The study employs the following approaches: 3D Lagrangian Pattern Tracking Velocimetry (3D-LPTV), 2DLagrangian Pattern Velocimetry (2D- LPTV), and Large-scale Particle Image Velocimetry (LSPIV), for surfacevelocity estimation. The outcomes revealed that all three approaches yielded consistent results in terms ofaveraged velocity. However, the LSPIV method produced about two times higher uncertainty in measured velocitiescompared to the other methods. A strategy to assess the quality of river surface patterns in velocityestimation is presented. Specifically, the sum of squared interrogation area intensity gradient (SSIAIG) was foundto be strongly correlated with measurement uncertainty. Additionally, a term related to the peak sidelobe ratio(PSR) of the cross-correlation map was found as an effective constraint, ensuring the image-tracking processachieves high reliability. The precision of measurements increases corresponding to the increase of image intensitygradient and PSR.

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