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Joybari, N. F. & Lundström, T. S. (2019). A Subgrid-Scale Model for Turbulent Flow in Porous Media. Transport in Porous Media, 129(3), 619-632
Open this publication in new window or tab >>A Subgrid-Scale Model for Turbulent Flow in Porous Media
2019 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 129, no 3, p. 619-632Article in journal (Refereed) Published
Abstract [en]

Given the analogy between the filtered equations of large eddy simulation and volume-averaged Navier–Stokes equations in porous media, a subgrid-scale model is presented to account for the residual stresses within the porous medium. The proposed model is based on the kinetic energy balance of the filtered velocity field within a pore; hence, when using the model, numerical simulations of the turbulent flow in the pores are not required. The accuracy of the model is validated with available data in the literature on turbulent flow through packed beds and staggered arrangement of square cylinders. The validation yields that the model successfully captures the effect of the pore-scale turbulent motion. The model is then used to study turbulent flow in a wall-bounded porous media to assess its accuracy.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Turbulence, Porous media, Volume average, LES, Subgrid-scale model
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-75782 (URN)10.1007/s11242-019-01296-w (DOI)000480750200001 ()2-s2.0-85070677723 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-08-30 (johcin)

Available from: 2019-08-30 Created: 2019-08-30 Last updated: 2019-08-30Bibliographically approved
Fallahjoybari, N. & Lundström, S. T. (2019). A Subgrid-Scale Model for Turbulent Flow in Porous Media. Transport in Porous Media, 129(3), 619-632
Open this publication in new window or tab >>A Subgrid-Scale Model for Turbulent Flow in Porous Media
2019 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 129, no 3, p. 619-632Article in journal (Refereed) Published
Abstract [en]

Given the analogy between the filtered equations of large eddy simulation and volume-averaged Navier–Stokes equations in porous media, a subgrid-scale model is presented to account for the residual stresses within the porous medium. The proposed model is based on the kinetic energy balance of the filtered velocity field within a pore; hence, when using the model, numerical simulations of the turbulent flow in the pores are not required. The accuracy of the model is validated with available data in the literature on turbulent flow through packed beds and staggered arrangement of square cylinders. The validation yields that the model successfully captures the effect of the pore-scale turbulent motion. The model is then used to study turbulent flow in a wall-bounded porous media to assess its accuracy.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Turbulence, Porous media, Volume average, LES, Subgrid-scale model
National Category
Engineering and Technology Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-76122 (URN)10.1007/s11242-019-01296-w (DOI)000480750200001 ()
Note

Validerad;2019;Nivå 2;2019-10-01 (johcin)

Available from: 2019-09-26 Created: 2019-09-26 Last updated: 2019-10-01Bibliographically approved
Wibron, E., Ljung, A.-L. & Lundström, S. (2019). Comparing Performance Metrics of Partial Aisle Containments in Hard Floor and Raised Floor Data Centers Using CFD. Energies, 12(8), Article ID 1473.
Open this publication in new window or tab >>Comparing Performance Metrics of Partial Aisle Containments in Hard Floor and Raised Floor Data Centers Using CFD
2019 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 8, article id 1473Article in journal (Refereed) Published
Abstract [en]

In data centers, efficient cooling systems are required to both keep the energy consumption as low as possible and to fulfill the temperature requirements. The aim of this work is to numerically investigate the effects of using partial aisle containment between the server racks for hard and raised floor configurations. The computational fluid dynamics (CFD) software ANSYS CFX was used together with the Reynolds stress turbulence model to perform the simulations. Velocity measurements in a server room were used for validation. Boundary conditions and the load of each rack were also retrieved from the experimental facility, implying an uneven load between the racks. A combination of the performance metrics Rack Cooling Index (RCI), Return Temperature Index (RTI) and Capture Index (CI) were used to evaluate the performance of the cooling systems for two supply flow rates at a 100% and 50% of operating condition. Based on the combination of performance metrics, the airflow management was improved in the raised floor configurations. With the supply flow rate set to operating conditions, the RCI was 100% for both raised floor and hard floor setups. The top- or side-cover fully prevented recirculation for the raised floor configuration, while it reduced the recirculation for the hard floor configuration. However, the RTI was low, close to 40% in the hard floor case, indicating poor energy efficiency. With the supply flow rate decreasing with 50%, the RTI increased to above 80%. Recirculation of hot air was indicated for all the containments when the supply rate was 50%, but the values of RCI still indicated an acceptable performance of the cooling system

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
data center, aisle containment, computational fluid dynamics (CFD), measurements, energy efficient
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-74676 (URN)10.3390/en12081473 (DOI)000467762600066 ()2-s2.0-85065507938 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-18 (johcin)

Available from: 2019-06-18 Created: 2019-06-18 Last updated: 2019-06-18Bibliographically approved
Xie, Q., Yang, J. & Lundström, S. (2019). Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load. Fluids, 4(1), Article ID 15.
Open this publication in new window or tab >>Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load
2019 (English)In: Fluids, ISSN 2311-5521, Vol. 4, no 1, article id 15Article in journal (Refereed) Published
Abstract [en]

Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both fresh-water flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2−5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with run-offs, lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
tidal meandering river, field measurements, 3D numerical model, flow features, sediment transport, erosion-deposition patterns
National Category
Water Engineering Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-72664 (URN)10.3390/fluids4010015 (DOI)000464467200002 ()
Note

Validerad;2019;Nivå 2;2019-01-31 (svasva)

Available from: 2019-01-24 Created: 2019-01-24 Last updated: 2019-04-30Bibliographically approved
Andersson, R., Hellström, J. G., Andreasson, P. & Lundström, S. (2019). Numerical investigation of a hydropower tunnel: Estimating localised head-loss using the manning equation. Water, 11(8), Article ID 1562.
Open this publication in new window or tab >>Numerical investigation of a hydropower tunnel: Estimating localised head-loss using the manning equation
2019 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 8, article id 1562Article in journal (Refereed) Published
Abstract [en]

The fluid dynamics within a water tunnel is investigated numerically using a RANS approach with the k-ε turbulence model. The computational model is based on a laser scan of a hydropower tunnel located in Gävunda, Sweden. The tunnel has a typical height of 6.9 m and a width of 7.2 m. While the average cross-sectional shape of the tunnel is smooth the local deviations are significant, where some roughness elements may be in the size of 5 m implying a large variation of the hydraulic radius. The results indicate that the Manning equation can successfully be used to study the localised pressure variations by taking into account the varying hydraulic radius and cross-sectional area of the tunnel. This indicates a dominant effect of the tunnel roughness in connection with the flow, which has the potential to be used in the future evaluation of tunnel durability. ANSYS-CFX was used for the simulations along with ICEM-CFD for building the mesh. 

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
ANSYS-CFX, Case-study, Head-loss, Hydropower, Rock tunnel, Surface roughness
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-75622 (URN)10.3390/w11081562 (DOI)000484561500036 ()2-s2.0-85070288117 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-08-21 (svasva)

Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-10-08Bibliographically approved
Bin Asad, S. M., Lundström, S., Andersson, A. G., Hellström, J. G. & Leonardsson, K. (2019). Wall shear stress measurement on curve objects with PIV in connection to benthic fauna in regulated rivers. Water, 11(4), Article ID 650.
Open this publication in new window or tab >>Wall shear stress measurement on curve objects with PIV in connection to benthic fauna in regulated rivers
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2019 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 4, article id 650Article in journal (Refereed) Published
Abstract [en]

The flow characteristics in the vicinity of a set of half-cylinders of different sizes simulating benthic objects were studied experimentally using particle image velocimetry (PIV). The cylinders were mounted on the bottom of an open channel, and the influence of the flow speed on the distribution of the shear stress along the bottom geometry was investigated. Of special interest was how the shear stress changes close to the wall as a function of the flow speed and cylinder arrangement. It was found that the shear stress varies significantly as a function of position. This implies habitat heterogeneity allowing benthic invertebrates with different shear stress tolerance exists when the bottom consists of differently sized stones. It was also shown that direct measurements of near wall velocity gradients are necessary to accurately calculate the wall shear stress for more complex geometries.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
shear stress, PIV, benthic fauna, river, stones
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-74913 (URN)10.3390/w11040650 (DOI)000473105700020 ()2-s2.0-85065032227 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-24 (johcin)

Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-08-16Bibliographically approved
Jouybari, N., Maerefat, M., Lundström, T. S., Eshagh Nimvari, M. & Gholami, Z. (2018). A General Macroscopic Model for Turbulent Flow in Porous Media. Journal of Fluids Engineering - Trancactions of The ASME, 140(1), Article ID 011201.
Open this publication in new window or tab >>A General Macroscopic Model for Turbulent Flow in Porous Media
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2018 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 140, no 1, article id 011201Article in journal (Refereed) Published
Abstract [en]

The present study deals with the generalization of a macroscopic turbulence model in porous media using a capillary model. The additional source terms associated with the production and dissipation of turbulent kinetic energy due to the presence of solid matrix are calculated using the capillary model. The present model does not require any prior pore scale simulation of turbulent flow in a specific porous geometry in order to close the macroscopic turbulence equations. Validation of the results in packed beds, periodic arrangement of square cylinders, synthetic foams and longitudinal flows such as pipes, channels and rod bundles against available data in the literature reveals the ability of the present model in predicting turbulent flow characteristics in different types of porous media. Transition to the fully turbulent regime in porous media and different approaches to treat this phenomenon are also discussed in the present study. Finally, the general model is modified so that it can be applied to lower Reynolds numbers below the range of fully turbulent regime in porous media.

Place, publisher, year, edition, pages
The American Society of Mechanical Engineers (ASME), 2018
National Category
Engineering and Technology Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-65199 (URN)10.1115/1.4037677 (DOI)000415379500009 ()2-s2.0-85029753712 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-10-03 (andbra)

Available from: 2017-08-19 Created: 2017-08-19 Last updated: 2018-11-27Bibliographically approved
Andersson, L. R., Larsson, S., Hellström, J. G., Andreasson, P., Andersson, A. G. & Lundström, S. (2018). Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations. Engineering, 10, 399-416
Open this publication in new window or tab >>Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations
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2018 (English)In: Engineering, ISSN 1947-3931, Vol. 10, p. 399-416Article in journal (Refereed) Published
Abstract [en]

High Reynolds number flow inside a channel of rectangular cross section is examined using Particle Image Velocimetry. One wall of the channel has been replaced with a surface of a roughness representative to that of real hydropower tunnels, i.e. a random terrain with roughness dimensions typically in the range of ≈10% - 20% of the channels hydraulic radius. The rest of the channel walls can be considered smooth. The rough surface was captured from an existing blasted rock tunnel using high resolution laser scanning and scaled to 1:10. For quantification of the size of the largest flow structures, integral length scales are derived from the auto-correlation functions of the temporally averaged velocity. Additionally, Proper Orthogonal Decomposition (POD) and higher-order statistics are applied to the instantaneous snapshots of the velocity fluctuations. The results show a high spatial heterogeneity of the velocity and other flow characteristics in vicinity of the rough surface, putting outer similarity treatment into jeopardy. Roughness effects are not confined to the vicinity of the rough surface but can be seen in the outer flow throughout the channel, indicating a different behavior than postulated by Townsend’s similarity hypothesis. The effects on the flow structures vary depending on the shape and size of the roughness elements leading to a high spatial dependence of the flow above the rough surface. Hence, any spatial averaging, e.g. assuming a characteristic sand grain roughness factor, for determining local flow parameters becomes less applicable in this case.

Place, publisher, year, edition, pages
Scientific Research Publishing, 2018
Keywords
CFD, Validation, Hydraulic Roughness, PIV, Hydropower
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71097 (URN)10.4236/eng.2018.107028 (DOI)
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2019-08-12Bibliographically approved
Wibron, E., Ljung, A.-L. & Lundström, S. (2018). Comparison of hard floor and raised floor cooling of servers with regards to local effects. In: IECON 2018: 44th Annual Conference of the IEEE Industrial Electronics Society. Paper presented at 44th Annual Conference of the IEEE Industrial Electronics Society, IECON 2018, October 21-23 2018, Washington D.C., USA..
Open this publication in new window or tab >>Comparison of hard floor and raised floor cooling of servers with regards to local effects
2018 (English)In: IECON 2018: 44th Annual Conference of the IEEE Industrial Electronics Society, 2018Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:ltu:diva-73013 (URN)10.1109/IECON.2018.8592885 (DOI)2-s2.0-85061534694 (Scopus ID)
Conference
44th Annual Conference of the IEEE Industrial Electronics Society, IECON 2018, October 21-23 2018, Washington D.C., USA.
Available from: 2019-02-26 Created: 2019-02-26 Last updated: 2019-02-26
Wibron, E., Ljung, A.-L. & Lundström, S. (2018). Computational Fluid Dynamics Modeling and Validating Experiments of Airflow in a Data Center. Energies, 11(3), Article ID 644.
Open this publication in new window or tab >>Computational Fluid Dynamics Modeling and Validating Experiments of Airflow in a Data Center
2018 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 11, no 3, article id 644Article in journal (Refereed) Published
Abstract [en]

The worldwide demand on data storage continues to increase and both the number and the size of data centers are expanding rapidly. Energy efficiency is an important factor to consider in data centers since the total energy consumption is huge. The servers must be cooled and the performance of the cooling system depends on the flow field of the air. Computational Fluid Dynamics (CFD) can provide detailed information about the airflow in both existing data centers and proposed data center configurations before they are built. However, the simulations must be carried out with quality and trust. The k–ɛ model is the most common choice to model the turbulent airflow in data centers. The aim of this study is to examine the performance of more advanced turbulence models, not previously investigated for CFD modeling of data centers. The considered turbulence models are the k–ɛ model, the Reynolds Stress Model (RSM) and Detached Eddy Simulations (DES). The commercial code ANSYS CFX 16.0 is used to perform the simulations and experimental values are used for validation. It is clarified that the flow field for the different turbulence models deviate at locations that are not in the close proximity of the main components in the data center. The k–ɛ model fails to predict low velocity regions. RSM and DES produce very similar results and, based on the solution times, it is recommended to use RSM to model the turbulent airflow data centers.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
data center, airflow, computational fluid dynamics (CFD), turbulence models
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-67958 (URN)10.3390/en11030644 (DOI)000428304300172 ()
Note

Validerad;2018;Nivå 2;2018-03-19 (andbra)

Available from: 2018-03-16 Created: 2018-03-16 Last updated: 2019-09-13Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-1033-0244

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