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B. Aupoix - One of the best experts on this subject based on the ideXlab platform.
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Improved heat transfer predictions on rough surfaces
International Journal of Heat and Fluid Flow, 2015Co-Authors: B. AupoixAbstract:Abstract The Equivalent Sand grain approach is the only approach to account for wall roughness in industrial CFD. As roughness effects are reproduced via a modification of the turbulence model in the wall region, the roughness correction preserves the Reynolds analogy. However, wall roughness increases much more the drag than the wall heat flux, so that the wall heat flux is overestimated with the Equivalent Sand grain approach. Due to the relative lack of detailed experimental data, the discrete element approach, which accounts for the different dynamical and thermal behaviours of wall roughness, was used to generate a large database to investigate thermal roughness effects. It turns out that, besides the Equivalent Sand grain height, another parameter has to be introduced to characterize roughness thermal effects. A correction of the turbulent Prandtl number was derived from the database and can be coupled to any roughness correction developed using ONERA’s technique. The thermal correction was validated for a wide range of roughness geometries, including academic roughness, in-service turbine blades and vanes and different ice shapes, for reduced Equivalent Sand grain heights k s + ranging from 10 to 6000, and for flows with pressure gradient. Heat transfer predictions are significantly improved, although heat transfer is generally still slightly overpredicted.
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Roughness Corrections for the k–ω Shear Stress Transport Model: Status and Proposals
Journal of Fluids Engineering, 2014Co-Authors: B. AupoixAbstract:Various corrections were previously proposed to account for wall roughness with the k–ω and shear stress transport (SST) models. A simplified analysis, based upon the wall region analysis, is proposed to characterize the behavior of these roughness corrections. As this analysis points out some deficiencies for each correction, two new corrections are proposed for the SST model, to reproduce different behaviors, mainly in the transition regime. The correction development is based upon a previously developed strategy. A large set of boundary layer experiments is used to compare the different roughness corrections, confirm the failures of previous proposals, and validate the present ones. Moreover, it assesses the proposed simplified analysis. It also evidences the difficulty to determine the Equivalent Sand grain roughness for a given surface. The Colebrook based correction is recommended while the Nikuradse based one can add information about the envelope of possible behaviors in the transition regime.
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Wall Roughness Modelling with k-w STT Model
2014Co-Authors: B. AupoixAbstract:Various corrections were previously proposed to account for wall roughness with the k-w and SST models. A simplified analysis, based upon the wall region analysis, is proposed to characterize the behaviour of these roughness corrections. As this analysis points out some deficiencies for each correction, two new corrections are proposed for the SST model, to reproduce different behaviours, mainly in the transition regime. The correction development is based upon a previously developed strategy. A large set of boundary layer experiments is used to compare the different roughness corrections, confirm the failures of previous proposals and validate the present ones. Moreover, it assesses the proposed simplified analysis. It also evidences the difficulty to determine the Equivalent Sand grain roughness for a given surface. The Colebrook based correction is recommended while the Nikuradse based one can add information about the envelope of possible behaviours in the transition regime.
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A general strategy to extend turbulence models to rough surfaces : Application to Smith's k-L model
Journal of Fluids Engineering, 2007Co-Authors: B. AupoixAbstract:A general procedure to extend turbulence models to account for wall roughness, in the framework of the Equivalent Sand grain approach, is proposed. It is based on the prescription of the turbulent quantities at the wall to reproduce the shift of the logarithmic profile and hence provide the right increase in wall friction. This approach was previously applied to Spalart and Allmaras one equation (1992, "A One-Equation Turbulence Model for Aerodynamic. Flows," 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA paper No. 92-0439; 1994, ibid, Rech. Aerosp. 1, pp. 5-21). Here, the strategy is detailed and applied to Smith's two-equation k-L model (1995, "Prediction of Hypersonic Shock Wave Turbulent Boundary Layer Interactions With The k-l Two Equaton Turbulence Model," 33rd Aerospace Sciences Meeting and Exhibit, Reno, NV, Paper No. 95-0232). The final model form is given. The so-modified Spalart and Allmaras and Smith models were tested on a large variety of test cases, covering a wide range of roughness and boundary layer Reynolds numbers and compared with other models. These tests confirm the validity of the approach to extend any turbulence model to account for wall roughness. They also point out the deficiency of some models to cope with small roughness levels as well as the drawbacks of present correlations to estimate the Equivalent Sand grain roughness.
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extensions of the spalart allmaras turbulence model to account for wall roughness
International Journal of Heat and Fluid Flow, 2003Co-Authors: B. Aupoix, Philippe R. SpalartAbstract:Abstract This paper describes extensions of the Spalart–Allmaras model to surface roughness, developed independently by Boeing and ONERA. They are rather simple and numerically benign, yield similar predictions, and are in fair agreement with experiments. They do not provide a description of the flow near the roughness elements, but rely instead on the “Equivalent Sand grain” approach. In that sense, they are not self-contained. The uncertain accuracy of the separate correlations, such as Dirling’s, needed to determine the Sand grain size presents a challenge, as always. The roughness height must be much smaller than the boundary layer thickness, but the full range of roughness Reynolds number is covered. Some test cases reveal an incompatibility between the predicted effect of roughness on heat transfer and on skin friction. i.e. if the Sand grain size is adjusted for skin friction, the heat transfer is too high.
Philippe R. Spalart - One of the best experts on this subject based on the ideXlab platform.
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extensions of the spalart allmaras turbulence model to account for wall roughness
International Journal of Heat and Fluid Flow, 2003Co-Authors: B. Aupoix, Philippe R. SpalartAbstract:Abstract This paper describes extensions of the Spalart–Allmaras model to surface roughness, developed independently by Boeing and ONERA. They are rather simple and numerically benign, yield similar predictions, and are in fair agreement with experiments. They do not provide a description of the flow near the roughness elements, but rely instead on the “Equivalent Sand grain” approach. In that sense, they are not self-contained. The uncertain accuracy of the separate correlations, such as Dirling’s, needed to determine the Sand grain size presents a challenge, as always. The roughness height must be much smaller than the boundary layer thickness, but the full range of roughness Reynolds number is covered. Some test cases reveal an incompatibility between the predicted effect of roughness on heat transfer and on skin friction. i.e. if the Sand grain size is adjusted for skin friction, the heat transfer is too high.
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EXTENSIONS OF THE SPALART–ALLMARAS TURBULENCE MODEL TO ACCOUNT FOR WALL ROUGHNESS
Engineering Turbulence Modelling and Experiments 5, 2002Co-Authors: B. Aupoix, Philippe R. SpalartAbstract:This paper describes extensions of the Spalart–Allmaras model to surface roughness, developed independently by Boeing and ONERA. They are rather simple and numerically benign, yield similar predictions, and are in fair agreement with experiments. They do not provide a description of the flow near the roughness elements, but rely instead on the “Equivalent Sand grain” approach. In that sense, they are not self-contained. The uncertain accuracy of the separate correlations, such as Dirling’s, needed to determine the Sand grain size presents a challenge, as always. The roughness height must be much smaller than the boundary layer thickness, but the full range of roughness Reynolds number is covered. Some test cases reveal an incompatibility between the predicted effect of roughness on heat transfer and on skin friction. i.e. if the Sand grain size is adjusted for skin friction, the heat transfer is too high.
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extensions of the spalart allmaras turbulence model to account for wall roughness
Engineering Turbulence Modelling and Experiments 5#R##N#Proceedings of the 5th International Symposium on Engineering Turbulence Modelling and Measure, 2002Co-Authors: B. Aupoix, Philippe R. SpalartAbstract:Extensions of the Spalart–Allmaras turbulence model to account for wall roughness were developed independently by Boeing and ONERA. They are rather simple, yield similar predictions, and are in fair agreement with experiments. Tests confirm the weakness of the “Equivalent Sand grain” approach, i.e. the uncertain accuracy of correlations to determine the Sand-grain size. Some test cases reveal an incompatibility between the predicted effect of roughness on heat transfer and on skin friction, i.e., if the Sand-grain size is adjusted for skin friction, the heat transfer will be too high.
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EXTENSIONS OF THE SPALART–ALLMARAS TURBULENCE MODEL TO ACCOUNT FOR WALL ROUGHNESS
Engineering Turbulence Modelling and Experiments 5, 2002Co-Authors: B. Aupoix, Philippe R. SpalartAbstract:Extensions of the Spalart–Allmaras turbulence model to account for wall roughness were developed independently by Boeing and ONERA. They are rather simple, yield similar predictions, and are in fair agreement with experiments. Tests confirm the weakness of the “Equivalent Sand grain” approach, i.e. the uncertain accuracy of correlations to determine the Sand-grain size. Some test cases reveal an incompatibility between the predicted effect of roughness on heat transfer and on skin friction, i.e., if the Sand-grain size is adjusted for skin friction, the heat transfer will be too high.
Zhixiang Xiao - One of the best experts on this subject based on the ideXlab platform.
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Distributed roughness induced transition on wind-turbine airfoils simulated by four-equation k-ω-γ-Ar transition model
Renewable Energy, 2019Co-Authors: Muchen Yang, Zhixiang XiaoAbstract:The fourth transport equation for “roughness amplification” factor Ar, which depends on Equivalent Sand grain roughness height ks, has been combined with the original three-equation k-ω-γ transition model. According to linear stability theory, the effective length scale of original k-ω-γ model is amplified through Ar, which could enhance the 1st mode time scale and lead to earlier transition. The new model is calibrated and validated by several cases with available experimental data, including flat plate, some wind-turbine airfoils with different patterns of distributed surface roughness. After careful comparisons with the measurements, the new four-equation transition model performs very well and satisfactory results have been achieved.
Karen A Flack - One of the best experts on this subject based on the ideXlab platform.
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velocity defect scaling for turbulent boundary layers with a range of relative roughness
Experiments in Fluids, 2006Co-Authors: Jonathan Connelly, Michael P Schultz, Karen A FlackAbstract:Velocity profile measurements in zero pressure gradient, turbulent boundary layer flow were made on a smooth wall and on two types of rough walls with a wide range of roughness heights. The ratio of the boundary layer thickness (δ) to the roughness height (k) was 16≤δ/k≤110 in the present study, while the ratio of δ to the Equivalent Sand roughness height (k s) ranged from 6≤δ/k s≤91. The results show that the mean velocity profiles for all the test surfaces agree within experimental uncertainty in velocity-defect form in the overlap and outer layer when normalized by the friction velocity obtained using two different methods. The velocity-defect profiles also agree when normalized with the velocity scale proposed by Zagarola and Smits (J Fluid Mech 373:33–70, 1998). The results provide evidence that roughness effects on the mean flow are confined to the inner layer, and outer layer similarity of the mean velocity profile applies even for relatively large roughness.
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experimental support for townsend s reynolds number similarity hypothesis on rough walls
Physics of Fluids, 2005Co-Authors: Karen A Flack, Michael P Schultz, Thomas A ShapiroAbstract:The Reynolds number similarity hypothesis of Townsend [The Structure of Turbulent Shear Flow (Cambridge University Press, Cambridge, UK, 1976)] states that the turbulence beyond a few roughness heights from the wall is independent of the surface condition. The underlying assumption is that the boundary layer thickness δ is large compared to the roughness height k. This hypothesis was tested experimentally on two types of three-dimensional rough surfaces. Boundary layer measurements were made on flat plates covered with Sand grain and woven mesh roughness in a closed return water tunnel at a momentum thickness Reynolds number Reθ of ∼14000. The boundary layers on the rough walls were in the fully rough flow regime (ks+⩾100) with the ratio of the boundary layer thickness to the Equivalent Sand roughness height δ∕ks greater than 40. The results show that the mean velocity profiles for rough and smooth walls collapse well in velocity defect form in the overlap and outer regions of the boundary layer. The Reyn...
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Experimental support for Townsend’s Reynolds number similarity hypothesis on rough walls
Physics of Fluids, 2005Co-Authors: Karen A Flack, Michael P Schultz, Thomas A ShapiroAbstract:The Reynolds number similarity hypothesis of Townsend [The Structure of Turbulent Shear Flow (Cambridge University Press, Cambridge, UK, 1976)] states that the turbulence beyond a few roughness heights from the wall is independent of the surface condition. The underlying assumption is that the boundary layer thickness δ is large compared to the roughness height k. This hypothesis was tested experimentally on two types of three-dimensional rough surfaces. Boundary layer measurements were made on flat plates covered with Sand grain and woven mesh roughness in a closed return water tunnel at a momentum thickness Reynolds number Reθ of ∼14000. The boundary layers on the rough walls were in the fully rough flow regime (ks+⩾100) with the ratio of the boundary layer thickness to the Equivalent Sand roughness height δ∕ks greater than 40. The results show that the mean velocity profiles for rough and smooth walls collapse well in velocity defect form in the overlap and outer regions of the boundary layer. The Reyn...
Muchen Yang - One of the best experts on this subject based on the ideXlab platform.
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Distributed roughness induced transition on wind-turbine airfoils simulated by four-equation k-ω-γ-Ar transition model
Renewable Energy, 2019Co-Authors: Muchen Yang, Zhixiang XiaoAbstract:The fourth transport equation for “roughness amplification” factor Ar, which depends on Equivalent Sand grain roughness height ks, has been combined with the original three-equation k-ω-γ transition model. According to linear stability theory, the effective length scale of original k-ω-γ model is amplified through Ar, which could enhance the 1st mode time scale and lead to earlier transition. The new model is calibrated and validated by several cases with available experimental data, including flat plate, some wind-turbine airfoils with different patterns of distributed surface roughness. After careful comparisons with the measurements, the new four-equation transition model performs very well and satisfactory results have been achieved.