UFR 3-34 Best Practice Advice: Difference between revisions
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* For the considered UFR characterized by high Reynolds numbers, no transition modelling is required. | * For the considered UFR characterized by high Reynolds numbers, no transition modelling is required. | ||
=== Near-wall modelling === | === Near-wall modelling === | ||
* For the considered test case, using hybrid methods with wall modelling is highly recommended, since they ensure a huge saving of computer resources compared to the WRLES and are not inferior to the latter in terms of accuracy, at least as far as the mean flow characteristics are concerned. However, this recommendation is not “general” since for other, more complex, flows belonging to the same UFR, WMLES may not be accurate enough (see, e.g., [39]). | * For the considered test case, using hybrid methods with wall modelling is highly recommended, since they ensure a huge saving of computer resources compared to the WRLES and are not inferior to the latter in terms of accuracy, at least as far as the mean flow characteristics are concerned. However, this recommendation is not “general” since for other, more complex, flows belonging to the same UFR, WMLES may not be accurate enough (see, e.g., [‌[[UFR_3-34_References#39|39]]]). | ||
* Effect of the specific choice of a wall model for LES has not been investigated, but the choice of the IDDES model may be safely recommended. | * Effect of the specific choice of a wall model for LES has not been investigated, but the choice of the IDDES model may be safely recommended. | ||
== Numerical Issues == | == Numerical Issues == | ||
* In the global WMLES/WRLES, in the focus region of non-zonal hybrid RANS-LES simulations including a separation bubble and an initial part of the reattached turbulent boundary layer, and in the part of the attached boundary layer prior to separation in the LES zone of the zonal approaches, use numerics with as low as possible numerical dissipation, particularly, pure or close to pure central difference schemes for convective fluxes with not less than 2nd order of accuracy. | * In the global WMLES/WRLES, in the focus region of non-zonal hybrid RANS-LES simulations including a separation bubble and an initial part of the reattached turbulent boundary layer, and in the part of the attached boundary layer prior to separation in the LES zone of the zonal approaches, use numerics with as low as possible numerical dissipation, particularly, pure or close to pure central difference schemes for convective fluxes with not less than 2nd order of accuracy. | ||
Revision as of 15:54, 30 November 2017
Semi-Confined Flows
Underlying Flow Regime 3-34
Best Practice Advice
Key Physics
Key physical features of the UFR in question are: separation of the turbulent boundary layer from a smooth surface driven by adverse pressure gradient, a rapid development of three-dimensional turbulent structures in the separated shear layer, its reattachment to the plane wall, and further relaxation of the reattached turbulent boundary layer to the “normal” state.
Physical Modelling Issues
Turbulence modelling
Based on the conclusions formulated above the following advice for the computations of the considered UFR may be given.
Use turbulence-resolving approaches (hybrid RANS-LES or global WMLES and WRLES), since none of the currently available RANS turbulence models, either linear eddy-viscosity or RSM, ensures capturing of the challenging physical features of the UFR indicated above. However a success of the scale-resolving approaches is also not guaranteed. In order to reach it:
- Impose as realistic as possible turbulent content at the inflow of LES zone within zonal RANS-LES approaches.
- Ensure a rapid development of 3D turbulent structures in the separated shear layer within nonzonal hybrid approaches (this demands employing of special grey-area mitigation tools).
- Use sufficiently wide domains for simulation of nominally 2D geometries with periodic boundary conditions in the uniform direction.
Whether all the above recommendations are really implemented in a simulation should be checked by:
- Visualizing the unsteady solutions (see, e.g., Figs. 13, 14) and getting a visual impression on the length of the RANS-to-LES transition in the zonal RANS LES simulations, on 3-dimensionality of the early region of the separated shear layer in the non-zonal hybrid simulations, and on sufficiency of the domain width for representation of large-scale structures showing up in a flow field.
Transition modelling
- For the considered UFR characterized by high Reynolds numbers, no transition modelling is required.
Near-wall modelling
- For the considered test case, using hybrid methods with wall modelling is highly recommended, since they ensure a huge saving of computer resources compared to the WRLES and are not inferior to the latter in terms of accuracy, at least as far as the mean flow characteristics are concerned. However, this recommendation is not “general” since for other, more complex, flows belonging to the same UFR, WMLES may not be accurate enough (see, e.g., [39]).
- Effect of the specific choice of a wall model for LES has not been investigated, but the choice of the IDDES model may be safely recommended.
Numerical Issues
- In the global WMLES/WRLES, in the focus region of non-zonal hybrid RANS-LES simulations including a separation bubble and an initial part of the reattached turbulent boundary layer, and in the part of the attached boundary layer prior to separation in the LES zone of the zonal approaches, use numerics with as low as possible numerical dissipation, particularly, pure or close to pure central difference schemes for convective fluxes with not less than 2nd order of accuracy.
Contributed by: E. Guseva, M. Strelets — Peter the Great St. Petersburg Polytechnic University (SPbPU)
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