# Workshop 2024

## Wall-modeled LES Workshop on Smooth-Body Separation, part of the High-Fidelity CFD Verification Workshop aimed for 2024

This workshop directly follows the 2022 workshop held at the AIAA Scitech meeting on Jan 8-9, 2022, as part of the High-Fidelity CFD Workshop.

#### Focus of the workshop

While there are many interesting and important aspects of LES, this particular workshop is focused on the application of LES to wall-bounded flows at very high Reynolds numbers. As is well known, this requires the turbulence in the inner part of the boundary layer to be modeled rather than resolved. One of the most important challenges in such “wall-modeled LES” is the ability to accurately predict separated flows, specifically flows where the separation is not dictated by abrupt geometric changes (e.g., sharp corners). Therefore, the workshop will focus on a central test case that features an equilibrium incoming boundary layer, separation over a smoothly curved ramp, and reattachment. The test case will be run at Mach 0.2 in order to enable both compressible and incompressible codes to be used. The test case has further been chosen to strike a balance between physical realism and computational cost, where it is recognized that a limited computational cost implies both that a broader group of research groups can participate and (equally importantly) that participants can afford to compute on a sequence of refined grids in order to carefully assess grid convergence (or the lack thereof).
The purpose of the workshop is to provide a snapshot of the predictive capability of modern WMLES methods on smooth-body separation flows at high Reynolds numbers, to build a comparative database for use in future method assessments, and to advance our ability to perform verification of WMLES solvers.

#### Monthly videoconferences

Participants (and prospective participants, and interested observers) meet on the first Monday of every month at 12pm (noon) US Eastern Time. The Zoom link for these calls is https://umd.zoom.us/j/4487374796.

#### Notes from telecons (continued from the old page):

• 07/05/2022 (mini-workshop, 10am-2pm US EDT):
• 06/06/2022 (telecon):
• 05/02/2022 (telecon):
• At least 7 groups (AFRL, Calgary, Colorado, KTH, KTH/USC/UMD, USC/UMD, UMD/OX) are running new cases and will submit some new data for the July workshop.
• 2-4 groups are refining the grid while keeping the modeling interface fixed at $$h_{\rm wm}=0.1\delta_{\rm ref}$$.
• 2 groups are exploring wider domains.
• 2 groups are exploring variations to the inflow.
• 2 groups are exploring changing the grid resolution in parts of the domain and/or boundary layer to see where resolution has the largest effect.
• Data submission will be similar as for the January workshop, with uploads to a shared Google drive. It is crucial for participants to keep the file format identical to last time!
• We will focus more of the analysis on the region immediately before separation.
• 04/04/2022 (telecon):
• The summer mini-workshop will be by Zoom on Tuesday July 5 between 10am and 2pm, US Eastern time.
• Timofey Mukha will run with fixed $$h_{\rm wm}=0.1\delta_{\rm ref}$$ and multiple grids, to compare with the equivalent cases from the Kansas and USC groups. Others are encouraged to do the same thing.
• Sparsh Ganju showed their experiments with changing the inflow turbulence generation on the boundary layer development, concluding that they need an even longer inflow domain (and thus smaller incoming BL thickness).
• Sean Walters showed results from an experiment where he modified the wall-normal grid-spacing, and found results that changed minimally for two cases that had the same grid-spacing near the wall but different in the upper part of the boundary/shear-layer.
• Don Rizzetta described how they are re-running their coarsest WRLES case with a different grid behind the ramp (less aggressive stretching). So far, the results seem to not change much.
• Ivan Bermejo-Moreno shared his plans for running additional cases before July, including the flat plates.
• 03/07/2022 (telecon):
• Will hold a follow-up mini-workshop during the week of July 5-8. This will be via Zoom, for 3-4 hours starting at 10am US Eastern time. The focus will be on the exact same smooth ramp problem definition as was considered at the January workshop, with the additional focus of the flat plate solutions.
• To maximize the utility of the July mini-workshop, we agreed to:
• Focus more on comparing the flat plate results, both in terms of streamwise development, grid-convergence, and comparisons between the groups.
• Perform additional parametric studies in hopes of understanding the differences among the results, including:
• inflow boundary layer thickness;
• downstream grid resolution and/or outflow location;
• run with fixed exchange location at $$h_{\rm wm}=0.1\delta_{\rm ref}$$ and different grids.
• Assess the convergence of averages by analysing time histories or averages computed over shorter times.
• We ask participants to present briefly at the next telecon (April 4), specifically addressing:
• What do you think caused the differences between all results (or in your own results)?
• What additional simulations and/or analysis do you plan on completing before July?
• We agreed to submit an abstract summarizing the workshop to the 2023 Scitech meeting.
• 02/07/2022 (telecon):
• Post-workshop discussion about how to move forward
• Will continue with the smooth ramp, but should we continue with exactly the same case specification (advantage: existing results don’t need to be re-run) or should we consider adjustments (advantage: if making adjustments creates a better case, then likelier to become a commonly used verification case in the future). Potential adjustments that were discussed:
• Double the spanwise width. Reduces/removes the strong 2D vortex shedding dynamics which are unrealistic. Does not produce results that are independent of the spanwise width (much wider domains required for that). Should increase the total cost by 20-40% due to shorter averaging time required.
• Change inflow turbulence to a steady inflow with a sharp step or similar to reduce variability due to synthetic turbulence methods.
• Lower Reynolds number. Reduces the cost of WRLES or DNS. Current finest grids are close to WRLES limit, though, which would be a worse problem with lower Re.
• Discussed whether to add the Boeing speed bump as an additional case. Three groups have (or are close to having) DNS results for Re=2M. Is likely a more expensive case than the smooth ramp due to the slightly wider domain (14 delta compared to 4 delta). Advantage: truth data exists. Disadvantage: quite low Re (Re_tau of 500) which makes good WMLES grids close to WRLES.
• Current plan is do a 3-hour mini-workshop (for the LES case alone) online in July or September 2022, then another in early 2023, and then a full workshop in-person in summer 2023. Will continue telecons on the first Monday of every month.
• Need to develop a better work flow for data processing and figure creation. Could involve more standardized data format, spreading the work among more people.
• Agenda for next meeting:
• Smooth ramp: keep the same or make adjustments?
• Speed bump: should it be added, and if so, when do we add it?
• Pick date for first mini-workshop.