WMLES

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

This workshop will be held at the AIAA Scitech meeting on Jan 6-7, 2024. It is part of the High-Fidelity CFD Verification 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).
The workshop will focus on 3 test cases, all 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. If you want to receive e-mail updates (approximately on a monthly basis), please add your name to this Google sheet.

Notes from telecons (continued from the old page):

• 02/04/2024 (post-workshop updates):
  • The slides describing the smooth ramp results from the workshop are available here. Note that the post-processing issue from David Flad has been fixed in these slides, but not yet the post-processing issue from the UMD-CHAMPS results — this will be added soon.
  • We will resume our telecons on Monday March 4, 2024, at noon Eastern time.
• 12/04/2023 (telecon):
  • The required averaging time for the smooth ramp was discussed, and it was agreed that averages converge very slowly in the separated region. Therefore, participants are encouraged to estimate the averaging standard error and include this in their submitted results. One method to estimate this to use the Matlab script stdErrorEstimate.m available at https://larsson.umd.edu/code/, which is based on the method of Russo & Luchini (JCP 2017).
• 11/06/2023 (telecon):
  • The data submission guidelines for the smooth ramp case are available at https://drive.google.com/file/d/1dekssSSpXL4vW28WBmHj3wHp1pL_uoy8/view?usp=share_link. Please follow this exactly to simplify the work on creating comparison plots. To submit data for the smooth ramp, go to https://drive.google.com/drive/folders/1PubyxF7lQw-sDJQiUq2PETYZAyYY7N-v?usp=share_link and create a directory with your name, and upload your files there.
  • The data submission guidelines for the airfoil case are simple: please submit the surface data and boundary layer profile data at the same locations and in exactly the same format (same columns etc) as the WRLES reference data available at https://www.klab.mech.tohoku.ac.jp/database/index.html. Submit the data by creating a directory with your name at https://drive.google.com/drive/folders/11aE5Ytt0vvRnMoaXr-wH3MQThhXRrC8M?usp=share_link.
• 10/02/2023 (telecon):
  • The two test cases (smooth ramp and A-airfoil) have six participants each at the moment.
  • Sparsh Ganju provided clarification on the angle-of-attack for the A-airfoil: the 13.3 degrees are measured from the x-axis of the geometry, NOT from the chord line of the airfoil. Sparsh also clarified that the XFoil calculations in the Tamaki paper were run at Mach 0.15.
  • The plan going forward is to discuss progress on these test cases on Nov 6, and then again on Dec 4.
  • The submission deadline will be around Dec 15. The exact format will be specified at the Nov 6 meeting, but ask for ASCII files of profiles with one file per location.
• 09/11/2023: MOVED TO 09/11/2023 DUE TO LABOR DAY
  • Sparsh Ganju described his early experiments with the airfoil case, including questions about the actual angle-of-attack of the problem (discrepancy between his XFoil results and those in the paper).
• 08/14/2023 (telecon):
  • Ali Uzun has finished his DNS of the smooth ramp case. He has released data at 4 locations (the inflow and 3 locations upstream of the ramp) that participants can use to tune their inflow boundary condition to reproduce the incoming turbulent boundary layer — the released data can be found at https://drive.google.com/drive/folders/1mSYS1ogIpKhdPyt9VlT_U1PDHTE3E8RP?usp=sharing.
  • The overall case description for the smooth ramp test case can be found here: https://drive.google.com/file/d/1umJCwcaeoqpn1ETCai4Q6i5f4vSYdUdt/view?usp=share_link.
  • Johan Larsson shared some preliminary results for the smooth ramp on a too-coarse grid, showing a very small separated region. Will pursue grid refinement during the coming month.
• 07/03/2023: CANCELLED DUE TO INDEPENDENCE DAY WEEKEND
• 06/05/2023 (telecon):
  • Ali Uzun gave an update on his DNS of the smooth ramp case. He started averaging after about 10 flow times (normalized by the inflow velocity and the ramp length) and has so far averaged for about 10 flow times. Ali will provide data in the incoming boundary layer (at multiple stations) but will keep the rest of the DNS data blind until the workshop.
• 05/01/2023 (telecon): Cancelled
• 04/03/2023 (telecon): next meeting
  • Ali Uzun described his DNS of the smooth ramp case. His slides can be found here. He expects to have results by early May, at which point he will extract profiles at multiple stations in the incoming boundary layer to enable people to match the incoming flow.
  • The inflow used in the DNS of the smooth ramp is taken from the KTH database at https://www.mech.kth.se/~pschlatt/DATA/, specifically the case at \( Re_\theta = 2000 \). Note that this inflow profile may *not* necessarily be suited to use by every participant, since the turbulent boundary layer will develop differently depending on synthetic turbulence generation method!
  • Some useful prior studies on the airfoil case: Tamaki and Kawai (WRLES at Re=10M), Tamaki et al (WMLES at Re=2M), Gao et al (WMLES at Re=2M).
• 03/06/2023 (telecon):
  • About 7 participants expressed commitment to participate in the next workshop, several others expressed hope or desire.
  • The final specifications for all test cases will be made during the Spring, after the DNS (smooth ramp) and preliminary WMLES (airfoil) results are in.
  • The official coordinates for the airfoil case are available at https://www.klab.mech.tohoku.ac.jp/database/2022TamakiAIAAJ/airfoil.txt.
  • We will aim for community grids for the airfoil; for the smooth ramp, the old community grids should still be valid.
  • Going forward, we will try to present preliminary results during each month.
• 02/06/2023: CANCELLED SINCE WE JUST MET AT SCITECH
• 01/02/2023: CANCELLED DUE TO NEW YEAR’S DAY
• 12/05/2022 (telecon):
  • Discussed the future airfoil case, essentially decided to go with the \( Re_c = 10^7 \) case of Tamaki and Kawai (AIAA J 2022).
  • Will meet next at Scitech, both at the LES DG (Monday Jan 23 at 6m) and at the Special Session focused on the workshop (Wednesday Jan 25 at 9:30am).
• 11/07/2022 (telecon):
  • Discussed the Scitech conference paper draft.
  • Decided to start focusing more on results and plots during the telecons, and thus decided that we don’t need the January 2023 virtual “mini-workshop”.
• 10/03/2022 (telecon):
  • Discussed desired characteristics of a potential DNS study of the smooth ramp. Main discussion point was: should the DNS mimic the set-up used during the past workshop (\( L_z = 4 \delta_{\rm ref} \)) or should it use a re-defined case specification with a larger spanwise domain to make the flow physics more healthy?
• 09/12/2022 (telecon, 2nd Monday of the month due to Labor Day):
  • Going forward we will start sending e-mail updates on an approximately monthly basis. If you want to receive these e-mails, please enter your details in this Google sheet.
  • We will attempt to write a Scitech 2023 paper documenting the main parts of the workshops in Jan and July of 2022. It will be written by a sub-set of of organizers and then circulated among the participants.
• 08/01/2022 (telecon): CANCELLED DUE TO VACATIONS
• 07/05/2022 (mini-workshop, 10am-2pm US EDT):
  • 6 groups submitted new data which was compared and discussed.
  • We will do our next virtual mini-workshop on Friday January 13 at 10am EST. The focus will be to really close the book on the flat plate test case, and to give participants an opportunity to submit new results on the smooth ramp with finer grids and inflows that better match at the reference station.
• 06/06/2022 (telecon):
  • Reminder to everyone to upload data to the Google drive (https://drive.google.com/drive/folders/1eX77eEBODNU-4z2LbCiEY4torAgsXQH-?usp=sharing) at least one week before the July 5 workshop and e-mail Ivan and Johan to let them know that your data has been uploaded.
  • When uploading data, please keep the flat plate case separate from the full ramp case.
  • We will repeat the same plots as in January, and will also plot additional profiles in the region of \( 0 \leq x/L \leq 0.4\) immediately before separation.
• 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.