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FREQUENTLY ASKED QUESTIONS

4th AIAA CFD High Lift Prediction Workshop

and

3rd Geometry and Mesh Generation Workshop


 
Logo 1 for HLPW-4

 
 

Q1: Why is HLPW-4 being conducted differently than previous high-lift workshops?

A1: Bottom line: to try to get more out of the workshop. In an attempt to accelerate the rate of progress, and to take better advantage of the collective brain-power focused on enhancing CFD prediction capability for practical high-lift aerodynamics, the committee decided to try something new for HLPW-4. Rather than working individually, active participants are asked to join one or more Technology Focus Groups during the months leading up to the workshop. Also, HLPW is again co-locating with the Geometry and Mesh Generation Workshop (GMGW) because of the important interplay of the technologies. We are hoping that the new way of operating the workshop will enhance this collaboration. As a side note, starting with HLPW-4 we are changing the workshop acronym from HiLiftPW to HLPW. However, the workshop's website name still uses "hiliftpw".


 

Q2: What are Technology Focus Groups, and how will they work?

A2: A Technology Focus Group (TFG) is made up of ACTIVE PARTICIPANTS working (remotely) together in a specific area, to try to answer questions related to CFD prediction of high-lift flows. These specific areas are currently defined to be the following: Each member of a TFG will contribute to team activities, as delineated by the TFG leaders. These activities will likely involve performing CFD or meshing work, and sharing it with the TFG team. (It is possible that each TFG may be conducted somewhat differently.) At the HLPW-4 workshop itself, each TFG will present a summary of its results. In other words, HLPW-4 is similar to previous workshops in that active participants will still perform research/work on their own. However, it is different in the following ways:
  1. The work is now part of a broader team's collective efforts.
  2. Rather than reporting results individually at the workshop, active participants are interacting regularly with their TFG, and sharing their results with the TFG in the months prior to the workshop. Individual active participants are no longer responsible for making a presentation at the workshop.
  3. Instead, the TFGs are responsible for summarizing their collective results and presenting them at the workshop.
It is hoped that this new experimental way of conducting the workshop will lead to (1) more up-front learning and (2) more time for discussions and rapid dissemination of ideas at the workshop.


 

Q3: How can I attend the workshop if I am not presenting my own work in an individual presentation?

A3: It is hoped that organizations will still encourage their employees to attend the workshop in order to see all of the TFG summaries, and to actively participate in the workshop discussions.


 

Q4: Do I have to join a TFG to attend HLPW-4?

A4: No. If you are not a member of a TFG, then your status is OBSERVER. Observers are free to run the test cases, use the grids, etc. But their results will not be included in the collective summaries presented at the workshop. Observers can also do nothing - just observe, because they are interested. Like everyone else, observers can attend the HLPW-4 workshop and join in the discussions there. In other words: Anyone, including active participants and observers, may attend HLPW-4 (subject to possible space availability/restrictions).


 

Q5: What do I need to do to attend the workshop?

A5: To attend the workshop, both active participants and observers must register for HLPW-4 with AIAA. AIAA will also give details regarding travel and accomodations, visas, etc. Note that registering for the AIAA Aviation conference alone does not give you access to HLPW-4 (and vice versa). You must specifically register for HLPW-4, and pay its additional registration fee.


 

Q6: How is COVID-19 affecting HLPW-4?

A6: Due to the inability to meet in person in 2021 because of the COVID-19 pandemic, the HLPW-4 workshop (as well as the concurrent GMGW-3 workshop) has been moved to the end of the AIAA SciTech meeting in early January 2022. The workshop will be IN-PERSON, occuring on Friday, January 7, 2022. Furthermore, leading up to the workshop, a new Workshop Progress Meeting (virtual format) is now planned for July 15-16, 2021. This meeting will bring all of the TFGs together for an assessment of progress and determination of areas requiring more focus prior to the January in-person event. In other words, even though COVID-19 has forced the in-person workshop to be delayed from June 2021 to January 2022, work within the TFGs associated with HLPW-4 is continuing without pause.


 

Q7: What happened at the first three high-lift prediction workshops?

A7: The first HiLift workshop took place in June 2010. See Webpage for HiLiftPW-1 for detailed information. The second HiLift workshop took place in June 2013. See Webpage for HiLiftPW-2 for detailed information. The third HiLift workshop took place in June 2017. See Webpage for HiLiftPW-3 for detailed information.


 

Q8: By what date will CFD results need to be completed?

A8: Because this workshop is being conducted so differently from the previous ones, this is a difficult question to answer. We now have the TFGs bringing participants together to collaborate well ahead of the workshop date. Each TFG has its own unique challenges, so may operate somewhat differently (and on potentially different schedules) from the others. However, all TFGs will no doubt require results from its members at intervals between now and the workshop. They will likely evaluate how things are going via regular team meetings, and will work interactively with all its members to answer a set of "key questions" prior to the workshop.


 

Q9: What should I do if I find "problem areas" in the geometry files?

A9: If you encounter any problems/issues as you build grids using the geometry files, please make your own fixes, DOCUMENT everything done, and plan to share your fixes with your TFG. Note that the geometry includes regions that may be very difficult to grid (and possibly solve), including very thin gaps at the bottom/inside edge of the slats. These were present in the wind tunnel configuration, so they were left in. It is up to you how to handle such areas; this is part of the challenge of the workshop. Again, be sure to document every assumption/change/fix that you make, as your grids are created.


 

Q10: It is confusing that the test cases use full-scale geometry yet wind-tunnel-scale Reynolds number (Re). How do I reconcile this, and how do I set Re if my code does not have Re as an input?

A10: It is particularly important that you set conditions to match the prescribed Reynolds number and Mach number of the test cases. These two parameters uniquely define the flow conditions. If your CFD code does not take Reynolds number (Re) as an input, then (preferably) kinematic viscosity should be adjusted in order to achieve the precise Re requested in the Test Cases Document. The Re is given based on mean aerodynamic chord (MAC): ReMAC=5,490,000. The MAC is 275.8 inches (full scale). Thus, when you run your CFD code on a grid based on the full scale configuration, the Re per inch should come out to ReMAC/275.8 = 19,905.7288. (If you scale the grid by 10% so that the configuration is at model scale instead, then the MAC would be one-tenth as large: 27.58 inches. The ReMAC would still be exactly the same, at 5,490,000, but now the Re per inch would be 199,057.288.)


 

Q11: Why do the meshing guidelines suggest minimum spacing y+ values greater than one for some of the grids?

A11: The current meshing guidelines are designed to try to achieve a "mesh family." This is done so that grid-convergence plots will be easier to interpret, and hence more meaningful and useful. Achieving a mesh family on unstructured grids can be challenging. It at least requires that the minimum spacing at the wall and boundary layer growth rate vary according to particular rules, as the spacings throughout the mesh are consistently refined. For the current workshop, we are trying to push toward use of significantly finer grids than in past workshops, in keeping with the goals of CFD Vision 2030. Rather than have y+ less than one on all mesh levels, which then pushes the finest grids to have y+ significantly smaller than one, we have chosen the D level mesh to be the coarsest level with estimated y+ less than one. As a result, the "average" y+ on the coarser A, B, and C meshes are expected to be one or greater by design. However, because the meshes are designed to be approximately a family, we hope to see consistent trends in the solutions on successively finer grids. (Note that the y+ values in the Gridding Guidelines are only estimates; the actual levels will vary significantly over the vehicle.) Participants are encouraged to explore the effect of this aspect of the meshing guidelines, by creating/using meshes with smaller minimum y+ values and reporting their findings.


 

Q12: Please describe how to obtain lift coefficient (CL), drag coefficient (CD), and pitching moment coefficient (CMy).

A12: Lift coefficient is defined as L/(qS), where L is the sum of the components of the pressure forces and viscous forces on all parts of the vehicle in the UP direction, taking into account the angle-of-attack. S is the reference area of the semi-span model (Sref/2). The q=dynamic pressure=0.5*rho_inf*U_inf2. Similarly, drag coefficient is defined as D/(qS), where D is the sum of the components of the pressure forces and viscous forces on all parts of the vehicle in the DOWNSTREAM direction, taking into account the angle-of-attack. Moment coefficient is defined as M/(qSc). Here, M is the moment about the moment reference center (MRC), and c is the mean aerodynamic chord (MAC). This workshop is asking for pitching moment coefficient, which is taken about the y-axis pointing out of the MRC. To get M, you need to sum up all of the local forces (pressure forces and viscous forces) times their moment arm about the MRC's y-axis. By convention, CMy is positive when it acts to pitch the aircraft nose UP. When computing the model in the wind tunnel, the model's standoff should not contribute to the computation of CL, CD, or CMy.


 

Q13: What is a PID and how do I use it?

A13: A PID is a Participant IDentification designation. It is the way we identify the submitted data for the workshop. The assigned PIDs can be found on the respective TFG pages from: https://hiliftpw.larc.nasa.gov/Workshop4/TFGlist.html. To keep them distinct, each TFG has its own letter designation:

If you are in more than one TFG, then you will have more than one PID. For example, if you are in the RANS TFG and the Adaptation TFG, you will have two PIDs: R-xxx and A-yyy, where xxx and yyy are 3 digit numbers. Generally, in a given TFG, each individual member has their own unique PID. However, if you are part of a team (and are only going to submit one set of results), then you should choose just one member's PID that your team will use, and let us know. When submitting results, you will be asked to include your PID as an identifier in all of your data submittal files. If you submit multiple sets of results to one TFG (for example, using different codes, different turbulence models, etc.), then append ".1", ".2", ".3", etc. in order to distinguish between them. For example, if you are on the Geometry TFG and your PID is G-001, then you would use G-001.1, G-001.2, G-001.3, etc. for multiple submission variants.


 

Q14: I tried to use the committee-provided RANS grids, but my solver would not work with them. Can I make modifications to these grids?

A14: Yes. Just as mentioned in A9 above, be sure to DOCUMENT everything done, and plan to share your changes and experiences with your TFG.


 

Q15: In what formats are the committee-provided RANS grids provided?

A15: The unstructured committee-provided RANS grids are provided in two widely-used formats: CGNS (.cgns) and AFLR3 (.ugrid). Most organizations today have the ability to read at least one of these formats. CGNS files are self-contained, whereas AFLR3 files include separate mapbc files. The structured overset committee-provided RANS grids are provided in PLOT3D (.p3d) format, with separate connectivity and BC files.


 

Q16: Can/should I submit an AIAA paper on my HLPW-4/GMGW-4 related work for SciTech 2022?

A16: The HLPW/GMGW committee prefers that participants wait until AIAA Aviation 2022 to write a paper. Here is the official position:

The GMGW and HLPW committee leadership strongly encourages the free interchange of ideas and results between workshop participants, and the publication of individual participant results in conference proceedings and peer-reviewed journals. With that in mind, we plan to have one or more Special Sessions at Aviation 2022, as well as a potential special edition of the AIAA Journal, dedicated to showcasing individual participant technical results. We would like to encourage participants in our workshops to use these planned opportunities to showcase their individual data and work from the workshop. This will enable us to first draw conclusions, establish consensus results and lessons learned as a community at the workshop to be held in conjunction with AIAA SciTech 2022, followed by more detailed technical information presented and published by individuals at AIAA Aviation 2022.

We will let everyone know the procedure for submitting a possible paper to AIAA Aviation 2022 well in advance of AIAA's abstract deadline in the Fall of 2021.


 

Q17: Is time-accurate balance data available from the wind tunnel test?

A17: The QinetiQ balance is mechanical, so any time-dependent data from it would not correlate directly to the aerodynamics. Therefore, the balance data are only reported as a time-averaged mean.
 
 

Return to: High Lift Prediction Workshop Home Page


 

Recent significant updates:
06/08/2021 - Minor modificatin to A6
05/14/2021 - Added Q17
04/14/2021 - Added Q16
03/16/2021 - Modified A6
02/19/2021 - Added a sentence to the end of A11
01/20/2021 - Added Q13 and Q14
12/18/2020 - Modified Q10/A10 because wind tunnel is being given in a size that matches full-scale geometry
12/17/2020 - Added Q12
12/07/2020 - Added Q11
10/21/2020 - Added Q10
10/20/2020 - Added Q9

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Responsible NASA Official: Li Wang
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Last Updated: 01/13/2022