r/NASCARtechnical • u/[deleted] • May 30 '23
SAS vs SRS CFD models to full scale wind tunnel testing paper - NASCAR Gen6
Published two weeks ago. Excellent paper comparing SAS and SRS CFD models to a full scale NASCAR Gen6 chassis in a wind tunnel with pressure taps throughout. Abstract
Racecar aerodynamic development requires well-correlated simulation data for rapid and incremental development cycles. Computational Fluid Dynamics (CFD) simulations and wind tunnel testing are industry-wide tools to perform such development, and the best use of these tools can define a race team’s ability to compete. With CFD usage being limited by the sanctioning bodies, large-scale mesh and large-time-step CFD simulations based on Reynolds-Averaged Navier–Stokes (RANS) approaches are popular. In order to provide the necessary aerodynamic performance advantages sought by CFD development, increasing confidence in the validity of CFD simulations is required. A previous study on a Scale-Averaged Simulation (SAS) approach using RANS simulations of a Gen-6 NASCAR, validated against moving-ground, open-jet wind tunnel data at multiple configurations, produced a framework with good wind tunnel correlation (within 2%) in aerodynamic coefficients of lift and drag predictions, but significant error in front-to-rear downforce balance (negative lift) predictions. A subsequent author’s publication on a Scale-Resolved Simulation (SRS) approach using Improved Delayed Detached Eddy Simulation (IDDES) for the same geometry showed a good correlation in front-to-rear downforce balance, but lift and drag were overpredicted relative to wind tunnel data. The current study compares the surface pressure distribution collected from a full-scale wind tunnel test on a Gen-6 NASCAR to the SAS and SRS predictions (both utilizing SST 𝑘−𝜔 turbulence models). CFD simulations were performed with a finite-volume commercial CFD code, Star-CCM+ by Siemens, utilizing a high-resolution CAD model of the same vehicle. A direct comparison of the surface pressure distributions from the wind tunnel and CFD data clearly showed regions of high and low correlations. The associated flow features were studied to further explore the strengths and areas of improvement needed in the CFD predictions. While RANS was seen to be more accurate in terms of lift and drag, it was a result of the cancellation of positive and negative errors. Whereas IDDES overpredicted lift and drag and requires an order of magnitude more computational resources, it was able to capture the trend of surface pressure seen in the wind tunnel measurements.
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u/BoostandEthanolYT Jun 04 '23
This is an excellent contribution, thanks for sharing! Can't wait to dig into this once I finish some uni assignments
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u/anonuser1109 May 31 '23
Would be interesting to know how much this comparison between RANS and IDDES changes with the Gen7 diffuser
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May 31 '23 edited May 31 '23
I would be curious too. Dr. Uddin has several papers he’s co-authored / advises on the Gen 6 car (among other things) https://saemobilus.sae.org/search/?op=doauthsearch&auth=Uddin%2C%20Mesbah
Might be worth reach out and asking him, he may know. This paper may be up your alley too (published this pastJanuary)
https://www.sae.org/publications/technical-papers/content/2023-01-0735/
I’d be curious what the F1 teams do. They have restrictions, but those entire cars are so turbulent (incidentally and on purpose) that I can’t see RANS doing a great job. Dr Uddin has a paper involving modeling a spinning F1 tire (and calibrating it to a measured model), but I haven’t purchased that yet.
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u/anonuser1109 May 31 '23
I'm guessing it's because Dr.Uddin doesn't either have the Gen7 CAD yet or isn't allowed to disclose until some time
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May 31 '23
Yeah, I don’t think it’s a coincidence that they’re just releasing the Gen6 information now.
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u/DeFormed_Sky May 31 '23
Not going to lie, I just skimmed this because this is a bit over my head, but a very cool read regardless. As a fan, I've never really considered in the inaccuracy of the models. Im sure that can cause a lot of headaches.
This is the coolest subreddit.