As I mentioned in some other posts, I am simulating the flow around ground vehicle models in a wind tunnel using version 8.0. After many runs and analyses, I managed to match the numerical results with the experimental data (https://ntrs.nasa.gov/citations/20010038028); however, upon visually inspecting the solution, I notice the presence of checkerboards in the pressure field (see image 1). It's a vehicle similar to the Ahmed body with a 14° yaw angle; the image corresponds to a top view of the midplane. Flow is from left to right.
Despite numerous runs and changes in the setup, this issue persists to varying degrees. Starting from a uniform velocity initial condition, this checkerboard does not appear initially, and the residuals are smooth (without significant oscillations). When the vehicle wake reaches the 1:2 mesh expansion near the outlet patch, the checkerboard starts to form, and pressure residuals begin to oscillate (see image 2; don't mind the oscillations seen in the omega variable; that was resolved with others advection schemes easily). This persists over time and even becomes more pronounced, though the simulation does not diverge or abort.
Analyzing drag values, side forces, pressure coefficients at specific positions, and friction coefficients at specific points yields very good results, but I suspect this checkerboard might be influencing these outcomes. Here is a list of different setups I tried without success:
- Variation of advection term schemes: From central differencing to high upwind blending and other intermediate schemes, none resolved the issue.
- Solver and preconditioner setup: Focused on those used for pressure; tried all available solvers for pressure without significant changes
- Gradient calculation strategies: Switched from least squares to Green-Gauss, with first and second-order limiters in the former, and varying climgr, without resolving the issue.
- Using a different turbulence model: Currently using SST DDES for transient history; switching to RANS SST still shows checkerboards.
- Increasing solver precision: Set all solver precision variables epsilon to 1e-8 and increased RHS iterations, but the issue persisted.
- Tried imvisf=0 and imvisf=1.
- Tried rcfact=1 and rcfact=0 (as Rhie-Chow is related to checkerboards), without success.
- Outlet boundary condition: Using BC outlet without manually setting pressure; using BC pressure outlet (setting outlet pressure) did not improve the issue. Attempting to set reference pressure position in cs_user_parameters.c is ineffective (seen in another post).
- Extending the refined mesh zone: To avoid a 1:2 transition in the wake region, currently in progress.
Thank you very much.