September 15, 2022 at 1:05 amjms26Subscriber
I am running a 3D steady hypersonic simulation with sharp (90 deg) external expansion corners and am curious if anyone has experienced a similar phenomena I'm getting. I'm using Fluent's density-based, implicit solver with energy on and SST k-omega viscous model. My Reynolds # is ~9.2e6. Density follows ideal gas, viscosity set to Sutherland's law, and thermal conductivity follows a Pr = 0.71.
In order to converge, I needed to reduce the solution limit for pressure minimum to be 1e-3 Pa, and thus the timestep there is sadly on the order of 1e-12 (to maintain a CFL = 1.0).
I would like to use this steady solution as an IC for an unsteady simulation, but (as entirely expected) the timestep in the unsteady runs are on the order of 1e-12 when CFL is slightly below 1.0.
Is this something that's actually expected in this situation? Can pressure/timestep truly be on this very low order of magnitude? It just seems a bit unphysical for this to be occurring. While trying to dive deeper into this, I also have completed numerous additional simulations on highly refined 3D structured grids modeling just a cuboid-shaped trailing edge (see below images):
The inlets are the left side - I used a compressible laminar BL solution as a profile for it. I am able to easily reproduce it this pressure/timestep issue with this above
I also am extremely confident it's not a mesh refinement issue.
Any input is extremely appreciated!!
September 30, 2022 at 2:33 pmKonstantine KourbatskiAnsys Employee
first question to ask is what would be the theoretical pressure downstream of the expansion? Assuming inviscid flow and 2D, you can calculate it by hand from the wave theory.
My recommendation would also be to use the density-based explicit solver with explicit time stepping. In this approach you set the CFL, and then the time step adjusts to that CFL. Also use ASUM for flux differencing. This alone may not resolve the issue with too small a time step. In this case you may consider 1st order spacial discretization which is more robust across strong shocks.
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