Learning Forum

[ae22b001]

Hi, I'm trying to simulate supersonic flow (M=1.6) through a duct with shocks, first in steady state, then as a transient simulation by varying outlet pressure.
 I was able to get a good and stable steady-state solution only after using high-speed numerics. After that i tried to run a transient simulation and used a udf to vary the outlet pressure sinusoidally. But I'm noticing a bizarre behaviour of residuals.

During each iteration it is continuously increasing. The timestep I used is 1e-6, which I think is small enough for the frequency of oscillations of the outlet pressure (1000hz).
Some of the other settings are: 

Some other settings- pressure inlet, k-w sst model, density based solver.
Here is a picture of the residuals


And here is the dp/dt contour from the exit,which looked pretty uniform at the start, looks weird and distorted after few iterations - 

I have been trying to figure out the reason for this problem since it seems like a pretty simple one but i haven't been able to fix it. Is it because of some numerical problem, or some scheme i chose ? Kindly help, Thanks.

[ae22b001]

Hi, any help would be greatly appreciated.

Thanks

[Essence]

Hello,

Please turn OFF the High Order Relaxation Factor. It is not recommended for transient cases. Could you check the report definition monitors? Which viscous model options did you choose? I cannot comment on debugging of UDFs, since it is out of scope of this Forum support.

[ae22b001]

Hi, I tried turning it off, but the residuals continuously increase. Im using k-w sst with sutherland viscosity model. The udf is fine, I'm just varying outlet pressure. I tried checking the mass flux but it too continuously increases but that is probably because I'm varying outlet pressure.

Kindly help, the residuals only increase when istart changing outlet pressure, it is running fine if i do an unsteady simulation without changing any properties.
Should i use non-reflective boundary conditions?

[Essence]

How do the report definitions look? Are they stable? Did you try using expressions instead of UDF to vary the outlet pressure? Try to increase the number of iterations per time step. Can you share the screenshot of the mesh? And what is the quality (orthogonal quality, aspect ratio, skewness) of the mesh?