Variable RMS Value fluctuating but not converging.
1 Attachment(s)
Hello, Everyone!
I am trying to simulate the 2D-flow around the cylinder with area blockage 0.01% in CFX (Structured Mesh). The Reynolds number is 0.5 Million. The boundary conditions I have is: Inlet : velocity (55.55 m/s) Outlet: static Pressure (0 atm) cylinder: wall (no-slip) farsides: wall (free-slip) sides: Symmetric Boundary conditions reference Pressure is the default 1 atm. Turbulence model: SST Time step: Physical time steps (0.0001 s) convergence criteria: 1e-6 The RMS values are fluctuating horizontally and not even close to convergence even after 3000 iterations. I need some advice to fix this problem. Thank you in advance! |
Since you wrote "Time step: Physical time steps (0.0001 s)" and you have periodic fluctuations in the residuals Im guessing you are running steady-state simulation where vortex shedding appears? Try a transient simulation instead.
|
Hello Lance, Thank you for the reply.
Yes, I am using steady state analysis because I want to find the mean drag force acting on the cylinder and there is vortex shedding in my case. Will I be able to calculate mean drag if I use transient simulation ? Thank you |
Your best chance for getting a converged steady-state solution in this case is by cutting your model in half and using a symmetry boundary condition. This will eliminate the large fluctuations from the vortex shedding without altering the solution from a RANS point of view.
If this still fails your only option is what Lance just wrote. |
Quote:
|
But due to vortex shedding, will not be any fluctuations in drag force too? I am following your approach but I see the drag plot line fluctuating too.
|
Quote:
|
Thank you, Could you please suggest if there is way in convergence criteria to monitor the time average of drag force (Normal force on cylinder (x))?
I would like to make it my convergence criteria. |
See "21.1.5.1.8.8. [Monitor Name]: Monitor Statistics" in the cfx-pre manual.
Make an expression on drag force, monitor the standard deviation over a certain time. Im not sure you can make CFX stop when the standard deviation is less than a threshold, but you can at least monitor it. |
Create an interruption control using a logical expression along the lines of
probe(ExpressionValue.Standard Deviation)@MyMonitorExpression < MyToleranceValue Please check documentation for accurate syntax |
Ansys posted video on its youtube channel about it. Here is the link:
ANSYS CFX: Using Derived Variables and Monitor Statistics to Set Up an interrupt Control |
Quote:
|
Only pseudo-2D with a volume mesh and a thickness of 1 cell. Fluent on the other hand has real 2D solvers. I don't think that a 2D solver will be added to CFX any time soon.
|
FAQ: https://www.cfd-online.com/Wiki/Ansy..._simulation.3F
With ANSYS AIM coming along there is no chance CFX will get real 2D simulations. I have not looked at ANSYS AIM in detail - can it do 2D simulations? |
All times are GMT -4. The time now is 19:01. |