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-   -   Residual Convergence on Meshes is poor (http://www.cfd-online.com/Forums/fluent/90897-residual-convergence-meshes-poor.html)

SleeperService July 23, 2011 20:54

Residual Convergence on Meshes is poor
 
Hello all, I'm trying to carry out a study involving a 2D axisymmetric simulation of a conical model in Mach 5 flow with a small spherical plasma spot some distance ahead of it. I've currently meshed the problem using a mix of quad pave and map meshing depending on the zones, but am seeing extremely poor convergence for my residuals. They tend to oscillate. I am lost as to why this is happening, and am a little bit out of my depth here - would someone be willing to look at my case files and help me out? I'll gladly PM them to any who offer their assistance. Thanks.

vxv July 23, 2011 21:58

".....but am seeing extremely poor convergence for my residuals. They tend to oscillate......"

As it is a Mach 5 flow, I assume you are using a limiter. Which limiter are you using?

Some limiters never allow the residual to go to machine zero, even though the solution looks ok. It may happen because of one bad cell, where the residual keeps oscillating. Try Venkatkrishnan limiter!

SleeperService July 24, 2011 03:30

I'm not familiar yet with the concept of limiters, but the residual oscillations are very high in magnitude - between 10^2 and 10^-1. I think if they were lower in magnitude, I could ignore them safely and assume the simulation has converged, but such a degree of uncertainty seems to be indicative of another problem - would you agree?

vxv July 24, 2011 10:52

I have never run a Mach 5 case but for supesonic cases around Mach 2, my residuals used to stall around 1e-2. But as you say, your residuals are order of 1e+2. In that case there certainly is something wrong elsewhere.

Which code are you using? What boundary conditions have you specified?

SleeperService July 25, 2011 06:17

Er...fairly new to this, so apologise in advance if this isn't the answers to the questions you're asking!

I know the Solver is density based, implicit, second order upwind. Simulations are being checked for grid dependency, laminar vs. turbulent cases (using K-omega SST), and finally steady vs. unsteady solutions.

Boundary Conditions at the inlet are:
P_inf = 1220 Pa
T_inf = 62 K
M_inf = 5.0

The flow is in the axial direction only, and enters from a pressure_far_field condition on the left (also present at the top of the grid, but of course, with zero flow contribution), and exits via a pressure_outlet condition with zero backflow.

The walls of the simulation are also set to the default condition. The small plasma spot, like the rest of the flow is simply modelled as air behaving as an ideal gas (viscous modelling is sutherland where applicable) but of course with a considerable energy source term totalling around 7W of power across it's 0.5 mm diameter.

I apologise if I've missed anything out - if you prefer, I'd be happy to send you my case or data files, or upload them to a filesharing account. Otherwise, please let me know what other information you need and I'll do my level best to find it - I just wasn't sure what you meant by Limiter or code.

vxv July 25, 2011 11:33

As the inlflow is supersonic, the flow at the outlet will also be supersonic after passing through the oblique shock wave created by the cone. So at the outlet use "extrapolation" boundary condition.

Let me know if this works!

SleeperService July 25, 2011 12:52

Hmm, I tried to do what you said, but it appears Fluent is already extrapolating that information. The outlet BC is of type pressure_outlet, and while this requires the specification of a outlet gauge pressure, this is ignored automatically when the local flow is supersonic as calculated from upstream values. It is also impossible to set the condition to force extrapolation in any case, since this setting is incompatible with compressible flow problems and fluent immediately reports an error.

vxv July 25, 2011 13:35

Can you send me your case and data files. Although I have not touched fluent in more than 3 yrs, I will give it a try. My id (varunvikas@gmail.com)

SleeperService July 25, 2011 14:32

Sure, sending you them now.


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