[mazhar16823]

Hi,

I am getting the above-mentioned warning on each iteration (from the start of simulation till now, it's been 600 iterations) while running steady MRF case using SST Transition Model.

First of all, I looked into the user guide, where it's recommended to increase the maximum ratio but that's for transient simulations. Although, I increased the max. ratio undet K-Omega Turbulent Viscosity from 100000 to 120000, but still this warning continues.

Could you please suggest anything on this?

[LuckyTran]

This happens easily when you have poor initial guesses for the transport variables for turbulence. Either make better guesses or iterate until they go away.

Less common but even more problematic is to have the wrong boundary conditions (intensity, length scale, etc.).

[mazhar16823]

Originally Posted by LuckyTran

This happens easily when you have poor initial guesses for the transport variables for turbulence. Either make better guesses or iterate until they go away.

Less common but even more problematic is to have the wrong boundary conditions (intensity, length scale, etc.).

How to ensure the better guess? I have not touched the variables you have mentioned they are there by-default. I only provided the initial conditions i.e. incoming wind velocity and rotation rate etc.

[LuckyTran]

You are doing a transient simulation which means your initial guess isn't an initial guess but a required initial condition. You should (in principle) know what these are or at least find a means to generate proper initial conditions for turbulence. I don't know your problem to say what is the right initial conditions for turbulence since I have no idea what you're solving. Just think about what your initial condition for velocity is, and consider what the proper turbulence variables should be for that velocity field. Maybe you can run a stationary case or steady case with that flow to get the turbulence variables.

You need to know your boundary conditions and initial conditions before your problem is even well defined. If you don't know what that BC's are, you are just doing CFD for fun.

[mazhar16823]

Originally Posted by LuckyTran

You are doing a transient simulation which means your initial guess isn't an initial guess but a required initial condition. You should (in principle) know what these are or at least find a means to generate proper initial conditions for turbulence. I don't know your problem to say what is the right initial conditions for turbulence since I have no idea what you're solving. Just think about what your initial condition for velocity is, and consider what the proper turbulence variables should be for that velocity field. Maybe you can run a stationary case or steady case with that flow to get the turbulence variables.

You need to know your boundary conditions and initial conditions before your problem is even well defined. If you don't know what that BC's are, you are just doing CFD for fun.

Thanks. But I am doing steady MRF case as already mentioned. I am using RANS CFD to account for the location of transition from laminar-turbulent BL; and provided a constant incoming wind velocity in the intial conditions under continua.

[arjun]

Chose the turbulent quantities such a way that turbulent viscosity comes out to be low. For example near zero k and very high epsilon would do for k-eps model. Similarly chose low k and higher omega for k omega model.

[mazhar16823]

Originally Posted by arjun

Chose the turbulent quantities such a way that turbulent viscosity comes out to be low. For example near zero k and very high epsilon would do for k-eps model. Similarly chose low k and higher omega for k omega model.

Where to impose this change in the solver?

[arjun]

Originally Posted by mazhar16823

Where to impose this change in the solver?

That depends on the solver, so you have to check on gui.

[LuckyTran]

Oh pardon me, for some reason I got confused and thought you were doing a transient case.


Well then either iteration it away or pick values of k and omega/epsilon that makes more sense. The default value of 1 for k is wayy to high for most problems.

[mazhar16823]

Originally Posted by LuckyTran

Oh pardon me, for some reason I got confused and thought you were doing a transient case.


Well then either iteration it away or pick values of k and omega/epsilon that makes more sense. The default value of 1 for k is wayy to high for most problems.

Thanks. But I am unable to figure out where they could be changed I checked with Continua>Initial Conditions and solver settings. Further, if you are aware with external aerodynamic problems, can you please tell whether I should specify free-stream velcoity at the inlet boundary or velocity containing induction effect i.e. U_inf*(1-0.33). What I assume that I need to specify the free-stream velocity whereas induction effect is taken into account by the solver itself.


However, now I have changed Continua>Physics Values>Initial Conditions>Velcocity from [0,0,6] to [0,0,0] and running the simulation again. For now, I don't see this error.

[arjun]

Originally Posted by mazhar16823

Thanks. But I am unable to figure out where they could be changed I checked with Continua>Initial Conditions and solver settings. Further, if you are aware with external aerodynamic problems, can you please tell whether I should specify free-stream velcoity at the inlet boundary or velocity containing induction effect i.e. U_inf*(1-0.33). What I assume that I need to specify the free-stream velocity whereas induction effect is taken into account by the solver itself.


However, now I have changed Continua>Physics Values>Initial Conditions>Velcocity from [0,0,6] to [0,0,0] and running the simulation again. For now, I don't see this error.

The problem is that from your post it is not clear to me which solver you are using. This information shall be in the first post. In my casual browsing i can't figure it out.



This is why it is hard for someone to point it out where you set it. Even if we are familiar with solver you are using. (now it looks like you might be using starccm).

[mazhar16823]

Originally Posted by arjun

The problem is that from your post it is not clear to me which solver you are using. This information shall be in the first post. In my casual browsing i can't figure it out.



This is why it is hard for someone to point it out where you set it. Even if we are familiar with solver you are using. (now it looks like you might be using starccm).

I am sorry for that. Yes, I am using STARCCM+

[mazhar16823]

Originally Posted by arjun

The problem is that from your post it is not clear to me which solver you are using. This information shall be in the first post. In my casual browsing i can't figure it out.



This is why it is hard for someone to point it out where you set it. Even if we are familiar with solver you are using. (now it looks like you might be using starccm).

Hi Arjun,


I have calculated the Turbulence Velocity Scale based on the turbulence intensity = 0.01 and I chose viscosity ratio as 1 because I believe that this turbulence intensity is considered as low so the viscosity ratio should also be taken lower which is 1<Mu_t/Mu<10. If you see in the attachments that "K" is very small, and omega is very high comparatively. So, in the end I get Turbulence Velocity Scale = 0.0088 m/s. Can you confirm if this is correct?


However, I tried using these values of TI, Velocity Scale and Viscosity Ratio in the Initial conditions and inlet boundary conditions but still the above warning persists with increasing number of cells.

[arjun]

Originally Posted by mazhar16823

Hi Arjun,


I have calculated the Turbulence Velocity Scale based on the turbulence intensity = 0.01 and I chose viscosity ratio as 1 because I believe that this turbulence intensity is considered as low so the viscosity ratio should also be taken lower which is 1<Mu_t/Mu<10. If you see in the attachments that "K" is very small, and omega is very high comparatively. So, in the end I get Turbulence Velocity Scale = 0.0088 m/s. Can you confirm if this is correct?


However, I tried using these values of TI, Velocity Scale and Viscosity Ratio in the Initial conditions and inlet boundary conditions but still the above warning persists with increasing number of cells.

Are you getting this warning from the start???? If this persists then you should lower momentum and continuity under-relaxations. Your turbulence equations is not been able to adjust to velocity changes.

[mazhar16823]

Originally Posted by arjun

Are you getting this warning from the start???? If this persists then you should lower momentum and continuity under-relaxations. Your turbulence equations is not been able to adjust to velocity changes.

Yes. I am getting it from the beginning and I am using steady MRF+Segregated Flow, so I tried reducing URFs for velocity and pressure to 0.5 and 0.2 respectively - but still it occurs continuously.