I have a control volume as attached. Left is inlet, right is outlet, top is symmetry and bottom is axis (it is a 2d axisymmetric analysis). What you see is a vehicle and there is a piston-cylinder mechanism inside it. The piston moves and creates pulses. There is no flow from the inlet, so the velocity of the inlet set to "0". When I initialize the case, it gives a warning as "turbulent viscosity ratio is limited to 1e+5 in xxx cells". When I plot turbulent k contours, I see that every turbulent k is over the limited value. What might be the problem?
I set inlet and outlet turbulent parameters to 0. The warning disappeared, but then it did not solve the case. I changed the boundary inlet to wall and the turbulent viscosity ratio warning still appears. I am really confused. Do you have any ideas?
A quick update: When I set any velocity at inlet, viscosity ratio problem disappears.
Hello
well you can increase your viscosity limits by going in to solution controls and there in limits. Increase your turbulent viscosity limits and see if that works.
Mr. Kumar, I think I have found an answer to my problem.
When I define zero velocity, there will be no turbulent viscosity. Because turbulent viscosity is a result of velocity. So no velocity means no turbulent viscosity.
Since turbulent viscosity ratio is the ratio of dynamic viscosity and turbulent viscosity, they both must have values other than zero. I don't exactly know which parameter is in nominator or denominator, but if one is zero and is in nominator, then the ratio will be zero. Otherwise, it is infinity.
In my case, I think, denominator becomes zero and the ratio goes is infinity.
I must say that I am not an neither expert in the subject of turbulence nor in fluid mechanics. So, my answer is not more than an idea.
Exactly your provided definition for turbulent velocity ratio is correct and its the ratio of turbulence viscosity and dynamic ( some person denote as laminar ) viscosity & denote as
turbulent viscosity
---------------------
laminar viscosity
But the error about viscosity ration comes because you might be provide wrong value of turbulence parameter which leads this problem so i suggest you that either you change the value of turbulence parameter or refine the grids .
The turbulence parameters defined at the inlet are turbulent intensity and viscosity ratio. Any value except zero causes the same problem for both values. On the other hand, when I define "zero" turbulence intesity and viscosity ratio, system gives another error and no solution is delivered.
The reason that I think the problem should be something about boundary condition, is the viscosity ratio exceeds the limiting value at every cell of the domain.
The answers you got before are correct, especially considering that the viscosity ratio is denoted as:
turbulent viscosity
---------------------
laminar viscosity
And there are different causes/interpretations/explanations for this "error" message. It can be linked to the quality/refinement of the mesh, to the solution methods, to the boundary conditions, to the typology of flow-field you are considering (multiphase flow, or big range of Reynolds Numbers and velocities in the domain, extension of the domain, etc. ). This limit in Fluent is automatically set to 1e+5, but it can be modified (namely improved) in the solution limits panel. This problem could be alleviated improving the mesh refinement in certain areas, changing the turbulence model whenever possible (it is said that RNG is better than standard k-eps for this aspect) or, sometimes, improving the solution order.
Changing limit is not the right way to solve this problem. Go by plotting turbulent viscosities for the flow domain and refine mesh for the areas with higher viscosities and try solving the problem again. Hope this helps!!
Hi,
I have a same problem , this warning does not disappear even i change my turbulent model, refine mesh and ..., I am very confused about my problem, because Fluent is stop suddenly after 2000 iterations. can anyone help me?
Dear Sajad Khodadadian,
If you are dealing with turbulence you may check and reduce the under relaxation factor (URF). For most cases default values for URF give best results but sometimes especially for compressible turbulent flows it can be reduced depends on your residual's stability during the solution. Also you might have other problems too as y plus value, mesh at the walls ... You need to couple your y plus value and your solution model (i.e. y plus should be less than 1 for k-w model).
Also you can try to increase the "turbulent viscsity ratio" value at the inlet and outlet boundries. It is 10 as default but can vary from 10 to 100 and impossible to predict. If you use pressure inlet and pressure outlet, you can set it as 50 or 60 for supersonic analyzes.
Hi , I have a VOF simulation for an open channel. I have used mass flow inlet and this error(turbulence viscosity limited to viscosity ratio.....) dose not disappear, can I fix it by under relaxation factor ?
Controlling the Courant number and defining proper turbulence boundary conditions may work. Thus refine the mesh where you suspect gradients are higher. Use higher gradient methods for calculations.
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