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Problem of convergence- simulation of flow wind around cylinder

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Old   October 19, 2015, 11:51
Default Problem of convergence- simulation of flow wind around cylinder
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Hi all,

I want realise a 2D simulation of a turbulent flow of a fluide, wind for example around a cylinder.
So the wind flow is turbulent. The turbulence model chosen (that i think is the best), is the Shear Stress Transport (SST) ; Reynolds-averaged NaviereStokes (RANS) model.
* the dimensions are :
- The solution domain => L=90 m ; H= 30 m
- the Cylindre has a diameter of D= 10 m ; and it is situated in midle of the domain
The problem is that I have no convergence amthough I give a big number of iterations
Is there a simulation that can never converge and that we can trust the resuts given by it ?

Thanks.
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Old   October 20, 2015, 09:02
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From what you write, most probably you have a problem without a steady state solution. Geomtries like this produce large vorticies, google for "von karman vortex street". That's why you don't get a converged solution with a steady state solver.

Of course there might be 1000 other possible solutions but you need to post some more information, such as a picture of the residuals, the latest flow (velocity) field and a picture of the grid.
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Old   October 20, 2015, 09:45
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It is possible that is unsteady, but for flow past a cylinder there are few unsteady cases based on the flow regime, thus you should also give details of the fluid and boundary conditions.
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Old   October 21, 2015, 19:42
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Thanks guys for replying,

In fact my main objective is that simulate a geometry partially cylindrical not a right cylinder (see pics below) I just begin by the cylinder the geometry that I thinked easy to simulate.
I remove actually to may main gemetry and as always no convergence !

RodriguezFatz I dont think is the problem of unsteady state you can see it in the velocity contour (pic below )
also I post a residual picture and a meshing picture.
- the fluide is the Air, the initial velocity imposed is of 17 m/s.
- the left boundary is the inlet .. the right and the high is defined as Pressure-outlet

thanks !
Attached Images
File Type: png Resid1.png (13.1 KB, 13 views)
File Type: png meshing1.png (46.9 KB, 14 views)
File Type: png sol1.png (36.6 KB, 15 views)
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Old   October 22, 2015, 01:20
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I think you can see from velocity contour that it is unsteady (most probably). It even looks like a beginning von Karman vortex street. Just the solver is not time-dependent so it tries to damp the vorticies as much as it can but it doesn't succeed. That's why your residuals don't decrease.
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Old   October 22, 2015, 22:08
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Hi, Thanks very much

So what can I do to fix this ? what's exactly the parameters that can be behind this kind of problem ?

I just checked also the effect of meshing on the convergence by using different fomrs of meshing but without any good results !
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Old   October 23, 2015, 01:57
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use transient simulation.

You can use strouhal number for the cylinder for equivalent Reynolds number and guess the time step. Then half the time step and see the effects.

compare results for different time steps and when they stop to change, that is your required time step.

Now you use these settings to run your required simulation and get data for analysis.
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