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October 19, 2015, 11:51 
Problem of convergence simulation of flow wind around cylinder

#1 
New Member
Badr badro
Join Date: Oct 2015
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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) ; Reynoldsaveraged 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. 

October 20, 2015, 09:02 

#2 
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Philipp
Join Date: Jun 2011
Location: Germany
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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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October 20, 2015, 09:45 

#3 
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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.


October 21, 2015, 19:42 

#4 
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Badr badro
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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 Pressureoutlet thanks ! 

October 22, 2015, 01:20 

#5 
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Philipp
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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 timedependent 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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October 22, 2015, 22:08 

#6 
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Badr badro
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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 ! 

October 23, 2015, 01:57 

#7 
Super Moderator

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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