Square_Cylinder Test Case

suman91 · October 11, 2014, 19:40

Hi ,
I am running the square cylinder test case for Unsteady, using SA turbulent model.
I have attached the config file here. The Mach Number is modified to M =0.0029 to have u = 1.0 ; v = 0 as initial conditions.

The plots of Cl and Cd are not giving me the oscillations that are suppose to come in unsteady simulation. Instead it flattens out to almost a steady value which is also different from the value that should actually come.

Another observation (from history file):
In iteration 1 : CDrag = 95.7139
In iteration 2 : CDrag = -189.37
In iteration 3 : CDrag = -10.8855

Then after a few iterations it settles down to 1.3 (approx) giving no oscillations but only variation in the second digit after decimal.
Why such high values? What is going wrong with the simulation ? Please suggest what needs to be corrected.

demanosalvas · October 13, 2014, 02:28

HI Suman91,

There are a couple things that I would like you to think about when running this case. First, the Mach number is too low for a compressible solver, and to solve this problem you either have to modify your conditions for the mach number to be 0.1 or higher, or you can use the incompressible solver that is available in SU2. The low mach number that you are using in a compressible solver is most likely the cause for the large values you are getting for the force coefficients.

Second, to be able to capture the vortex shedding that is characteristic of bluff body aerodynamics, you have to make sure that the time stepping you have chosen is small enough to be able to resolve the periodic behavior. To do this you need to calculate your expected shedding frequency and period, and use this information to have at least 25 sample points per period ( t = T/25 ).

This should be able to get you the result that you are expecting,

David

suman91 · October 13, 2014, 05:48

Hi David,

Thanks for the reply. I will surely try this out. I have a few questions to what you suggested:

1. SU2 has a low mach number preconditioner inbuilt to tackle low mach problems when
run as compressible flows right ?

2. Do I have to some how switch on the preconditioner if I want to run low mach
problems as compressible only or is it tuned on some how by default?
If I have to switch it on, then please mentioned how to use this feature.

Thanks.

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October 13, 2014, 02:28		#2
demanosalvas New Member David Manosalvas-Kjono Join Date: Feb 2014 Posts: 25 Rep Power: 12	HI Suman91, There are a couple things that I would like you to think about when running this case. First, the Mach number is too low for a compressible solver, and to solve this problem you either have to modify your conditions for the mach number to be 0.1 or higher, or you can use the incompressible solver that is available in SU2. The low mach number that you are using in a compressible solver is most likely the cause for the large values you are getting for the force coefficients. Second, to be able to capture the vortex shedding that is characteristic of bluff body aerodynamics, you have to make sure that the time stepping you have chosen is small enough to be able to resolve the periodic behavior. To do this you need to calculate your expected shedding frequency and period, and use this information to have at least 25 sample points per period ( t = T/25 ). This should be able to get you the result that you are expecting, David

October 13, 2014, 05:48		#3
suman91 Member Suman Chakraborty Join Date: Sep 2014 Location: Mumbai, India Posts: 42 Rep Power: 12	Hi David, Thanks for the reply. I will surely try this out. I have a few questions to what you suggested: 1. SU2 has a low mach number preconditioner inbuilt to tackle low mach problems when run as compressible flows right ? 2. Do I have to some how switch on the preconditioner if I want to run low mach problems as compressible only or is it tuned on some how by default? If I have to switch it on, then please mentioned how to use this feature. Thanks.