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No recirculation zone, why??

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Old   February 19, 2013, 03:30
Default No recirculation zone, why??
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I have developed a code to simulate a flow past a circular cylinder using Immersed Boundary Method.
I set Re=40, and the free stream inlet velocity to 1 and the diameter of the cylinder 0.1.
The resultant streamlines results have no recirculation zone behind the cylinder .
Could any one help me to know what is the reason?
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Old   February 19, 2013, 04:42
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Quote:
Originally Posted by o_mars_2010 View Post
I have developed a code to simulate a flow past a circular cylinder using Immersed Boundary Method.
I set Re=40, and the free stream inlet velocity to 1 and the diameter of the cylinder 0.1.
The resultant streamlines results have no recirculation zone behind the cylinder .
Could any one help me to know what is the reason?
At Re=40 you should have a steady, symmetric separation. Have you reached a full steady state? Are you using upwind discretization? what about your grid?
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Old   February 19, 2013, 05:12
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i am using AB-CN discretisation on a staggered grid, the mesh size 0.1D, and a time step of 0.01.
I think i reached the steady state because there is no change in the velocity contours or the vorticity contours is taking place.
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Old   February 19, 2013, 05:16
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an intersting thing happened when i changed the Re to 500 i could watch a steady state recirculation zone ????

is my inlet velocity has to be adjusted according to the Re ??
I mean
if Re =40 then the inlet velocity should be

U=viscosity*Re/(D)
because i specified the inlet velocity to be 1, with no relation to Re
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Old   February 19, 2013, 07:53
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Fix your Re number, then you have to fix two among the parameters U, D, viscosity.

Furthermore, AB-CN is a time integration scheme, but says nothing about the spatial discretization
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Old   February 19, 2013, 08:19
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sorry, i used second order centeral difference for the spatial discretization.
I will repeat what you say from my understanding to it:
So, for example if i want to make a simulation at Re= 40 around a cylinder with a diameter 0f 0.1 using the kinematic viscosity of water, then the free stream velocity (inlet velocity) will be equal to: (fixing the diameter and viscosity)
U=40*1.04e-6/0.1=4.16e-4 m/s
and in the case of Re=150, the free stream velocity will be:
U=150*1.04e-6/0.1=1.56e-3 m/s
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Old   February 19, 2013, 08:25
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Quote:
Originally Posted by o_mars_2010 View Post
sorry, i used second order centeral difference for the spatial discretization.
I will repeat what you say from my understanding to it:
So, for example if i want to make a simulation at Re= 40 around a cylinder with a diameter 0f 0.1 using the kinematic viscosity of water, then the free stream velocity (inlet velocity) will be equal to: (fixing the diameter and viscosity)
U=40*1.04e-6/0.1=4.16e-4 m/s
and in the case of Re=150, the free stream velocity will be:
U=150*1.04e-6/0.1=1.56e-3 m/s

correct

be sure you respect the stability constraint for the AB-CN scheme with central second order discretization
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Old   February 19, 2013, 08:31
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Thanks a lot for your help.
About the stability condition, i think because i am using an implicit way for the treatment of the viscous term then, the restrict constrain of time will be relaxed, so:
dt<=dx/U
However, i am not sure about the IBM, may be it add additional constrain on the time step!!!
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Old   February 20, 2013, 03:19
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I have a very strange results:
When i set the kinematic viscosity as 1.04e-6 i have no recirculation zone.
however, when i set it to 1e-2 i clearly see the recirculation zone.
i calculate the free stream velocity according to the relation i mentioned above in both cases of viscosity.
could you please explain to me why this happened???
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Old   February 20, 2013, 06:46
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by retaining the same reynolds number?
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Old   February 20, 2013, 07:25
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I am using the non-dimensional form of N-S equation, So i have to set the Reynolds no. and the inlet velocity explicitly, the viscosity does not appear explicitely in the momentum equations.
So, lets say i want to make a simulation of flowing water past a circular cylinder with a diameter D=0.1 at Re=40 and the kinematic viscosity of water is 1.04e-6, then the calculated inlet velocity will be U=40*1.04e-6/0.1=4.16e-4 m/s.
I set the parameters like that in my problem, however i didn't have any circulation.
However, when i changed the kinematic viscosity to 1e-2 (i do not know what fluid have such a value) i got circulation zone!!!!!!!!!!!!
So, my question why i do not have a circulation zone for water???
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Old   February 20, 2013, 07:29
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Diameter=0.1; % cylinder diameter
%kinematicviscosity=1.04e-6; %water at 20 'C
kinematicviscosity=0.01;
ineltvelocity=Re*kinematicviscosity/Diameter;
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Old   February 20, 2013, 09:25
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I do not understand what you are done... if you have non-dimensional equations (and a non dimensional domain), the only parameter to set is the Reynolds number!
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Old   February 20, 2013, 09:31
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So, how can i specify the inlet velocity (Dirchilit B.C) on a recatangular domain ?
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Old   February 20, 2013, 09:36
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Quote:
Originally Posted by o_mars_2010 View Post
So, how can i specify the inlet velocity (Dirchilit B.C) on a recatangular domain ?
The non-dimensional inlet is the same as well as the non-dimensional diameter!
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Old   February 20, 2013, 09:46
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So, if i have a rectangular domain of 40D * 30D, and a grid size 0.05D
then, the non dimensional diameter will be equal to 1
and the non dimensional inlet velocity will be equal to 1
and the rectangular domain will be equal to 40*30
and the grid size will be equal to 0.05
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Old   February 20, 2013, 09:55
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x*=x/D, u*=u/U, t*=t.(U/D)

Last edited by o_mars_2010; February 20, 2013 at 11:12.
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Old   February 20, 2013, 11:21
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.... yes !
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Old   February 20, 2013, 11:23
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Thank's a lot.
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Old   March 18, 2013, 23:42
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Dear Dr. Filippo
i have some strange results:
when i set the mesh size to 0.1D, i have a good results for the length of the recirculation zone and the drag coefficient.
l/D=2.33 & Cd=1.55
On the other hand, when i decrease the size of the mesh to 0.05D, which is supposed to give better results, it gives worth results:
l/D=2.03 & Cd=1.39

Could you tell me please what could be the reason ???
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