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February 19, 2013, 02:30 |
No recirculation zone, why??
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#1 |
Member
Osman
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Location: Japan
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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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February 19, 2013, 03:42 |
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#2 | |
Senior Member
Filippo Maria Denaro
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Quote:
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February 19, 2013, 04:12 |
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#3 |
Member
Osman
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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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February 19, 2013, 04:16 |
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#4 |
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Osman
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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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February 19, 2013, 06:53 |
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#5 |
Senior Member
Filippo Maria Denaro
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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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February 19, 2013, 07:19 |
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#6 |
Member
Osman
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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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February 19, 2013, 07:25 |
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#7 | |
Senior Member
Filippo Maria Denaro
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Quote:
correct be sure you respect the stability constraint for the AB-CN scheme with central second order discretization |
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February 19, 2013, 07:31 |
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#8 |
Member
Osman
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Location: Japan
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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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February 20, 2013, 02:19 |
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#9 |
Member
Osman
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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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February 20, 2013, 05:46 |
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#10 |
Senior Member
Filippo Maria Denaro
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by retaining the same reynolds number?
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February 20, 2013, 06:25 |
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#11 |
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Osman
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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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February 20, 2013, 06:29 |
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#12 |
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Osman
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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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February 20, 2013, 08:25 |
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#13 |
Senior Member
Filippo Maria Denaro
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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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February 20, 2013, 08:31 |
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#14 |
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Osman
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So, how can i specify the inlet velocity (Dirchilit B.C) on a recatangular domain ?
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February 20, 2013, 08:36 |
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#15 |
Senior Member
Filippo Maria Denaro
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February 20, 2013, 08:46 |
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#16 |
Member
Osman
Join Date: Oct 2012
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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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February 20, 2013, 08:55 |
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#17 |
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Osman
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x*=x/D, u*=u/U, t*=t.(U/D)
Last edited by o_mars_2010; February 20, 2013 at 10:12. |
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February 20, 2013, 10:21 |
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#18 |
Senior Member
Filippo Maria Denaro
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.... yes !
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February 20, 2013, 10:23 |
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#19 |
Member
Osman
Join Date: Oct 2012
Location: Japan
Posts: 53
Rep Power: 13 |
Thank's a lot.
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March 18, 2013, 22:42 |
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#20 |
Member
Osman
Join Date: Oct 2012
Location: Japan
Posts: 53
Rep Power: 13 |
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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