# No recirculation zone, why??

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 February 19, 2013, 03:30 No recirculation zone, why?? #1 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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?

February 19, 2013, 04:42
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by o_mars_2010 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?

 February 19, 2013, 05:12 #3 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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.

 February 19, 2013, 05:16 #4 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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

 February 19, 2013, 07:53 #5 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,692 Rep Power: 33 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

 February 19, 2013, 08:19 #6 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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

February 19, 2013, 08:25
#7
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Filippo Maria Denaro
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Quote:
 Originally Posted by o_mars_2010 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

 February 19, 2013, 08:31 #8 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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!!!

 February 20, 2013, 03:19 #9 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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???

 February 20, 2013, 06:46 #10 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,692 Rep Power: 33 by retaining the same reynolds number?

 February 20, 2013, 07:25 #11 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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???

 February 20, 2013, 07:29 #12 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 Diameter=0.1; % cylinder diameter %kinematicviscosity=1.04e-6; %water at 20 'C kinematicviscosity=0.01; ineltvelocity=Re*kinematicviscosity/Diameter;

 February 20, 2013, 09:25 #13 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,692 Rep Power: 33 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!

 February 20, 2013, 09:31 #14 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 So, how can i specify the inlet velocity (Dirchilit B.C) on a recatangular domain ?

February 20, 2013, 09:36
#15
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Filippo Maria Denaro
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Quote:
 Originally Posted by o_mars_2010 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!

 February 20, 2013, 09:46 #16 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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

 February 20, 2013, 09:55 #17 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 x*=x/D, u*=u/U, t*=t.(U/D) Last edited by o_mars_2010; February 20, 2013 at 11:12.

 February 20, 2013, 11:21 #18 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,692 Rep Power: 33 .... yes ! o_mars_2010 likes this.

 February 20, 2013, 11:23 #19 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 Thank's a lot.

 March 18, 2013, 23:42 #20 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 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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