# No recirculation zone, why??

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March 19, 2013, 12:06
#21
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Lefteris
Join Date: Oct 2011
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Quote:
 Originally Posted by o_mars_2010 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???
Water's kinematic viscosity at T=293K is 1.004e-6(m^2/s) and not 1.04e-6.
(http://www.engineeringtoolbox.com/wa...ity-d_596.html)

Another thing that I always ensure is to scale the grid. For example, if you have created the grid with the dimensions in mm, then before running your simulation you should convert your grid's dimensions from mm to m.
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Lefteris

 April 11, 2013, 05:53 #22 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 The recirculation length L/D at Re=40 is around 2, however, from literature i found that it is in the range 2.2~2.4. Could you please tell me what could be the reason?? (I used different grid size, however i got the same results) In addition, i am using Tecplot to calculate the drag coefficient. For example: If the Cylinder radius is for example R=0.05 & the grid size is dx=0.01 I could not get accurate results for the drag coefficient if the the calculation is taking place with the same radious. But, i get accurate results for the drag coefficient if i used a radious smaller than the actual radious such as R=0.0485. What could be the reason????

April 28, 2013, 08:32
#23
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Lefteris
Join Date: Oct 2011
Location: UK
Posts: 199
Rep Power: 7
I would use much smaller grid spacing. dx=0.01 is too large I think compared to diameter of the cylinder.

Quote:
 Originally Posted by o_mars_2010 The recirculation length L/D at Re=40 is around 2, however, from literature i found that it is in the range 2.2~2.4. Could you please tell me what could be the reason?? (I used different grid size, however i got the same results) In addition, i am using Tecplot to calculate the drag coefficient. For example: If the Cylinder radius is for example R=0.05 & the grid size is dx=0.01 I could not get accurate results for the drag coefficient if the the calculation is taking place with the same radious. But, i get accurate results for the drag coefficient if i used a radious smaller than the actual radious such as R=0.0485. What could be the reason????
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Lefteris

 April 28, 2013, 09:33 #24 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 Thanks a lot for your reply. what about the 2nd half of my question about tecplot!! i am using Tecplot to calculate the drag coefficient. For example: If the Cylinder radius is for example R=0.05 & the grid size is dx=0.01 I could not get accurate results for the drag coefficient if the the calculation is taking place with the same radious. But, i get accurate results for the drag coefficient if i used a radious bigger than the actual radious such as R=0.055. What could be the reason????

 April 28, 2013, 12:27 #25 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 199 Rep Power: 7 I don't really get it. Why don't you just compute in your code the force on the cylinder? __________________ Lefteris

 April 29, 2013, 07:35 #26 Member   Osman Join Date: Oct 2012 Location: Japan Posts: 51 Rep Power: 6 Actually, now, i am calculating it in my code (Immersed Boundary Code). The drag coefficient is excellently matching the reference data at Re=185 However, the lift coefficient is small around 0.302 In reference data it is around 0.42 Do you have any suggestion??

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