
[Sponsors] 
November 23, 2011, 06:16 
Reynolds Number Similarity not applicable in Fluent?

#1 
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Sorry for the dramatic title...
I'm running the same mesh, with the same time step, with the same boundary conditions, with these inlet values: First I achieved by Re=150 by setting U=L=Miu=1 and rho=150. (As per a tutorial I found online..) Second, I kept the real properties of air, and calculated the corresponding velocity. For the first case, I got my vortex shedding and transient behaviour, with correct results for Cd and St when compared to the literature. For the second case I got a steady flow...! Does anyone know what is going on? Surely the Reynolds number similarity should be valid for these two cases. Any thoughts? 

January 14, 2012, 08:37 

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Any thoughts?


January 15, 2012, 15:27 

#3 
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Lucky Tran
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I would love to help, but I have no idea what you are doing. Can you perhaps describe it more clearly?


January 18, 2012, 08:38 

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Hi there,
Sorry for the lack of information. What I was trying to do was to simulate the flow over a square cylinder in a channel. I was using reference "Numerical and modeling influences on large eddy simulations for the flow past a circular cylinder", Breuer 1998. Simulation Details So ran the same mesh, with the same time step, with the same boundary conditions, with these inlet values: First I achieved by Re=150 by setting U=L=Miu=1 and rho=150. (As per a tutorial I found online..) Second, I kept the real properties of air, and calculated the corresponding velocity. Results: For the first case, I got my vortex shedding and transient behaviour, with correct results for Cd and St when compared to the literature. For the second case I got a steady flow...! My question is that my Reynolds number is identical, yet I get very different results. This does not sound physical to me, which is why I'm trying to find an explanation for this. Many thanks for your help in advance. 

January 18, 2012, 10:26 

#5 
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Lucky Tran
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You are running LES? If so, did your solution relaminarize? The laminar solutions would produce very steadylike behaviour.
How did you setup the initial conditions? 

January 20, 2012, 05:09 

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Sorry for the lack of information again.
I was running Re=150, so definitively in the laminar flow regime. My only guess is that there maybe some mysterious Buckingham Pi group other than Reynolds number that is being ignored here. Or it could be a bug in the code (Ansys Fluent ), but doubt it... The initial conditions were as normal as possible, with my inlet velocity as the initial velocity prescribed in the domain. No turbulence (Laminar flow). Look forward to your comments! 

January 20, 2012, 09:00 

#7 
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duri
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Change in velocity would have changed frequency of vortex shedding. I guess you need to estimate frequency based on strouhal number and fix the time step.


January 20, 2012, 11:12 

#8  
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Lucky Tran
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Quote:
As duri stated, the other grouping is Strouhal number. Since you already know the shedding frequency from your first simulation, you should be able to calculate the new shedding frequency (by keeping Strouhal number constant). From the change in properties, it is highly unlikely that you can run the simulation again with the same time step. Your viscosity is 56 orders of magnitude off and density is off by factor of 150. Velocity is therefore different by 4 orders of magnitude. For the same grid, that means you would need to reduce your timestep by at least 4 orders of magnitude to capture the same flow physics in your simulation. 

January 22, 2012, 16:20 

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This is very interesting stuff!
Yeah, I think that the Strouhal number is definitively something that needs to be taken into account! Makes perfect sense now. I'll try to run this again and see what happens. Many thanks to "LuckyTran" and "duri" for your helpful inputs. I foolishly thought that just maintaining the Reynolds number would be enough: clearly not. Many thanks! 

Tags 
fluent, fluid mechanics, reynolds number, similarity 
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