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October 27, 2016, 07:08 
low speed compressible flow

#1 
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Abolfazl
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Hi everyone.
I am trying to simulate compressible viscous flow around a circular cylinder by implementing VANLEER scheme. for supersonic flow the results are satisfactory but for low mach number (mach=0.3 and Reynolds=300) I can't simulate the vortex shedding. I only get two stationary small vortexes. can anybody suggest me what should I do!?? thanks. 

October 27, 2016, 07:24 

#2  
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Filippo Maria Denaro
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Quote:


October 27, 2016, 08:22 

#3  
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Abolfazl
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but my mesh is fine enough. the problem is as i increase the mach number the length of steady symmetrical vortexes increase and as i decrease the mach the length decrease. I don't think this is related to mesh. 

October 27, 2016, 08:26 

#4 
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Filippo Maria Denaro
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Assuming the speed of sound is the same, you are changing both the Mach and the Reynolds number. If you decrease it and your scheme has numerical viscosity, your actual Re number can be as low as to produce a laminar steady solution. Ensure that at low Re you have no turbulence model active in your code.


October 27, 2016, 08:35 

#5  
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Abolfazl
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my numerical method is VanLeer and it is first order upwind. and i don't have turbulence model. 

October 27, 2016, 08:51 

#6 
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Filippo Maria Denaro
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the solution with first order upwind is full of artificial viscosity...check your grid is so fine to get the cell Reynolds number =O(1) everywhere


October 27, 2016, 09:24 

#7  
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Abolfazl
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I have checked with really fine grid and I'm almost certain the grid is fine enough. but don't you think that Van Leer doesn't work properly for low mach numbers or for viscous flows!? 

October 27, 2016, 17:29 

#8 
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Martin Hegedus
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Can't say if Van Leer will work or not, but, since your original post reads as if you may have written the code, did you divide the Reynolds number by the Mach number? For a nondimensionalized compressible solver the Reynolds number is, in general, calculated using the speed of sound rather than the velocity therefore the inputed Reynolds number (which in general is calculated using velocity) is divided by the Mach number internal to the code.


October 30, 2016, 04:18 

#9  
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Abolfazl
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Thank u for your reply. No, I didn't divide the Reynolds number with Mach number. I just set Mach and Reynolds number. and the Reynolds number is base on free stream velocity. 

October 30, 2016, 09:47 

#10 
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Martin Hegedus
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Given your reply, I'm not sure my post was understood correctly. In general, the Reynolds number that goes into compressible solvers is based on the freestream velocity. So your post reads as if your input is correct. However, internal to a compressible code the Reynolds number will be modified to be based on the speed of sound by dividing by the Mach number. So, if you have written your own code, make sure your equations are nondimensionalized consistently.


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