# CFD Analysis of Airfoil at Mach = 0.0

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 October 8, 2012, 04:39 CFD Analysis of Airfoil at Mach = 0.0 #1 Senior Member   Ali Join Date: Jan 2012 Location: Pakistan Posts: 135 Rep Power: 10 Hi folks I want to perform a 2-D airfoil CFD analysis at Reynolds number of 9.0M and Mach number 0.0. Is this possible? __________________ Best Regards Ali

 October 8, 2012, 04:47 #2 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 No. Check your numbers, there must be a typo. Mach number = 0.0 means your velocity is zero.

 October 8, 2012, 07:46 #3 Senior Member   Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 19 Technically yes, as the Mach number is just a function of the compressibility of the fluid. If you have not turned the energy equation on, the physics modelled disregard any compressibility effects, and therefore Ma = 0. Effectively, the bulk modulus and ratio of specific heats go to infinity; but your velocity can still be non-zero. You can set your Re by changing the fluid viscosity, scaling the model (to achieve a specific L) or modifying the velocity. Assuming Ma = 0 for low Ma numbers (~<0.3) is common and generally accepted within the industry. Hope this helps, Stu

 October 8, 2012, 07:57 #4 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 Ok, thats true. If you have M = v/a with a = infinity. But if you artificially remove the speed of sound from your equations, it does not seem appropriate to me to use the phrasing "Mach number" at all. Maybe this is the common practice .

 October 8, 2012, 08:07 #5 Senior Member   Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 19 Rodriguez, You're right, I think it is quite illogical to think of modelling a flow with Ma = 0.0; but I bet if Ali had said "incompressible flow", you would have no problem realising what he meant. Ma = 0.0 is just a bad (or at least, unnatural) way of saying flow is incompressible! Stu RodriguezFatz likes this.

 October 8, 2012, 08:11 #6 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 Absolutely! I'm no "airodynamics"-guy, so thanks for the free lecture

 October 8, 2012, 10:19 #7 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 What if, you put the very very very large enclosure (order of 500 as compared to body characteristic length) and specify the same static and total pressure (101325 pa to be specific)? In fact we did this for the Fan simulation, by specifying this condition so that we can truly model the engine and atmosphere. Although the flow in the vicinity of fan was not with Mach = 0.0. Therefore it is just a technique to model the flow with real boundary conditions. What you guys think about this?

October 10, 2012, 22:49
#8
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Ali
Join Date: Jan 2012
Location: Pakistan
Posts: 135
Rep Power: 10
Quote:
 Originally Posted by stuart23 Technically yes, as the Mach number is just a function of the compressibility of the fluid. If you have not turned the energy equation on, the physics modelled disregard any compressibility effects, and therefore Ma = 0. Effectively, the bulk modulus and ratio of specific heats go to infinity; but your velocity can still be non-zero. You can set your Re by changing the fluid viscosity, scaling the model (to achieve a specific L) or modifying the velocity. Assuming Ma = 0 for low Ma numbers (~<0.3) is common and generally accepted within the industry. Hope this helps, Stu

Flow is incompressible (M < 0.3), So what will be the velocity then for M=0.0.
__________________
Best Regards
Ali

October 11, 2012, 05:40
#9
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Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
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Quote:
 Originally Posted by Hybrid Flow is incompressible (M < 0.3), So what will be the velocity then for M=0.0.
Write down the definition of Reynolds number, solve for velocity and insert all your variables. (density, viscosity, Reynolds number and length-scale)

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