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-   -   CFD Analysis of Airfoil at Mach = 0.0 (http://www.cfd-online.com/Forums/fluent/107848-cfd-analysis-airfoil-mach-0-0-a.html)

 Hybrid October 8, 2012 04:39

CFD Analysis of Airfoil at Mach = 0.0

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?

 RodriguezFatz October 8, 2012 04:47

No.
Check your numbers, there must be a typo. Mach number = 0.0 means your velocity is zero.

 stuart23 October 8, 2012 07:46

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

 RodriguezFatz October 8, 2012 07:57

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 :(.

 stuart23 October 8, 2012 08:07

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 October 8, 2012 08:11

Absolutely! I'm no "airodynamics"-guy, so thanks for the free lecture :)

 Far October 8, 2012 10:19

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?

 Hybrid October 10, 2012 22:49

Quote:
 Originally Posted by stuart23 (Post 385518) 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.

 RodriguezFatz October 11, 2012 05:40

Quote:
 Originally Posted by Hybrid (Post 386071) 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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