Diffusor testcase
 

Hello,

I have a question concerning the testcase 8.2 of the 8th ERCOFTAC workshop in Helsinki, 1999. It's a 2D asymmetric diffusor flow.

I'm not quite sure about the inlet condition. There's said the inlet condition is a fully developed channel flow with a Reynolds-Number of 20000 based on the centreline velocity and the channel height.

Is the centreline velocity the max. velocity of the channel velocity profile ?

If my channel has a height of 1m and the fluid is air am I right if the the centreline velocity is:

v=Re*vis/rho/height=20000*1.8e-5/1.16/1=0.3134 m/s.

In the testcase there was separation but in my calcs there's attached flow. That why I'm so doubtful towards my boundary conditions.

Thanks for your help, Marat

Re: Diffusor testcase
 

(1). I don't know what you do with your calculation, the formulation?, the turbulence model?, etc. (2). The accurate prediction of separated diffuser flow is very difficult, because the existing two-equation k-epsilon turbulence model performs poorly in adverse-pressure gradient conditions, which exists in diffuser flows.

Re: Diffusor testcase
 

Hi John,

thanks for your advise. But what's your opinion towards my two questions ?

Regards, Marat

Re: Diffusor testcase
 

> Is the centreline velocity the max. velocity of the channel velocity profile ?

The answer is yes.

>If my channel has a height of 1m and the fluid is air am I right if the the centreline velocity is:

> v=Re*vis/rho/height=20000*1.8e-5/1.16/1=0.3134 m/s.

The answer is yes again.

> In the testcase there was separation but in my calcs there's attached flow.

As John said, this is a very difficult test case for turbulence models (so it's a good one in fact). I'm not so surprise that the numerical result doesn't show the correct behavior.

The proceedings of this workshop show how it was difficult to correctly predict the recircultaion zone.

Re: Diffusor testcase
 

hi there,

i don't exactly know all the details of your calculation, but you should be able to find some very usefull informations in the following publication:

H.-J. Kaltenbach, M. Fatica, R. Mittal , T. S. Lund, and P. Moin. "Study of flow in a planar asymmetric diffuser using Large Eddy Simulation" J. Fluid Mech. 390, 151-185, 1999.

I hope this helps. Sincerely,

Frederic Felten CFD Lab, UT Arlington. http://utacfdb.uta.edu/

Re: Diffusor testcase
 

(1). I don't see anything wrong with the maximum centerline velocity, if it is fully-developend. Sometimes, people use the averaged inlet velocity. (2). My estimate of the velocity is around 1m/sec, which is close to your number.

Re: Diffusor testcase
 

John,

your estimate is around 1m/sec. But that 3 times higher than my estimate ! Where does that difference come from ?

Regards, Marat

Re: Diffusor testcase
 

(1). It comes from my estimate of the kinematic viscosity of the air.

Re: Diffusor testcase
 

So, what is your estimate for the kinematic viscosity of air ?

Regards, Marat

Re: Diffusor testcase
 

(1).In the book of Boundary Layer Theory, by Schlichting, the kinematic viscosity of air at 68degreeF, 14.7psi, is 160x10E-06 FT*FT/SEC. (2). You get 3.2 FT/SEC when U is calculated from U=Re*nu/L, with RE=20,000. So, U is around 1m/sec.

Re: Diffusor testcase
 

I'm sorry, but you get U=3.2 FT/SEC if L=1FT but if L=1m=3.2FT, it comes U=0.097FT/SEC, which leads to U=0.31m/s.

Best regards,

Sylvain

Eureka!
 

(1). I think, you are right. (2). That's why I said, it was an estimate. It's important to double check the result. Thank you. By now everybody knows how to compute the Reynolds number.

Re: Diffusor testcase
 

Marat, as John pointed out your issue is probably with your turbulence model. I guess you're using k-e (because it's so popular) unfortunately k-e is known to be poor for adverse pressure gradients. you may want to use the k-omega. See David Wilcox's "Turbulence Modelling for CFD"