CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   Main CFD Forum (http://www.cfd-online.com/Forums/main/)
-   -   problem in Cf in flow over flat plate (http://www.cfd-online.com/Forums/main/115397-problem-cf-flow-over-flat-plate.html)

mb.pejvak March 29, 2013 23:46

problem in Cf in flow over flat plate
 
2 Attachment(s)
hi friends;
I am working on modeling flow with a non-zero pressure gradient (NZPG) over flat plate. Gamma-Retheta and SST K-omega are used as transition and turbulent models. flow with zero-pressure gradient was solved well and its results were accurate according to ERCOFTAC prediction. but in NZPG flow, cf is not consistent with the experimental results. I used TVD (van leer limiter, mid-mod limiter) and QUICK schemes in advection descritization and my grid (as it is shown in picture) has about 53000 cells and first cell near wall is located 1e-6 m from the wall.
does anyone know where the problem stems from?
if any information is needed in order to clarifying more, please ask to explain more?
thanks in advance

Far March 30, 2013 23:58

consider this paper:


http://cfd.mace.manchester.ac.uk/twi...a-re-theta.pdf

mb.pejvak April 2, 2013 03:26

Quote:

Originally Posted by Far (Post 417384)

Dear Far;
thank you so much for your answer. I had read it, but I read it again. unfortunately, it does not anything that can help me to solve the problem. I generated finer grid with about 68000 cells, and change convection scheme, but results did not improve. I think I should find results of numerical modeling by using this transition model (such as intermettency and Retheta contour over NZPG flat plate). do you know where I can find them? I have tried, but I can find anythings.

Far April 2, 2013 11:05

Did you implement Gamma-theta model in your own code?

are you solving 2nd order scheme for turbulence quantities as well?


What are the inlet turbulence boundary conditions?

mb.pejvak April 2, 2013 23:36

1 Attachment(s)
Quote:

Originally Posted by Far (Post 417874)
Did you implement Gamma-theta model in your own code?

are you solving 2nd order scheme for turbulence quantities as well?


What are the inlet turbulence boundary conditions?

Yes I implement Gamma-Retheta in self-develop code. I applied SST K-Omega and simulated ZPG and NZPG over a flat plate.
the boundary condition I used set based on the date of T3C4 (ERCOFTAC data). other boundary condition are set like in "Correlations for modeling transitional boundary layers under influences of freestream turbulence and pressure gradient". tu. intensity in leading edge calculated based on try and error and the formula Langtry introduced in the end of his thesis. U in far field changes based on change of the cross-sectional area in order to impose pressure gradient (slip wall; wall shear=0).
following figure can explain better the geometry and boundary condition. If anything is vague, please ask to explain it.

Far April 2, 2013 23:44

Do you have access to some commercial code : Fluent/CFX? Try first there with same mesh and see the results...

I can also try on your mesh in Fluent or CFX if needed... Let me know as now a days I am also going to publish our resutls on LPT turbine transition study under various Reynolds number, turbulence intensities and upstream wakes.

mb.pejvak April 3, 2013 01:19

Quote:

Originally Posted by Far (Post 417966)
Do you have access to some commercial code : Fluent/CFX? Try first there with same mesh and see the results...

I can also try on your mesh in Fluent or CFX if needed... Let me know as now a days I am also going to publish our resutls on LPT turbine transition study under various Reynolds number, turbulence intensities and upstream wakes.

Dear Far;
Thank you so much for your kind suggestion. I worked a little with Fluent, but I think I am not experienced enough for applying this problem, so it may take a lot of time to do, although if it is necessary, I will do it. after some investigation on my result, I found out intermittency is not simulated correctly. I had faced this problem when I simulate transition on ZPG flat plate, but with some methods (using under relaxation and source term linearization based on Patankar's suggestion) it was fixed, and results were consistent with benchmark data.

omario April 5, 2013 12:11

Detecting transition
 
Dear users,

Your help would be kindly appreciated as I am really stuck on my dissertation.

I am struggling to plot the Cf vs Re.x graph and need this to check the location of transition. I am uncertain when it comes to the reynolds number as I have tried using an expression/variable but still could not get the graph.

I want to see if there is turbulence and transition on my flat plate but I am not sure if it has been set up correctly. There is a boundary layer on my model but remains uniform across a long distance.

All surfaces have been specified as symmetry other than the inlet, outlet and flat plate is obviously a wall. I specified a roughness height hoping that it would show more turbulence/transition but I cannot see anything yet.

Thank you for your time to read this

Kind regards

Omar

desmoge October 31, 2013 19:50

Hey Mehdi(mb.pejvak), have you finally got the skin friction validation for your problem?
I am currently working with the same geometry and I think that your poor validation might be related to the shape of the upper boundary. How did you get that? Have you compared the freestream velocity data with the results of your simulation?

rajeshamech November 22, 2013 00:13

Dear Sir,

I am Rajesh. A from India, I am doing research in the modelling of transitional flows in the RANS framework. I am trying to simulate T3C2 flat plate test case for which i am not finding the geometric details in internet. In ERCOFTAC website also they have provided only the experimental results. If you can please provide me with the geometric details of T3C2 flat plate test case, which will be very use full for my work.

desmoge November 22, 2013 01:01

Hey Rajesh, try with equation (23) from the article "Correlations for modeling transitional boundary layers under influences of freestream turbulence and pressure gradient" K.Suluksna, P.Dechaumphai, E. Juntasaro.
They claim that equation (23) should give you the cross section height trend for all cases T3C but the T3C4 (for that you should use equation (22) ). I'm working on T3C4 case and unfortunately that didn't work for me.

rajeshamech November 22, 2013 01:10

Thank you so much for the reply..

desmoge December 1, 2013 19:38

Hey Rajesh, have you finally tried the eq.(23) to shape your domain?
Can you actually get turbulence decay validation as well as the same free stream velocity profile by using the equation suggested in that article? I'm curious about it since that did not work for me.

rajeshamech December 2, 2013 01:13

Hi, i tried eq.(23) for the domain to simulate T3C2 test case, the turbulence decay initially didn't match. Then i tried for different viscosity ratios. Finally for viscosity ratio 8 the turbulence decay matched. The free stream velocity profile is matching in favorable pressure gradient region but in adverse pressure gradient its under predicted.


All times are GMT -4. The time now is 19:46.