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-   -   Which Descritisation Scheme to be used? (http://www.cfd-online.com/Forums/fluent/32205-descritisation-scheme-used.html)

HN Das September 25, 2003 06:30

Which Descritisation Scheme to be used?
 
For a high Reynolds Number (in the order of 10^8) incompressible (steady) flow problem, I have used different decritisation schemes - 1st order Upwind, 2nd order Upwind and QUICK. I am surprised that 1st order upwind scheme is giving more accurate result in terms of drag when validated against experimental results. As higher order upwind schemes are supposed to be more accurate, the result is bit confussing to me. The trend of result is consistant and compared with number of experimental results.

I understand, by default, fluent uses central difference scheme for defussive fluxes and the scheme user chooses is applied for convective fluxes.

Experts may please comment and advise.

thomas September 25, 2003 07:29

Re: Which Descritisation Scheme to be used?
 
Hey, Sorry i am not an expert of discretization scheme but I could give you an opinion. By default I never use the 2nd order upwind scheme (not well developped in fluent). Then I do think that the Quick scheme cannot be applied to every type of mesh. You should check on the Fluent online-documentation if your mesh fits to a quick scheme use.

Hope this help. thomas

ap September 25, 2003 07:58

Re: Which Descritisation Scheme to be used?
 
I don't think it's a problem of implementation, maybe of stability. Also, as said in the manual, the second order discretization scheme is implemented in order to reduce numerical diffusion, so it should improve accuracy.

However, the QUICK scheme implemented in FLUENT can be used on all kinds of grids (structured and unstructured). It's more accurate on structured grids, if they are aligned with the flow direction.

If you have an unstructured grid or a hybrid one, FLUENT applies the second order discretization scheme at the faces of non hexahedral cells.

ap September 25, 2003 08:07

Re: Which Descritisation Scheme to be used?
 
First order is suitable only in simple cases, with grids aligned with flows. But if the flow doesn't follow this behaviour, second or higher order schemes should give better results.

Try doing a calculation using first order (you can use the one you've already done). When converged switch to second order scheme to see what happens.

Hi :)

ap

Tom September 25, 2003 11:00

Re: Which Descritisation Scheme to be used?
 
I agree with ap. I believe from my experince the 2nd order scheme should give better results. What turbulence model are you using? this will also change your results considerably.


HN Das September 26, 2003 00:11

Re: Which Descritisation Scheme to be used?
 
Thanks Mr Thomas. My mesh is aligned to flow and fluent on line documentation says that 1st order upwind can be used for this type of mesh.

My problem is when I use second order upwind or QUICK for the same problem (same mesh), my results deviate from experimental results. I am unable to understand why 1st order upwind scheme is giving better result than 2nd order.

Das

HN Das September 26, 2003 00:19

Re: Which Descritisation Scheme to be used?
 
I have used k-e (all three standard, RNG and realizable), k-w and RSM models. Except k-w all the models have given similar results.

In all the instances, when I use 2nd order upwind scheme my results starts deviating from experimental results.( Results with 1st order upwind scheme is close to experimental findings).

I am using a structured grid aligned to flow. It is a convection dominated flow, Reynold's No is very high. Das

Helge September 26, 2003 07:45

Re: Which Descritisation Scheme to be used?
 
General speeking results from 1st order scheems must be the same as from 2nd order scheems when you have an infinite fine mesh.

From this it is clear that a coarser mesh will give different results using different discretisation scheems if the solution is mesh-dependent.

So before looking into turbulence models you should try to obtain a mesh independent solution. Using a 2nd order scheme, you will reach this state on a coarser mesh compared to a 1st order scheme.

If you have access to solution dependent mesh refinement this feature will help you saving elements.

If a finer mesh brings you into trouble regarding the validity of your turbulence model (i.e. y+ gets too small) you have of course to use an appropriate turbulene model which can handle high resolution grids.

zwdi September 26, 2003 10:50

Re: Which Descritisation Scheme to be used?
 
Hi Helge, Do you know how to get mesh independent solution? Thanks. Zwdi

ap September 26, 2003 14:56

Re: Which Descritisation Scheme to be used?
 
A solution is said to be grid independent if a further refinement of the grid doesn't implicate a change in the solution.

So, to obtain a grid independent solution, you have to refine your grid until the solution stops to change in consequence of grid refinements.

Hi :)

ap

zwdi September 27, 2003 19:39

Re: Which Descritisation Scheme to be used?
 
Thanks.Is mesh independent solution equal to true solution? Thanks again. Zwdi

zwdi September 27, 2003 19:45

Re: Which Descritisation Scheme to be used?
 
Hi Helge, As you said, "If a finer mesh brings you into trouble regarding the validity of your turbulence model (i.e. y+ gets too small) you have of course to use an appropriate turbulene model which can handle high resolution grids". Could you give me guides which kind of turbulence model efficiently handle high resolution grids? Thanks. Zwdi

ap September 28, 2003 08:14

Re: Which Descritisation Scheme to be used?
 
Just check turbulence models limitations in FLUENT manual.

For example, if you adopt k-eps models with wall-functions, y+ have to be between 30 and 60. So you can't refine your grid too much in order to respect that condition.

Hi :)

ap

ap September 28, 2003 08:27

Re: Which Descritisation Scheme to be used?
 
The grid-independent solution is just the solution you obtain when your space discretization doesn't influence the solution anymore.

This doesn't mean it's the physical solution of your problem.

Hi :)

ap

HN Das September 30, 2003 06:53

Re: Which Descritisation Scheme to be used?
 
Turbulence Models are not spesific to resolution of grid. I mean, a same turbulence model can be used for a coarse mesh or a fine mesh. You have to look for an appropriate WALL FUNCTION to match with your resolution (in terms of y+). For example, standard wall function works well within a range of y+ between 30 to hundred. Any wall function, based on log law, is suitable for that y+ range. For mesh of higher resolution ( Y+ < 5), you can use two layered wall function like enhenced wall treatment.

You can also use turbulence model like, v2f, to avoid use of turbulence model. I hope this info may be useful for you.

Das

zwdi September 30, 2003 10:34

Re: Which Descritisation Scheme to be used?
 
Thanks. I could not find v2f model available in the VISCOUS list. Do you know how to make it work by using text commands in Fluent? Zwdi

HN Das September 30, 2003 22:42

Re: Which Descritisation Scheme to be used?
 
I have never used v2f model while working with fluent. As far as I know, Fluent 6.0 does not contain this model. You have to ask support group of fluent, if it is available with any of Fluent version.

Helge October 1, 2003 04:49

Re: Which Descritisation Scheme to be used?
 
I would recommend the SST model. It blends the k-e with a k-w model near the wall. The k-w behaves well on fine grids (y+ around or below 1). The normal k-e low reynolds model from my experience converges very slow.

zwdi October 1, 2003 11:02

Re: Which Descritisation Scheme to be used?
 
Hi Helge, I tried SST before. But there are many large shoots and vibration during converge process. Could you give me good suggestions about using SST model? So that I can made solution smoothly converged. Thanks a lot. Zwdi


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