CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   FLUENT (http://www.cfd-online.com/Forums/fluent/)
-   -   why I can only use SIMPLE of 1 order? (http://www.cfd-online.com/Forums/fluent/29831-why-i-can-only-use-simple-1-order.html)

 quain tchew May 29, 2002 04:21

why I can only use SIMPLE of 1 order?

I'm computing a combustion chamber of an engine which has two subsonic air inlets, a fuel inlet and a nozzle.

I set 1 the air inlets as pressure inlets with totol pressure of about 950000 Pa; 2 the fuel inlet as mass flow inlet with mass flux of 0.09kg/s; 3 the nozzle as pressure outlet with 26000Pa.

I use ideal-gas model, K-E turbulence model with standard wall function.

Only with SIMPLE method of one order, I got a converged result. But when I switch to two order simple method, it didn't converge.

I don't know why? Any one give some advices?

 Jin-Wook LEE May 29, 2002 04:49

Re: why I can only use SIMPLE of 1 order?

It is very natural that 1st order is cheaf(CPU time) and easy to converge and 2nd order is expensive and difficult to converge.

1. 1st order is easy to converge because of numerical diffusion which has the effect of smoothing the result.

2. To enhance the convergence, there are couple of approaches such as,

- good mesh(low skewness)

- low under-relaxation

- step by step approach(e.g., mixing only --> reaction)

- and so on

Sincerely, Jinwook

 Quain Tchew May 30, 2002 05:54

Re: why I can only use SIMPLE of 1 order?

Thanks

I have tried low under-relaxation, but....

Yes, maybe my grid is not good.

 moiami June 3, 2002 02:34

Re: why I can only use SIMPLE of 1 order?

Try this: do 1st order then after it is converged carry on the simulation but replace 1st with 2nd (like in the tutorial). You'll see a rise in peak for the residual plot b4 it peters down to a convergence.

But b4 that you have to be clear on why you're using 2nd order. You may not have to, you know (see message below: Posted By Gavin <gavinbrady85@hotmail.com> Date: Wed, 29 May 2002, 4:12 a.m. in Response To: convergence problems (jeremy))

 All times are GMT -4. The time now is 04:18.