CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   Main CFD Forum (http://www.cfd-online.com/Forums/main/)
-   -   Oscillation at the boundary (http://www.cfd-online.com/Forums/main/437-oscillation-boundary.html)

arief January 7, 1999 09:10

Oscillation at the boundary
 
I use MacCormack+FCT to solve the conservation law equations. My problem is I got an oscillation at the boundary. I suspect this is caused by the FCT algorithm can not be fully implemented at the boundaries (FCT needs some grids which are beyond the boundaries). Has someone any idea to solve this problem ?

Thanks in advance,

arief

Thanasis January 13, 1999 08:32

Re: Oscillation at the boundary
 
I gave solution to such problem. I used the MacCormack-Explicit and an Implicit Scheme, which comes from the Lerat family to solve two dimensional Subcritical and Supercritical, open-channel flows. Maybe I could help you if you gave me more information about your problem. What type of problems are you trying to simulate, what is "FCT"? Have you added some artificial viscosity to your equations? Are your equations applied on a transformed grid or not?

Mail me if you want to Klonidis@otenet.gr

Thanks

Dan Williams January 16, 1999 21:24

Re: Oscillation at the boundary
 
Hi there,

I see you still haven't figured your problem out arief.

Since I (and many others) have done a plethora of computations with it, and it works fine, it must be an implementation problem. I have also implemented a MacCormack+FCT code as well and I've seen no problems at boundaries, as you mention. What sort of calculations were you doing again??

You could try the free LCPFCT routines, these ones definitely work.

By "needing grids beyond the boundaries", I assume you mean you're using ghost cells to set the boundary conditions. It is common to do this for many schemes, not only FCT, so you must have an implementation problem.

Dan W.

Thanasis January 19, 1999 07:45

Re: Oscillation at the boundary
 
I finally found what FCT is. It's a good way of damping your physical quantities of your problem. As I mentioned in previous report, I used the Mac Cormack explicit technique but with different smoothing and it worked. Lately I've used the Beam & Warming Implicit, which introduces second order differences, and it also worked. So I definitely agree with Mr. Williams. It seems to be an implementation problem. I also point out that not all the damping techniques do they work perfectly. I think that you should pay attention to the way you handle the solid boundaries. Of course the method of ghost cells, which is classical, works in many problems. As far as I am concerned, I didn't use it because it didn't work very well in problems with compound geometry. But tell as about the problems you are trying to solve.

arief January 19, 1999 08:51

Re: Oscillation at the boundary
 
Thank you for your responds,

I am now working on simulation of fuel injection systems. my case occurs for the internal flow in the fuel pipe line.

Like I said, FCT works very well to eliminate the oscillations but only in the internal grids. If shocks reach the boundaries oscillations come out again.

I have tried to use the ghost cells and it doesn't produce a satisfactory results. Perhaps you're right, this is an implementation problems, i will try to find the errors.

Thanks,

arief


All times are GMT -4. The time now is 13:42.