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tranhophu September 12, 2011 23:45

"Spurious oscillation"
 
Dear all,
I’m a PhD student in France and my question is about the problem “spurious oscilation” in Solid Rocket Motor.
I’m using the source code of my group to solve Navier-Stokes compressible equation 3D (written in DENSITY BASE form) with collocated grid and Explicit Scheme Euler 1st order with Upwind TVD 2nd order and Splitting Flux with ROE approximation scheme used.


We’re looking forward to Mach number which is equal to 1 at throat. After some iteration, we have had the Mach number which is equal to 1 at throat and the mach number in the chamber is about 0.3. In addition, the field of vorticity seems also ok. But when we observe the field of pressure into small range near the mean pressure in chamber (1.7 bar) , we have the problem . There is “spurious oscilation”. On the contrary, into large range of pressure the “spurious oscilation” doesn’t occur. Oscillation pressure in the chamber is like the following figure :
http://img849.imageshack.us/img849/2184/image0202.jpg




I don’t know how to define the problem which I met and how to resolve it. I wonder if my problem is “pressure-velocity decoupling” on collacated grid or anything else?
Many thanks,
Best regards, and thank you very much for your helpful advices.

davide_c September 13, 2011 03:34

I'm absolutely not expert of finite volumes - more accostumed to finite elements - but usually this problem occurs for a matter of localization of the variables (in finite elements, spurious oscillation occur if you use not compatible elements for velocity and pressure) so probably you have to check for the collocation of your variables on the grid - staggered grids are often used , if i remember correctly...

leflix September 14, 2011 17:52

Hi Tranhophu,

As suggests David this kind of spurious oscillations may arise from the collocation of velocity and pressure. But generally it occurs for incompressible flows.
For compressible flows as you have the state law which links density, temperature and pressure you can compute the pressure from this expression and thus you are not subjected for this kind of spurious oscillations.
Density is computed from mass conservation equation, and temperature from energy equation.

But it is possible also, even for compressible flows, to use SIMPLE like algorithm to compute the pressure and then you use the state law to compute the density.

In using this pressure correction equation, if the variables are collocated, it is possible that spurious oscillations arise in the pressure field. In this case the problem is solved while using Rhie and Chow interpolation technique.

But if it is not the reason, check your TVD scheme, because if it is not well implemented oscillations may arise due to dispersion error.

good luck


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