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Problem with UGS NX 6 advanced flow

Problem with UGS NX 6 advanced flow

Problem with UGS NX 6 advanced flow

(OP)
I am working on model that will simulate the flow inside a rocket combustion chamber and nozzle.  I know NX has the capability to model high speed flow but I am having trouble getting even a basic model working.  I get my highest velocities at the nozzles throat and they are subsonic.  Obviously the mach number at the exit of the nozzle should be higher then 1.  The part file that i am using for my model was used successfully with cosmos flowworks in the past so its not the geometry of the file thats causing the problem.

Any suggestions on what kinds of boundary conditions i should use to specify the flow velocity, pressure, and temperature at the combustion chamber inlet and nozzle exit??

RE: Problem with UGS NX 6 advanced flow

Hello!,
If you are simulating a flow with a very high speed you will have to decrease the solution time step. Edit solver parameters > 3D Flow Solver > Time Step = 0.001 sec.

If you prescribe a velocity at the inlet simply stimate the distance a particle will flow from the inlet to the outlet and compute the time spend based in the inlet velocity, then the time step should be smaller than this value.

Also, instead of using the default "mixing length" turbulent model use "the standard two-equation k-ε model", takes longer to solve but convergence is more robust.

Another important suggestion: ALWAYS prescribe at inlet mass flow rates instead velocity or volumen flow rate, you will note that convergence will be faster.

Also, make sure that mass equilibrium exist between mass inlet & outlet, this is critical to assess a convergence solution.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
 
IBERISA
48011 BILBAO (SPAIN)
WEB: http://www.iberisa.com

RE: Problem with UGS NX 6 advanced flow

(OP)
Blas,
Thanks for your reply.  My university just upgraded to nx 7.5 so I'll be using that from now on.

Changing the time step to 0.001 improved the convergence for my model - prior to doing that i had to pause the solution otherwise it would keep cycling through an endless pattern.  

I've prescribed the mass flow rate at the inlet and exit with the inlet flow and outlet flow boundary condition but am having trouble with setting the pressure and temperature of my fluid.  Do you have any advice for setting those properties at the inlet/exit?  Right now it seems like the only way to have control over those is to set them in the ambient conditions.

When I get a solution that converges I'm still getting my maximum Mach numbers at the throat and they are still subsonic.  Its possible that my mesh isn't correct.  I created the mesh with a fluid domain with a relative size of 0.05.  I'm convinced that I will need to add a fluid surface  boundary layer mesh but am getting fatal errors in my solution when using this option.  

Do you have any advice for creating a good mesh for a high speed flow?

again thanks for your input any assistance is greatly appreciated.
Glen

RE: Problem with UGS NX 6 advanced flow

Hello!,
The exact boundary conditions depends of the problem to solve, basically if you prescribe in the inlet a mass flow rate then in the outlet you must prescribe ambient pressure, then the CFD code will compute the static pressure at both inlet/outlet, as well as mass flow at outlet.

Also, you must mesh the model correctly, a minimun of six elements across the passage should be used, if not the fluid flow will not be developed correctly. Mesh density is critical, repeat the solution using different mesh densities, this will help you to achived convergenced solutions, and to see how the mesh density affect results.

Best regards,
Blas.
 

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
 
IBERISA
48011 BILBAO (SPAIN)
WEB: http://www.iberisa.com

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