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Supersonic Nozzle Exhaust Simulation

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Old   May 1, 2014, 21:28
Default Supersonic Nozzle Exhaust Simulation
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Mike H
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I am having difficulty setting up a FLUENT13 simulation of a supersonic nozzle. I am relatively new to CFD and have had some success with simple convergent-divergent nozzle geometries, however, I find it very difficult to get convergent repeatable results with even small changes to the problem setup.

My problem is basically a convergent divergent Venturi tube with a fixed inlet pressure. The nozzle exhausts into atmosphere at supersonic velocities and I am particularly interested in modelling the flow in the output domain.

From tutorials for 2d-axisymetric nozzles I have had limited success in modelling the flow within the nozzle without including the output domain in the simulation, but simply putting a pressure outlet boundary at the nozzle end.

Presently I am attempting to use 3D solid model of my part reduced to a 45 sector and using periodic rotational boundaries in the simulation. I have enclosed my entire cylindrical nozzle in a large cylindrical domain that is many times the diameter and length of the part with one end aligned to the nozzle inlet port.

My simulation setup is:
  • Density based solver, 3D, steady state
  • Standard k-e model
  • Default mesh with inflation at the part boundaries (looks reasonable to me). 352k cells.
  • Fluid is ideal gas air
  • Pressure inlet defined (with a supersonic guess pressure) with 1% turbulent intensity
  • All far-field boundaries defined as "outlet-vent" at 100kPa, turbulent intensity set to 0.
  • Solution method: Explicit, Roe-FDS, Least squares, second order flow, first order turbulence.

The simulation residuals are quite unstable and eventually it will diverge. If stopped at a minima the results look somewhat reasonable.

I would greatly appreciate any advice on how to get my simulation to converge more reliably without oscillation. I am hoping that I can find a setup that will allow me to vary nozzle geometry without disturbing the sim parameters.
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