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help with divergence in jet flow simulation

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Old   April 7, 2017, 07:47
Default help with divergence in jet flow simulation
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Marco Seid
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hi all,

ive searched and read over 20 threads on this forum trying to figure out my problem but to no avail. hopefully i can get some help here.

i am using ansys 18.0 and im trying to simulate a jet flow of air (at 50 m/s, 500K) in a coflow of air (at 5m/s, 1500K). theres no combustion or chemical reactions involved.

i made my geometry and mesh (see attached pics) and in the setup i am solving for the energy eq and standard k-e eq. i use hybrid initialisation for all my calculations. when the properties of air (density, cp, k, mu) are constant, the solution eventually converges. when i change the density to ideal gas, it still works fine. however, when i change cp, k and mu to kinetic theory (which is what i need), the solution always diverges.

ive tried running with constant properties for 50 iterations, then changing to kinetic theory and continuing but it didnt work.

ive tried turning off the energy eq for 50 iterations, then turning it on with kinetic theory air properties and continuing but it didnt work.

ive tried decreasing the under-relaxation factor of energy (under solution controls) from 1 to 0.9 but it didnt work. (should i decrease it more?)

ive tried decreasing the relaxation factor of energy and turbulent viscosity from 1 to 0.9, turning off energy eq for first iterations, then turning it back on with kinetic theory air properties but it didnt work.

what should i do? any suggestions/advice is greatly appreciated
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Old   April 9, 2017, 06:37
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Marco Seid
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update: I used the coupled solver (under solution > methods > pressure-velocity coupling) and the solution didnt diverge.
  • air density = ideal gas
  • air mu, cp, k = kinetic theory
  • did not need to reduce under-relaxation factors, or turn off energy eq for first 50 iterations
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