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Why is my flow choking at two points?

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Old   July 19, 2016, 12:08
Default Why is my flow choking at two points?
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Sabomb
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I have trouble simulating a straight nozzle (exit area 340 mm≤). The plenum to the nozzle has an inlet of 260 mm≤.

According to the simulation result, as shown in the figure, I should be getting a choked flow at the inlet to the plenum. Instead I get about mach 1.07 at the plenum inlet and about mach 1 at the nozzle exit.

I use a mass flow inlet condition with 0.145 kg/s air with ideal gas assumption; Coupled solver, Steady state solution, 2D Axisymmetric and RSM Turbulence model.

The solution I expect to achieve is a sub-sonic flow at the nozzle exit, and hence blame the choking at the plenum inlet for the loss of pressure. Am I not using the right physics? Or is it my understanding of choked flow incorrect?

Thanks in advance!
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choked flow, compressible flow, fluent, gas flow, nozzle

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