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2D: Nozzle Inlet Mach No. Affected By Downstream Injector

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Old   September 18, 2015, 16:59
Default 2D: Nozzle Inlet Mach No. Affected By Downstream Injector
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Carlos
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Hello all,

As the title suggests, I'm modeling a 2D nozzle with an injector modeled downstream of the throat.

My baseline, axisymmetric case yields the proper nozzle inlet Mach number of ~0.5.

However, upon running my 2D planar injector case, the nozzle inlet Mach number increases to ~0.6.

I've checked the BCs and reference values time and time again and cannot figure out why the discrepancy is occurring. The reference area of the nozzle inlet is the same as my axisymmetric case, and only modify the reference depth to get the proper nozzle mass influx (changing said value doesn't affect the nozzle inlet Mach #).

Any and all input would be greatly appreciated.

- Carlos
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Old   September 18, 2015, 19:24
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Are you referring to the reference values pane? The reference area and reference depth in the values pane do not affect the simulation results (it only affects lift, drag coefficients, etc). Changing these references values cannot change your mass-influx.
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Old   September 18, 2015, 21:24
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Quote:
Originally Posted by LuckyTran View Post
Are you referring to the reference values pane? The reference area and reference depth in the values pane do not affect the simulation results (it only affects lift, drag coefficients, etc). Changing these references values cannot change your mass-influx.
But it does affect my mass-influx. The default value gave me ~3.5x more mass at the nozzle inlet than my baseline case.

But that's not what concerns me; it's the increase in nozzle inlet Mach number when incorporating the downstream injector that concerns me.
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Old   September 22, 2015, 07:20
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I had the same problem while analyzing a de laval nozzle in 2D and i could not achieve appropriate mach number at the outlet. Fluid was pressurized oxygen.
Upon converting the problem to 3D and then analyzing, i could get appropriate mach number, density, pressure etc
I do not know if it could help in your case but might as well give it a try.
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Old   October 1, 2015, 18:41
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I've learned that a 2D planar case with only the nozzle, no injector, and the extended domain for the exhaust flow yields the same increase in Mach number at the nozzle inlet.

Any and all ideas as to why the nozzle inlet Mach number increases when switching from 2D axisymmetric to 2D planar would be greatly appreciated!
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Old   October 2, 2015, 08:20
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What are the inlet&outlet B.C.'s? Mass-flow inlet and pressure outlet? You are specifying the same mass flow rate for each and adjusting the reference depth to achieve the same mass-flux for both simulations? Why are you not using a mass-flux boundary condition if you want to fix both inlet mass-fluxes to be the same? Or are you using a pressure inlet?

You are using the exact same geometry & mesh for both the axisymmetric and 2D planar simulation? Presumably, instead of an axis boundary condition you have a symmetry boundary condition for the planar case?
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Old   October 2, 2015, 14:59
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Quote:
Originally Posted by LuckyTran View Post
What are the inlet&outlet B.C.'s? Mass-flow inlet and pressure outlet? You are specifying the same mass flow rate for each and adjusting the reference depth to achieve the same mass-flux for both simulations? Why are you not using a mass-flux boundary condition if you want to fix both inlet mass-fluxes to be the same? Or are you using a pressure inlet?

You are using the exact same geometry & mesh for both the axisymmetric and 2D planar simulation? Presumably, instead of an axis boundary condition you have a symmetry boundary condition for the planar case?

The BCs are Pressure In and Pressure Out for both the baseline (axisymmetric) case and 2D planar (full nozzle with or without injector). I've attached some figures for clarification.

I modeled the full geometry for the planar case in order to see the full effect of secondary injection on the flow past the nozzle exit. But like I said, the nozzle inlet Mach number increases with or without an injector present.

I adjust the reference depth for the planar case in order to obtain the proper mass flow valuesóchoosing a mass flow inlet BC doesn't yield the correct value for whatever reason.
Attached Images
File Type: png Baseline Axisymmetric.png (13.2 KB, 8 views)
File Type: png 2D Planar Case (no injector).png (17.9 KB, 8 views)
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2d planar, injector, mach number, nozzle

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