Pressure jump too late in (laminar, turbulence free) convergent-divergent nozzle

Simser · July 19, 2019, 13:59

Hello Guys,

sorry for any mistakes, this is my first post.

For University I am trying to model a convergent-divergent nozzle with CFX (quasi 2D ICEM CFD Mesh).

For now I just want to simulate it without viscous Forces and laminar, turbulence free flow.

My boundary conditions are p_exit = 0.7bar (static pressure at outlet), p_0 = 1bar (total pressure at inlet). I assume Air ideal Gas (kappa=1.4, T_0 = 300K). The crosssections are 51mm^2 at inlet, 24mm^2 at minimal crosssection and 42.28mm^2 at Outlet.

From these I calculated the pressure drop to be at the crosssection with approx. 28.5mm^2 (see attached file "calculate_shock").
But when simulating with CFX, the shock occurs further downstream at approx. 35mm^2. (Arrow shows location where it should be)

For the simulation I used free-slip walls and symmetry conditions at the sides. Turbulence model is none. I use Total Energy Equations without viscous terms.

Has somebody a hint what I might have missed?
Or what further information do you need?

Best Regards
Simser

ghorrocks · July 19, 2019, 21:50

Before comparing your results to anything you need to check your simulation is accurate. Comparing an inaccurate simulation to anything is pointless. This FAQ has some explanation: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

With the key bits being you need to check your convergence criteria is tight enough and your mesh fine enough.

Simser · July 20, 2019, 06:15

Thank you for your answer, I already looked through these guidelines. I startet with coarse meshs and refined them stepwise until cells with 0.25mm width globally and then refined the location of the pressure drops further. I got max. Residuals under 10^-5 and the pressure drop was always at the same position.

But a colleague informed me today that I had a wrong assumption in my theoretical calculations and now both Simulation and Theory correlate well.

July 19, 2019, 21:50		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Before comparing your results to anything you need to check your simulation is accurate. Comparing an inaccurate simulation to anything is pointless. This FAQ has some explanation: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F With the key bits being you need to check your convergence criteria is tight enough and your mesh fine enough. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 20, 2019, 06:15		#3
Simser New Member Simon Join Date: Jul 2019 Location: Germany Posts: 3 Rep Power: 7	Thank you for your answer, I already looked through these guidelines. I startet with coarse meshs and refined them stepwise until cells with 0.25mm width globally and then refined the location of the pressure drops further. I got max. Residuals under 10^-5 and the pressure drop was always at the same position. But a colleague informed me today that I had a wrong assumption in my theoretical calculations and now both Simulation and Theory correlate well.