Need for energy equation at low speeds to prevetn divergence

Adithya_11 · September 15, 2021, 02:53

Hello
I am simulating an aircraft flying at an altitude, with Re=8.8e6 and Mach=0.26

Settings- SA model, Green gauss node based, coupled pressure-velocity with time 0.1, second order.

The aircraft has this cooling intake situated above the fuselage. When I simulate it it without the intake, the solution either converges or residuals drop to 2e-6.

When I have the intake, the solution diverges at high angles of attack.

One way to prevent the divergence is by using ideal gas and sutherland' law, and hence the energy equation as well, instead of using constant density and viscosity. This prevent divergence, but my residuals now drop to 1e-5 only and the force files show greater oscillation.

Why would the energy equation stabilize this case? Does it mean that the flow is compressible at these conditions? Is there another recommended way to prevent divergence?

LuckyTran · September 15, 2021, 11:07

Energy equation isn't relevant. You just got lucky.

You can also "stabilize" the case by simply doing a better initialization. You can also stabilize it using a smaller time scale factor. There's lots of things that can be done. CFD is not a fire and forget approach.

September 15, 2021, 02:53	Need for energy equation at low speeds to prevetn divergence	#1
Adithya_11 Member Adithya Join Date: May 2021 Posts: 73 Rep Power: 4	Hello I am simulating an aircraft flying at an altitude, with Re=8.8e6 and Mach=0.26 Settings- SA model, Green gauss node based, coupled pressure-velocity with time 0.1, second order. The aircraft has this cooling intake situated above the fuselage. When I simulate it it without the intake, the solution either converges or residuals drop to 2e-6. When I have the intake, the solution diverges at high angles of attack. One way to prevent the divergence is by using ideal gas and sutherland' law, and hence the energy equation as well, instead of using constant density and viscosity. This prevent divergence, but my residuals now drop to 1e-5 only and the force files show greater oscillation. Why would the energy equation stabilize this case? Does it mean that the flow is compressible at these conditions? Is there another recommended way to prevent divergence?

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September 15, 2021, 11:07		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	Energy equation isn't relevant. You just got lucky. You can also "stabilize" the case by simply doing a better initialization. You can also stabilize it using a smaller time scale factor. There's lots of things that can be done. CFD is not a fire and forget approach.