# Boundary Conditions for Supersonic Flow

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August 14, 2017, 16:39
Boundary Conditions for Supersonic Flow
#1
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Hi,

I have been struggling with creating a correct solution for the simulation of a retrorocket in supersonic flow. Before I add the retrorocket boundary condition, I am trying to run it with a mach 2 flow to see a zero thrust case that I can use for an initial guess later.

My initial attempt to do this followed the example found here and uses similar boundary conditions and rhoCentralFoam albeit with different values for velocity, pressure, and temperature. I suspect my boundary conditions are incorrect since the back patch is behaving very oddly with flow reversing and the flow fluctuating. Is there a more appropriate boundary condition for the top and back patches besides zero gradient? Or perhaps some other insight that could help me simulate this more correctly?

I've attached the result of my simulation.

Thanks
Attached Images
 Domain.png (39.5 KB, 147 views) flow.png (78.6 KB, 188 views)

 August 15, 2017, 02:02 #2 Member   Taiwan,new north city Join Date: Aug 2017 Location: Taiwan Posts: 74 Rep Power: 8 hello, there The reversed flow occurs because of the inlet and outlet pressure setting. many people saying that the reversed flow will not affect the results much but it will affect the convergence rate. i think i read somewhere, we need to set the outlet pressure initially then once the solution converged then you can choose the appropriate pressure for the inlet and outlet. someone suggest about this. you can check it in this forum.

August 15, 2017, 12:50
#3
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When you say to set the outlet pressure initially, what value should I be using? Should I set the outlet pressure to be a value based on the freestream or the pressure as though the flow went through a shock?

My intent was to leave the top and outlet unspecified so that that the solution would acquire these values for me. Would it be better to specify the outlet pressure or use a waveTransmissive boundary condition?

I've attached my u,T,and p files in case they are useful.

Thanks
Attached Files
 files.zip (1.5 KB, 49 views)

 August 15, 2017, 20:20 #4 Member   Taiwan,new north city Join Date: Aug 2017 Location: Taiwan Posts: 74 Rep Power: 8 Hi..there, check the following, its available in fluent manual. Pressure outlet boundary conditions require the specification of a static (gauge) pressure at the outlet boundary. The value of the specified static pressure is used only while the flow is subsonic. Should the flow become locally supersonic, the specified pressure will no longer be used; pressure will be extrapolated from the flow in the interior. All other flow quantities are extrapolated from the interior. A set of "backflow'' conditions is also specified should the flow reverse direction at the pressure outlet boundary during the solution process. Convergence difficulties will be minimized if you specify realistic values for the backflow quantities. Several options in FLUENT exist, where a radial equilibrium outlet boundary condition can be used (see Section 7.8.1 for details), and a target mass flow rate for pressure outlets (see Section 7.8.4 for details) can be specified. For an overview of flow boundaries, see Section 7.2.

 August 16, 2017, 20:36 #5 New Member   Join Date: Jun 2016 Posts: 9 Rep Power: 9 I'm not sure how this applies to my particular case. In my case I'm trying to leave the outlet unspecified by using a zeroGradient condition so that I do not need to specify outlet flow properties such as pressure. Is my assumption that I can use a zeroGradient here correct? Considering how close the back patch is to the body I suspect it may not be.

 August 17, 2017, 01:17 #6 Member   Taiwan,new north city Join Date: Aug 2017 Location: Taiwan Posts: 74 Rep Power: 8 yes i think you can