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Subsonic Outlet Boundary Condition for Diffuser Simulation |
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September 6, 2023, 09:29 |
HELP: Subsonic Outlet Boundary Condition
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#1 |
Member
K
Join Date: Jul 2017
Posts: 97
Rep Power: 9 |
I have a diffuser where the flow is supersonic in the converging part and subsonic in the divergent part. I am using my own 1D solver built based on rhoCentralFoam, which has been verified and tested for supersonic nozzles, however this is time I'll use it for a diffuser (geometry the same, different inlet and outlet boundary conditions are required). For the supersonic inlet, I specify three boundary conditions: fixedValue for pressure (p), temperature (T), and velocity (U). However, for the subsonic outlet, I have encountered issues as the solver does not converge towards the correct solution. According to theory, at the subsonic outlet, only one boundary condition needs to be fixed. Therefore, I have tried various combinations for the subsonic outlet, such as:
1) did not work U_outlet fixedValue p_outlet zeroGradient; T_outlet zeroGradient; T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 2) wrong solution p_outlet fixedValue; T_outlet zeroGradient; U_outlet zeroGradient; T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 8) wrong solution p_outlet & U_outlet zeroGradient; T_outlet fixedValue; T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 37) U < 0 T_outlet zeroGradient p_outlet fixedValue U_outlet pressureInletOutletVelocity T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 38) U < 0 T_outlet zeroGradient p_outlet freestream U_outlet zeroGradient T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 39) crashed very early T_outlet freestream p_outlet freestream U_outlet zeroGradient T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 40) crashed very early T_outlet freestream p_outlet freestream U_outlet freestream T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 41) crashed very early T_outlet zeroGradient p_outlet freestream U_outlet freestream T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue 41) converges towards supersonic and U <0 T_outlet inletOutlet p_outlet zeroGradient U_outlet inletOutlet T_inlet fixedValue p_inlet fixedValue U_inlet fixedValue etc. Could someone please tell me what the appropriate subsonic outlet boundary condition to impose is? P.S.: some of the combinations lead to negative values for velocity. I tried to use inletOutlet and freestream BCs, but it did not help. Last edited by mkhm; September 6, 2023 at 22:02. Reason: typo |
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September 7, 2023, 10:34 |
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#2 |
Member
K
Join Date: Jul 2017
Posts: 97
Rep Power: 9 |
I think the problem for OpenFOAM is how to go from a low pressure to high pressure. While I am expecting it to follow the isentropic relationship, the fact of having higher pressure at output than the one at input seems problematic for OpenFOAM. That is the reason why velocity becomes negative (flow goes from high pressure to low pressure).
Has anyone encountered this problem before? |
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boundary condition, diffuser, supersonic/subsonic flow |
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