# 3-D Compressible Flow Boundary Conditions

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 February 4, 2009, 16:07 3-D Compressible Flow Boundary Conditions #1 Tyler Guest   Posts: n/a I am having problems with convergence on a 3-d compressible external flow problem. I am trying to model Mach 0.8 axial flow around a solid cylinder. The flow field is external, so I have tried using "pressure far-field" boundary conditions on the 6 faces of the flow field volume. However, whenever I run the simulation, I can never get convergence with the continuity equation. I also get errors stating the pressure and temperature have been limited to a certain value. The flow volume is a rectangle with a cylinder in the middle. No elements are even close to being skewed, so I don't think it is a mesh problem. Are there better/more appropriate boundary conditions for external flow? Any help or assistance is greatly appreciated. Please let me know if more details are needed. -Tyler

 February 4, 2009, 20:24 Re: 3-D Compressible Flow Boundary Conditions #2 NRD Guest   Posts: n/a How about specifying non-reflecting boundary conditions?

 February 5, 2009, 07:53 Re: 3-D Compressible Flow Boundary Conditions #3 VAMSIDHAR Guest   Posts: n/a Dear Tyler, Getting convergence with compressible flow is not easy .first see whether the domain you have conseidered is enough or not.Then try with pressure inlet or mass flow inlet boundary condtion at inlet and pressure outlet condtion at outlet.Also you can use wall with large doamin or pressure far feild or opening.but initially try with pff.Also see that the domain is properly considered.that is very important for convergence. cheers, vamsi

 February 5, 2009, 13:29 Re: 3-D Compressible Flow Boundary Conditions #4 sa Guest   Posts: n/a might be the probelm is essentially unsteady, so you wont be able to get convergene with steady state sover

 February 5, 2009, 19:58 Re: 3-D Compressible Flow Boundary Conditions #5 zongtwi Guest   Posts: n/a My personal experience for converging compressible cases is to start slow. Use the pressure far field and start from a relatively low Mach number. Say 0.3. Then run the simulation until it is converged, and increase the spped to Mach 0.5 say...and then to 0.7 and 0.8. Another reason it is not converging well is that there might be some local areas where the speed has reached sonic or supersonic conditions, and thus a shock might occur locally, but your mesh size is too coarse to capture it. Try to use mesh adaption with a criteria of pressure gradient say, and see what you get. Hope that helps.