3 Replies Latest reply on Nov 3, 2013 1:58 PM by Jared Conway

    Supersonic flow through pipe - how to define outlet boundary conditions

    Michael Oswald

      I am modelling a very simple converging/diverging nozzle - just a tube with a restriction in the middle.  I've specified the inlet static pressure (and ambient pressure) as 1 Torr, and I have a volumetric flow at the outlet of 70 L/s.  I should be achieving about Mach 3 at the exit. 

       

      I ran the simulation first without the 'high Mach flow' option.  Solidworks gave me warnings and came up with a Mach number just over 1.  But it did converge.  When I selected the 'high Mach flow' option I received this warning: Maximum Mach number exceeded dV/V=267.036.  It also did not converge, but simply stopped when the number of iterations hit 3200. 

       

      When I graphed the results, a contour plot of the Mach number over the cross section gave a maximum of 2.3 (pretty close to what I expected).  But the default scale went all the way up to Mach 15.  When I did a surface plot of Mach number on the inside face of the exit lid, the entire thing was red.  So that's where my high mach number is.

       

      I'm wondering what the problem is with my model.  The lids were generated automatically by Solidworks, so I'm assuming they are ok.  Is it the way I defined the outlet boundary condition?  Is it that my ambient pressure is 1 Torr but the pressure inside the tube at the exit is in the mTorr range (and Solidworks is trying to expand the flow as it exits the tube?).  I already submitted a question about how ambient pressure interacts with the inlet and outlet boundaries, because that's confusing me also.  Any help would be greatly appreciated.  Thanks!

       

      Michael

        • Re: Supersonic flow through pipe - how to define outlet boundary conditions
          Jared Conway

          A couple of things:

          Outlet volume flow rate is not recommended but should work

          The error you can look up in the swx kb

          The auto lids have an overlap that I don't usually like to keep but shouldn't affect the sim for flow only

          See other post about completion criteria and pressure

           

          The problem setup sounds fine. A couple tweaks and you should be ok. Then improve the mesh and see if your results converge

            • Re: Supersonic flow through pipe - how to define outlet boundary conditions
              Michael Oswald

              Thanks Jared.  I left the simulation running over the weekend so I'll see where it ends up.  You mentioned that an outlet volume flow is not recommended - why not?

               

              The error indicates that somewhere in my model Solidworks calculated a Mach number > 10.  I need to investigate more where the problem lies.

               

              I noticed that about the auto-lids as well.  Don't understand why Solidworks overlaps instead of just matching the diameter.  I left it, though, thinking that maybe I just didn't understand something

               

              Thanks for the explanation on the other post.  Don't know how I didn't clue in on the initial conditions.  I think you're right, a few tweaks and some mesh refinements and I should be good.  I appreciate your help.

               

              Michael