3 Replies Latest reply on Feb 1, 2013 3:35 PM by Jared Conway

    Why surface parameters results are unconsistent ant cut plots works fine?

    Isaac Llorens

      I'm designing a nozzle for space with Flow Simulation. I had some problems with vacuum until i detected that SW dosen't run properly in this conditions.

      I'm trying to simulate a nozzle with 20bar in input and 1atm  at output. When i plot the results using cut plots tool i obtain that results.

      Presure.png

      Mach Number.png

      Temperature.png

      All this results are completely consistence according my conditions. High temperature, high pressure low Mach number at the entrance  and low temperature, low pressure and high Mach number at the exit. However when I use Surface Parameters tool something happens. Using Surface Parameters tool the average output temperatures and pressure are bigger than input temperature and pressure.

      Can anybody tell me what is happening?

        • Re: Why surface parameters results are unconsistent ant cut plots works fine?
          Jared Conway

          can you give us the full details of the input and output boundary conditions and the setup of the problem?

          what results are you expecting?

          what goals have you set and did they converge? were there any error messages in the solver window at the end of the solution?

          what specifically were the outputs you saw at the inlet and outlet with the surface parameters?

            • Re: Why surface parameters results are unconsistent ant cut plots works fine?
              Isaac Llorens

              can you give us the full details of the input and output boundary conditions and the setup of the problem?

              Yes, ther it is.

               

              First i have to say that all this simulation is using butane as fluid.

              In my inlet I have this conditions.

              butane pressure: 20bar

              temperature : 400k

              mass flow rate 7.5g/s

               

              Outlet boundary conditions:

                   Envorimental pressure 1atm

                   Temperature 100k

               

              An this is my problem setup.

              2.png

              what results are you expecting?

              I'm expecting to obtain a consistent results that means high temperature high pressure an low Mach number at the intake and low temperature, low pressure and high Mach number at the output. But as i explained there is and inconsistence between the plots and the surface parameter results.

              what goals have you set and did they converge?

              I didn't set any goal. In my opinion the result was convergent.

               

              were there any error messages in the solver window at the end of the solution?

              I don’t have error messages

              what specifically were the outputs you saw at the inlet and outlet with the surface parameters?

              my surface parameters.

              https://www.dropbox.com/s/bjrf3ihnc2ra6dz/3_Surface%20Parameters%20In.xls

              https://www.dropbox.com/s/vc68b625ie6p8fe/3_Surface%20Parameters%20Out.xls

               

               

              And thanks for all

                • Re: Why surface parameters results are unconsistent ant cut plots works fine?
                  Jared Conway

                  isaac, i setup my own test model in 2013 sp1.0 to confirm the software was working as I would expect and to get an idea of where you might be running into an issue.

                   

                  regarding the surface parameters not matching the cut plots, i wouldn't expect them to match exactly. the surface parameter is over the whole inlet/outlet area, the cutplot is only what is in that plane. a better comparison would be to do surface plots on both the inlet and outlet and compare them against the surface parameters. also, you might want to set the min/max for the plot vs the global min/max in the cutplot so that you can better see what is happening specifically in that cut-plane. However that may cause them to deviate from the surface parameters further. The only real comparison is surface plot to surface parameter though.

                   

                  But, I might be missing what you think is there. If you can give some more details I might be able to provide more insight.

                   

                  Regarding your outputs not matching your inputs (i think that is the bigger concern), I think it is because you've over constrained your model. Or I should say you've made an attempt and the software foiled you.

                   

                  You can either enter 2 pressures and get the mass flow rate or you can enter a pressure on one end and mass flow on the other. The pressure you're entering on the mass flow side is an approximate pressure only. It isn't used unless the estimated mach number at the inlet is above 1. Do you have access to the SolidWorks KB? There is a good discussion about it in S-040099.

                   

                  Also, remember that environmental pressure may be a static pressure or may be a total pressure depending on whether it is an inlet or outlet. Based on which it is, you may need to use pressure (static pressure) or total pressure output plots to match the value you entered to what you see in the plot or surface parameter.

                   

                  Another tip, you may want to think about extending the length on both ends of your nozzle to "soften" the effects of those boundary conditions in the area that you're interested in.