8 Replies Latest reply on Sep 23, 2014 7:37 PM by Jared Conway

    flow freezing vs mach number

    Chris Michalski

      I've got a relatively length heat transfer simulation I'm working with.  I wondered if perhaps I could initiate some flow freezing to speed things up.

       

      I get a warning "that Flow Freezing may cause problems" followed by a comment "Maximum Mach Number = 0.3142"

       

      What interaction is there between Flow Freezing and Mach Number?  Are they just indicating that because temperature effects on fluids plays such a large role in determining Mach Number that it might prevent accurate convergence?

       

      I can't find anything in the help files that links Flow Freezing and Mach Number.

        • Re: flow freezing vs mach number
          Txon Aguirre

          Hi Chris:

          As far as I understand how Flow freezing works on Solidworks flow simulation, it "stops" the fluid in the achieved conditions for the selected calculation stage range.

          Mach number, is a function of the specific heat at constant pressure to specific heat at constant volume, ratio. So, if during the freezing range, one of those variables has a significant change, Mach number would also change, following that the heat transfer conditions would not be accurate.

          For steady flows it would work fine, but for other conditions, it would have to be checked.

           

          I think the key would be checking how the pressure to volume rate changes along the simulation before making the choice.

           

          Regards

          • Re: flow freezing vs mach number
            Chris Michalski

            So based on the 400+ views and only 1 reply... Nobody has ever seen any problem other than SW making sure you realize there is a POSSIBILITY of incorrect convergence due to flow freezing really messing with Mach Numbers?

            Has anyone ever had a problem where convergence didn't seem to get back on track if periodic flow freezing was used to re-verify that Mach Numbers were stable?

             

            Because this is a heat exchanger simulation the deviation should reduce as time goes on, the velocities stabilize quickly (hence the warning) but it takes longer for the heat transfer and radiation to converge (so I would assume that warning would go away).  By the time temperatures stabilize the Mach Number should be quite a bit lower (~1/2 of initial condition).

             

            What steps would people take to verify that Flow Freezing didn't save time but lead to suspect data?

            My only thought is to run a typical simulation both ways and see if they differ by any measurable amount.

              • Re: flow freezing vs mach number
                Jared Conway

                i personally have not had an application where flow freezing was necesssary

                i looked through the help and the kb and all the tech references and it looks like there are a lot of things to consider for implementation

                 

                my first thoughts are:

                1. what type of cooling is going on? natural? forced?

                2. is that high mach number flow expected?

                3. what settinsg are you using for the flow freezing

                4. how many cells are we talking about? and how long is it taking to solve?

                 

                i'm sure you've seen this in the technical ref already:

                 

                Flow Simulation treats Flow Freezing for the High Mach number flows differently. All flow parameters are frozen, but the temperature of the solid is calculated using these fixed parameters at the outer of the boundary layer and user defined time step. Temperature change on the solid's surface and relevant variation of the heat flows are accounted in the boundary layer only. It is impossible (and makes no sense) to use Flow Freezing for calculation of concentration propagation in the High Mach number flow. If custom time step is not specified, the steady-state temperature of solid will be reached in one time step assumed to be infinite.

                  • Re: flow freezing vs mach number
                    Chris Michalski

                    I'm not sure if Flow Freezing will be helpful or not, this is the first time I considered using it.  I am running a simulation with a lot of velocity goals and temperature goals to see if flow freezing could even be activated enough before temperatures stabilize to be worthwhile.

                     

                    1) The only cooling is natural convection - it is a high temperature ceramic heat exchanger contained in an insulating shroud. (Heat Transfer, Radiation, Gravity included)

                    2) Mach 0.4 isn't unreasonable based on the volumetric expansion of the active fluid and the dimensions of the interior cavities.

                    3) I was considering using a periodic freezing so that it could reactivate in case things got out of line.

                    4) The model is 474k fluid, 423k solid, 775k partial and takes about 28 hours typically.

                     

                    I could likely reduce that by adding a lot more initial conditions for various components of the heat exchanger.  At this point I was hoping a single click (Flow Freezing) could be used instead of taking the time to setup ICs as I'm comparing several completely different models so I can't easily create and re-use the ICs.