10 Replies Latest reply on Oct 22, 2015 9:16 AM by Amit Katz

    car downforce

    Mo Jalaly

      I am playing around with the flow simulation for a car to see how best to create down force. I keep having a few issues pop up and wanted to see if any body here could help me out.

       

      1) When I look at the the force normal to y (as in the attached pic) I am seeing that when "better" aero is applied, this number goes from negative to positive. Is that correct? Does a positive number indicate more down force?

       

      2) For some reason when i do a cut plot towards the center of the vehicle, it doesn't show, move a bit outside the center and it does. What could be causing this?

       

      3) Not matter how i change the parameters, all the selected surfaces show one color when looking at pressure on the surface plot. I dont know why

        • Re: car downforce
          Amit Katz

          Look at your delta and convergence criteria compared to the calculated value, something is way off here. How does your goal plot look?

            • Re: car downforce
              Mo Jalaly

              Sorry i am new to this and not quite following. Attached is the goal plot for a newer run

               

              I am basically i am trying to figure out the over all down force/ lift and how a diffuser changes the values.

                • Re: car downforce
                  Amit Katz

                  Just from a first impression that goal plot has a reasonable convergence, but I am concerned with the very low number of iterations. It seems like the solution converged after just 40 iterations, which is not what I would expect from such a complicated looking model. Can you upload your project? Your mesh may be too coarse, which would give you a low cell count and bad results.

                    • Re: car downforce
                      Mo Jalaly

                      Here it is. I took a little time to simplify it a bit so that it would attach. I believe that the ground and tire conditions are correct (moving wall) but not sure if in reference to the correct axis. Yes the mesh may not be too course.

                        • Re: car downforce
                          Mo Jalaly

                          Thoughts??

                            • Re: car downforce
                              Amit Katz

                              When I unpacked your project file, there were some components missing, so this may affect what I'm about to say. Looking at your Flow Sim project options, it seems like the meshing tool was left at the default "Level 3" mesh with no other options adjusted. When I meshed this project and looked at the cut plots, this is what I saw:

                               

                              Side mesh:

                              mesh side.PNG

                              Front mesh:

                              mesh front.PNG

                               

                              Note that I turned off "use CAD geometry" for these plots, because that option will make the software 'cheat' and not show you the actual meshed solid geometry that will be used in the simulation.

                               

                              I think you can see the problem here. The software isn't seeing the car model, it's just a bunch of floating plates and bars. The mesh needs to be worked on thoroughly, there are good meshing tutorials available in Flow Sim's "Help" menu.

                                • Re: car downforce
                                  Mo Jalaly

                                  Ok. So do you think the issue has more to do with the model?

                                   

                                  Even when i change the mesh settings to 6.. the results are not much different.

                                    • Re: car downforce
                                      Amit Katz

                                      Because I got an geometry error when I first opened the assembly, I can't say for sure that I saw the same model that you did. But I can tell you that Flow Sim does not like to work with surfaces. The only type of geometry it will read in is solids, so you need to make sure you model is as "filled out" as it can be. I would also try to simplify it down for the first pass. By that I mean doing things like replacing the wheels with simple cylinders, removing small details from the body, etc... Luckily the software doesn't mind if solid geometry overlaps, so you can save a copy of your model and use the existing geometry as a reference for simpler shapes. As long as you keep everything as unmerged solid bodies you can easily suppress problematic geometry in the Flow Sim project options.

                                        • Re: car downforce
                                          Mo Jalaly

                                          i think the biggest issue is that it doesn't like the chassis since it is a weldment. I tried to clean it up and save it as a part but that didn't work out so well. Anyway to simply the whole thing instead of having each weldment profile as an individual body?

                                            • Re: car downforce
                                              Amit Katz

                                              I don't have a lot of experience working with weldments. But if the software is not recognizing solid geometry, it's possible that exporting it to a parasolid may help. I'm sorry I can't offer a better solution.