9 Replies Latest reply on Jun 28, 2014 2:05 AM by Jared Conway

    "Use CAD Geometry"?

    Travis Storm

      Can someone explain what this option does and why it affects the displayed results? When should this option be enabled vs disabled?

       

      In the case I'm looking at now (an external flow around an airplane) the streamline results are drastically different with and without this option.

        • Re: "Use CAD Geometry"?
          Jared Conway

          Use cad geometry will "interpolate" the results to the geometry. Disabling it is exactly what the solver sees and calculates.

            • Re: "Use CAD Geometry"?
              Travis Storm

              Thanks Jared. As to the second part - when would you recommend using this option? It seems to me that it should always be disabled so you're looking at the result as it was calculated rather than a glossed-over solution, but the default is to have this option enabled.

                • Re: "Use CAD Geometry"?
                  Jared Conway

                  its a great tool for checking your mesh and your solution

                  it is something we teach in our more advanced training/mentoring for users that want to deep dive into the software

                    • Re: "Use CAD Geometry"?
                      Travis Storm

                      Consider me one of these people Though most of my background is with more traditional CFD packages like FLUENT and OpenFOAM where meshes are manually generated, so I'm still struggling to understand how to get Flowsim to generate an acceptable mesh.

                       

                      From your comments I gather that a good mesh will show the same result with and without the "use CAD geometry" option. Is that correct?

                        • Re: "Use CAD Geometry"?
                          Jared Conway

                          would be interested to hear your concept of an "acceptable" mesh

                           

                          but something to consider is that use cad geometry helps show how well the geometry is being captured, not necessarily a mesure of the mesh. but they are interrelated. make a finned heatsink, and make the mesh really coarse, you'll see what i mean. corners will be cut, sometimes it will turn into a block. the mesh will still be there, but hte geometry won't be captured.

                            • Re: "Use CAD Geometry"?
                              Travis Storm

                              In this case, my geometry is well captured (in that I can plot surface contours with and without "use CAD geometry" and the geometry is similar). But the streamlines are drastically different. As much as I'd love to show you a picture, I can't for legal reasons. Seems to me that one measurement of a "good" mesh should be that the streamlines plotted with and without "use CAD geometry" are the same.

                               

                              My measurement of a good mesh is different, but since SW can't do things I'd like it do there's no real point in going deeper into that (things like building boundary layer cells).

                                • Re: "Use CAD Geometry"?
                                  Jared Conway

                                  can you create a sample file that shows the same behavior? the streamlines really shouldn't change, the only thing that should really change is the shape of the geometry

                                   

                                  what are the rules of thumb that you use for a "good mesh"? how the mesh looks is certainly important and there are rules of thumb that people use but my experience is mesh convergence ends up being the most effective way to balance solution time with solution accuracy.

                                   

                                  not sure if you've seen the articles but there are some good articles on why boundary layer elements aren't as important in flow. they are on solidworks' website.

                                    • Re: "Use CAD Geometry"?
                                      Travis Storm

                                      Jared,

                                       

                                      I disagree on boundary layer cells; an effort should be made to create a mesh with gridlines that are parallel and orthogonal to streamlines to minimize numerical error. But that's a different discussion and irrelevant since it's not an option in FlowSim.

                                       

                                      In FlowSim, I generally insert local initial meshes wherever I expect regions of high gradients or in regions I'm particularly interested in. I start with reasonably fine meshes ('reasonably' defined by experience) and set refinement level to the maximum, then manually refine when my goals level off. I consider a case converged when the solution is grid-independent (when the goal values don't change after a refinement).

                                       

                                      Best I can do at the moment is to upload some screenshots - I don't have the time to scrub a model to make it OK to upload. I've removed the air vehicle which is upstream of the propeller. The streamlines are coming out of the back side of the vehicle and turning through the prop wake. The streamlines are calculated for the same distance downstream in both screenshots (a distance of about 2.5 propeller diameters).

                                       

                                      With CAD geometry:

                                      With CAD Geometry.PNG

                                       

                                      Without CAD geometry:

                                      Without CAD Geometry.PNG