14 Replies Latest reply on Nov 13, 2014 3:46 PM by Andreas Svindt

    Floating sphere fixture?

    Andreas Svindt

      Hi.

       

      First of all I would like to refer to my drawing. Imagine this case:

      simplyfied.png

      A float made of 316SS is connected to a wire, which connected to the bottom. The scenario is under water. I don't really care about the wire nor the bottom-connector, I am only looking at the float.
      I would like to do following simulations: Static, Buckling and fatique.
      My question is: How do I connect the float, so it is realistic? I tried different type of fixtures, for instance, Fixed geometry at the bottom, but that doesn't give me realistic results. Can anyone help me in which fixture type I should use to get realistic results?

       

      Thanks on forehand.

       

      Br,

      Andreas

        • Re: Floating sphere fixture?
          James Riddell

          Andreas,

           

          It appears as if your model is only restrained in the 'Y' direction.  As such you should only fix that linear direction leaving the other 5 DOFs free.  If it still won't run you could try applying soft-springs to help with the solve.  Is the sphere small enough that you can consider a uniform hydro-static loading on the surface or does the differential pressure come into play?  You might also try a 2D solution since it's a symmetric model.

          • Re: Floating sphere fixture?
            Shaun Densberger

            Can you give a a more detailed description of the problem?

            • Re: Floating sphere fixture?
              Andreas Svindt

              As James says, it is actually a buoy. It is to be inside a hot water tank.

              Now this is the float in solid work:

              explain.png

              The picture on the left is the float cut in half.

              Now if I use the "Fixed Geometry" function and apply and outside pressure on the float, it gives me unrealistic results. What type of fixture should I use to get realistic results?

                • Re: Floating sphere fixture?
                  James Riddell

                  What are the 'unrealistic' results?  I'd just restrain the inside bottom circular surface in the 'Z' axis only, split the outer surface and apply the calculated buoyant force upwards, apply a hydrostatic pressure to the entire outside surface and gravity downward.  You should probably also have an atmospheric pressure on the inside as well of 14.7 psia.

                   

                  Do you have a picture of your current loads and constraints?

                    • Re: Floating sphere fixture?
                      Andreas Svindt

                      First of all, thanks alot.

                       

                      Yes I do, I will find it in a second.


                      First can you answer this:

                       

                      How do you apply a buoyant force?

                       

                      Thanks

                        • Re: Floating sphere fixture?
                          James Riddell

                          The simplest way to apply a buoyant force (and if the community knows better, please, jump in here) would be to calculate the difference in weight between your structure and the displaced volume of fluid, split the surface of the sphere around the middle in the X-Y plane and apply a total upward force on the lower surface along the 'Z' axis equal to the difference in weight as a force.

                        • Re: Floating sphere fixture?
                          Andreas Svindt

                          Here is the forces applied. I must say, after running the simulation again, the results are better. The stress and strain results looks fine, but the displacement, i dont really get. Why would it not be more distrubeted? Why is it mainly in the top of the float, the biggest displacement is? That must be because of the fixture or am I wrong?

                          222.png22222.png

                            • Re: Floating sphere fixture?
                              James Riddell

                              Andreas,

                              Of course your displacement at the top would be greatest since there is less force holding it down.  The bottom should have the least because it is restrained (although you could calculate the 'stretch' of the anchor cable and allow that much displacement - but the top would still move more due to the buoyant force.) 

                               

                              I may be missing it but I don't see where you have applied your upward force or did you incorporate that into the fluid force on the outer surface using formulas?

                              • Re: Floating sphere fixture?
                                Shaun Densberger

                                As James pointed out earlier, there is symmetry that can be taken advantage of here; specifically, 2D axisymmetric. This is even more important as failure of this will be buckling, which is best done with a non-linear study.

                                 

                                Regarding the hydro-static pressure on the sphere, it'd probably be easier to do this using a couple force loads (assuming that you want to capture is right up to the point that buckling occurs at). However, we might not need to account for this depending on how large the sphere is. How big is this sphere?

                                • Re: Floating sphere fixture?
                                  Jared Conway

                                  These look like junky soft spring results. I think I would expect them to be symmetric based on the way you describe The analysis. From a learning perspective, if you can't simplify the restraints, just add the parts into the analysis. Once you are confident there, you can remove and replace.

                                   

                                  fixed really should work fine.

                            • Re: Floating sphere fixture?
                              Andreas Svindt

                              Thanks for all the answers, I really appreciate it!

                              Sorry for not answering any quicker, I have a lot to do.

                               

                              Anyway, to answer Shauns question:
                              The sphere is 3.5" in diamater around 89mm.

                               

                              I will try to work with all your inputs, and I will get back to you later today.


                              Thanks.

                               

                               

                               

                              Br,

                              Andreas

                              • Re: Floating sphere fixture?
                                Andreas Svindt

                                That's exactly what I did. Got some great results, will post them later, if you want to Actually change the dimensions of the sphere to a radius of 2.75".

                                Thanks a lot for all your help.