3 Replies Latest reply on Jul 21, 2009 1:24 PM by Michael Buckley

    Modal Frequency varying significantly with type of mesh

    Mike Lowinske
      Hi all,

      I have been playing around with the frequency simulation, doing a study on a part using different approaches and meshes. What I am doing a study on is a hollow core metal door with a uniform wall thickness of .125". I have approached it in the following ways:

      1. As a solid body with a very fine, standard mesh.
      2. As a solid body with a very fine curvature based mesh.
      3. As a 3D surface feature, with an assigned shell thickness of .125" and specified as "thin" with a very fine standard shell mesh.
      4. As a 3D surface feature, with an assigned shell thickness of .125" and specified as "thin" with a very fine curvature based shell mesh.
      5. As a 3D surface feature, with an assigned shell thickness of .125" and specified as "thick" with a very fine standard shell mesh.
      6. As a 3D surface feature, with an assigned shell thickness of .125" and specified as "thick" with a very fine curvature based shell mesh.

      Simulating these ways gives significantly different results. The frequencies at a certain mode are varying by almost 900 hertz between the highest and lowest frequency from the six results. Both of the solid body simulations with either mesh give results that are similar to each other but on the higher end and the shell mesh results are similar to each other but on the lower end.

      So I have some questions. When doing a frequency analysis, why do the frequencies vary so much between shell and solid meshes? What is the real difference between specifying the surface body as either thin or thick and when would one be used over the other? Which of the methods I did would be the most accurate method of finding the frequencies (I honestly have no idea what they would be for this door)?

      *edit* forgot to mention that I am applying no forces or restraints to the body

        • Modal Frequency varying significantly with type of mesh
          Ameer Chilakala
          May be this is due to Solid elements having only three degree of freedom per node(i.e. traslations). But the shell elements having six degree of freedom per node (i.e. 3 rotational & 3 translations). Due to this you are getting different frequencies between the solid mesh and shell mesh while running the frequecy analysis.

          As per my knowledge, first we need to decide which type of mesh we need to use. when the L/t ratio is more than 15 we need to use shell elements.

          Kindly let me know if i mentioned any this wrong.
          • Modal Frequency varying significantly with type of mesh
            Bill McEachern
            With the right mesh density you will get the same answers more or less regardless of element choice. I suggest that you do something that you know the answers to like a cantilever beam, fixed at one end. Do experiments till you figure out under what settings all of the analysis produce more or less the same results. They should all get the same lower modes and be reasonably close. Then you will know what it takes to get an answer with whatever elements and geometry you are considering. If this doesn't happen then there are some serious issues. I am pretty confident that it will though.
            • Re: Modal Frequency varying significantly with type of mesh
              Michael Buckley

              Mike,

               

              Have you figured out what is going on?  Or at least the best approach to the most accurate solution?

              I'm experiencing the same problem now. I have an assembly with sheet metal components.

              By the guidelined, they should be treated as shells, but...

              I have the option to treat the components as solids or shells. Using the same mesh settings, the two give vastly different frequency values.

              Doing a mixed mesh, some solid and some shell, gives even different results. I'm using 09 sp4.0