Lecture 32: Sysnoise and Structural Models Computational Acoustics, c V. Sparrow/PSU, 2000 References: - Sysnoise Rev. 5.4User s Manuals, LMS Numerical Technologies, Leuven, Belgium. - Sysnoise Rev. 5.4Examples Manuals, LMS Numerical Technologies, Leuven, Belgium. Last time: Began fluid-structure interaction, Sysnoise option COUPLED. Reading assignment: Skim read The I-DEAS Master Series Sysnoise Interface in Sysnoise manuals packet from Gnomon Copy Getting Structural Information into Sysnoise: Last time we mentioned that you could use Sysnoise s internal FE modeling capability for simple structural models. However, for realistic problems you will want to import the structural model or structural results into Sysnoise from an external FE program. Let s now examine the different ways that Sysnoise interfaces with external FE programs. L32R4, p. 1
Lets suppose you have a BEM model and you want to set the velocity B.C. based on the results from an external structural model. You could set the velocity B.C. manually as before (time consuming) or you could... Get FEA Structural Results onto Mesh into Sysnoise: structural displacements velocities = Velocity B.C. accelerations > GENERATE... Model/Vibrating Panels/FEA Has several interpolation algorithms if external and acoustic meshes are different. This method assumes you have enough FEA information to actually prescribe the boundary conditions exactly for the frequencies of interest. If you only have modes from your FE model and you also have some experimental measurements at the frequencies of interest, you can instead use... L32R4, p. 2
Modal Expansion of Limited Experimental Data The goal here is to create velocity boundary conditions from 1. limited experimental data, and 2. FE structural modes. One rarely can take enough experimental data for velocity boundary conditions all over the surface of a structure to get good acoustic radiation results. Hence, use your limited experimental data with what you know about the modes of the structure to generate adequate velocity B.C. information. > EXPAND... Model/Vibrating Panels/Measurements L32R4, p. 3
If you have a structure modeled in finite elements in your favorite FE program (NASTRAN, I-DEAS, ANSYS, ABAQUS, etc.) and you want to use that structure in Sysnoise for a coupled analysis, you have only a few options for getting that structure in. There are two ways of doing this... 1. Use FEM Matrices Directly: Not in Rev. 5.4documention or in GUI, but still works in SCL. Here we take the FEM system structural matrix and read it directly into Sysnoise for a coupled solution. > INPUT MATRIX... You will probably need to contact LMS for instructions on how to get this right. However, most people... L32R4, p. 4
2. Get FE Modes (uncoupled) from External FE Solution: Here we take the uncoupled modes from another FE program and use Sysnoise to calculate the coupled modes. > INPUT MODES STRUCTURE... File/Import/Modes Structure What if the acoustic and structural meshes are different? > TRANSFER... File/Import/Modes Structure and click Incompatible Mesh Caveats: Sysnoise can only read real (lossless) structural modes. The modes must all be normalized with respect to the corresponding mass matrix. L32R4, p. 5
For coupled analyses, you also want... Compatible Structural Meshes: Sysnoise only cares about structural elements that are directly in contact with the fluid for a structural mesh. Your structural mesh probably has many more elements than this, so you can get rid of all elements in your structural model that don t touch the fluid. Recall from last time, Sysnoise only understands thin shell elements. If you are using solid elements in your model, you must convert them to shell elements before Sysnoise can run a coupled analysis. The easiest way to do this is take the envelope of the structural model. This will give a mesh of shell elements. Sometimes need to coarsen a mesh, convert element types, or remove ribs to make surface meshes compatible with Sysnoise. To do this for structural meshes, you will need to do this outside of Sysnoise. You can use your own FEM program, or you can purchase another program from LMS, Pre-Sysnoise, that is used for this task. L32R4, p. 6
Coarsening Meshes for BEM: You may want to use a structural mesh AS a mesh for performing a BE analysis. This is another case where you will need to modify the mesh for use with BEM. In Sysnoise you use > COARSEN... Geometry/Coarsen Mesh See the Sysnoise User s Manual for how to do this. L32R4, p. 7
An additional Sysnoise feature... Combining and Manipulating Sysnoise Results Can manipulate Sysnoise results in a big calculator. Uses reverse-polishnotation, RPN. Example: x ENTER y + > COMBINE... Tools/Combine Can take FFTs, add A or C frequency weighting, etc. Questions? L32R4, p. 8
Example Coupled Problem in Sysnoise: A box with rigid walls except for a flexible plate for the top. Coupled Cavity Example 8 in the Sysnoise Examples Manual Uses acoustic FEM in box and structural FEM for vibrations of plate. We solve by creating - FEM model for plate (structure.sdb) - FEM for fluid in box (acoustic.sdb) This problem is coupled. We put a forcing function on the plate, and it creates sound in the box. Use Sysnoise BUTTON files. - Sparrow never got modal superposition to work, but did get physical coordinates to work. - Sparrow thinks B.C.s on plate are not right, but we use anyway to get the same answer they do. L32R4, p. 9