October 17, 2014

Frequency Analysis: Super Frequency Study

“Super Frequency” study type

There are many different types of studies that you can run in SOLIDWORKS Simulation, such as Linear Static, Thermal, Fatigue, Nonlinear and Drop Test, but there is one study type that is sort of a “super” type.  The super study that I’m talking about is a Frequency study type.

Why it’s super is that it has multiple applications where it is useful:

1 – Determining natural frequencies

         The typical usage of a Frequency analysis is to determine the natural resonances of a system, see example listing below.  You all may be familiar with some famous examples, such as the Tacoma Narrows bridge, where a system’s vibration will be amplified when it is excited by an external force at (or near) one of its resonant points.  And every system theoretically has an infinite number of resonant frequencies that are at increasingly higher values, so everything you design has a lot of points where it can fail.  If you don’t think that your design will undergo vibration during use, you should consider how your products get transported often by many vehicles, either by land or air, that subject it to a wide range of external frequencies getting to its final destination.
list modes on frequency analysis

2 – Characteristic movement of system

      Another great result to obtain from a Frequency simulation is the characteristic shapes that the system wants to move.  This gives you an indication of the relative stiffness in each direction, and where some areas of the design may be “weaker” (or more susceptible to stress) than others.  As you can see in the example below for an engine mounting frame, where the engine is replaced by a remote mass point, the first mode shape shows that has a tendency to want to rock to-and-fro and thus tear itself away from the base.  While it cannot be determined from this analysis whether it will actually tear away, but you can plainly see that special attention should be placed on the horizontal braces and its bolted connections to the base.  And there are multiple characteristic shapes that can be evaluated

solidworks simulation

3 – Troubleshooting tool

      One might not think of running a simulation as a troubleshooting measure for the FEA modeling, but a Frequency analysis can also help in this regard.  You can setup the model similarly as you would in say a linear Static analysis; the advantage of starting it as a Frequency study is that even if you have issues with insufficient restraints or connecting all the parts in your assembly, it will still solve albeit with some rigid body motions yielding a nearly 0 (zero) Hz frequency.  This information is valuable to working out the model issues.  Once you have it all figured out in the Frequency study, all the setup features can be copied to another study by ctrl+mouse select plus drag-and-drop them into the tab (at the bottom) for that study.
frequency anlaysis

4 – Basis for many other types of analysis

     A Frequency study is also the underlying basis for the following study types: linear Buckling and the four Linear Dynamic studies (Time History, Harmonic, Random Vibration (or PSD), and Response Spectrum analysis).  The definition of a linear Buckling study is when the transverse stiffness goes to zero under a compressive loading case, and this can be represented by the first natural frequency, which for a simple column example is a half-sine wave.  The combined response from summing up all of the resonant frequencies and associated characteristic mode shapes provides a very efficient way of performing a Linear Dynamic analysis.

solidworks simulation

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