peteroznewman
Subscriber

Hello Geovanna, I will answer the questions and invite others with more expertise to also reply.

1.       Different mathematical equations are used for MSUP analysis than Full Harmonic.  I can’t point to the specific reason why displacements require the use of a full harmonic solution, but does it matter? Applied forces are better for harmonic response because they can use a MSUP analysis. The difference between a Displacement and a Nodal Displacement is simply the scope: geometry vs nodes.

2.       You applied a 2.5 mm Nodal Displacement, but you didn’t mention the start and end frequencies for the analysis. This is important because it directly affects the acceleration applied to that node. A displacement describes the motion of the node. The acceleration of that node has a frequency squared term.

Consider a SDOF oscillator with a base excitation frequency of 10 Hz. For the base to have a displacement of about 2.5 mm, the base acceleration is 0.1 G.

But at an excitation frequency of 100 Hz, the base must have an acceleration of 100 G to maintain a 2.5 mm displacement.  Can the vibration source really generate those accelerations?

Now consider the alternative, using an applied force instead of a displacement. In the example above, at an excitation frequency of 10 Hz, the force causes a displacement of 2.5 mm and an acceleration of 0.1 G. But now at 100 Hz, the force is constant, so the displacement must go down to 0.0025 mm and the acceleration remains at 0.1 G.

3.       I don’t know what a longitudinal loss factor is. Constant Structural Damping Coefficient is divided by the excitation frequency and multiplied by the Stiffness Matrix to form the Damping Matrix.  Damping Ratio is not used in Full Harmonic Analysis. Damping Ratio is used in MSUP Harmonic Analysis. This is because these to analyses use different governing equations. In Full Harmonic, you can select values of the Rayleigh Damping coefficients of Alpha and Beta, also called the Mass and Stiffness Coefficients, and use an equation to compute the equivalent Damping Ratio.

4.       Fixed BC add a lot of stiffness to the model and are to be avoided. Remote Displacements, Behavior = Deformable allows a collection of edges to be constrained, in the average, without adding stiffness.  Use one on each major cutting plane.

5.       If you do MSUP Harmonic Response, then the BC come from the Modal analysis.  If you do Full Harmonic Response, then you must add the BC yourself.

6.       What is the source of vibration? How is it specified or measured?  Over what frequency range is it measured?