1 Reply Latest reply on Mar 13, 2012 4:20 PM by Kevin Corr

    Reaction force vs frequency plot

    Sylvian Braibant

      Hi there,

       

      I am doing a dynamic simulation on what I've attached a picture of. Meaning a membrane excited by a cycling force (10 N) between 0 and 60 kHz, while held at another place on the structure (fixed geometry). I got actually 2 questions about the reaction forces. The fixed geometries are the 2 faces of the flange and I would expect the reaction forces on those surfaces to be opposite. And they are not : for the lower face X : 37 N ; Y : 27 N ; Z : 37 N // on the upper face X : 47 N ; Y : 83 N ; Z : 46 N (the cylindrical axis is Y, and so is the exciting force's). That's my first question.

      My second one is : is this possible to plot these reaction forces along the whole frequency range ? The values given above are for the step 60, at 49259 Hz. My goal is to be able to spot the frequency peak on this reaction force plot.

      I hope I am being clear.... thanks for any hint !

       

      Sylvian

        • Re: Reaction force vs frequency plot
          Kevin Corr

          comments ...in reverse order...I think you mean to plot a response graph as a result in a harmonic linear dynamic study. Its known as a gain or a bode plot in audio circles. As for what to expect the reactions to be ...as in equal and opposite...I'd sooner expect them to be unequal and not opposite at step 60 ie. 50 khz. At high  frequency the step may be near a resonance that is excited by the load. So check the mode list to see what modes lie around 50khz. Then, display the mode shape to get an idea of whether the load you apply would excite the mode shape. My lean is toward there being reaction forces to a surprising behavior of the geometry at 50khz which in order to keep the fixed items fixed, the reactions diverge. On the other end of the spectrum, in the low frequency range below the fundamental frequency ie. in a quas- static  case, I'd expect the simulation to approximate reactions as equal and opposite. If experimental measurement confirms the reactions are equal and opposite at 50khz, one possibility is adjusting damping to null out the resonance peak and realize your equal and opposite expectation.