Higher harmonics in vortex generation

[hynatec]

Actually we investigate the vortex generation passed a bluffbody. The bluffbody has a triangle shape and is placed in a circular tube. Behind the bluffbody a small plate element is placed with a small sensor that gives a voltage output proportional with the bending force of the plate. With this experimental set-up we can detect very nicely the vortices, that pass the plate element. However we see also - very significantly - an oscillation that has a frequency 3 times the normal frequency. At the present time we cannot find a explanation for that.
Who has also found such a phenomenon?
What could be the explanation?

 

[israelkk]

Maybe the plate is entring its own resonant frequency?

 

[m777182]

The vortex generation is described, if I remember correctly, by a third degree differential equation of nonlinear type so that higher harmonics of the base frequency that is given by Strouhal number may be involved. But exclude vibrations of the pump and the pipe system first.It would be nice to have two sensors- for pipes and the pump and for vortex deltaP to eliminate system noise.
m777182

 

[ApC2Kp]

hynatec,
An earlier study of vortex formation and the resonant frequencies of a thermowell resulted in a realization that the in-line frequency was twice the lateral frequency. Perhaps the superposition of the two may result in what seems to be the 3rd mode. David Bartran has software for thermowell vibration analysis at tempsensor.net (temperatures.com), search the site for "bartran", or contact him by email at primary.net