Dear All
It is known that turbulance and cavitation (or any problem that make broadband random vibration) may cuase resonance problems. Usually the natural frequency that excited by this way, lie inside of broadband vibration generated. But I have a question, is there anyone who has been countered a case that for example the cavitation occurred in 3~5KHz but it excite a natural freq. of say 70Hz or anywhere outside the broadband vibration?
Regards.
Farahani

Farahani,

I would say the answer is No, since the force level at 70 Hz from Cavitation is very low. The 70 Hz may be excited by a pulsation frequency that would have to be a multiple of shaft speed; assuming we are talking about a pump.

Walt

Your assumption that the cavitation occurs at 3 to 5 kHz is overly restrictive. While 3 to 5 kHz may be a good frequency range to detect it, particularly in the incipient stage, there can be effects at lower frequencies. Depending on the amount of vapour present, and head loss, there can be unsteady flow / pressure behaviour in much lower frequency ranges.
Also, keep in mind that there doesn't have to be a periodic excitation right at 70 Hz to excite a system natural frequency at 70 Hz. All transient responses of a system are at natural frequencies - so if you whack it with broadband or impulsive flow / pressure effects, you'll see natural frequencies in the response.

Dear Ron
Let me confine the discussion into a special problem that I encounter it nowadays. It is a pump that have resonating case. The peak is at 62.5Hz and has value of about 40 mm/s rms. It is directional and almost disappear along coupling. The pump work in cavitation situation but in freq. range between 3~5 khZ. I seek anything that have possibility to produce 62.5Hz vibration, but until now I coud not find any justification. How do you think about this problem?
Regards.