if I would like to obtain resonance due to rotation what is the conditions I should set?
is it enough to only assume that the rotation frequency need to be equal to the natural frequency?
if I added a force to excite the rotating disk, is the condition will remain the same, excitation frequency need to be equal to be equal to rotation frequency and to the natural frequency?
if my machine can t achieve the necessary speed, do I will have resonance at half speed?
I think it might be resonant when the natural resonance is 1, 3 , 5 etc times the rotation frequency, depending whether the coupling is linear or not.
Probably not at twice rotation frequency.
The rotation frequency is already 5000 so do I need to rotate it further more?
You start slow and slowly increase the speed using a microphone or accelerometer to measure the vibration and detect when it is highest.
I start with low rotation speed until I find speeds that gives resonance
By 5000 I expect you mean 5000 rpm. This is less than 100 Hz. What frequencies are you expecting?
Malcolm White the natural frequency of my blade is 347 hz for 1st mode.
and the max speed of the turbine is 5000 rpm.
I know that to reach resonance I need that the speed be equal to the natural frequency value but I thought I might have a pseudo resonance by using sub-speed (frequency of rotation/2*pi).
No. That can't work. You probably mean frequency of resonance/(2*pi). Frequency of rotation/2/pi is less than 1000. it looks like you don't understand how the rotation makes the object vibrate. See if you can find slow motion You-tube of objects being vibrated, or something like that.
347 Hz requires just over 20000 rpm. At around 4000 rpm there is a chance that the 5th harmonic might be excited, but probably not much. You might get the third harmonic of something around 6700 rpm.
Malcolm White yes I am new to this subject.
the subharmonic is to devide the resonance frequency to a lower value to obtain the Nth harmonic... devide on 5 to get the 5th harmonic, in the same time we devide on the speed of rotation to 5 to excite the 5th harmonic.??
It is possible that if the resonant frequency is 5 times the rotation frequency you may get resonance, more probable or stronger when the resonant frequency is 3 times the rotation frequency, and you should get a strong response when the rotation frequency is equal to the resonant frequency. You probably won't get anything when the resonant frequency is 2 or 4 times the rotation frequency.
Malcolm White I Know that I already disturbed you a lot but still did not get it
The rotating object shakes the thing you are interested in from side to side each time it goes round. If they don't shake at the same rate then very quickly they will be going in opposite directions and the rotating object will stop the shaking instead of increasing it. If the rates are very different then there will almost be no effect.
If the rotation is 1/3 of the resonant frequency, then the object will start shaking, and will shake 1 1/2 time and be going in the opposite direction when the rotation is also going in the opposite direction, so may get kick in that direction. After another 1 1/2 cycles of vibration they will both be going in the original direction, so there is a chance of resonance building up. This works for 1/3, 1/5, 1/7 ..... but not for the even numbers. I think this may require that the coupling in non-linear, but I think there is a good chance it might be.
Read something about forced oscillations and do some mathematics. If you are serious you need to do some work.
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