Source of Vibration for Gas Turbine Engine Fan

[desimontreal]

A question arised about broadband frequency noise and vibration induced by a gas turbine engine Fan. I have some experience doing some analysis on vibration caused by interaction between static and rotating components but no experience at all on the sort of vibration that may arise from unsteady aerodynamic forces from a fan due to possible some geometric differences between fan blades (manufacturing or untwist due to CF). A colleaged asked about high broadband vibration eminating from the front of the engine possibly the source was the fan. The fan was balance as per specs twice. The question really is such induce vibration from unsteady aerodynamic flow significant to generate high levels of vibration and if so how does one determine if it is the fan unsteady flow is the source. What would be the range of frequencies that this sort of aerodynamics flow generate. So far, i have search the net for answers and have found only noise reductions...but not much on dentrimental effect to structures and how does one determine if it is a flow induced vibration. Can someone enlighten me on the subject and perhaps a good reference on the subject would be appreciated

 

[spciesla]

For typical HVAC fans, flow problems manifest themselves in the vibration spectrum with a dominant peak at the vane passing frequency (VPF) or a harmonic, 2xVPF. For example, a 16 blade fan might show large peaks at 16X and 32X, if the fan is not centered in the housing. This vibration data would be seen on the motor/housing. If you are using proximity probes to view the shaft relative vibration, I don't know if you would see it.

 

[SomptingGuy]

I guess turbocharger compressor noise is similar in concept. Here we have 3 distinct sources:

Whistle - A narrow peak at blade-passing frequency.
Whine - A narrow peak at the rotation frequency, caused by imbalance.
Whoosh - Broad-band noise caused primarily by turbulence, worse near stall.

From your description it sounds like you have whoosh.

 

[desimontreal]

Thank you so far for the reply...I understand the blade passing frequencies...more interested in vibration source that can cause significant level of vibrations that is out of the ordinary or something typical to large fans...for example would tip clearance (a local inferfection due to asymetric wear)local disturbance cause signifant vibration. Differnce is geometry of the fan blade.

Ive have done some analysis on compressor blades before to ensure the blade passing frequencies are no where near the structure natural frequencies. At the time, the work was done on a smaller compressor blades. I am more interested if irregular airflow can cause significant vibration other than just generating significant level of noise in similarly large fan blades on gas turbines...

The only instrumentation used was an accellerometer. My colleague suspect broadband noise/vibration due to difference in geometry of the blades and deformation of fan blade due to CF loading that is either causing irregular airflow patterns that is causing the vibration. I believe, if it irregular then how can it hit the structures resonance to cause high vibration levels. Unless, the level of aerodynamic forces is sufficient to cause such levels.

Aero dynamic stall...i know that one, but never had the oppotunity to have to detect a stall...unless of course a full surge accur and it becomes obvious. Is there a way of estimating frequencies from such disturbance.

What is a woosh ? Thank you.
- Can irregularities in the flow path like perhaps irregular tip clearance around the circumference generate sufficient aerodynamic forces to activate some of the rotor natural frequencies.
- How about if one of the blades is just not geometrically the same...would that cause significant rotor dynamic vibration or induce aerodynamic forces down stream..

Does anyone know of any site or forum where they deal with Aircraft turbofan gas turbine engine vibration...or operational problems related to gas turbine engine.

 

[SomptingGuy]

What is a whoosh?

This is an onomatopoeic word used to describe the sound of turbulence-generated noise. Think of the wind blowing through trees. It is broadband in nature as you said your noise is.