Expert: Paul Soderman - 2/13/2010

Question
QUESTION: Dear Sir,

Some time ago i read an article called "Theoretical and experimental response of panels to turbulent boundary layer pressure fluctuations and separated flow. Some preliminary results" from NASA's website. If im not mistaken, the article concluded that separated flow increased the strength of pressure fluctuation on the wall of the panel.

My question is:

1.Can this increase of pressure fluctuation cause significant vibration on the panel?
If for example this separated flow occured  at the flow over a propeller fan blade, can this vibration cause any harm such as fatigue, over a period of time?

2.Will this pressure fluctuation also contributes to increase in aerodynamic noise?

Thank You very much,
Akbar

ANSWER: Yes, flow separation can cause high pressure fluctuations on a panel or fan blade.  The flow is chaotic and will cause vibrations that can lead to fatigue depending on the nature of the structure.  Flow separation on a fan blade will lead to an increase in aerodynamic noise because that type of noise is a function of the steady and unsteady loading.

---------- FOLLOW-UP ----------

QUESTION: Thank you very much Mr.Soderman,

I have a few more question related to this :

1. In a propeller fan, can the wake caused by this separated flow on one blade, cause a negative effect on the aerodynamic performance of the downstream blade/blades? If any,what kind of effect will that be?

Thank you in advance,
Akbar

Answer
Yes, flow separation on a fan blade will thicken the wake and increase the perturbation on any downstream airfoil.  That could cause a noise increase depending on the distance to the airfoil.  I assume you are talking about a downstream stator, which is non-rotating.  The only job it has is structural and perhaps some flow straightening.  So poor flow on the rotor blades won't do much to the stator performance.  The bigger problem is the performance of the rotor.