I am not so clear the use of freestream velocity in FWH equation when helicopter rotor was in forward flight condition and observer position was fixed relative to the rotor.
The description of the problem is that when i use the CFD information on blade surface as the input for non-porous FWH equation, where is the freestream velocity used, is it used for the correction of surface velocity?
Dear Fu Jinbin,
We know that forward flight velocity has large impact on acoustic emission of a helicopter rotor. If the observer is stationary in relation to the rotation axis, the impact of its forward movement is many-fold (non-porous FW-H):
1) it modifies the velocity vector of each acoustic source panel (by adding a constant velocity vector) - as a result Un is modified as well
2) speed of sound propagation c becomes u+c (downstream waves) or u-c (upstream waves) - the acoustic waves are bunching-up together in front of the advancing side of the rotor - also the retarded time tau=t+r/c is different...
3) Mach number of the surface towards the observer Mr is not affected
4) Pressure distributions over each blade are modified, hence the loading term of FW-H changes
5) In high-speed forward flight, shock waves may appear leading to quadrupole emission of high-speed impulsive noise (porous FW-H advised).
Best wishes,
Oskar
Dear Dr. Oskar Szulc
Thanks a lot for your patient reply. By your answer, I can understand the mechanism of sound propagation in forward flight rotor clearly, but in your answer, the second aspect, confused me a lot, since the observer position moved with the rotor hub, the relative speed between observer and rotor hub is zero. In general, the sound speed should be the normal speed, why there is a difference with upstream and downstream wave?
Kind Regards
Jinbin Fu
Dear Fu Jinbin,
The speed of sound difference is there. Please imagine a source and an observer (both stationary in space). If you put them in an external "wind", the disturbances will travel downstream faster than upstream. This is because the speed of sound is local and relative to the medium that may be stationary or in movement.
I hope it is helpfull.
With my best regards,
Oskar
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