| Abstract | Some preliminary measurements of noise from a highly loaded fan-in-wing configuration are reported. Measurements of the spectra are presented for fan speeds of 7500, 9750, and 13, 125 rpm (corresponding to tip Machnumber 0.35, 0.45, and 0.62) at an angle of 20° from the axis of the fan and at 5 feet from the inlet and efflux faces of the fan.
The experimental results show a discrete peak at blade-passing frequency, superimposed on a broad band noise that extends from 1000 cs to 15,000 cs. An analysis of the duct transmission of higher order modes at the above rotational speeds, as given by Tyler and Sofrin, reveals high decay rates. This explains the absence of discrete tones at the harmonics of the blade-passing frequencies. The presence of high intensity broad band noise may be attributed to the turbulence in the wake and free stream turbulence ahead of the rotor blades. |
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