Sound Generated by Rotor Wakes Interacting with a Leaned Vane Stator

An unsteady three-dimensional lifting surface theory to predict the sound field of a stator with leaned, i.e., nonradial, vanes in an annular duct is presented. The duct carries a uniform subsonic main flow and is assumed to be anechoic. The sound is generated by the interaction of velocity disturbances with the stator vanes. The problem is formulated as an integral equation for the pressure jump across the vanes. This equation is solved by a Fourier series expansion, followed by a collocation procedure. The effect of vane lean on the sound field of a typical stator exposed to the viscous wake system of a rotor is studied. The modal distribution proves to be very sensitive to lean variation. Unless the rotor speed is very low (one mode cut-on), no reduction in the acoustic power at the blade passing frequency is found for any lean angle. On the contrary, even a moderate amount of lean raises the power significantly.