Mean flow interactions of a counter-rotating propeller

The aerodynamic interaction between the forward and rear rotors in a counter rotating (CRP) system, has been examined using a double conditional sampling methodology applied to 3-D thermal anemometer data. The technique effectively freeze the rotors in any desired relative position and provides the inter-rotor flow field. Axial, radial and circumferential steady mean flow between rotors is shown relative to the 'fixed' forward rotor for various 'fixed' rear rotor positions. The effects of the upstream disturbance from the rear rotor on the wakes of the forward rotor blades have been documented. This disturbance occurs in all three flow components and varies with distance from the rear rotor and with radial location. Modulations of the forward wakes depend upon the relative rear rotor locations. As the rear rotor continues to rotate past the forward blade, the location of the peak velocity from the forward blades moves circumferentially and springs back as the cycle repeats for the next rear blade. This is a 'wiggling' motion. Double conditional sampling is also used to evaluate the determining CRP flow field by superposing the flow that would occur as a result of the forward and rear rotors operating separately. Simulation of conditions on both rotors that occur in the CRP configuration is attempted by duplicating alternately RPM's, inflow conditions, angles of attack and power and thrust coefficients. The superposition for the various single rotor operating conditions does not yield an acceptable accuracy of wave forms and magnitude simultaneously. The velocity vectors of the cross-stream component show that superposition does not produce the identical flow direction and magnitude of the actual flow.