Measurement of the interaction between a rotor tip vortex and a cylinder

Abstract

The transient interaction between a cylinder and the trailing vortex from a rotor in forward flight is studied. Phase-averaged laser velocimetry and surface pressure measurements made with flush-mounted microphones are used to study the velocity and pressure variations during such an interaction. Vorticity contours constructed from the velocity measurements exhibit the presence of a secondary structure with vorticity opposite in sense to that of the primary tip vortex. This structure moves rapidly around the tip vortex from upstream to downstream. The pressure variations caused by the tip vortex on the surface of the cylinder are smooth as the vortex core passes by, and no evidence is found of fine structure inside the vortex core region. After vortex interaction, the secondary structure causes large variations in the surface pressure before being dissipated. Calculations using measured vortex strength and speed data indicate that the distortions and deflections of the vortex immediately prior to impingement on the surface differ significantly from those computed using two-dimensional potential flow concepts. Nomenclature Cpu = unsteady pressure coefficient: difference between the instantaneous pressure and the local mean static pressure, normalized by the tunnel dynamic pressure R - rotor radius r = radial position along the rotor radius U = velocity component along the tunnel axis, positive going downstream Um = freestream velocity V = vertical velocity component, positive downward X = distance parallel to the tunnel axis, origin is at the rotor hub center Xb - distance parallel to the tunnel axis, measured from the nose of the cylinder Yb = distance along the lateral direction from the cylinder axis Zb = vertical distance from the cylinder axis, positive