On the Relationship Between Blade Circulation and Tip Vortex Characteristics

Measurements were made of the three-dimensional roll-up of the tip vortices generated by a small-scale rotor operating in hover. The measurements were made using three-component laser Doppler velocimetry. A boundary layer trip was used to simulate the e®ects of a higher blade Reynolds number. The detailed structure of the tip vortex was investigated, including estimates of the core growth resulting from viscous di®usion. A general logarithmic core growth curve is suggested, which for older wake ages is proportional to the square-root of time. The spanwise circulation loading on the blade was estimated from the velocity field measurements by contour integration. The connection between blade circulation loading and the tip vortex properties was examined. It is shown that for rotors it is difficult to exclude trailed wake circulation from contours that enclose the blade, making circulation measurements possible only under some conditions.