Fluid Dynamics of the Helicoidal Wake Sheets Trailed from a Hovering Rotor

摘要

High-resolution particle image velocimetry (PIV) masurements were made to understand the behavior of the helicoidal wake sheets trailed by a rotor system, with and without blade twist. PIV measurements were made in hover at two representative blade loading coefficients. The wake sheets were initially laid down behind the span of the blades as small-scale, counterrotating vortical pairs of unequal strengths. This initial concentrated vorticity, however, quickly diffused into zones of concentrated turbulence, which progressively became a significant source of overall turbulence in the downstream rotor wake. Several types of fluid dynamics were documented inside the rotor wake, including interactions of the sheets with the tip vortices, which depended on both blade twist and rotor thrust. At earlier wake ages, a wave was seen to form on the wake sheets, although the growth of wave amplitude was limited as the sheets were stretched in the downstream wake.