Rotor wake interaction with separated flow

Abstract

Aerodynamic interactions represent one of the toughest challenges to computational methods for the prediction of rotorcraft performance, loads, and noise. Efforts over the past ten years using a simple hemisphere/cylinder airframe under a teetering two-bladed rotor have shown that most of the dominant effects can be predicted using potential-flow concepts, in the absence of massive flow separation and strong vortex-surface collision. Here, the prospects for modeling the separated flow interaction are studied. The rotor wake interacts with the separated flow over a pressure-instrumented boom, downstream of a backstep cut into the original cylinder. The undisturbed backstep flow is characterized using spectral analysis of the velocity and surface pressure fields. The interaction between the tip vortex and the backstep free shear layer is visualized using laser sheet videography, and measured in detail using laser velocimetry and pressure sensing. The tip vortex dominates the interaction, causing periodic destruction and reconstruction of the shear layer, and large-amplitude longitudinal motion of the reattachment zone. However, the characteristics of the undisturbed shear layer are still detected in the surface pressure spectra. The tip vortex and its collision with the boom surface appear to be unaffected by the shear layer interaction. It is argued that this experiment is a conservative representation of the interaction, so that occurrences on real rotorcraft should be less complex. Thus, while the separated flow interaction appears to be extremely complex at first sight, the dominant effects are surprisingly simple, and may be easily modeled.