Frequency Content in the Wakes of Rotating Bluff Body Helicopter Hub Models

Abstract

It is estimated that the rotor hub of a helicopter is responsible for up to 30% of the parasite drag of a helicopter. This is because the hub is a group of rotating bluff-body shapes exposed to high-velocity flow, which may produce hub revolution-dependent flow structures in the hub wake. These structures also interact with a helicopter’s empennage and tail rotor, negatively impacting stability and performance. While some specific helicopter hubs have been studied, no study of a generalized hub shape has taken place. Because the hubs that have been studies are very geometrically complex, computational prediction of hub flow physics is not yet mature enough to be of use to industry. The objective of this study is to characterize the long-age wake of a geometrically simple configuration of canonical bluff bodies to model a helicopter hub. Three scale models were examined, each with four larger arms to represent rotor blade shanks and two smaller arms to represent scissor links. The models were identical in dimension, but one had a smoother arm profile and another had a variation in phase angle between the two sets of arms. The models were mounted in the Experimental Flow Physics Laboratory Large Water Tunnel, and tested at a hub diameter-based Reynolds number of 7.6 × 10?. Time-resolved velocity measurements were taken 14 hub radii downstream of one model, while phase-averaged velocity measurements were taken 7 hub radii downstream of the other two. Similar trends to previous works were observed, including two-per-hub revolution, four-per-hub revolution, and 6-per-hub revolution frequency content in the velocity spectra. This study therefore aids in the uncovering of fundamental flow physics of rotor hubs, creating a baseline case to which further parametric variations may be compared.