Embedded Laser Doppler Velocimetry Boundary-Layer Investigation on a Tilt-Rotor Blade

Abstract

A fundamental study is made of a rotating boundary layer (BL) in the inboard region of a tilt-rotor blade, using an embedded laser Doppler velocimetry probe inside a rotating blade. Velocity measurements have been performed to get insight into the BL behavior around a blade section close to the rotor hub.,BL transition and separation phenomena are investigated on the basis of both chordwise and spanwise velocity profiles measured for different collective pitch angles on a two-bladed hovering rotor. The velocity. experimental data bank appears to be in good agreement with theoretical velocity profiles in attached How conditions (laminar, transitional, or turbulent). Moreover, validation of a transitional criterion based on the second thickness of energy delta'(3) of the BL is extended to the rotational flow configuration. Usually such a configuration classically exhibits a separation of the BL for high collective pitch angles. However, in the particular case of a high twisted tilt-rotor blade operating in, hover, centrifugal effects are shown to be responsible for an important delay in the BL separation process.