Deformation and wake of a flexible rotor in water

A S Eldemerdash,T Leweke

doi:10.1088/1742-6596/1934/1/012006

Abstract

In this paper, we study experimentally the fluid-structure interaction of a simplified small-scale rotor operating in a water channel. The rotor consists of a single untwisted rectangular blade and is tested with various pitch angles at different rotation speeds and free stream velocities. Flapwise blade deformations are measured using image processing, and flow fields are obtained by Particle Image Velocimetry. The blade exhibits extreme downstream and upstream bending at high negative and positive pitch angles, respectively. In addition, for negative pitch angles, unsteady flapwise fluctuations are observed, accompanied by large-scale recirculation zones. These zones are formed and shed periodically behind the rotor, resembling the dynamics of the Vortex Ring State known for helicopters in steep descent. The induced axial velocity near the blade tip changes direction from upstream to downstream, in accordance with the bending direction, reaching values that exceed twice the free-stream velocity in extreme bending conditions. The blade deformation has a strong dependence on the tip speed ratio for low pitch angles, and a more pronounced dependence on the tip Reynolds number for high pitch angles. Results for a two-bladed rotor are also shown and compared to the behaviour observed for a single blade.

Highlights

The blades of a rotor often exhibit some degree of flexibility, due to their material properties or sheer size
We have investigated the fluid-structure interaction of a generic flexible rotor in water, where the large fluid forces can lead to strong blade deformations, focussing on flapwise bending
We have illustrated the effects of varying blade pitch angle, rotation frequency and free stream velocity, and presented a comparison between the deformation behaviour of single- and double-bladed rotors

Summary

Introduction

The blades of a rotor often exhibit some degree of flexibility, due to their material properties or sheer size. For the current set of experiments, involving a single fluid and only one blade material, the Reynolds number and tip speed ratio, which effectively represent the flow velocity and rotor frequency, are sufficient to characterise the various configurations.

Results

Conclusion