The distortion of turbulence by a low Reynolds number UAV propeller and the effect on aeroacoustics

An analytical and experimental study

Master thesis (2021)

Authors

J.C.M. van Beek Aerospace Engineering

Contributors

F. Avallone Wind Energy - Aerospace Engineering (supervisor 1)
D. Ragni Wind Energy - Aerospace Engineering (supervisor 1)
E. Grande Wind Energy - Aerospace Engineering (supervisor 1)
Faculty
Aerospace Engineering
Copyright: © 2021 Luc van Beek
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Published Date

14-07-2021

Language

English

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Faculty

Aerospace Engineering

Abstract

This work
presents an experimental and analytical investigation of the distortion of grid
generated turbulence and its relation to the aeroacoustics of a low Reynolds
number propeller. The propeller has a chord based Reynolds number of 104≤
Rec  ≤ 105 and
operates at an advance ratio of 0.225. The mean flow model of Hough and Ordway
(H&O) is used to compute the streamtube upstream of the propeller. This
allows the distortion of turbulence due to streamtube contraction to be
predicted by means of Paterson and Amiet's distortion model. The distortion of
turbulence by the leading edge of the blade is modelled with the asymptotic
solution of Hunt. Amiet's propeller turbulence ingestion noise model is used to
couple the distorted turbulence to noise. The models are validated by means of
an experiment. A square bar grid with mesh size 0.1m and bar width 0.01m is
placed upstream to generate turbulence. A load-torque cell measures the thrust
which serves as input for the analytical mean flow model. Using particle image
velocimetry (PIV) and hotwire anemometry the turbulence statistics upstream and
up to the propeller plane are measured. PIV is also used to measure the
distortion of turbulence by the leading edge of the propeller blade. The
experiment is done in the TU Delft A-tunnel, where acoustic measurements are
performed using a microphone array. Results reveal that the model of H&O
overpredicts the induced velocities and a correction has to be applied such
that the experimental results are matched. The analytical model and flow
measurements show that the isotropic turbulence is not distorted due to the
contraction of the streamtube and isotropy is conserved. The turbulence
intensity and integral lengthscale do change and this is ascribed to the decay
of turbulence behind a grid. Turbulence is distorted by the leading edge and an
increase of the turbulent upwash velocity of 25% is observed. It is found that
by using the decayed turbulence statistics, the tones at the blade passing
frequency harmonics are best resembled. The correction for distortion by the
leading edge does not improve the predictions.