Modelling lidar volume-averaging and its significance to wind turbine wake measurements

A R Meyer Forsting,N Troldborg,A Borraccino

doi:10.1088/1742-6596/854/1/012014

A R Meyer Forsting, N Troldborg + Show 1 more

Open Access

https://doi.org/10.1088/1742-6596/854/1/012014

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Abstract

Lidar velocity measurements need to be interpreted differently than conventional in-situ readings. A commonly ignored factor is “volume-averaging”, which refers to lidars not sampling in a single, distinct point but along its entire beam length. However, especially in regions with large velocity gradients, like the rotor wake, can it be detrimental. Hence, an efficient algorithm mimicking lidar flow sampling is presented, which considers both pulsed and continous-wave lidar weighting functions. The flow-field around a 2.3 MW turbine is simulated using Detached Eddy Simulation in combination with an actuator line to test the algorithm and investigate the potential impact of volume-averaging. Even with very few points discretising the lidar beam is volume-averaging captured accurately. The difference in a lidar compared to a point measurement is greatest at the wake edges and increases from 30% one rotor diameter (D) downstream of the rotor to 60% at 3D.

Highlights

Lidars rely on the reflection of emitted light from aerosols back to the receiver to measure wind speed
To determine the velocity at the desired measurement location, the beam can be focused or the backscattered signal split by the time of flight
The measured line-of-sight velocity vlos at a point in space xF is given by the convolution of the radial velocities sampled along the beam and the weighting function vlos(xF ) = − e(xF, xL) · V(r)W (r) dr

Summary

Introduction

Lidars rely on the reflection of emitted light from aerosols back to the receiver to measure wind speed. To determine the velocity at the desired measurement location, the beam can be focused (continous-wave) or the backscattered signal split by the time of flight (pulsed). The measured line-of-sight velocity vlos at a point in space xF is given by the convolution of the radial velocities sampled along the beam and the weighting function vlos(xF ) = − e(xF , xL) · V(r)W (r) dr

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