Modal Analysis of the Wake Shed Behind a Horizontal Axis Wind Turbine with Flexible Blades

TL;DR

This study uses proper orthogonal decomposition to analyze wake characteristics behind a full-scale wind turbine, revealing the effects of blade flexibility and tower presence on wake flow structures.

Contribution

It provides new insights into the wake dynamics of flexible versus rigid turbines and highlights the importance of tower effects in wake modeling.

Findings

Flexible blades show similar modal behavior to rigid blades in wake regions.

Wake structures include vortices and fluctuating velocities, especially far from the rotor.

Excluding the tower omits significant wake features, affecting downstream turbine conditions.

Abstract

The proper orthogonal decomposition has been applied on a full-scale horizontal-axis wind turbine to shed light on the wake characteristics behind the wind turbine. In reality, the blade tip experiences high deflections even at the rated conditions which definitely alter the wake flow field, and in the case of a wind farm, may complicate the inlet conditions of the downstream wind turbine. The turbine under consideration is the full-scale model of the National Renewable Energy Laboratory 5MW onshore wind turbine which is accompanied by several simulation complexities including turbulence, mesh motion and fluid-structure interaction. Results indicated an almost similar modal behaviour for the rigid and flexible turbines at the wake region. In addition, more flow structures in terms of local vortices and fluctuating velocity fields take place at the far wake region. The flow structures due to the wake shed from the tower tend to move towards the center and merge with that of the nacelle leading to an integral vortical structure 2.5 diameter away from the rotor. Also, it is concluded that the exclusion of the tower leads to missing a major part of the wake structures, especially at far-wake positions.