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

TL;DR

This work investigates wake shed behind a large horizontal-axis wind turbine with flexible blades using a high-fidelity two-way fluid–structure interaction framework and proper orthogonal decomposition. The NREL 5MW onshore turbine is modeled with overset mesh and mesh morphing to capture aeroelastic blade deflection and wake dynamics under rated conditions. The main finding is that blade flexibility has negligible impact on wake modal behavior compared with tower and nacelle effects, with near-wake differences up to about $2$ m/s and far-wake structures dominated by wake shedding and tip vortices. The study demonstrates that tower wake contributions are essential for accurate far-wake predictions, informing wind-farm design and turbine aeroelastic considerations.

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.

Figures (22)

• Figure 1: Geometrical properties of the NREL 5MW
• Figure 2: Fluid domain and boundary conditions considered for the onshore NREL 5MW
• Figure 3: Mesh of the background and overset regions
• Figure 4: Polyhedral and tetrahedral meshes for the blade in the fluid and solid regions, respectively
• Figure 5: Comparison of the edge-wise deflection in one cycle for three meshes