Effect of freestream turbulence on the coherent dynamics of a wind turbine wake

TL;DR

This study experimentally probes how freestream turbulence with varied intensity $T_i$ and integral length scale $L_v$ influences the coherent dynamics of a wind turbine wake using time-resolved PIV in a controlled lab setup. Through spectral analysis, Optimal Mode Decomposition with mode clustering, and a multi-scale triple-decomposed coherent kinetic energy budget, it shows that increased FST accelerates the breakdown of tip vortices and advances wake recovery, while near-wake meandering remains turbine-driven and far-wake meandering becomes more energetic and broadband under FST. The work reveals that FST modifies energy pathways by enhancing mean-flow production of wake-meandering modes but weakening triadic exchanges among coherent modes, and it identifies new frequency interactions ($3f_r\pm f_{wm}$) arising from nonlinear coupling. Overall, the findings demonstrate that even short-$L_v$ turbulence can significantly alter wake dynamics, with implications for turbine spacing and wake modelling in wind farms.

Abstract

The wake of a model wind turbine exposed to incoming freestream turbulence (FST) with a variety of turbulent characteristics is studied through Particle Image Velocimetry experiments. The FST cases were produced using different passive turbulence generating grids. The cases spanned turbulent intensities (T_i) in the range 1.3% < T_i < 14% and only considered short integral length scales L_v<0.2D (where D is the turbine diameter). Increasing T_i and L_v in this range resulted in an earlier breakdown of the tip vortices which in turn resulted in an earlier onset of wake recovery. For all the FST cases considered, the initiation of wake meandering was found to be related to an intrinsic instability of the turbine, even for the cases with the highest FST levels. The amplitudes of wake meandering were similar for all the cases in the near wake (x<2D), but the amplitudes in the far wake (x>4D) were discernibly higher for all the FST cases compared to the no grid case (lowest T_i), primarily due to the early break down of the tip vortices. Deeper insights into the origins, and subsequent evolution, of the various coherent motions (characterised by particular frequencies) in the presence of FST are obtained through analysis of the multi-scale triple-decomposed coherent kinetic energy budgets. The wake meandering modes in the presence of FST are shown to better utilize the mean velocity shear, extracting more energy from the mean flow while other sources such as non-linear triadic interactions and diffusion also become important.

Figures (26)

• Figure 1: Schematic of the experimental setup.
• Figure 2: Profiles of mean streamwise velocity at the rotor plane (with the turbine removed). The case IDs are shown on the top right.
• Figure 3: Turbulent intensity profiles at the rotor plane (with the turbine removed). The case IDs are shown on the bottom right.
• Figure 4: Profiles of integral length scales at the rotor plane (with the turbine removed). The case IDs are shown on the bottom right.
• Figure 5: (a) Parameter space of turbulence intensity ($T_i$) and integral length scale ($L_v$) produced by different turbulence generating grids. The symbols show the values averaged over the rotor radius. (b) shows the effective tip speed ratios ($\lambda_{eff}$) for $\lambda_{\infty} = 6$. $\lambda_{eff}$ is measured based on the rotor averaged bulk velocity, $U_b$ instead of $U_{\infty}$ and hence is lower than $\lambda_{\infty}$. The same symbol is used for a particular type of grid used as indicated in (a). The case IDs are also indicated in (a).