Dissipation of turbulence in the wake of a wind turbine

The wake of a wind turbine is characterized by increased turbulence and decreased wind speed. Turbines are generally deployed in large groups in wind plants, and so the behavior of an individual wake as it merges with other wakes and propagates downwind is critical in assessing wind plant power production and equipment lifetimes. This evolution depends on the rate of turbulence dissipation in the wake.

In an experiment at the US Department of Energy's National Wind Technology Center, in situ measurements of winds and dissipation from the wake of a multi-MW turbine were collected using a tethered lifting system (TLS) carrying a payload of high-rate turbulence probes. Ambient flow measurements were provided from sonic anemometers on a meteorological tower located near the turbine.

These first direct measurement of turbulent kinetic energy (TKE) dissipation rate in the wake of a multi-MW wind turbine document the significant enhancement of dissipation rate downwind of a wind turbine: we observe enhancements of approximately 1-2 orders of magnitude at a distance ~ 160 m (2 rotor diameters) downwind of the turbine as compared with tower measurements upwind of the turbine. The direct measurements of dissipation rate from the TLS show good agreement with inertial dissipation estimates from sonic anemometers on the upwind stationary tower when both platforms sample undisturbed flow.

These wake measurements suggest that a local balance between turbulence production and dissipation does not hold in wind turbine wakes, and that it may be useful to pursue modeling approaches that account for enhanced dissipation. To enable incorporation of these results into simulations, these observations of dissipation rate variability are expressed in terms of variability with wind speed, with fluctuations in the horizontal wind speed (as a proxy for turbulent kinetic energy), and with turbulence intensity. The present dataset can provide useful benchmarks for turbine-resolving modeling studies.