Effect of thermal stability on wind turbine wakes: an experimental and analytical study

We present first results from the thermally stratified boundary layer wind tunnel at EPFL, Switzerland. Stable, neutral, and convective thermal stability conditions are developed in the wind tunnel using temperature control of the floor and the air at the inlet. Two turbines in fully aligned conditions are then used to study the effect of thermal stability on the wake flow. The effect of stability on the recovery of the single and cumulative wake is shown, with a faster (slower) recovery in the convective (stable) conditions compared to the neutral ones. Two-point spatial correlations are performed to characterize the effect of thermal stability on turbulent length scales in turbine wakes. The correlations are found to be stronger in the convective case, with a hint towards enhanced meandering in this case. The stable case shows smaller correlations compared to the neutral one, with a strong signature of tip vortices behind the first turbine. The experimental data is used to test the Gaussian analytical wake model. As the streamwise turbulence intensity is the same in all cases, the model is unable to differentiate between different stability conditions. To resolve this, the relations for wake growth rate, near wake length, and wake added turbulence are re-formulated in terms of the vertical turbulence intensity. The re-formulated analytical framework yields reasonable predictions of the single and cumulative wake velocity deficit for all stability conditions.