Wind-Turbine Wakes in a Convective Boundary Layer: A Wind-Tunnel Study

Abstract

Thermal stability changes the properties of the turbulent atmospheric boundary layer, and in turn affects the behaviour of wind-turbine wakes. To better understand the effects of thermal stability on the wind-turbine wake structure, wind-tunnel experiments were carried out with a simulated convective boundary layer (CBL) and a neutral boundary layer. The CBL was generated by cooling the airflow to 12-15 • C and heating up the test section floor to 73-75 • C. The freestream wind speed was set at about 2.5 m s −1 , resulting in a bulk Richardson number of −0.13. The wake of a horizontal-axis 3-blade wind-turbine model, whose height was within the lowest one third of the boundary layer, was studied using stereoscopic particle image velocimetry (S-PIV) and triple-wire (x-wire/cold-wire) anemometry. Data acquired with the SPIV were analyzed to characterize the highly three-dimensional turbulent flow in the near wake (0.2-3.2 rotor diameters) as well as to visualize the shedding of tip vortices. Profiles of the mean flow, turbulence intensity, and turbulent momentum and heat fluxes were measured with the triple-wire anemometer at downwind locations from 2-20 rotor diameters in the centre plane of the wake. In comparison with the wake of the same wind turbine in a neutral boundary layer, a smaller velocity deficit (about 15 % at the wake centre) is observed in the CBL, where an enhanced radial momentum transport leads to a more rapid momentum recovery, particularly in the lower part of the wake. The velocity deficit at the wake centre decays following a power law regardless of the thermal stability. While the peak turbulence intensity (and the maximum added turbulence) occurs at the top-tip height at a downwind distance of about three rotor diameters in both cases, the magnitude is about 20 % higher

Wind turbine wake properties: Comparison between a non-rotating simplified wind turbine model and a rotating model

“…On the other hand, no signature of the tip vortex remains visible at x/D = 3, even for the low turbulent inflow conditions. These results are consistent with Zhang et al [23,24] works. They showed that the tip vortex signatures are not distinguishable anymore from x/D = 3 in a neutral or convective incoming boundary layer with a turbulence intensity of 8% at hub height.…”

“…The values of maximum velocity deficit (40 to 50% of the mean hub velocity) are representative of full scale wind turbine wakes and are in agreement with literature [21,24,22]. Since the wake diffusion is dependent of the boundary layer conditions, it is difficult de precisely compare the wake evolution obtained in the latter references.…”

Influence of atmospheric stability on wind-turbine wakes: A large-eddy simulation study

“…This behavior was reported in previous numerical and experimental studies by Wu and Porté-Agel 40 , Rados et al 41 , Chamorro and Porté-Agel 42 , and Zhang et al 10 . Recently, Bastankhah and Porté-Agel 17 developed a new analytical model to predict the wind velocity distribution downwind of a wind turbine by applying conservation of mass and momentum and assuming a self-similar Gaussian distribution for the velocity deficit in the wake.…”

“…They showed that the wakes recover faster under the convective conditions compared with the neutral cases. Recently, Zhang et al 10 carried out wind-tunnel experiments of the down-scaled wind-turbine models to investigate the effect of thermal stability on wind-turbine wakes under the convective and neutral conditions. They found a smaller velocity deficit (around 15% at the wake center) in the CBL compared with the wake of the same wind turbine in the neutral boundary layer.…”

The impact of the diurnal cycle of the atmospheric boundary layer on physical variables relevant for wind energy applications

“…There are experimental studies considering different atmospheric stratifications (NBL, SBL, CBL) (e.g. Medici and Alfredsson (2006), Chamorro and Porté-Agel (2010), Zhang et al (2012), Tian et al (2013), Zhang et al (2013)). In most of the numerical simulations of an individual wind turbine, however, an NBL is assumed (e.g.…”