Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

Abstract

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.

Effects of freestream turbulence on the wake growth rate of a model wind turbine and a porous disc

“…It is observed that the wake growth rate for model wind turbines and porous discs are different especially at low ambient turbulence intensity levels. One should note in here that aside from the current study and the porous disc wind tunnel data reported in Abdulrahim et al [16], the other wind turbine and porous disc data have different thrust coefficients. The scatter in the data can be attributed to different levels of thrust coefficients presented for wind turbine or porous disc.…”

“…For more details regarding the estimation of wake spreading characteristics one can refer to Abdulrahim [16]. The variations of wake width ( ⁄ ) with downstream distance for the wind turbine and porous disc at different ambient turbulence intensity levels are shown in Figure 9.…”

“…The wake growth rate for the wind turbine is higher compared to the porous disc at both ambient turbulence intensity levels. [14], field measurements [15], from small wind turbine measurements in wind tunnels [6,19,20,21,22,23], and from porous disc wind tunnel measurements [6,16,24] including current measurements for wind turbine and porous disc. In this respect, k* values of other porous discs and for small wind turbines have been calculated by the methodology described above using the published wake velocity profile data in the literature.…”

“…The linear variation relations are mainly based on LES results and field measurement data for full scale wind turbines. Abdulrahim et al [16] show, using porous disc wind tunnel measurement data, that the wake growth rate for model wind turbines and porous discs can be significantly different, especially at low ambient turbulence intensity levels. This point is critical and needs to be kept in mind especially when using porous discs or small wind turbines to study various wake related phenomena in wind tunnels and when relating observed results to various wake models in the literature.…”

The far wake of porous disks and a model wind turbine: Similarities and differences assessed by hot-wire anemometry

“…They used a disk with ribs protruding from the center, connecting several solid circles at different radial positions. Very recently, Abdulrahim et al 31 investigated which effects an incoming boundary layer has on the wake of a PD of similar design. Unsteady loading on wire mesh disks was studied by Yu et al 32,33 PDs have also been used as wind turbine models in wind farm experiments.…”