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

Abstract

The wakes of two different porous disks have been evaluated experimentally. Such disks are commonly used as physical actuator disk analogs for wind turbines. One disk is made of a uniform wire mesh, while the other has a nonuniform design with radial spars connected by rings. The disks have the same solidity and produce approximately the same drag. The wakes have also been compared to the wake of a model wind turbine and a solid disk. In contrast to earlier studies, the far wake, up to 30 diameters downstream, is included in the comparison. In the near wake, the velocity deficit and turbulence intensity profiles of the disk wakes differ significantly. High levels of turbulence intensity in the wake of the nonuniform disk increase the transverse transport in the wake, which leads to faster spreading and lower velocity deficits in the far wake, compared to the uniform disk and the wind turbine. High velocity gradients in the wake of the uniform disk give rise to turbulence production farther downstream, maintaining higher turbulence levels in the far wake. In addition, coherent vortex shedding is only identified in the wake of the nonuniform disk. None of the disks were able to replicate the asymmetric features of the wind turbine wake. Nonetheless, the results highlight important flow physics that should be considered in the design process of a porous disk used as a wind turbine surrogate.

Characterizing porous disk wakes in different turbulent inflow conditions with higher-order statistics

“…The two PDs both have a diameter of D = 200 mm, yielding a Reynolds number of Re = U ∞ D ≈ 1.1 × 10 5 for the experiments, where U ∞ = 8.1 ms −1 is the freestream velocity and is the kinematic viscosity. The disks are the same as the ones used by Vinnes et al (2022). The schematics of them are shown in Fig.…”

“…The disks were mounted on a tower and positioned in the center of the tunnel. More details on the disks and mounting can be found in Vinnes et al (2022).…”

“…The data acquisition and post-processing were done in the same manner as by Vinnes et al (2022), and the methodology is described in detail there. Therefore, only a summary will be presented here.…”

“…In case of the ND, a distinct peak with a reduced frequency of fD∕U = 0.15 (low TI) and fD∕U = 0.16 (high TI) is visible particularly at outer radial positions. Vinnes et al (2022) showed that the frequency in the low TI case is the shedding frequency of the ND. With only a minor change of frequency, the peak for the high TI case is ascribed to the same phenomenon.…”

“…For porous disks, two major types have been used for wake investigations: mesh disks and disks with varying degrees of blockage that can also change radially. Porous disks are interesting to study for their practical use to replicate the flow past, for example, parachutes (Higuchi et al 1998) or more often wind turbines (España et al 2011(España et al , 2012Aubrun et al 2013;Theunissen et al 2015;Lignarolo et al 2016;Howland et al 2016;Camp and Cal 2016;Bossuyt et al 2017;Yu et al 2017Yu et al , 2019Camp and Cal 2019;Abdulrahim et al 2021;Helvig et al 2021;Neunaber et al 2021;Vinnes et al 2022). To replicate the flow past a wind turbine, PDs are often mounted to towers which breaks the axisymmetry of the wake by shifting it in the direction of the tower (Pierella and Saetran 2017).…”