Wind Energy and Wind-Energy-Inspired Turbulent Wakes: Modulation of Structures, Mechanisms and Flow Regimes

• Main Author: Camp, Elizabeth H.
• Format: Others
• Published: PDXScholar 2018
• Subjects: Mechanical Engineering
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/4391; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=5436&context=open_access_etds

The interaction of turbulent wakes with one another and with the adjacent fluid directly impacts the generation of electricity in wind turbine arrays. Computational modeling is well suited to the repeated iterations of data generation that may be required to inform understanding of the function of wind farms as well as to develop control schemes for plant function. In order to perform such computational studies, a simplified model of the turbine must be implemented. One of the most computationally efficient parametrizations of the blade utilizes a stationary disk which has a prescribed drag and produces a wake. However, since accurate estimates of wake properties and the interaction with the surrounding fluid is critical to the function of wind farms, a comparison of the wakes emitted from a stationary disk model should be compared to that of a model with a rotating blade. Toward this end, an array of model rotating wind turbines is compared experimentally to an array of static porous disks. Stereo particle image velocimetry measurements are done in a wind tunnel bracketing the center turbine in the fourth row of a 4$\times$3 array of model turbines. Equivalent sets of rotors and porous disks are created by matching their respective induction factors. The similarities and differences in the wakes between these two cases are explored using time-averaged statistics. The primary difference in the mean velocity components was found in the spanwise mean velocity component, which is much as 190\% different between the rotor and disk cases. Conditional averaging of mean kinetic energy transport in wake from these two models reveals that a differing mechanism is responsible for the entrainment of mean kinetic energy in the near wake. In contrast, results imply that the stationary porous disk adequately represents the mean kinetic energy transport of a rotor in the far wake where rotation is less important. Proper orthogonal decomposition and analysis of the invariants of the Reynolds stress anisotropy tensor is done in order to examine large scale structure of the flow and characterize the turbulent wake produced by the porous disks and rotors. The spatial coherence uncovered via the proper orthogonal decomposition in the rotor case and its absence in the disk case suggests caution should be employed when applying stationary disk parametrization to research questions that are heavily dependent on flow structure. Motivated by questions on the impact of freestream turbulence on wakes in wind energy, a study of pairs of cylinders subject to varying levels of inflow turbulence is undertaken. Time-averaged statistics show a modification of the symmetry and development of the wakes originating from the pairs of cylinders in response to freestream turbulence. Recurrence-based phase averaging allowws examination of the many configurations of the wake and the modification of these topologies due to varying inflow turbulence. Results show the changes in vortex shedding synchronization as well as large scale cross stream advection in response to elevated levels of incoming turbulence.