University of Massachusetts Amherst

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Mean kinetic energy budget and conditional averaging in an model wind turbine array versus an array of porous disks


Friday, April 8, 2016 - 10:00am


Betsy Camp, Portland State University


Kellogg Room, Elab II


Wind energy is an increasingly valuable renewable. As new wind farms are planned and installed globally, it becomes ever more important to understand as well as describe the flow physics in such wind turbine arrays. The wake of a wind turbine modeled experimentally using a porous disk versus a rotor is compared in a wind farm scenario in order to quantify the similarities and differences in mean kinetic energy transport in these two cases. This comparison has implications on the use of an actuator disk to model the wind turbine rotor in computational simulations. Stereo particle image velocimetry measurements are done in a wind tunnel bracketing the center turbine in the fourth row of a 4x3 array of model turbines. An equivalent set of rotors and porous disks are created by matching induction factor. The streamwise and wall-normal components of the mean velocity are quite similar for the disk and rotor cases while the spanwise mean velocity is as much as 190% different. Horizontal averages of mean kinetic energy transport terms in the region where rotation is most important show percent differences in the range 3-41% which decrease to 1-6% at streamwise coordinates where rotation is less important. Conditional averaging via octant analysis is performed on the most significant term related to vertical mean kinetic energy flux, <u'v'>U. The average percent difference between corresponding octants is as much as 68% different in the near wake and as much as 17% different in the far wake. Furthermore, octant analysis elucidates the three dimensional nature of sweeps and ejections in the near wake of the rotor case.  Together, these results imply that a stationary porous disk adequately represents the mean kinetic energy transport of a rotor in the far wake where rotation is less important while significant discrepancies exist at streamwise locations where rotation is a key phenomenon. Proper orthogonal decomposition and vortex identification techniques are applied in order to gain insight on the structure of the wakes produced by the disk and rotor cases.

Elizabeth Camp is a Ph.D. candidate in the Department of Mechanical and Materials Engineering at Portland State University. She holds dual B.S. degrees in Chemistry and Mechanical Engineering from obtained from Oregon State and Portland State University, respectively. After receiving her M.S. in Chemistry from the University of Pennsylvania, she served as a New York City Teaching Fellow for three years in Bronx, NY where she taught high school physics and chemistry. Her research in fluid dynamics is focused on turbulent flows. Particular attention is placed on topics such as interactions of wind turbine arrays with the atmosphere and on turbulent wakes of groups of cylinders in crossflow. She uses particle image velocimetry and hot-wire anemometry as her primary experimental tools to quantify such flows in scaled settings.