[CCoE Notice] Cullen College Dissertation Defense Announcement - Masoumeh Gharaati

[Hutchinson, Inez A]

[Dissertation Defense Announcement at the Cullen College of Engineering]

Investigating the effects of helical-shaped blades on the wake characteristics and performance of vertical axis wind turbines using large eddy simulations
Masoumeh Gharaati
July 20, 2023; 9 AM – 11 AM (CDT)
Location: Mechanical Engineering Large Conference Room, RM 202 Engr. Bldg. 1
Committee Chair:
Di Yang, Ph.D
Committee Members:
Dong Liu, Ph.D.| Kamran Alba, Ph.D.| Daniel Floryan, Ph.D.| Ben Xu, Ph.D.|
Abstract
Turbulent wake flows behind helical- and straight-bladed vertical axis wind turbines (VAWTs) are studied numerically using the large-eddy simulation (LES) method combined with the actuator line model (ALM). The effects of blade geometry on turbine wake characteristics are explored at both laboratory and field scales differed primarily by their operational tip-speed ratios (TSRs). At the laboratory scale, a small-size 5-blade VAWT base design is considered, which operates at a relatively low TSRs of about 0.4-0.6 at the wind speed of 13 m/s.  At the field scale, a commercial VAWT design is considered, which is composed of 3 blades and operate at a higher TSR of 1.19 at the wind speed of about 11-12 m/s.  In both cases, the wake flows behind straight- and helical-bladed VAWTs were simulated, and the flow characteristics were quantified and compared. At low TSRs, the simulation results show that the wake behind the straight-bladed VAWT expand considerably in the spanwise directly due to the quasi-2D vortex shedding effect from the straight blades. In contrast, the helical-bladed VAWT generates highly 3D wake flow structures to produce a relatively narrow wake with faster decay of turbulent intensity. At high TSRs, the helical-bladed VAWTs generate a screw driver effect to induce mean vertical flow motions at the spanwise edges of the wake flow, which are balanced by a mean vertical counter-flow (i.e., with reversed direction) near the center of the wake. As a result, the wake flows behind helical VAWTs exhibit vertical tilting that affects the turbulent intensity and the Reynolds transport of momentum in the shear layers around the VAWT wake region, leading to faster wind speed recovery than the wake behind straight-bladed VAWTs. If the different VAWT designs are used in wind farm applications, the field-scale helical-bladed VAWTs may improve the mean power production rate for the fully developed region of the wind farm by up to about 6.8% compared with the corresponding straight-bladed VAWT. More significantly, using the helical-bladed VAWT designs can reduce the temporal fluctuations of the power coefficient and the structural bending moment by about 50% relative to the straight-bladed VAWT cases, resulting in smoother wind power production and improved longevity of the VAWT system.
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