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Dynamic Stall on Vertical Axis Wind Turbines

Citation

Dunne, Reeve (2016) Dynamic Stall on Vertical Axis Wind Turbines. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z92Z13FX. http://resolver.caltech.edu/CaltechTHESIS:09042015-152813860

Abstract

In this study the dynamics of flow over the blades of vertical axis wind turbines was investigated using a simplified periodic motion to uncover the fundamental flow physics and provide insight into the design of more efficient turbines. Time-resolved, two-dimensional velocity measurements were made with particle image velocimetry on a wing undergoing pitching and surging motion to mimic the flow on a turbine blade in a non-rotating frame. Dynamic stall prior to maximum angle of attack and a leading edge vortex development were identified in the phase-averaged flow field and captured by a simple model with five modes, including the first two harmonics of the pitch/surge frequency identified using the dynamic mode decomposition. Analysis of these modes identified vortical structures corresponding to both frequencies that led the separation and reattachment processes, while their phase relationship determined the evolution of the flow.

Detailed analysis of the leading edge vortex found multiple regimes of vortex development coupled to the time-varying flow field on the airfoil. The vortex was shown to grow on the airfoil for four convection times, before shedding and causing dynamic stall in agreement with 'optimal' vortex formation theory. Vortex shedding from the trailing edge was identified from instantaneous velocity fields prior to separation. This shedding was found to be in agreement with classical Strouhal frequency scaling and was removed by phase averaging, which indicates that it is not exactly coupled to the phase of the airfoil motion.

The flow field over an airfoil undergoing solely pitch motion was shown to develop similarly to the pitch/surge motion; however, flow separation took place earlier, corresponding to the earlier formation of the leading edge vortex. A similar reduced-order model to the pitch/surge case was developed, with similar vortical structures leading separation and reattachment; however, the relative phase lead of the separation mode, corresponding to earlier separation, necessitated that a third frequency to be incorporated into the reattachment mode to provide a relative lag in reattachment.

Finally, the results are returned to the rotating frame and the effects of each flow phenomena on the turbine are estimated, suggesting kinematic criteria for the design of improved turbines.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Dynamic Stall, Unsteady Aerodynamics, Vertical Axis Wind Turbines
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Mechanical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • McKeon, Beverley J.
Thesis Committee:
  • Colonius, Timothy E. (chair)
  • Dabiri, John O.
  • Hunt, Melany L.
  • McKeon, Beverley J.
Defense Date:24 August 2015
Non-Caltech Author Email:reeve.dunne (AT) gmail.com
Funders:
Funding AgencyGrant Number
Gordon and Betty Moore Foundation2645
Record Number:CaltechTHESIS:09042015-152813860
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:09042015-152813860
DOI:10.7907/Z92Z13FX
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.2514/6.2015-2882DOIAdapted for ch. 5
http://dx.doi.org/10.2514/6.2014-3142DOIAdapted from: Dynamic separation on a pitching and surging airfoil as a model for ow over vertical axis wind turbine blades
http://dx.doi.org/10.1007/s00348-015-2028-1DOIAdapted for ch. 3, 6
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9140
Collection:CaltechTHESIS
Deposited By: Reeve Dunne
Deposited On:05 Oct 2015 23:05
Last Modified:21 Jan 2016 22:34

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