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Unsteady Aerodynamics and Optimal Control of an Airfoil at Low Reynolds Number


Choi, Jeesoon (2016) Unsteady Aerodynamics and Optimal Control of an Airfoil at Low Reynolds Number. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9J1014Q.


As opposed to conventional air vehicles that have fixed wings, small birds and insects are known to flap their wings at higher angles of attack. The vortex produced at the tip of the wing, known as the leading-edge vortex (LEV), plays an important role to enhance lift during its flight. In this thesis, we analyze the influence of these vortices on aerodynamic forces that could be beneficial to micro-air vehicle performance and efficiency. The flow structures associated with simple harmonic motions of an airfoil are first investigated. The characteristics of the time-averaged and fluctuating forces are explained by analyzing vortical flow features, such as vortex lock-in, leading-edge vortex synchronization, and vortex formation time. Specific frequency regions where the wake instability locks in to the unsteady motion of the airfoil are identified, and these lead to significant changes in the mean forces. A detailed study of the flow structures associated with the LEV acting either in- or out-of-phase with the quasi-steady component of the forces is performed to quantify the amplification and attenuation behavior of the fluctuating forces. An inherent time scale of the LEV associated with its formation and detachment (LEV formation time) is shown to control the time-averaged forces. With these results, several optimal flow control problems are formulated. Adjoint-based optimal control is applied to an airfoil moving at a constant velocity and also to a reciprocating airfoil with no forward velocity. In both cases, we maximize lift by controlling the pitch rate of the airfoil. For the former case, the static map of lift at various angles of attack is additionally examined to find the static angle that provides maximum lift and also to confirm whether the optimizations perform according to the static map. For the latter case, we obtain a solution of the optimized motion of the flapping airfoil which resembles that of a hovering insect.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Unsteady aerodynamics, Optimal flow control, Vortex dynamics
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Mechanical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Colonius, Tim
Thesis Committee:
  • Blanquart, Guillaume (chair)
  • McKeon, Beverley J.
  • Williams, David R.
  • Colonius, Tim
Defense Date:14 April 2016
Non-Caltech Author Email:jeesoonchoi (AT)
Funding AgencyGrant Number
Air Force Office of Scientific ResearchFA950-09-1-0189
Kwanjeong Educational FoundationUNSPECIFIED
Record Number:CaltechTHESIS:05272016-220450949
Persistent URL:
Related URLs:
URLURL TypeDescription DOIArticle adapted for Chapter 3 adapted for Chapter 3
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9808
Deposited By: Jeesoon Choi
Deposited On:31 May 2016 19:17
Last Modified:26 Oct 2023 19:38

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