Shift - flap : Shear-layer instabilities and flow transition – A fundamental link to airfoil performance

Patel, Yogi

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https://hdl.handle.net/2142/129416 Copy

Description

Title

Shift - flap : Shear-layer instabilities and flow transition – A fundamental link to airfoil performance

Author(s)

Patel, Yogi

Issue Date

2025-04-23

Director of Research (if dissertation) or Advisor (if thesis)

Ansell, Phillip

Doctoral Committee Chair(s)

Ansell, Phillip

Committee Member(s)

• Chamorro, Leonardo
• Saxton-Fox, Theresa Ann
• Villafane Roca, Laura

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Aerodynamics
• Fluid Mechanics
• Laminar Separation Bubble
• Linear Stability Analysis
• Airfoil Performance
• Rotors
• UAVs
• Low-Re

Abstract

The aerodynamic efficiency of airfoil geometries operating at transitional Reynolds numbers is intricately linked to the behavior of laminar separation bubbles (LSBs), which can significantly impact performance coefficients. This study investigates the fluid instability mechanisms within LSBs through a combination of experimental measurements and analytical methods. Particle image velocimetry was employed to acquire detailed shear layer profiles, which were subsequently used in Linear Stability Analysis (LSA) to determine the spatial amplification rates of flow instabilities at various chordwise locations. Two pivotal scenarios were explored: A laminar separation region with off-body transition and reattachment, and a laminar separation process without boundary-layer reattachment. It was observed that variations in transitional flow features across the airfoil surface significantly affect drag characteristics and the linearity of the lift curve. The greatest decrease in lift-curve linearity and increase in drag occurred when laminar boundary-layer separation without reattachment was observed. Upstream movement in the reattachment position with increased Reynolds number was linked to a faster breakdown of the Kelvin-Helmholtz (KH) instability that governs the transition process in the free shear layer, resulting in a shorter streamwise length of the separation bubble and reduction in viscous decambering of the airfoil performance. The study also discusses the influence of Reynolds number on the isotropy of turbulent flows. For very low Reynolds numbers (low Re), the turbulence displayed two- component isotropy, suggesting energy transfer from streamwise to transverse Reynolds stress components. As the Reynolds number increased, the flow shifted towards one-component turbulence, emphasizing the predominant role of the streamwise component in turbulent production. LSA was found to be an effective tool for predicting the growth rate of instability in the initial transition region. The spatial wavenumber estimates from LSA closely match those obtained using the continuous wavelet transform. Additionally, it was observed that, at the most dominant frequency, the spatial growth rate of instabilities is directly proportional to the streamwise spatial wavenumber.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129416

Copyright and License Information

Copyright 2025 Yogi Patel

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