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

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Aerospace Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Patel, Yogi

Contributors dc:contributor

• Ansell, Phillip J.
• Ansell, Phillip J
• Chamorro, Leonardo
• Saxton-Fox, Theresa Ann
• Villafane Roca, Laura

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

Statement dc:rights

• Copyright 2025 Yogi Patel

Language dc:language

en, eng

Handle dc:identifier

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

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129416