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Title:Unsteady flow physics of airfoil dynamic stall
Author(s):Gupta, Rohit
Director of Research:Ansell, Phillip J
Doctoral Committee Chair(s):Ansell, Phillip J
Doctoral Committee Member(s):Elliott, Gregory S; Dutton, J. Craig; Mulleners, Karen
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):dynamic stall
unsteady flows
laminar separation bubble
boundary layer separation
vortex-dominated flows
transitional Reynolds number
surface pressure
vortex merging
empirical mode decomposition
hilbert transform
spectral analysis
flow timescales
dominant frequency
Strouhal number
hydrodynamic stability
K-H instability
Abstract:A series of wind tunnel experiments were conducted on an NACA 0012 airfoil undergoing a linear pitch ramp maneuver at a fixed dimensionless pitch rate of 0.05 and across three transitional Reynolds numbers, Re = 200,000, 500,000, and 1,000,000. The primary objectives of these experiments were to perform a detailed analysis of the flow evolution, with particular emphasis on the underlying physical mechanisms, and to extract the dominant scales associated with the flow perturbations, for a canonical dynamic stall process. A series of unsteady surface pressure measurements, with a high sampling frequency, were acquired in order to investigate the time-dependent behavior of the flow in the immediate vicinity of the airfoil. These surface pressure measurements were used to identify the region of boundary layer transition during the initial stages of the dynamic stall process. A spatially-contracting laminar separation bubble was also identified near the airfoil leading edge from the characteristic pressure plateau in the surface pressure distribution. The dominant frequencies associated with the laminar separation bubble were extracted using a continuous wavelet transform technique. These frequencies were observed to span a wide range of chord-based Strouhal numbers between St = 50 and St = 105, at Re = 500,000. The off-body flow evolution was inferred and described using a combination of surface pressure measurements and time-resolved particle image velocimetry. For Re = 200,000 and Re = 500,000, the dynamic stall vortex was observed to emerge from a collective interaction of the discrete vortices that were ejected from the leading edge of the airfoil. At Re = 1,000,000, however, the near-wall vortices were observed to amalgamate into two regions, forming a distinct primary and a secondary coherent structure. After formation, these two structures were observed to interact with each other, following a co-rotating vortex merging process and resulting in the emergence of a single, coherent dynamic stall vortex. The process of emergence of the dynamic stall vortex at Re = 1,000,000, observed from the present experiments, is therefore quite distinct from the classical understanding of the dynamic stall vortex formation, which was observed at the lower Reynolds numbers. The time-dependent spectra of the velocity field were calculated using a combination of empirical mode decomposition and Hilbert transformation. From the velocity spectra, the fluctuations in the flow were observed to attain an amplified state during the initial ejection of vorticity from the leading-edge region of the airfoil. During this amplified phase, the most dominant velocity fluctuations were found to conform to a range of displacement-thickness based Strouhal scales between 0.09 and 0.14. Finally, a numerical implementation of the Orr-Sommerfeld equation was used to extract the spatially-unstable modes associated with the phase-averaged velocity measurements near the airfoil leading edge. The most unstable frequencies from linear stability analysis were found to be consistent with those determined directly from the velocity acquisition during the amplified shedding phase of the dynamic stall process.
Issue Date:2020-07-17
Rights Information:Copyright 2020 by Rohit Gupta. All rights reserved.
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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