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Title:Post-stall hysteresis and flow field unsteadiness on an NACA 0012 airfoil
Author(s):Hristov, Georgi Kalinov
Advisor(s):Ansell, Phillip J
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Unsteady flows
Separated flows
Abstract:The current study was conducted to understand flow field unsteadiness associated with static stall hysteresis on an NACA 0012 airfoil at Rec = 1.0 × 10^6. Unsteady pressure measurements were acquired to evaluate the performance of the airfoil, and a hysteresis loop was identified in the vicinity of the airfoil Cl,max. Two fundamentally different flow regimes were observed at post-stall angles of attack for the airfoil during the upstroke and downstroke branches of the hysteresis loop. A Fourier analysis of the surface pressure distributions was used to attribute the flow field unsteadiness to a low-frequency, high-amplitude oscillation across the leading-edge region during the upstroke, along with a regular bluff-body shedding frequency across the separated region of the airfoil. The low-frequency oscillations were observed to be more dominant for the upstroke branch, while the bluff-body shedding process was more dominant in the downstroke branch. The flow field unsteadiness was observed to become more energetic at lower post-stall angles of attack. In addition, time-resolve particle image velocimetry data were acquired across the leading-edge region of the airfoil to qualitatively and quantitatively describe the unsteadiness in the flow. These data were used to link the low-frequency oscillations across the leading edge of the airfoil during the upstroke to a quasi-periodic surging of the flow, which was also associated with an advancing and retreating of the separation location across the surface.
Issue Date:2017-04-25
Rights Information:Copyright 2017 Georgi Hristov
Date Available in IDEALS:2017-08-10
Date Deposited:2017-05

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