Post-stall hysteresis and flow field unsteadiness on an NACA 0012 airfoil

Hristov, Georgi Kalinov

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https://hdl.handle.net/2142/97458 Copy

Description

Title

Post-stall hysteresis and flow field unsteadiness on an NACA 0012 airfoil

Author(s)

Hristov, Georgi Kalinov

Issue Date

2017-04-25

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

Ansell, Phillip J.

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Unsteady flows
• Separated flows
• Stall
• Hysteresis

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.

Graduation Semester

2017-05

Type of Resource

text

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http://hdl.handle.net/2142/97458

Copyright and License Information

Copyright 2017 Georgi Hristov

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