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Low-Frequency Flow Oscillations on Stalled Wings Exhibiting Cellular Separation Topology

Disotell, Kevin James
http://orcid.org/0000-0001-8565-609X
http://rave.ohiolink.edu/etdc/view?acc_num=osu1449162356

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Aero/Astro Engineering.
One of the most pervasive threats to aircraft controllability is wing stall, a condition associated with loss of lift due to separation of air flow from the wing surface at high angles of attack. A recognized need for improved upset recovery training in extended-envelope flight simulators is a physical understanding of the post-stall aerodynamic environment, particularly key flow phenomena which influence the vehicle trajectory. Large-scale flow structures known as stall cells, which scale with the wing chord and are spatially-periodic along the span, have been previously observed on post-stall airfoils with trailing-edge separation present. Despite extensive documentation of stall cells in the literature, the physical mechanisms behind their formation and evolution have proven to be elusive. The undertaken study has sought to characterize the inherently turbulent separated flow existing above the wing surface with cell formation present. In particular, the question of how the unsteady separated flow may interact with the wing to produce time-averaged cellular surface patterns is considered. Time-resolved, two-component particle image velocimetry measurements were acquired at the plane of symmetry of a single stall cell formed on an extruded NACA 0015 airfoil model at chord Reynolds number of 560,000 to obtain insight into the time-dependent flow structure. The evolution of flow unsteadiness was analyzed over a static angle-of-attack range covering the narrow post-stall regime in which stall cells have been observed. Spectral analysis of velocity fields acquired near the stall angle confirmed a low-frequency flow oscillation previously detected in pointwise surface measurements by Yon and Katz (1998), corresponding to a Strouhal number of 0.042 based on frontal projected chord height. Probability density functions of the streamwise velocity component were used to estimate the convective speed of this mode at approximately half the free-stream velocity, in agreement with Yon and Katz. Large-amplitude streamwise Reynolds stresses in the separated shear layer were found to be manifested by the low-frequency oscillation through inspection of the spectral energy distribution. Using the method of Proper Orthogonal Decomposition to construct reduced-order models of the acquired time sequences, the low-frequency unsteadiness appeared to be linked to an interaction between the separated and trailing-edge shear layers, in contrast to a bubble-bursting mechanism which has been observed for different stall behaviors. As the static angle of attack was increased further, the separated flow structure was seen to transition to a faster eddy motion expected for bluff-body wakes. A novel scaling study was conducted to evaluate the potential role of low-frequency unsteadiness in producing the spanwise wavelengths associated with cell formation, which was found to be in favorable agreement with scaling trends in the literature. Finally, instantaneous pressure-sensitive paint measurements were demonstrated on a DU 97-W-300 wind turbine airfoil at chord Reynolds number of 225,000 with leading-edge trip applied, in which the development of spiral node structures associated with cell formation were captured in the trailing-edge separation. The contributed work suggests that further study into the influence of large-scale unsteadiness on the three-dimensional organization of stall cells is merited.
James Gregory, Ph.D. (Advisor)
Jeffrey Bons, Ph.D. (Committee Member)
Mo Samimy, Ph.D. (Committee Member)
Jen-Ping Chen, Ph.D. (Committee Member)
259 p.
Aerospace Engineering; Fluid Dynamics
wing aerodynamics; stall cells; high-angle-of-attack airfoil aerodynamics; turbulent separated flows; three-dimensional separation; large-scale unsteadiness; vortical wakes; low-frequency flow oscillations; time-resolved particle image velocimetry

Recommended Citations

Citations

  • Disotell, K. J. (2015). Low-Frequency Flow Oscillations on Stalled Wings Exhibiting Cellular Separation Topology [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449162356

    APA Style (7th edition)

  • Disotell, Kevin. Low-Frequency Flow Oscillations on Stalled Wings Exhibiting Cellular Separation Topology. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1449162356.

    MLA Style (8th edition)

  • Disotell, Kevin. "Low-Frequency Flow Oscillations on Stalled Wings Exhibiting Cellular Separation Topology." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449162356

    Chicago Manual of Style (17th edition)

osu1449162356
341
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This open access ETD is published by The Ohio State University and OhioLINK.