Acoustic and turbulence influences on stall hysteresis

Marchman, James F.; Sumantran, V.; Schaefer, Christof

doi:10.2514/6.1986-170

Cited by 16 publications

(6 citation statements)

References 4 publications

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“…Without PSJ actuation, the airfoil stalls at = 15.5 • , signified by an abrupt change of the lift and drag coefficients. A hysteresis loop is exhibited between = 13 • and = 15 • , which is expected for this type of airfoil at a chord-based Reynolds number of O(10 5 ) (Marchman et al 1987;Timmer 2008). When PSJ actuation is applied, the stall angle is postponed to approximately 22 • , and the hysteresis loop observed in the baseline case is completely eliminated, which is consistent with the observations of Post and Corke (2004b) where the same airfoil (NACA-0015) is tested with SDBDAs at Re c = 1.6 × 10 5 .…”

Section: Balance Measurement Resultsmentioning

confidence: 64%

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

2018

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An array of 26 plasma synthetic jet actuators (PSJA) is flush-mounted on a NACA-0015 airfoil model to control the leadingedge flow separation at moderate Reynolds number (Re c = 1.7 × 10 5). The stall angle of this airfoil is postponed from 15.5 • to approximately 22 • , and the peak lift coefficient is increased by 21%. PSJAs exhibit distinctive separation control mechanisms depending on the relative location between actuation and separation and reduced frequency of actuation (F *). At an angle of attack of = 15.5 • , the non-actuated flow separates approximately 4% chord length downstream of the jet orifices. Plasma synthetic jets (PSJs) applied at F * ≥ 0.5 can displace the separation point downstream to mid-chord position, as a result of the energizing of the incoming boundary layer through mixing enhancement. As a comparison, with actuation frequency of F * ≤ 0.25 , the separation point at = 15.5 • remains near the leading edge and the zero-velocity line is periodically swept towards the suction surface by the convecting spanwise vortices generated from PSJ actuation, leading to a reduction of timeaveraged backflow area. For the case of separation control at = 22 • , the separation point resides always upstream of the actuation position, regardless of actuation frequency. The peak lift coefficient is attained at F * = 1 , and the decreasing lift at high actuation frequency (F * = 2) is ascribed to the severe interaction between adjacent spanwise vortices at short spacing.

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Section: Balance Measurement Resultsmentioning

confidence: 64%

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

2018

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“…Mueller 1985). The relatively high turbulence intensity in the tunnel (about 0.25 %) could be a reason why i t was not observed (Marchman, Sumantran & Schaefer 1987). However, the exact cause remains unknown as hysteresis has been observed under comparable or even higher ambient turbulence levels; on the other hand, the phenomenon is known to depend on other factors like airfoil shape, aspect ratio, Reynolds number, etc.…”

Section: Resultsmentioning

confidence: 99%

Effect of acoustic excitation on the flow over a low- Re airfoil

1987

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Wind-tunnel measurements of lift, drag and wake velocity spectra were carried out under (tonal) acoustic excitation for a smooth airfoil in the chord-Reynolds-number (Rec) range of 4 × 104−1.4 × 105. The data are supported by smoke-wire flowvisualization pictures. Small-amplitude excitation in a wide, low-frequency range is found to eliminate laminar separation that otherwise degrades the airfoil performance at low Rec near the design angle of attack. Excitation at high frequencies, scaling as $U_{\infty}^{\frac{3}{2}}$, eliminates a pre-stall, periodic shedding of large-scale vortices; U∞ is the free-stream velocity. Significant improvement in lift is also achieved during post-stall, but with large-amplitude excitation. Wind-tunnel resonances strongly influence the results, especially in cases requiring large amplitudes. It is shown that large transverse velocity fluctuations, induced near the airfoil by specific cross-resonance modes, lead to the most effective separation control; resonances inducing only large-amplitude pressure fluctuations are demonstrated to be less effective.

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“…A fully attached or massively separated flow is observed for the same angle of attack, depending on whether the configuration is reached by increasing or decreasing (in a quasistatic way) the angle of attack. This hysteresis was subsequently observed for various airfoils, mostly for transitional flow regimes Re ∼ 10 5 (Mueller et al 1983;Pohlen & Mueller 1984;Marchman, Sumantran & Schaefer 1987) and more recently for turbulent flow regimes (Re ∼ 10 6 ) (Broeren & Bragg 2001;Hristov & Ansell 2018). The low-frequency oscillation of the aerodynamic coefficients of an airfoil near stall was thoroughly investigated by Zaman, Bar-Sever & Mangalam (1987), who first confirmed that it was due to a natural flow oscillation rather than a structural vibration.…”

Section: Introductionmentioning

confidence: 92%

Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

et al. 2021

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We numerically investigate stalling flow around a static airfoil at high Reynolds numbers using the Reynolds-averaged Navier–Stokes equations (RANS) closed with the Spalart–Allmaras turbulence model. An arclength continuation method allows to identify three branches of steady solutions, which form a characteristic inverted S-shaped curve as the angle of attack is varied. Global stability analyses of these steady solutions reveal the existence of two unstable modes: a low-frequency mode, which is unstable for angles of attack in the stall region, and a high-frequency vortex shedding mode, which is unstable at larger angles of attack. The low-frequency stall mode bifurcates several times along the three steady solutions: there are two Hopf bifurcations, two solutions with a two-fold degenerate eigenvalue and two saddle-node bifurcations. This low-frequency mode induces a cyclic flow separation and reattachment along the airfoil. Unsteady simulations of the RANS equations confirm the existence of large-amplitude low-frequency periodic solutions that oscillate around the three steady solutions in phase space. An analysis of the periodic solutions in the phase space shows that, when decreasing the angle of attack, the low-frequency periodic solution collides with the unstable steady middle-branch solution and thus disappears via a homoclinic bifurcation of periodic orbits. Finally, a one-equation nonlinear stall model is introduced to reveal that the disappearance of the limit cycle, when increasing the angle of attack, is due to a saddle-node bifurcation of periodic orbits.

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Acoustic and turbulence influences on stall hysteresis

Cited by 16 publications

References 4 publications

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

Effect of acoustic excitation on the flow over a low- Re airfoil

Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

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