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

Zaman, K. B. M. Q.; Bar‐Sever, A.; Mangalam, S. M.

doi:10.1017/s0022112087002271

Cited by 153 publications

(77 citation statements)

References 20 publications

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“…The force measurement indicated that the vortex generators increased the maximum lift by up to 25 % as they sliced the laminar separation bubble into segments. Zaman, Bar-Severs & Mangalam (1987) used acoustic excitation to control the flow over an LRN-(1)-1007 airfoil at Reynolds numbers of 40 000-140 000, where small-amplitude excitation in a wide low-frequency range was found to eliminate laminar separation. Yarusevych, Sullivan & Kawall (2005) indicated that the optimum acoustic excitation frequency should match that of the most amplified disturbance in the separated shear layer, which promoted the transition to bring earlier reattachment.…”

Section: Flow Separation Controlmentioning

confidence: 99%

Flow control over an airfoil using virtual Gurney flaps

2015

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Flow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin-Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.

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Section: Flow Separation Controlmentioning

confidence: 99%

Flow control over an airfoil using virtual Gurney flaps

2015

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“…In comparison, the excitation at large a, i.e., in the poststalled case, consistently increased the C Q but the effect was noted to be strongly dependent on the excitation amplitude. 2,10 In the present paper, attention is focussed on the post-stalled flows. As indicated earlier, these flows are defined as ones that are fully stalled, i.e., fully separated from near the leading edge.…”

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confidence: 99%

Effect of acoustic excitation on stalled flows over an airfoil

Zaman

1992

AIAA Journal

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The effect of acoustic excitation on poststalled flows over an airfoil, i.e., flows that are fully separated from near the leading edge, is investigated. The excitation results in a tendency towards reattachment, which is accompanied by an increased lift and reduced drag, although the flow may still remain fully separated. It is found that with increasing excitation amplitude, the effect becomes more pronounced but shifts to a Strouhal number which is much lower than that expected from linear, inviscid instability of the separated shear layer.

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“…1 -12 In Ref. 10, the general effect over a large a-range was investigated addressing such questions as the roles of the tunnel resonance and the instability of the boundary layer in the process. Differences were observed in the effect depending on the a-range.…”

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confidence: 99%