Effects of Periodic Excitation on Turbulent Flow Separation from a Flap

Nishri, B.; Wygnanski, I.

doi:10.2514/2.428

Cited by 137 publications

(39 citation statements)

References 9 publications

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“…This provides the minimum drag, and in turn, yields the best lift-todrag ratio. The non-dimensional optimal frequency in this study is f act D/u ∞ ( ≡ F + ) ≈ 1, which has been observed for various flow geometries in many previous experiments (Seifert et al 1996;Nishri & Wygnanski 1998;. Figure 22 similarly plots the centroid of vorticity production.…”

Section: Dependence On Actuation Frequencysupporting

confidence: 74%

“…Much effort has been put into finding the optimal forcing frequency. It has been reported (Seifert, Darabi & Wygnanski 1996;Nishri & Wygnanski 1998;) that the best performance is obtained when a non-dimensional frequency defined by F + ≡ f L/u ∞ (f being the forcing frequency, L the chord length, and u ∞ the free-stream velocity) is approximately unity. At this frequency, the length of the vortical structures over the airfoil becomes about one-third to one-half of the chord length.…”

Section: Introductionmentioning

confidence: 99%

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Effects of a synthetic jet acting on a separated flow over a hump

Suzuki

2006

J. Fluid Mech.

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The effects of an oscillatory zero-net-mass-flux jet (i.e. synthetic jet) acting on a separated flow over a hump are investigated in terms of two actuation parametersactuator position and forcing frequency. By considering the vorticity flux balance and introducing a centroid of vorticity production over the hump surface, lift and drag acting on the hump can be expressed as a function of this centroid and the rate of vorticity production. To study the parametric dependence of lift and drag, direct numerical simulation (DNS) is performed by solving compressible, unsteady, laminar flows over a half-cylindrical hump in two dimensions. The DNS results show that periodic actuation significantly reduces the rate of vorticity production at the wall and shifts the centroid upstream so that the drag is reduced and the lift is increased, respectively. When the actuation parameters are varied, it is found that the lift is governed by the horizontal coordinate of the vorticity-production centroid, while the drag is determined by both the vertical coordinate of the centroid and the rate of vorticity production over the hump. This paper explains by using ideal flow models that the vorticity-production centroid is controlled by two factors: one is the actuator position at which clockwise vorticity is generated, and the other is the point where the separation vortex is pinched off, thereby the clockwise vorticity being absorbed.

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Section: Dependence On Actuation Frequencysupporting

confidence: 74%

Section: Introductionmentioning

confidence: 99%

Effects of a synthetic jet acting on a separated flow over a hump

Suzuki

2006

J. Fluid Mech.

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“…Thus an interaction with ultrasound is not plausible. All studies where active flow control using acoustic sound or oscillating wall structures was investigated, were carried out in the lower audible frequency range (Yarusevych et al 2006, Nishri et al 1998, Ahuja et al 1984, Greenblatt et al 2000and Sinha 2001.…”

Section: Operating Frequency Rangementioning

confidence: 99%

Optically interrogated MEMS pressure sensor array

et al. 2011

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A novel pressure measurement technique is developed with the objective to simplify wind tunnel investigations while providing full field measurement capability with highest measurement performance. The static pressure distributions are recorded by interferometric means using an array of silicon diaphragm microresonators as pressure sensing elements. They are remotely excited into free vibration using a capacitive ultrasound transducer (CUT) transmitting high power narrow band (50 -80kHz) acoustic noise. Dependent on their quasi-static deflection caused by a pressure load the resonance frequency varies with an average pressure sensitivity of 3Hz/Pa at 70kHz. Further requirements for the system performance are a maximum pressure, temporal and spatial resolution of 0.2% of FS (10kPa), 1Hz and 5mm, respectively.

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“…With regard to active flow control, Nishri and Wygnanski [9] reveal that an oscillatory jet introducing momentum flux with a net zero mass flow is more effective than a steady blowing jet in delaying flow separation over a flapped airfoil. They are concerned primarily with increasing lift by means of oscillatory suction/blowing.…”

Section: The Drag Coefficient Is Defined Asmentioning

confidence: 99%

Base Flaps and Oscillatory Perturbations to Decrease Base Drag

Hsu

Hammache

Browand

2004

The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains

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The objective of this investigation is to study possible means for reducing the base drag of a tractor-trailer. The experiments are conducted in the Dryden wind tunnel at the USC Ground Vehicle Aerodynamics Laboratory. A roughly 1/15 scale model resembling a trailer is utilized for the study. The model is fitted with a shaped nose-piece to ensure attached flow over the forward portion of the model. The model is equipped with a force balance to measure drag. In addition base pressures are measured, and hot-wire wake surveys are conducted downstream from the model base. The Reynolds numbers (based on the square-root of the model cross-sectional area), range from 0.1 x 10 6 to 0.4 x 10 6 . Drag reduction is effected by means of flaps attached along the edges of the model base, and inclined inward to decrease the size of the downstream wake. In addition, an oscillatory perturbation is applied at the flap origin in an attempt to maintain attached flow for larger angles of flap inclination.The present study has found that a simple, passive base-flap deflection-no forcing whatsoever-produces significant drag saving. The maximum drag reduction is 0.06 -0.08 at an angle of 9-10 degrees. The magnitude of the saving is in accord with both early and recent measurements in other laboratories.The present results also show that oscillatory momentum addition has little effect on drag reduction unless the net oscillatory momentum flux coefficient is equal or greater than 0.1%. Increasing the oscillatory momentum perturbation to a coefficient value of 0.3% produces drag savings at angles greater than 9-10 degrees, but has very little effect upon the maximum saving at 9-10 degrees.To further study this anomalous behavior, follow-on experiments are planned to investigate a larger range of forcing amplitudes, and a variety forcing-function duty cycles. In addition, Digital Particle Image Velocimetry will be used to capture the detailed flow-field in the vicinity of the flap. 304T.-Y. Hsu, M. Hammache, and F. Browand

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Effects of Periodic Excitation on Turbulent Flow Separation from a Flap

Cited by 137 publications

References 9 publications

Effects of a synthetic jet acting on a separated flow over a hump

Effects of a synthetic jet acting on a separated flow over a hump

Optically interrogated MEMS pressure sensor array

Base Flaps and Oscillatory Perturbations to Decrease Base Drag

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