Longitudinal vortices imbedded in turbulent boundary layers. Part 1. Single vortex

Shabaka, Ibrahim; Mehta, Rabindra D.; Bradshaw, P.

doi:10.1017/s0022112085001707

Cited by 97 publications

(28 citation statements)

References 7 publications

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“…Vane-type VGs typically have height of the order of the boundary-layer thickness, h/δ = 1 (δ-scale VGs), although recent studies suggest smaller devices with h/δ = 0.1-0.5 minimize the device drag without hindering performance (low-profile VGs, see Lin (2002)). Detailed measurements of the flow structure of streamwise vortices and vortex pairs have been made by Shebaka, Mehta & Bradshaw (1985), Mehta & Bradshaw (1988), Pauley & Eaton (1988), Yao, Lin & Allen (2002) and Angele & Grewe (2007). Godard & Stanislas (2006) optimized VG geometry over a bump and found peak performance with triangular vanes of height h/δ = 0.37, length l/h = 2, yaw angle β = 18…”

Section: Vortex Generatorsmentioning

confidence: 99%

On the formation of streamwise vortices by plasma vortex generators

Jukes

Choi

2013

J. Fluid Mech.

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The streamwise vortices generated by dielectric-barrier-discharge plasma actuators in the laminar boundary layer were investigated using particle image velocimetry to understand the vortex-formation mechanisms. The plasma vortex generator was oriented along the primary flow direction to produce a body force in the spanwise direction. This created a spanwise-directed wall jet which interacted with the oncoming boundary layer to form a coherent streamwise vortex. It was found that the streamwise vortices were formed by the twisting and folding of the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet, which added its own vorticity to increase the circulation along the actuator length. This is similar to the delta-shaped, vane-type vortex generator, except that the circulation was enhanced by the addition of the vorticity in the plasma jet. It was also observed that the plasma vortex was formed close to the wall with an enhanced wall-ward entrainment, which created strong downwash above the actuator.

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Section: Vortex Generatorsmentioning

confidence: 99%

On the formation of streamwise vortices by plasma vortex generators

Jukes

Choi

2013

J. Fluid Mech.

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“…There have been several investigations into the interaction of relatively weak vortices, generated by half-delta wings on a flat plate, with turbulent boundary layers (Shabaka et al, 1985, Westphal et al, 1985, 1987, and Pauley and Eaton, 1988. Measurements of Shabaka et al (1985) show that circulation around the embedded vortex is nearly conserved, with a slight streamwise decay due to the spanwise component of the wall shear which opposes the rotation of the vortex.…”

Section: Embedded Vortex Flowsmentioning

confidence: 99%

Structure of 2-D and 3-D Turbulent Boundary Layers with Sparsely Distributed Roughness Elements

George¹

2005

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The present study deals with the effects of sparsely distributed three-dimensional elements on two-dimensional (2-D) and three-dimensional (3-D) turbulent boundary layers (TBL) such as those that occur on submarines, ship hulls, etc. This study was achieved in three parts: Part 1 dealt with the cylinders when placed individually in the turbulent boundary layers, thereby considering the effect of a single perturbation on the TBL; Part 2 considered the effects when the same individual elements were placed in a sparse and regular distribution, thus studying the response of the flow to a sequence of perturbations; and in Part 3, the distributions were subjected to 3-D turbulent boundary layers, thus examining the effects of streamwise and spanwise pressure gradients on the same perturbed flows as considered in Part 2. The 3-D turbulent boundary layers were generated by an idealized wing-body junction flow.Detailed 3-velocity-component Laser-Doppler Velocimetry (LDV) and other measurements were carried out to understand and describe the rough-wall flow structure. The measurements include mean velocities, turbulence quantities (Reynolds stresses and triple products), skin friction, surface pressure and oil flow visualizations in 2-D and 3-D rough-wall flows for Reynolds numbers, based on momentum thickness, greater than 7000. Very uniform circular cylindrical roughness elements of 0.38mm, 0.76mm and 1.52mm height (k) were used in square and diagonal patterns, yielding six different roughness geometries of rough-wall surface. For the 2-D rough-wall flows, the roughness Reynolds numbers, + k , based on the element height (k) and the friction velocity ( τ U ), range from 26 to 131. Results for the 2-D rough-wall flows reveal that the velocity-defect law is similar for both smooth and rough surfaces, and the semi-logarithmic velocity-distribution curve is shifted by an amountdepending on the height of the roughness element, showing thatis a function of + k and the wall geometry. For the 3-D flows, the data show that the surface pressure gradient is not strongly influenced by the roughness elements. Higher roughness elements cause greater turbulence intensities near the wall, which cause the pressure-driven mean flow three-dimensionality to propagate or diffuse more rapidly from the wall region. In general, for both 2-D and 3-D rough-wall TBL, the differences between the two roughness patterns (straight and diagonal), as regards the mean velocities and the Reynolds stresses, are limited to about 3 roughness element heights from the wall.For the single elements, the values of + k range from 23 to 92. The study on single elements revealed that the separated shear layers emanating from the top of the elements form a pair of counter rotating vortices that dominate the downstream flow structure. These vortices, termed as the roughness top vortex structure (RTVS), in conjunction with the mean flow, forced over and around the elements, are responsible for the production of large Reynolds stresses in the neighborhood of th...

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“…This creates new common-flow up arrangements that then lift from the surface and hinder the momentum transfer process. Detailed measurements of the flow structure of streamwise vortices and vortex pairs have been taken by Shabaka et al (1985), Mehta and Bradshaw (1988), Pauley and Eaton (1988), and Angele and Grewe (2007) and analysed numerically by You et al (2006).…”

Section: Introductionmentioning

confidence: 99%

Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control

Jukes

Choi

2011

Exp Fluids

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We investigated the use of dielectric-barrierdischarge plasma actuators as vortex generators for flow separation control applications. Plasma actuators were placed at a yaw angle to the oncoming flow, so that they produced a spanwise wall jet. Through interaction with the oncoming boundary layer, this created a streamwise longitudinal vortex. In this experimental investigation, the effect of yaw angle, actuator length and plasma-induced velocity ratio was studied. Particular attention was given to the vortex formation mechanism and its development downstream. The DBD plasma actuators were then applied in the form of co-rotating and counter-rotating vortex arrays to control flow separation over a trailing-edge ramp. It was found that the vortex generators were successful in reducing the separation region, even at plasma-to-freestream velocity ratios of less than 10%.

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Longitudinal vortices imbedded in turbulent boundary layers. Part 1. Single vortex

Cited by 97 publications

References 7 publications

On the formation of streamwise vortices by plasma vortex generators

On the formation of streamwise vortices by plasma vortex generators

Structure of 2-D and 3-D Turbulent Boundary Layers with Sparsely Distributed Roughness Elements

Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control

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