Theoretical prediction and design for vortex generators in turbulent boundary layers

Smith, F. T.

doi:10.1017/s0022112094004210

Cited by 31 publications

(17 citation statements)

References 6 publications

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“…The geometry is defined by four parameters : the length L, base width w b , and upper edge width w e of the trapezoidal blade and the angle a between the blade and the wall. The thickness of the blade is constant and equal to 10 -3 m. In most of the following study, the length L of the VG will be L = 10 -2 m and the angle a = 60°or a = 120°, so that the apparent height of the VG is h ¼ L sinðaÞ ¼ 8:5 Â 10 À3 m: The geometry of the VGs is rather unusual compared to classic vane type VGs used in aeronautics (Smith 1994;Betterton et al 2000;Angele and Grewe 2002). The nature of the perturbations induced by the VG in a flat-plate boundary layer can not be easily guessed.…”

Section: Vortex Generators For Flow Controlmentioning

confidence: 99%

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Drag and lift reduction of a 3D bluff-body using active vortex generators

2009

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In this study, a passive flow control experiment on a 3D bluff-body using vortex generators (VGs) is presented. The bluff-body is a modified Ahmed body (Ahmed in J Fluids Eng 105:429-434 1983) with a curved rear part, instead of a slanted one, so that the location of the flow separation is no longer forced by the geometry. The influence of a line of non-conventional trapezoïdal VGs on the aerodynamic forces (drag and lift) induced on the bluffbody is investigated. The high sensitivity to many geometric (angle between the trapezoïdal element and the wall, spanwise spacing between the VGs, longitudinal location on the curved surface) and physical (freestream velocity) parameters is clearly demonstrated. The maximum drag reduction is -12%, while the maximum global lift reduction can reach more than -60%, with a strong dependency on the freestream velocity. For some configurations, the lift on the rear axle of the model can be inverted (-104%). It is also shown that the VGs are still efficient even downstream of the natural separation line. Finally, a dynamic parameter is chosen and a new set-up with motorized vortex generators is proposed. Thanks to this active device. The optimal configurations depending on two parameters are found more easily, and a significant drag and lift reduction (up to -14% drag reduction) can be reached for different freestream velocities. These results are then analyzed through wall pressure and velocity measurements in the near-wake of the bluff-body with and without control. It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant. Investigation of the velocity field in a crosssection downstream the model reveals that, in the same time, the intensity of the longitudinal trailing vortices is strongly reduced, suggesting that the drag reduction is due to the breakdown of the balance between the separation bubble and the longitudinal vortices. It demonstrates that for low aspect ratio 3D bluff-bodies, like road vehicles, the flow control strategy is much different from the one used on airfoils: an early separation of the boundary layer can lead to a significant drag reduction if the circulation of the trailing vortices is reduced.

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Section: Vortex Generators For Flow Controlmentioning

confidence: 99%

“…There are many ways to produce longitudinal vortices leading to a large set of mechanical or fluidic vortex generators that could be appropriate (Betterton et al 2000;Smith 1994). In this study, an original vortex generator geometry is proposed and will be discussed later in the paper.…”

Section: Introductionmentioning

confidence: 99%

Drag and lift reduction of a 3D bluff-body using active vortex generators

2009

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“…Liu, Piomelli & Spalart 1996;You et al 2006). The model of Smith (1994) predicts the flow field induced by low-profile triangular vanes (extending approximately to the logarithmic region of the boundary layer) in a zero pressure gradient boundary layer. The method modifies the governing equations based on the scales of the geometry and the oncoming flow.…”

Section: Introductionmentioning

confidence: 99%

Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow

2009

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Embedded vortices in turbulent wall-bounded flow over a flat plate, generated by a passive rectangular vane-type vortex generator with variable angle β to the incoming flow in a low-Reynolds-number flow (Re = 2600 based on the inlet grid mesh size L = 0.039 m and free stream velocity U ∞ = 1.0 m s −1 ), have been studied with respect to helical symmetry. The studies were carried out in a low-speed closed-circuit wind tunnel utilizing stereoscopic particle image velocimetry (SPIV). The vortices have been shown to possess helical symmetry, allowing the flow to be described in a simple fashion. Iso-contour maps of axial vorticity revealed a dominant primary vortex and a weaker secondary one for 20 • 6 β 6 40 • . For angles outside this range, the helical symmetry was impaired due to the emergence of additional flow effects. A model describing the flow has been utilized, showing strong concurrence with the measurements, even though the model is decoupled from external flow processes that could perturb the helical symmetry. The pitch, the vortex core size, the circulation and the advection velocity of the vortex all vary linearly with the device angle β. This is important for flow control, since one thereby can determine the axial velocity induced by the helical vortex as well as the swirl redistributing the axial velocity component for a given device angle β. This also simplifies theoretical studies, e.g. to understand and predict the stability of the vortex and to model the flow numerically.

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“…A theoretical investigation was conducted of three-dimensional turbulent flow provoked in a boundary layer by an array of low-profile vortex generators on the surface [23]. Triangular type vortex generators, with various span wise spacing were considered.…”

Section: Introductionmentioning

confidence: 99%

Enhancing the Performance of an Axial Compressor Cascade using Vortex Generators

Am¹,

Mf²,

Ma³

2016

J Aeronaut Aerospace Eng

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Axial flow compressors have a limited operation range due to the difficulty of controlling the secondary flows. Therefore, a new design of vortex generators is considered in the current investigation to control the secondary flow losses and consequently enhance the compressor's performance. Different sets of curved side vortex generators with varying configurations are studied to find their effect on the secondary flow losses. Numerical simulations of a three-dimensional compressible turbulent flow have been performed to explore the effect of vortex generators on the reduction of secondary flow losses. Based on the simulation results, the pressure, velocity, and streamline contours are presented to track the development of secondary flows in the compressor cascade. Thus, the total pressure loss and static pressure rise coefficients, blade deflection angles, and diffusion factors are estimated. Results indicate that vortex generators have a significant impact on secondary flow losses such as reducing the corner vortices, and improving the location of separation lines which are moving toward the trailing edge. At the cascade design point, it is found that vortex generators have a significant effect on the reduction of normalized total pressure loss which is evaluated to be up to 20.7%. Using vortex generators do not lead to a significant change in flow deflection and accordingly the off-design conditions will still be far from reached.

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Theoretical prediction and design for vortex generators in turbulent boundary layers

Cited by 31 publications

References 6 publications

Drag and lift reduction of a 3D bluff-body using active vortex generators

Drag and lift reduction of a 3D bluff-body using active vortex generators

Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow

Enhancing the Performance of an Axial Compressor Cascade using Vortex Generators

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