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

Abstract

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.

Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

“…An approach using vortex generators to produce coherent streaks that increase or decrease the separation bubble was also carried out by Aider et al (2009) and Pujals et al (2010) leading, respectively, to a 12 and 10% drag reduction. However, apart from the active vortex generators proposed by Aider et al (2009), all these passive techniques introduce quite unsightly appendages on the body, which is in contradiction with design constraints.…”

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body

“…1999;Spohn & Gillieron 2002;Lienhart & Becker 2003). Recently, the model has been subjected to intensive research for the pursuit of flow control methods capable of reducing the aerodynamic drag on the model, both passive control (Beaudoin & Aider 2008;Krajnović 2013), and active control (Brunn et al 2008;Pastoor et al 2008;Krajnović & Fernandes 2011;Aider et al 2010). In the present study, we focus on the square-back variant of the Ahmed body, which is essentially a bluff body with curved front edges placed in the proximity of ground.…”

Influence of Short Rear End tapers on the Base Pressure of a Simplified Vehicle.

“…This can be achieved through passive optimization, for example geometry changes, vortex generators, flaps, and surface roughness, or using active control, for example, suction, blowing, oscillated suction and blowing, moveable vortex generators or flaps [2][3][4][5][6][7][8][9][10][11]. Active flow control is an attractive option because of the potential freedom it allows for vehicle styling as all that is required externally is the jet orifices.…”