Drag reduction of a slanted-base cylinder using sweeping jets

In this work, a pair of sweeping jet actuators is installed underneath the endplate of a slanted-base cylinder at ReD = 200 000. The sweeping jets form a 30° inclined angle with the endplate and are placed at different streamwise locations, and their strength is varied with a momentum coefficient, Cμ, ranging from 3.8 × 10-3 to 6.0 × 10-2. For all the cases examined in this paper, it is found that while a higher Cμ produces a higher drag reduction, the flow control energy efficiency decreases rapidly as Cμ increases. A net energy saving is achieved when Cμ is less than 0.01, and the highest energy efficiency obtained in the present study is 2.8% when the actuator pair is placed at the most upstream location tested. The drag reduction is attributed to the reaction force and an increase in the surface pressure force acting on the endplate produced by the jet pair. The contribution from the former constitutes an increasing proportion of the total drag reduction as Cμ increases leading to lower energy efficiency in flow control. Depending on the relative positions between the trajectory of the sweeping jet and afterbody vortex, sweeping jets are not only capable of altering the surface pressure distributions via directly imposing a footprint of high pressure on the surface, but also affecting the roll-up of the afterbody vortex and/or reducing its strength via injecting turbulence into the afterbody vortex.