|Abstract: ||This work of thesis is part of a wider research project with the aim of developing an aerodynamic active device for drag reduction of ground vehicles. The system, previously studied on a bullet-shaped body by Qubain (2009) and Oxlade (2013), is applied to a bluff body that idealises a long vehicle, such as an articulated lorry or a coach. The model, tested in the Honda wind tunnel of the Department of Aeronautics at Imperial College, is equipped with a synthetic jet, or zero net-mass-flux actuator, composed of a cavity, a plate with a slot, and an oscillating diaphragm, placed at the rear end of the body. The effects produced by the actuator are studied by monitoring the base pressure on the model, and by measuring the aerodynamic forces and the moments acting on the body. During the experiments, performed at a constant ReH=UH/ν=4.1x10^5, a parametric study of the response of the mean base pressure, forces and moments to changes in the forcing parameters (frequency and amplitude), and slot width is performed.
The unforced wake is characterised by two main structures: the bubble-pumping mode, with Strouhal number StH≈0.08, and the vortex shedding, with StH≈0.17 and StH≈0.20 on the vertical and horizontal plane, respectively. These structures, still visible in the forced wake at low forcing amplitudes, are almost completely suppressed when the forcing amplitude is increased. The suppression of the structures in the wake corresponds to a decrease in the integrated energy of the wake itself, associated to base pressure recovery and drag reduction. The optimal values achieved corresponds to 27.3% gain in base pressure and -13.1% reduction of drag. The higher sensitivity to changes in forcing amplitude rather than in frequency displayed by the system confirms the existence of a plateau of optimal base pressure recovery/drag reduction at frequencies around 5 times the characteristic shear layer frequency.|