The interaction of synthetic jets with cross flow and the modification of aerodynamic surfaces.

摘要

The interaction of synthetic jets with a cross flow, and the effect of this interaction on the aerodynamic characteristics of a circular cylinder are investigated experimentally using particle image velocimetry (PIV). In the vicinity of the actuator orifice, the interaction can lead to the formation of either a closed recirculating region or a finite train of discrete vortices, depending on a local dimensionless frequency. These flow structures locally modify the streamwise pressure gradient, leading to the establishment of a thinner boundary layer downstream. When synthetic jets are properly positioned on the cylinder higher than normal adverse pressure gradients can be overcome, resulting in the delay of boundary layer separation and thus augmented aerodynamic performance (lift up to C_L = 0.69, and pressure drag reductions of up to 56% have been observed in the present experiments). The dependence of these effects on the free stream Reynolds number, as well as actuation frequency, amplitude, and location are presented.; Small obstructions (similar to trip-wires) mounted upstream from separation are observed to produce global flow changes that are similar to those achieved by jet actuation. The presence of a separation bubble behind an obstruction creates a new dividing streamline and thus alters the apparent aerodynamic shape of the cylinder. The flow mechanisms involved appear to be similar to those that occur at the drag crisis (Re_D ≈ 3 × 105), where the formation of laminar separation bubbles leads to extremely low drag.; The interaction of a synthetic jet and cross flow is also investigated within a fully turbulent boundary layer developing along a flat plate. The local pressure gradients induced by the passage of discrete vortices formed at the jet orifice, explicitly computed from PIV data using the two-dimensional Navier-Stokes equations, are qualitatively similar in both geometries. Since the dynamics of the interaction domain are only weakly dependent on the global flow, it is expected that synthetic jet actuation can be exploited to improve aerodynamic performance in other separated flows.