A high-speed, compressible synthetic jet

Abstract

The operation of a high-speed, compressible synthetic (zero net mass flux) jet is investigated experimentally using a small (nominally 6.5 cm3) piston/cylinder actuator driven by a variable speed motor. The actuator performance is characterized using phase-locked cavity pressure measurements, particle image velocimetry of the jet flow, and Schlieren flow visualization. Cylinder pressure ratios (relative to the ambient) measured over the experimental limits range as high as 8 and as low as 0.2, during the blowing and suction phases, respectively (producing sonic flow velocities at the orifice during both phases). Owing to compressibility effects, the shapes of the time-periodic pressure curves during the blowing and suction cycles are dissimilar (the suction duration is longer) and there is a clear phase shift between the peak piston displacements and the corresponding pressure extrema. It is shown that the performance is primarily affected by the characteristic velocity (defined by the product of the operating frequency and the normalized piston stroke length) and the compression ratio. A simple numerical model of the gas in the cylinder that treats the volume change due to the piston motion as a series of isentropic and adiabatic compressions and expansions and flow through the orifice as inviscid and isentropic is presented and is in good agreement with the measured system performance. The model indicates that the cylinder pressure curve grows to approximate a closed system as the characteristic velocity is increased beyond the experimental range. Finally, it is shown that the duty cycle of the pressure wave form (between blowing and suction) can be modified by increasing the flow area during the suction stroke using self-actuated valves in the cylinder head, resulting in higher peak pressures in blowing.