SparkJet Actuators for Controlling Flows

Supersonic and hypersonic flows would be controlled without moving parts.

December 1, 2008

Air Force Office of Scientific Research

SparkJet actuators are under investigation as means of controlling flows — especially supersonic and hypersonic flows. In one important class of potential applications, SparkJet actuators would be used to effect rapid and flexible steering of advanced aerospace vehicles. Effective manipulation of flow fields in aerospace systems could afford significant benefits, including increases in performance, maneuverability, payload, and range, as well as reductions in overall costs. These macro-scale benefits would be achieved through the use of SparkJet actuators to alter such phenomena as laminar-to-turbulent transition, turbulence, and flow separation on a micro scale.

In a Basic SparkJet Actuator, an electric discharge heats and pressurizes a gas in a chamber, causing rapid expulsion of gas. After a recovery phase during which gas flows back into the chamber relatively slowly, the actuator is ready for another discharge.

SparkJet actuators are related to (but not the same as) the piezoelectrically actuated flow-control devices known by the similar name, "synthetic jet actuators." SparkJet actuators are compact, simple, robust devices that contain no moving parts. A basic SparkJet actuator includes a chamber fitted with electrodes and a discharge orifice, as shown in the figure. Typically, the dimensions of the chamber, electrodes, and orifice are of the order of millimeters.

A cycle of operation is initiated by means of an electrical discharge between the electrodes. The energy deposited in the chamber by the discharge causes sudden heating and pressurization of the gas in the chamber. A significant portion of the pressurized gas is expelled through the orifice, giving rise to a rapid outgoing jet and a sudden reduction of pressure in the chamber. During the final recovery phase of the cycle, ambient gas flows back into the chamber, re-establishing equilibrium between the ambient and chamber gas pressures.

The jet transfers momentum to the ambient gas, even though there is no net mass flow into and out of the chamber over the complete cycle. The amount of mass expelled during the heating phase of the cycle is large in comparison with that expelled during the corresponding phase of the cycle of a piezoelectric flow-control device. Whereas piezoelectrically actuated flow-control devices can be effective for controlling flows at subsonic speeds but do not develop sufficient driving pressures for penetrating supersonic boundary layers, SparkJet actuators show promise for developing the higher pressures and jet velocities needed for controlling supersonic flows.

It is envisioned that in typical applications, SparkJet actuators would be energized with repeated discharges to create pulsed jets capable of exerting macro-scale effects. The actuators would be positioned and operated so that the flow disturbances associated with the jets would alter micro-scale flow phenomena (e.g., laminar-to-turbulent transition, turbulence, and/or flow separation as mentioned above) to obtain the desired macro-scale effects. Conversely, the disturbances could be used to study micro-scale flow phenomena.

Thus far, SparkJet actuators have been studied theoretically on the basis of a first-order energy-conservation mathematical model in simulations using commercial computational fluid dynamics software that solves the Navier-Stokes equations of time-dependent flow, and by experiments using a variety of diagnostic techniques. It is planned to utilize the knowledge gained from these studies to develop SparkJet-actuator flow-control systems to enhance the aerodynamic performances of high-speed aerospace vehicles.

This work was done by B. Z. Cybyk, D. H. Simon, H. B. Land III, and J. T. Wilkerson of the Johns Hopkins University Applied Physics Laboratory for the Air Force Office of Scientific Research.

AFRL-0091