Array Of Synthetic Jets Research Articles

Influence of a synthetic jet excitation on the development of a turbulent mixing layer

This paper concerns the experimental study of the control of a turbulent plane mixing layer by a single array of synthetic (loudspeakers) jets deployed through circular orifices. The displacement of the mixing layer towards the wall partially mimics the flow control scenario of the reattachment of a massively separated flow with the jets positioned after the separation location. The experimental setup is the mixing layer downstream of a splitter plate positioned at a small distance away from a flat plate. PIV observations show that the synthetic jet abruptly destroys the large scale coherent structures. There is a large modification to the turbulence field due to the distortion in the mean velocity field. An overall increase in the mixing of the flow between the flat plate and the mixing layer which cause the deviation of the flow towards the wall surface. Lastly, a parametric study of the synthetic jet trajectory is realized by varying the velocity of the air stream that generates the mixing layer. The ratio of the peak jet velocity to the lower velocity of the mixing layer should not be overly excessive such that the jet penetrates the mixing layer completely.

A random-jet-stirred turbulence tank

We report measurements of the flow above a planar array of synthetic jets, firing upwards in a spatiotemporally random pattern to create turbulence at an air–water interface. The flow generated by this randomly actuated synthetic jet array (RASJA) is turbulent, with a large Reynolds number and a weak secondary (mean) flow. The turbulence is homogeneous over a large region and has similar isotropy characteristics to those of grid turbulence. These properties make the RASJA an ideal facility for studying the behaviour of turbulence at boundaries, which we do by measuring one-point statistics approaching the air–water interface (via particle image velocimetry). We explore the effects of different spatiotemporally random driving patterns, highlighting design conditions relevant to all randomly forced facilities. We find that the number of jets firing at a given instant, and the distribution of the duration for which each jet fires, greatly affect the resulting flow. We identify and study the driving pattern that is optimal given our tank geometry. In this optimal configuration, the flow is statistically highly repeatable and rapidly reaches steady state. With increasing distance from the jets, there is a jet merging region followed by a planar homogeneous region with a power-law decay of turbulent kinetic energy. In this homogeneous region, we find a Reynolds number of 314 based on the Taylor microscale. We measure all components of mean flow velocity to be less than 10% of the turbulent velocity fluctuation magnitude. The tank width includes roughly 10 integral length scales, and because wall effects persist for one to two integral length scales, there is sizable core region in which turbulent flow is unaffected by the walls. We determine the dissipation rate of turbulent kinetic energy via three methods, the most robust using the velocity structure function. Having a precise value of dissipation and low mean flow allows us to measure the empirical constant in an existing model of the Eulerian velocity power spectrum. This model provides a method for determining the dissipation rate from velocity time series recorded at a single point, even when Taylor's frozen turbulence hypothesis does not hold. Because the jet array offers a high degree of flow control, we can quantify the effects of the mean flow in stirred tanks by intentionally forcing a mean flow and varying its strength. We demonstrate this technique with measurements of gas transfer across the free surface, and find a threshold below which mean flow no longer contributes significantly to the gas transfer velocity.

Two-Dimensional Synthetic Jet Simulation

Direct numerical simulation was used to model an array of two-dimensional synthetic slot jets pulsing into initially quiescent e ow. Such zero-net mass-e ux jets can represent an array of microelectromechanical systems. Fluid ejected by the pulsing jets rolls up into vortex pairs that propagate as a result of their mutually induced velocity andtheinitial momentum given by thepulse.Theeffectsofe uidand geometricparametersontheresulting e owe eld were studied. The results near the slots were compared with experimental data and found to be in good agreement.

Effects of Zero-Mass “Synthetic” Jets on the Aerodynamics of the NACA-0012 Airfoil

A numerical study was conducted to investigate the effect of an array of zero-mass synthetic jets on the aerodynamic characteristics of the NACA-0012 airfoil. Flowfield predictions were made using a modified version of the NASA Ames ARC2D unsteady, two-dimensional, compressible Navier-Stokes flow solver. An unsteady surface transpiration boundary condition was enforced over a user-specified portion of the airfoil's upper or lower surface to emulate the time variation of the mass flux out from and into the airfoil's surface. Here, a sinusoidal function which describes the approximate time behavior of the mass flux across the airfoil's surface was used. Our numerical results have indicated that zero-mass jets can, with the careful selection of their peak amplitude and frequency, enhance the lift characteristics of airfoils (helicopter rotor blades, wings, etc.). Effects of the jet peak suction and blowing velocities, oscillation frequency and, jet placement on the time histories of the sectional lift, drag and moment are presented for two angles of attack and one free stream Mach number. Flowfield results which provide more insight into the mechanics of the interaction between the array of jets and the developing boundary layer over the airfoil are presented.