Flow Characteristics and Novel Applications of Synthetic Jets - A Review

Synthetic jets, often abbreviated as 'SJs' or 'Synjets' are a type of Zero Net Mass Flux (ZNMF) jets which have gained popularity due to their ability to transfer momentum without transferring mass. This property distinguishes SJs from their counterpart traditional continuous jets. Over a stretch of time, SJs have been employed extensively in applications such as active flow control, impingement heat transfer, miniature electronics cooling, etc. However, researchers are now slowly extending the frontiers of SJ research and making efforts to utilize excellent properties of SJs in novel applications such as AUVs, marine systems, bio-inspired propulsion systems, etc. This paper strives to identify the gaps in the current research and the areas which have remained unexplored yet. The article begins with a discussion of the origin of SJs i.e. 'acoustic streaming'. The resulting segments talk about the formation criteria and the recent works employing various configurations of orifices and cavities. The paper discusses critical concepts such as JIF, volumetric efficiency, and propulsive efficiency of synthetic jets. It is followed by the introduction of novel flow control systems employing SJs such as gurney flaps, bumps, and electroactive synthetic jets. Finally, the most innovative applications of SJs such as AUVs, UAVs, jetting cavities. Pulsatile jet flows found in jellyfish, cephalopod, squid and salp have been discussed in great detail. The exhaustive discussion on future prospects strives not only to provide reader with comprehensive insights into SJs, but also to motivate future researchers to overcome the gaps identified in this paper.

Modal Reduction of Synthetic Jet Actuator Based Separation Control With Spectral POD

A synthetic jet actuator (SJA) is a zero-net-mass-flux device that imparts fluid momentum and is useful for active flow control (AFC). In many applications, airfoil performance is often limited or degraded by flow separation which is usually associated with loss of lift, increased drag, and kinetic energy losses. Therefore, it is of interest to investigate methods of separation region suppression with the forcing control of SJA. This paper studies the flow behavior of cross flow over an airfoil and how the addition of SJA influences flow characteristics. Using the Spectral Proper Orthogonal Decomposition and LES simulation, flow instabilities in the wake region are analyzed in their different temporal and spatial scales. The objective of this study is to explore the viability of SPOD for separation control and correlating the decomposed flow modes to the aerodynamic performance of airfoil.

Synthetic jet for underwater flow control applications

The formation, propagation, and interaction of vortex rings have been the subject of various researchers due to its wide application in nature and day-to-day life. Some of the important applications of vortex rings are volcanic eruption, propulsion of aquatic creatures, sewerage outfalls. As an active device, synthetic jet with a train of large scale coherent structures and zero net mass flow have many practical and industrial applications in flow control.A synthetic jet is generated using a simple cavity with a small orifice at one end and a deformable flexible diaphragm at the opposite end oscillating at different frequency and amplitude. It generates a series of vortex rings due to the changes in the cavity size with diaphragm oscillation. The vortex ring moves away from the orifice exit plane under the influence of self-induced velocity and thus synthesize a jet. Synthetic jet is also called zero net mass flux (ZNMF) system, the net mass transfer through the opening in one cycle is zero but the jet imparts a net transport of momentum to the surroundings.In this work, the aim is to study the characteristics of synthetic jet for various orifice shapes, actuation frequencies and diaphragm displacements. A series of experiments were performed to understand the behavior of circular and rectangular shape synthetic jet in quiescent flow environment. To achieve this, a synthetic jet generation unit mounted on the sidewall of water tank is used. Further, a torpedo shape model with built-in circularsynthetic jet has been designed and developed and the behavior of synthetic jet in crossflow has been investigated. For qualitative study, the bulk flow visualization and laser induced fluorescence (LIF) technique has been used whereas for quantitative measurement, the velocity is measured using hot-film anemometry and Laser Doppler Velocimetry (LDV). Circular orifices of 8, 10 and 13 mm diameter operating at actuation frequencies of 1, 2, 4 and 6 Hz have been used for characterization of circular synthetic jet generation.

Passive Bristling of Shark Skin Scales at the Micro-Level: A Fundamental Viscous Flow Study to Understand the Separation Control Mechanism

In this paper, we use a CFD analysis of a simplified, 2D geometry to study the ability of mako shark denticles to mitigate flow separation. We represent the viscous sublayer below a turbulent boundary layer streak as a Couette flow. Incipient separation is simulated by balancing upper wall velocity and adverse pressure gradient to achieve zero net mass flow, and we add various denticle geometries to study their effects. Each modeled denticle protrudes at an angle from 15° to 85° and sublayer blockage ratio from 0.05 to 0.85. Through variation of fluid properties and boundary conditions, we show that the anti-flow-reversal abilities of a single, bristled shark denticle are independent of Reynolds number, and we investigate the effect of the denticle at cases other than zero net mass flux. Based on these results, we create a new relationship to predict separation inhibition. These conclusions are highly generalizable and represent previously undiscovered universal behavior.