Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs On the flow physics and vortex behavior of rectangular orifice synthetic jets Abhay Kumar a , Arun K. Saha b , Pradipta Kumar Panigrahi b , Ashish Karn a, ⁎ a Department of Mechanical Engineering, University of Petroleum and Energy Studies, Energy Acres, Bidholi, Dehradun, Uttarakhand 248007, India b Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh 248007, India ARTICLE INFO Keywords: Synthetic jet Vortex rings Rectangular orifice Bifurcation Vortex ABSTRACT Synthetic jet actuators possess a continuous jet like behavior in its far region and have found wide-scale en- gineering applications since it allows momentum transport to the flow system without any net mass transfer across the flow boundaries. The case of a non-axisymmetric synthetic jet is particularly significant since it is affected by the differential shear layer at the orifice exit, that depends on its aspect ratio. However, despite exhaustive research on both continuous and synthetic jets, very few studies have experimentally investigated the case of rectangular orifice synthetic jets, focusing on the effect of aspect ratio of the orifice as well as the actuation frequency upon the vortex behavior and the flow physics. In particular, the intriguing phenomenon of vortex bifurcation has mostly been reported only for an individual vortex or for a plain jet. Yet, in a train of vortex rings, such as that obtained in a synthetic jet, the occurrence of vortex bifurcation can be expected, although the flow physics in the wake of individual vortex rings is significantly different. The present study experimentally investigates a rectangular orifice synthetic jet at different orifice aspect ratios and actuation frequencies, focusing on exploring the conditions at which vortex bifurcation occurs, through LIF imaging and Hot-film measurements. The primary objective of these experiments is to provide a qualitative physical insight into the synthetic jet ejected from a rectangular orifice (through LIF imaging), as well as to quantitatively explore the experimental conditions that promote different flow structures (through velocity time trace, time- averaged velocity profiles and power spectral density measurements), particularly the bifurcation of vortex rings. Our experiments indicate that the phenomenon of vortex bifurcation is observed during the axial switching of vortex rings, but only in a narrow range of experimental conditions. Further, the velocity mea- surements have ascertained that the two prominent reasons behind this bifurcation process are a large disparity in the velocities of the vortex core and the center of vortex ring, as well as the time lag in which the separation distance between the counter-rotating vortices decrease gradually to zero. 1. Introduction A synthetic jet can be defined as a train of vortex rings that origi- nates completely from the working fluid, carries no net mass flux and yet transfers linear momentum to the fluid flow. A synthetic jet ac- tuation system that typically consists of a mechanism to change the volume of chamber either through a piston or through a flexible dia- phragm. As the oscillating boundary of the cavity moves away from the orifice, thus increasing the volume, fluid from the surrounding is sucked into the cavity. The fluid is subsequently ejected out when the oscil- lating boundary moves towards the orifice reducing the volume of the chamber. During the ejection period, the shear layer formed around the orifice circumference due to finite thickness of the orifice plate sepa- rates at the sharp edges of the orifice and rolls up into vortex rings. These vortex rings move away from the orifice under their self-induced velocity thus synthesizing a jet of fluid through the entrainment of the ambient fluid. It is also called a Zero Net Mass Flux (ZNMF) jet, since it is formed entirely from the ambient working fluid of the flow system allowing momentum transport to the flow system without any net mass transfer across the flow boundaries. Consequently, in contrast to other jet flows, it is a preferred choice in many engineering applications such as flow separation control over bluff bodies [2], stalled airfoils [3], duct flow [1,11], maneuverability of Unmanned Aerial Vehicles [9,17], jet- vectoring [20], mixing enhancement [8] and in electronic cooling [4]. In the existing literature, a plethora of work on synthetic jets has already been reported. Many of these studies underline the importance of the orifice shape and dimensions upon the flow physics of the jet, including the process of vortex formation and propagation. For in- stance, Utturkar et al. [21] reported that depending upon orifice shape, aspect ratio and curvature, the attainment of a threshold Strouhal https://doi.org/10.1016/j.expthermflusci.2019.01.020 Received 13 October 2018; Received in revised form 23 December 2018; Accepted 16 January 2019 ⁎ Corresponding author. E-mail address: akarn@ddn.upes.ac.in (A. Karn). Experimental Thermal and Fluid Science 103 (2019) 163–181 Available online 17 January 2019 0894-1777/ © 2019 Published by Elsevier Inc. T