Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Abstract

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

Control of High-Temperature Supersonic Impinging Jets Using Microjets

“…A recent approach to suppress the feedback mechanism of supersonic impinging jets using an array of high-momentum microjets appropriately placed near the nozzle exit has shown highly promising results [15][16][17][18][19]. This control-on-demand technique has many advantages over traditional passive and active control methods and has proven to be successful over a range of geometric and flow conditions.…”

“…This control-on-demand technique has many advantages over traditional passive and active control methods and has proven to be successful over a range of geometric and flow conditions. With the help of particle image velocimetry (PIV) measurements, Alvi et al [19] have shown that one of the main mechanisms at work is the introduction of streamwise vorticity at the expense of azimuthal vorticity of the main jet. A reduction in the primary shear-layer instability, attenuation of upstream propagating acoustic waves, and disruption of spatial coherence between largescale structures and the acoustic field lead to an overall attenuation in the feedback loop.…”

Supersonic flow control

“…Both acoustic and PIV measurements were conducted on control of a Mach 1.5 ideally expanded free and impinging jets using an array of circumferentially placed 16 steady MJs 400 μm in diameter and with injection angles of 60 • [57]. The combined mass flow from the MJ configuration is only 0.5 % of the main jets [58,59]. The velocity field measurements indicate that the MJs result in a rapid nonlinear thickening of the shear-layer growth, particularly near the nozzle exit, as opposed to the uncontrolled case.…”

Flow Field Characteristics of Oblique Shocks Generated Using Microjet Arrays

“…Mahesh [4] provides a recent, comprehensive review of single jets in crossflow. Some applications of jet injection include fuel injectors in scramjets [5][6], active flow control of cavities in supersonic flow using micro-orifice jets [7][8], suppression of jet noise in high-speed flows among others (Alvi et al [9]). Interaction of the injectant with the supersonic crossflow creates a shock wave, where a sufficiently strong shock wave (due to high injection pressures or momentum flux) is capable of separating the incoming boundary layer.…”

“…The cross-stream velocity (V/U o ) contours show significant negative velocities with a magnitude of -0.09 near jet exit, and close to the edge of the boundary layer (0.1H), which is an indication of the strong penetration of the microjets into the crossflow. The significant penetration of the injectant into the freestream results in generating streamwise vorticity (see Alvi et al [9]; Mahesh [4]; Fernandez et al [23]) and leads to higher mixing with the freestream flow. This feature was exploited in another study where microjet arrays are used as mixing enhancement actuators upstream of a backward facing step in a supersonic flow (Ahmed et al [24]).…”