An experimental investigation into the structure of a supersonic jet impinging on a large plate is presented. Digital particle image velocimetry (DPIV), shadowgraph photography and acoustic measurements are used to understand the relationship between the unsteady jet structure and the production of tones for nozzle-to-plate spacings between 1 and 5 nozzle exit diameters at a nozzle–pressure ratio equal to 4. Results indicate that the instability of the jet depends on the location of the plate in the shock cell structure of the corresponding free jet and the strength of the standoff shock wave, rather than on the occurrence of recirculation zones in the impingement region. Phase-locked studies show streamwise displacements of the stand-off shock wave, a moving recirculation zone in the subsonic flow in front of the plate, and significant oscillations of both the compression and expansion regions in the peripheral supersonic flow when tones are produced. Sound is shown to be generated by periodic pulsing of the wall jet boundary resulting from periodic motion of the flow in the impingement and near-wall regions of the flow.

An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets

The fluid dynamics of microflows has recently commanded considerable attention because of their potential applications. Until now, with a few exceptions, most of the studies have been limited to low speed flows. This experimental study examines supersonic microjets of 100–1,000 μm in size with exit velocities in the range of 300–500 m/s. Such microjets are presently being used to actively control larger supersonic impinging jets, which occur in STOVL (short takeoff and vertical landing) aircraft, cavity flows, and flow separation. Flow properties of free as well as impinging supersonic microjets have been experimentally investigated over a range of geometric and flow parameters. The flowfield is visualized using a micro-schlieren system with a high magnification. These schlieren images clearly show the characteristic shock cell structure typically observed in larger supersonic jets. Quantitative measurements of the jet decay and spreading rates as well as shock cell spacing are obtained using micro-pitot probe surveys. In general, the mean flow features of free microjets are similar to larger supersonic jets operating at higher Reynolds numbers. However, some differences are also observed, most likely due to pronounced viscous effects associated with jets at these small scales. Limited studies of impinging microjets were also conducted. They reveal that, similar to the behavior of free microjets, the flow structure of impinging microjets strongly resembles that of larger supersonic impinging jets.

/pdf/experiments-on-free-and-impinging-supersonic-microjets-3ozl0mpt2j.pdf

Experiments on free and impinging supersonic microjets

This paper presents a computational study of the flow and flow-induced acoustic fields of a supersonic jet impinging on an inclined flat plate. For the numerical simulations, we solved three-dimensional compressible Navier-Stokes equations with a modified weighted compact nonlinear scheme. We analyzed the simulation results mainly from the viewpoint of the acoustic emission and propagation mechanism, and we investigated the acoustic field characteristics such as directivity, their spectra, and acoustic wave source positions. The acoustic fields indicate that there are at least three types of acoustic waves in all the cases considered in the study: (i) Mach waves generated from the shear layer of the main jet, (ii) acoustic waves generated from the impingement region, and (iii) Mach waves generated from the shear layer of the supersonic flow downstream of the jet impingement. The indication of the second type of wave (ii) is important because the commonly used empirical method for the estimation of the aco...

Aeroacoustic waves generated from a supersonic jet impinging on an inclined flat plate

The behavior of supersonic impinging jets is dominated by a feedback loop due to the coupling between the fluid and acoustic fields. This leads to many adverse effects when such flows occur in short takeoff and vertical landing aircraft, such as a significant increase in the noise level, very high unsteady loads on the nearby structures, and an appreciable loss in lifting during hover. In earlier studies, it was demonstrated that by using supersonic microjets one could disrupt the feedback loop that leads to substantial reductions in the aforementioned adverse effects. However, the effectiveness of control was found to be strongly dependent on the ground plane distances and the jet-operating conditions. The effect of various microjet control parameters are investigated in some detail to identify their influence on control efficiency and additional insight is provided on the physical mechanism behind this control method. Parameters studied include microjet angle, microjet pressure, and the use of microtabs instead of microjets. These results indicate that by choosing appropriate control parameters it should be possible to devise a control strategy that produces optimal control for the entire operating range of conditions of the supersonic impinging jet. Moreover, the experimental results provide convincing evidence of the generation of significant streamwise vorticity by the activation microjets. It is postulated that the generation of streamwise vorticity and its evolution in the jet flow might be one of the main physical phenomena responsible for the reduction of flow unsteadiness in impinging jets.

/pdf/active-and-passive-control-of-supersonic-impinging-jets-36mevqfrtz.pdf

Active and Passive Control of Supersonic Impinging Jets

Supersonic impinging jets, such as those occurring in the next generation of STOVL aircraft, generate a highly oscillatory flow with very high unsteady loads on the nearby aircraft structures and the landing surfaces. These high pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover. Previous studies of supersonic impinging jets suggest that the highly unsteady behavior of the impinging jets is due to a feedback loop between the fluid and acoustic fields which leads to these adverse effects. In this paper a unique active control technique was attempted with the aim of disrupting the feedback loop, diminishing the flow unsteadiness and ultimately reducing the adverse effects of this flow. Flow control was implemented by placing multiple 40 supersonic microjets around the periphery of the main jet. This control approach was very successful in disrupting the feedback loop in that the activation of the microjets led to a dramatic reduction in the lift loss (40%), unsteady pressure loads (11 dB) and nearfield noise (8 dB). This relatively simple and highly effective control technique makes it a suitable candidate for implementation in practical aircraft systems.

/pdf/control-of-supersonic-impinging-jet-flows-using-microjets-4uo5jjyfi2.pdf

K. Iyer

Papers

Mean and unsteady flowfield properties of supersonic impinging jets with lift plates