Sound source mechanisms in under-expanded impinging jetsExp Fluids

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Authors
Giorgia Sinibaldi, Luca Marino, Giovanni Paolo Romano
Year
2015
DOI
10.1007/s00348-015-1967-x
Subject
Mechanics of Materials / Fluid Flow and Transfer Processes / Physics and Astronomy (all) / Computational Mechanics

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Exp Fluids (2015) 56:105

DOI 10.1007/s00348-015-1967-x

RESEARCH ARTICLE

Sound source mechanisms in under‑expanded impinging jets

Giorgia Sinibaldi1 · Luca Marino1 · Giovanni Paolo Romano1

Received: 14 November 2014 / Revised: 13 March 2015 / Accepted: 3 April 2015 © Springer-Verlag Berlin Heidelberg 2015 and S/VTOL aircraft jet engines, as well in other fields, such as electronic equipment cooling, paper drying and laser cutting. The interaction between the jet and a solid surface makes the structure of the flow very complex. In particular, these kinds of flows can be characterized by the simultaneous presence of subsonic and supersonic regions, shock wave oscillations and instabilities, as well as the occurrence of regions of turbulent shear and the interaction of shock waves with the jet shear layer. In addition, a recirculation zone in front of the impinging surface possibly occurs (Henderson et al. 2005; Kim and Park 2005).

The noise spectrum of an under-expanded impinging jet is characterized by discrete tones which can be the dominant noise contribution. The unsteady oscillatory behavior of the jet and the related “impinging tones” develop a lot of undesirable effects, such as significant acoustic emission, sonic fatigue on the structures surrounding the nozzle, aerodynamic and thermal loads and a possible lift loss during hover (Margason et al. 1996; Levin and Wardwell 1997;

Krothapalli et al. 1999).

In Fig. 1, a sketch of the geometry of the jet issuing from a circular convergent nozzle and impinging on a perpendicular plate is shown. The main parameters of the problem are the nozzle pressure ratio, NPR = p0/pa, where p0 is the stagnation pressure and pa the ambient pressure, the nozzle exit diameter d, the nozzle-to-plate distance h and the impinging plate characteristic size D.

As sketched in Fig. 2, the flow field of an axisymmetric supersonic jet impinging on a solid surface has a complex topology which can be roughly divided into three regions (Donaldson and Snedeker 1971; Alvi and Iyer 1999): (1) an upstream region with a “free jet” behavior; (2) an impingement region where the structure of the flow is influenced by the presence of the impinging surface; (3) a wall jet region with radial flow along the impingement surface. The

Abstract Experiments on the aeroacoustics of an underexpanded supersonic jet impinging on a flat plate are presented and thoroughly discussed. A wide range of nozzle pressure ratios and of nozzle-to-plate distances has been analyzed with particular attention to the behavior of the discrete component of the noise. The investigation has been carried out by means of acoustic, particle image velocimetry and wall pressure measurements. The analysis of the relationship between the acoustic data and the fluid dynamic fields allows to examine the different source mechanisms of the discrete component of the noise and to evaluate the link between the jet flow structure and the acoustic tone features. Specifically, two ranges of nozzle pressure ratio have been observed showing different acoustic behaviors, characterized by distinct mechanisms of discrete noise generation. These regions are separated by a range of nozzle pressure ratios where impinging tones are not observed. The present experimental data extend previously published results, improving the analysis of the connection between fluid dynamic and acoustic fields and leading to a better comprehension of the impinging tone source mechanisms. 1 Introduction

Phenomena involving high-speed impinging jets are present in several applications in the aerospace industry, such as space launch vehicle systems, rocket motors * Giorgia Sinibaldi giorgia.sinibaldi@uniroma1.it 1

Department of Mechanical and Aerospace Engineering,

La Sapienza, Via Eudossiana 18, 00184 Rome, Italy

Exp Fluids (2015) 56:105 1 3 105 Page 2 of 14 relative extent of these regions depends on both NPR and h/d (Sinibaldi et al. 2013a).

Following Powell (1988), Henderson (2002) and Henderson et al. (2005), we report a brief description of the flow field of moderately and highly under-expanded circular jets, perpendicularly impinging on a plate.

For a convergent nozzle, the sonic speed at the exit is reached at the ideal critical nozzle pressure ratio, i.e.,

NPR = 1.893. For NPR > 1.893, the flow continues to be sonic at the throat and expansion waves begin to spring from the nozzle boundary. These waves reflect upon the jet constant pressure boundary as compression waves merging into a shock, whose shape and location change with the nozzle pressure ratio and the nozzle-to-plate distance.

For NPR  3.8, the shock is conical and the entire flow behind it is supersonic. The conical shock wave reflects on the jet boundary as expansion waves which extend to the entire jet. Then, they reflect again as compression waves that merge to form a standoff shock wave in front of the plate. For NPR  3.8, a Mach disk cuts off the apex of the conical shock wave, characterizing the flow with a subsonic and a supersonic region, separated by a curved sonic line.

The shock wave reflects on the jet boundary as expansion waves, which, differently from the lower NPR case, reflect on the slip stream as compression waves while evanescent waves form in the subsonic region. The evanescent waves and the mixing across the shear layer, dividing the supersonic and subsonic regions, cause the acceleration of the subsonic flow, while in the supersonic region, the compression waves merge to form an annular standoff shock wave.

Behind the standoff shock wave, there are a subsonic and a supersonic regions, divided by a sonic line (see Fig. 2).

The total pressure loss by the flow passing through the oblique shock is lower than the loss through the Mach disk, so that there is a low-pressure region at the center of the solid surface and a maximum value of pressure away from the center. If the pressure in the center of the impingement region is sufficiently low and the flow cannot overcome the pressure maximum, the outwards radial flow separates from the solid surface and forms a stagnation bubble and the primary jet passing through the Mach disk is diverted and impinges at a certain distance from the center of the plate. Although such a recirculation region has been largely investigated in the last decades, e.g., in Donaldson and Snedeker (1971), Carling and Hunt (1974), Kalghatgi and Hunt (1976), Lamont and Hunt (1980), Alvi and Iyer (1999), Krothapalli et al. (1999) and Henderson et al. (2005), it is still not completely clear under which flow conditions it fully establishes.