Effect of impingement surface roughness on the noise from impinging jets

Sound source mechanisms in under-expanded impinging jets

Abstract: Experiments on the aeroacoustics of an under-expanded 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.

A review of impinging jets during rocket launching

Abstract: During rocket launching, the engine exhaust impinges on the launch structures, such as the launch platform and the jet deflector, leading to a complex flowfield and generating strong noise. The engine exhaust impingement induces large aerothermodynamic and acoustic loads on the launch structures and the rocket, the overload of which is a potential risk to the rocket launch. Thus, it is important to predict and reduce the loads. Although the jet impingement during rocket launching has been studied extensively, the jet impingement during rocket launching is a complex phenomenon, where multiple mechanisms are coupled. In most researches, several mechanisms are studied separately or in a partially-coupled way. Still, there are a lot of issues on the jet impingement remaining to be solved, especially when quantitative relations are taken into account. In this article, several issues involved are reviewed, to present an overview on progresses and perspectives of the jet impingement research for rocket launching. It starts with a summary of the results accessed for normal and inclined jets impinging on flat plates, then the interactive mechanisms between impinging jets and the launch platform or the deflector system, together with relevant factors that affect the flowfield and noise. Additionally, discussions on the interaction of impinging jets with multiphase flows are also presented.

Analysis of the performance of a new developed shear stress transport model in a turbulent impinging jet flow

Abstract: In this work, a developed Shear Stress Transport (SST) model has been used for numerically simulating the problem of turbulent round jet impingement heat transfer. Based on the cross-diffusion correction activated in the logarithmic and wake parts of a region by using a blending function in the destruction term of turbulent kinetic energy k, the developed SST model is capable of recovering the effect of the pressure gradient ignored by the standard SST model. Also, the Kato-Launder model is added in the production term of k to consider the stagnating flows. The developed model has been investigated for turbulent round jets with the nozzle-plate spacing of 2, 4, and 6. The model is verified by comparing with the measurements and the results of the standard SST model, the SST with low-Re model, the Launder and Sharma model with the Yap model, the k-ω model, and the Reynolds-averaged Navier-Stokes/large eddy simulation model. Comparing with other referred methods, the developed model obtains accurate prediction in terms of velocity and pressure. As for heat transfer, it also possesses appropriate performance. Moreover, the developed model is sensitive to the pressure gradient, which helps the model be capable of reproducing accurate flow structures. By using the present model, it has been found that the velocity profiles are dominated by the turbulent kinetic energy away from walls. Meanwhile, the results show that the inner peak of heat transfer is connected with the radial pressure gradient at the stagnation point.

Experimental investigation of heat transfer of impinging jet on a roughened plate by a micro cubic shape

Abstract: An experimental study of average heat transfer from air jet impinging on a rough plate by a micro cubic pin is investigated. The main objective of the present study is to investigate the effect of ...

Instability modes of millimeter-scale supersonic jets

Abstract: The appearance of instability modes on subsonic and supersonic impinging jets is a phenomenon of great interest that has been observed in jets produced by nozzles of different sizes. The present study describes these instabilities in millimeter-sized supersonic jets produced by convergent and convergent–divergent nozzles ( $$D_\mathrm{throat} = 2 \hbox { mm}$$ ). The smaller nozzle sizes allowed for a comprehensive parametric study involving 378 impingement distance values between $$H{/}D=0.5$$ and $$H{/}D=9.9$$ , as well as 30 nozzle pressure ratio values for both nozzles, totaling a set of more than 22,000 experiments. The detailed resolution on the impingement distance parameter allowed for the observation of acoustic resonant tones that alternate between two or three distinct frequency bands. It was observed that very small changes in the impingement distance ( $$\Delta H{/}D \le 0.5$$ ) are sufficient for resonant mode switching, which is related to the instability mode switching phenomenon. The complementary high-speed schlieren images, captured for a set of 36 cases, allowed for the effective observation and classification of the jet resonant modes via feature tracking. A correlation between the resonant mode shape (helical or axisymmetric) and the resonant tone was clearly observed and quantified. Further experiments with high-resolution schlieren images showed the shape of the acoustic waves produced, connecting the acoustic wave packets with the production of shear layer instabilities. In the helical instability mode, it was possible to observe what appears to be a helix-shaped acoustic wave.

On Sound Generated Aerodynamically. I. General Theory

Abstract: A theory is initiated, based on the equations of motion of a gas, for the purpose of estimating the sound radiated from a fluid flow, with rigid boundaries, which as a result of instability contains regular fluctuations or turbulence. The sound field is that which would be produced by a static distribution of acoustic quadrupoles whose instantaneous strength per unit volume is ρv i v j + p ij - a 2 0 ρ δ ij , where ρ is the density, v i the velocity vector, p ij the compressive stress tensor, and a 0 the velocity of sound outside the flow. This quadrupole strength density may be approximated in many cases as ρ 0 v i v j . The radiation field is deduced by means of retarded potential solutions. In it, the intensity depends crucially on the frequency as well as on the strength of the quadrupoles, and as a result increases in proportion to a high power, near the eighth, of a typical velocity U in the flow. Physically, the mechanism of conversion of energy from kinetic to acoustic is based on fluctuations in the flow of momentum across fixed surfaces, and it is explained in § 2 how this accounts both for the relative inefficiency of the process and for the increase of efficiency with U . It is shown in § 7 how the efficiency is also increased, particularly for the sound emitted forwards, in the case of fluctuations convected at a not negligible Mach number.

On the Mechanism of Choked Jet Noise

Abstract: The character of jet noise undergoes a marked change above choking, the noise due to turbulent mixing being dominated by a powerful whistle or screech whose wavelength is related to the regular shock wave spacing. The mechanism in two-dimensional flow is further examined (by the aid of a dynamic Schlieren apparatus), verifying the suggested mechanism and showing the similarity to that in axially symmetric flow where discontinuities in frequency, partly analogous to edge tones, occur. The resultant sound emitted as the periodic eddy system traverses the regular shock wave pattern is highly directional, producing a powerful beam at doubled frequency normal to the jet and an intense beam at eddy frequency in the upstream direction adjacent to the jet, resulting in fluctuations in jet velocity direction at the orifice which initiate new stream disturbances. A gain criterion for the self-maintained cycle is given, enabling certain qualitative deductions concerning the intensity to be made, and use will be made of this in considering methods of reducing the noise level.

A study of free jet impingement. Part 1. Mean properties of free and impinging jets

Abstract: In this, the first part of a two-part experimental study of the behaviour of impinging jets, the mean properties of the flow field are established. Velocity profiles are given for three types of jet flow issuing from a circular convergent nozzle. Measured distributions of surface pressure are given which result when the jets impinge both normally and obliquely at various distances on several surface shapes. The pressure distributions are used to compute the radial velocity gradient at the impingement stagnation point. It is found that for normal impingement this gradient correlates with the free jet centreline velocity and half-radius at the same axial location. A fall-off in the correlated value is noted as the impingement is made oblique. Measurements of the azimuthal distribution of momentum flux in the resulting wall jet are also given. The general behaviour of all three types of jet is found to be similar at locations downstream of any local effects due to the shock waves present in the under-expanded types. A special study of the close-range impingement of an under-expanded jet containing a normal shock disk reveals a region of separated flow surrounding the stagnation point. This condition results in a negative value of the radial velocity gradient at the centreline.

Dynamics of an impinging jet. Part 1. The feedback phenomenon

Abstract: In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle (x0/d < 7·5). In the present study extensive measurements of the near-field pressure provide solid support for the hypothesis that a feedback mechanism is responsible for the sudden change observed in the pressure fluctuations at the onset of resonance. The feedback loop consists of two elements: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate. The upstream-propagating waves and the coherent structures are phase-locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and produce periodic coherent structures. The period is determined by the convection speed of the coherent structures, the speed of the upstream-propagating waves as well as the distance between the nozzle and the plate. An instability process, herein referred to as the ‘collective interaction’, was found to be critical in closing the feedback loop near the nozzle lip.

On the Edgetone

Abstract: The feedback mechanism of classical low‐speed edgetones in which the action at the edge is interpreted as an acoustical source is developed in detail. A theoretical development indicating that the acoustic field is primarily due to the dipole associated with the fluctuating fluid force on the edge has been verified. It is the hydrodynamic field of the dipole which disturbs the jet, whose instability characteristics are shown to depend acutely on the Reynolds and Strouhal numbers, and the orifice‐edge distance. The gain criterion is developed in detail, it being shown how the eigenfrequencies (which can form no algebraic sequence) arise; the lower limit to the orifice‐edge distance is discussed, yielding an estimate of the “linear” instability of the stream. The amplitude of the established edgetone depends on the nonlinear behavior of large‐amplitude stream disturbances and the corresponding upper limit to the edge force proves to be in satisfactory agreement with measurements, thus yielding acceptable expressions for the sound pressure. Multiple tones and the circumstances of the hysteretic frequency jumps are discussed. The basic action depends only on Reynolds number for geometrically similar systems, while the sound power depends on the cube of the Mach number also.