Flow structures and noise from a supersonic impinging jet

It was observed in an earlier study [Lim, H. D., New, T. H., Mariani, R., & Cui, Y. D. (2019). Effects of bevelled nozzles on standoff shocks in supersonic impinging jets. Aerospace Science and Technology, 94, 105371.] that standoff shocks in M=1.45 supersonic impinging jets produced by non-bevelled and bevelled nozzles under small separation distances may undergo large and abrupt fluctuations in their positions above the impingement points. This occurs when the separation distance is h/d=1.5 and for nozzle-pressure-ratio (NPR) between NPR=4 and 5, where h is the distance between the nozzle exit and flat-wall and d is the jet diameter. It has to be highlighted that the observed behaviour is unlike the more minor positional fluctuations typically associated with supersonic impinging jets. To reveal further details on the unsteadiness of the standoff shock, color schlieren results will be presented here to shed more light on the flow phenomenon, while initial modelling efforts to reproduce these standoff shock behaviour will be elaborated. Present results reinforce the notion that the large standoff shock fluctuations are due to the standoff shock trying to achieve pressure equilibrium through abrupt displacements when variation in the NPR changes the location of the shock reflection point. Considering the small separation distance used, small NPR changes are able to trigger abrupt standoff shock unsteadiness.

The underexpanded jet is well known as one of the supersonic jets, and it is formed when the pressure ratio across a convergent nozzle from which the jet is exhausted is more than the critical value. The underexpanded jet has a characteristic barrel-shaped cellular structure. When the underexpanded jet impinges on a flat plate, a detached shock wave called “plate shock” is formed in front of the flat plate. Such flow fields are utilized in industrial applications such as laser processing and tempered glass manufacturing processes. The impinging jet oscillates and induces the high frequency sound called “screech tone”. The present study focused on the screech tone generated from the underexpanded jet impinging on the flat plate and aimed to examine a relation between the high-frequency sound and the jet structure. In the experiment, the jet impinged on a flat plate or a mesh, and the flow characteristics of each case were compared. The pressure ratio was kept constant at 3.0 and the distance between the nozzle and the object being impinged by the jet was changed. Visualization of the flow field and acoustic measurement were carried out under each condition. As a result, integral multiples of the fundamental frequency in the impingement on the mesh agreed with the frequency of the screech tone generated in the impingement on the flat plate. Since the screech tone frequency is based on the feedback loop between the nozzle and the flat plate, this agreement indicates the screech tone is caused by the agreement of the harmonic frequency of the feedback loop between the nozzle and the plate shock with that between the nozzle and the flat plate.

A large eddy simulation of a highly underexpanded supersonic jet impinging on a large plate is reported. The Favre filtered compressible Navier–Stokes equations in cylindrical coordinates have been solved by using a finite difference method. The large eddy simulation code employs a fifth-order weighted essentially nonoscillatory scheme known asWENO for the convective fluxes, a fourth-order centered difference approach for the viscous fluxes, and a four-stage third-order strong-stability-preserving Runge–Kutta technique in time. The numerical simulation successfully captures the shockwave structure in the jet plume and vortex structureswith different scales in the inner and outer shear layers. The evolution of annular shock waves and the vortex in the inner shear layer has been observed. Under the impact of the inner and outer shear layers, the interlacing vortex structures in the wall jet are formed. From this simulation, it can be concluded that the generation and disappearance of the annular shock arise from the actions of theMach disk, the oblique shock wave, vortices in the inner shear layer, and the impinging plate, all combined.

Multiple jet impingement is a widely implemented convective process for enhancing heat transfer over target surfaces. Depending on the engineering application, the impinging plate can have different configurations. However, the increased complexity of the surface induces complicated thermal behaviors that must be analyzed. In that sense, this study consisted of the experimental and numerical analysis of multiple jets impinging on a step surface. A particle image velocimetry technique was applied to measure velocity fields, while a heat flux sensor was mounted on the surface to determine the heat transfer. Numerical simulations, for both flat and non-flat plates, were conducted in ANSYS FLUENT applying the SST k-ω model, and experimental results were used to validate the model. Three surface configurations were analyzed, flat, 1 D, and 2 D steps, and the results show an increase in the average Nusselt number compared with the flat plate, 9% and 20%, respectively. This increase was mainly due to the intensification of the flow turbulence induced by the step. Numerical results were in good agreement with the experiments, but the heat transfer was slightly underpredicted for the 2 D step case due to the difficulty of predicting with accuracy the velocity field near the step.

The flow field associated with a jet impinging onto a surface at an inclined angle is investigated using pressure-sensitive paint (PSP) and particle image velocimetry. The PSP yields continuous measurements of pressure on the jet impingement surface. The jet footprint on the impingement surface is visualized using the half-maximum pressure contour. The results indicate that the impingement angle of the jet is the dominant parameter in determining the footprint of the jet on the impingement surface. This contour is similar in shape to an ellipse that is created by projecting the nozzle through the impingement surface. The ellipse is centered at the location of maximum pressure and the width of the minor axis is just over one jet diameter. The location of maximum pressure is found upstream of the geometric impingement point and this location is a strong function of the impingement angle. A curve fit for the location of maximum pressure can be constructed using an exact solution of the Navier–Stokes equations for a non-orthogonal stagnation flow. The maximum value of pressure is a function of impingement angle and varies as the sine of the impingement angle squared; the maximum pressure is also a function of jet impingement distance. Using these results, a simple procedure for predicting the overall structure of the jet on the impingement surface is presented.

Heat transfer characteristics of natural gas/air swirling flame impinging on a flat surface

Experimental investigation of heat transfer characteristics and wall pressure distribution of swirling coaxial confined impinging air jets

Impinging jets on the curves surfaces and flat plate are frequently used in industrial practice requiring studying the subatmospheric zone. It is important to understand the structure of behavior impinging jet on the flow fields. Same of prior researchers have concluded that a recirculation zone occurs on the impingement flat plate. This recirculation zone creates subatmospheric pressure at the impingement surface. This study has been made to investigate the effects of nozzle to plate spacing, the Reynolds number and changing of the surface shape on subatmospheric region which occurs on these impingement surfaces. The subatmospheric region could be very important in engineering application. It is hoped that this study will provide data for the further research on this subject. In the present study the air jets from convergent circular nozzle of 10-mm exit diameter have been allowed to impinge normally on a flat plate, hemispherical, and conical surfaces. Experiments were done with varying the stag...

Present study simulates oscillatory supersonic impinging jet flows using an axisymmetric Navier -Stokes code. We focus on the staging behavior of the oscillation frequency of the impinging surface pressure associated with the variation of the nozzle -to -plate distance and nozzle pressure ratio. These two distinct staging behaviors of the oscillat ion frequency are found to be well correlated when the frequency and the distance are normalized by the length of the first shock cell. We show that the staging behavior is in line with the changing pattern of large coherent structures of the jet shear lay er . have clearly established that the self -sustained, highly unsteady behavior o f the jet and the resulting impinging tones is governed by a feedback mechanism. The energy of the feedback loop is provided by the instability waves in the shear layer of the jet. This instability wave usually generates large -scale vortical structures in the shear layer. The feedback loop is the coupling between the downstream -traveling instability waves (coherent structures) and the upstream -propagating acoustic waves. It has been reported that the frequencies of the dominant peak in both of sound and pre ssure signals of a supersonic impinging jet coincide. It is also well known that the frequencies of the impinging tone and the surface pressure oscillation exhibit staging behavior as the distance from the nozzle exit to the plate gradually increases. 1,4 This staging behavior for moderately underexpanded jets is typical of the class where disturbances from the jet interaction feed back to the nozzle, and so cause periodic self -induced oscillations. To predict the frequency and amplitude of the tones, many t heoretical modeling by using linear stability analysis have been attempted. However, the amplitude prediction and staging feature of the frequencies still remains elusive. 1-4

The effect of jet location and duty cycle on the fluid mechanics of an unconfined free jet and its heat transfer on an impinging plate

Heat transfer performance of a single cylindrical orthogonal jet impinging on a flat plate was obtained through steady-state infrared (IR) thermometry. One Reynolds (Re) number of 23,000 based on pipe exit diameter was considered. The distance of the jet exit plane from the shim varied from two to ten times the impingement jet diameter in increments of two diameters. The observed temperature and constant heat flux boundary condition allowed for the calculation of a Nusselt (Nu) number distribution to estimate the heat transfer performance of the impingement jet. At the smallest separation distance of two diameters, the relative maximum heat transfer performance is found at the stagnation point followed by a second peak occurring at a radial distance of approximately two diameters from the stagnation point. Compared to all jet exit separation distances studied, the distance of six diameters exhibited the greatest magnitude Nusselt number at the stagnation point. A paired fluids experiment using Magnetic Resonance Velocimetry (MRV) techniques collected hydrodynamic data of a single impinging jet at a matching Re number of 23,000 and jet exit plane distances. This work provides relevant data for correlation of impingement cooling design through unique analysis of the fluid mechanics and heat transfer characteristics of a single impinging jet. Measurement uncertainty was assessed to range from +/−1% to +/−10% for Nusselt number and +/−7% for velocity.

Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets

The broadband and discrete sources of sound in small propeller and centrifugal type cooling fans were investigated to understand the turbulent vortex structures from multi-bladed fans using an ANSI test plenum for small air-moving devices (AMDs). The noise measurement method uses the plenum as a test apparatus to determine the acoustic source spectral density function based on acoustic similarity laws at each operating condition similar to real engineering applications. The characteristics of fans such as the head rise vs. volumetric flow rate performance were measured using a performance test facility. The sound power spectrum is decomposed into two non-dimensional functions: an acoustic source spectral distribution function F(St, ϕ) and an acoustic system response function G(He, ϕ). Here St, He, and ϕ are the Strouhal number, the Helmholtz number, and the volumetric flow rate coefficient, respectively. The autospectra of radiated noise measured for the fan operating at several volumetric flow rates, ϕ, are analyzed using acoustical similarity. The rotating stall in a small propeller fan with a bell-mouth guide is mainly due to a leading edge separation. It produces a blockage in the passage and reduction in flow rate. The sound power levels were measured with respect to the rotational speed to reveal the mechanisms of stall and/or surge for different loading conditions and geometries, for example, fans installed with an impinging plate. Lee and Meecham (1993) studied the effect of large-scale motions, like impinging normally on a flat plate, using Large-Eddy Simulation (LES) and Lighthill’s analogy. Using cross-correlations of the hot-wire and microphone signals, the dipole and quadrupole sources in the fans tested are shown to be closely related to vortex structures.

: The flow interaction effects from a jet issuing into a supersonic crossflow were investigated computationally for the case of a flat plate and a generic fin-stabilized projectile. For both configurations, simulations were performed at several Mach numbers and jet total to freestream static pressure ratios (PRs). In the flat plate case, the jet force was generally amplified, with a strong dependence on PR and the freestream Mach number. In the projectile configuration, seven jet locations along the missile axis were investigated at three supersonic crossflow Mach numbers and five angles of attack in the range 10 deg /= a /= 10 deg. The jet force was generally attenuated, unless the jet was located very close to the tail fins. In the latter case, this results from (1) a combination of little or no projectile surface area for the detrimental jet interaction effects to act on, and (2) the high pressures developed on the fin surfaces. The choice of turbulence model was found to affect the local pressure distribution due to the flow interaction, but the jet interaction force parameters varied less than 15% and 6% for the flat plate and missile configurations, respectively. The effect of angle of attack on jet interaction forces was more prevalent as a became more negative and the counter-rotating vortex pair in the jet plume was pushed closer to the tail fins. Flight simulations using reaction jet squibs were performed to evaluate the sensitivity of the control maneuver to the differences in jet thrust and jet actuation location. These results showed that significant differences in the projectile maneuver control were obtained if the effective jet thrust acting at the effective jet location were used instead of the ideal jet thrust acting at the jet exit location.

An experimental study of a turbulent jet impinging on a flat surface