Jet Mixing Layer Research Articles

Jet centrifugal pump internal flow numerical simulation and structural optimization

Abstract Jet centrifugal pump is essential equipment in multiple systems such as agricultural irrigation and energy production. This paper investigates the internal flow characteristics of jet centrifugal pumps at different flow rates and the cavitation behavior under high flow conditions by numerical simulation methods. Additionally, two performance optimization strategies are proposed based on simulation results. The findings indicate that high-velocity regions and low-pressure zones exist within the nozzle. As flow rate increases, the outlet velocity of the nozzle gradually decreases, and large areas of low pressure are observed near the inlet of the impeller. Cavitation occurs to varying degrees at flow rates of 4.056 m3/h and 3.840 m3/h. The turbulent kinetic energy contours for these conditions reveal uneven distribution within the jet nozzle, with higher values near the jet mixing layer, inner wall of the jet nozzle and impeller inlet. Structural optimization using hemispherical bolts significantly reduces the turbulent kinetic energy around the bolts, resulting in an increase in head ranging from 1.68% to 6.70% across various flow rates. Shortening the clearance between the impeller and the rear cover reduces energy losses, leading to an increase in head ranging from 2.72% to 5.86%.

Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.

Numerical simulation study of the injection characteristics affecting trans-/supercritical sprays based on orthogonal experiment design

A mixture of numerical models and orthogonal experiments was used to better understand the mixing behavior and heat/mass transfer in jets operating under supercritical conditions. The goal is to investigate the effects of injector diameter (Dinj), ambient pressure (Pamb), injection temperature (Tinj), injection velocity (Vinj), and ambient temperature (Tamb) on jet evolution. The findings highlight that, among these parameters, Tinj and Dinj exert the most substantial impact on jet mixing characteristics. Higher Tinj and Dinj enhance the jet spreading angle (θhsa) and mixing layer thickness (δ) for improved mixing efficiency. Increasing Tinj and reducing Dinj effectively shorten the potential core length (Lp). Moreover, higher Tinj enhances the temperature rise rate (Tr), effectively raising fuel temperature over a shorter distance. The velocity decay rate (Vd) increases with Vinj and Tinj. Furthermore, this study explores the effects of injection density ratio on evaluation indexes (θhsa, Lp, Vd, Tr and δ) for jet mixing efficiency and trends of jet structural development (Lp, turbulent chaos length (CL) and self-similarity length (SL)). These findings are important for understanding supercritical spray development, guiding engine chamber design and setting operational conditions based on spray behavior.

Simulation of supersonic jet flow past a blunt body in a laboratory experiment using computer vision

The complex flow of a supersonic jet (Mach number 2.4–2.5) around the blunt aerodynamic body (consisting of a cylinder with a diameter of 16 mm and a truncated cone) was studied using the high-speed shadowgraph technique and computer vision-based visual data processing. The problem considered is directly related to the rocket and space systems stages safe separation problem in a sufficiently dense atmosphere. For systems with a sequential arrangement of stages, their separation often occurs under the rocket engine jet action. The occurrence of lateral forces in violation of axial symmetry can lead to overturning of the discarded stage and undesirable interaction with the outgoing stage. The goals of the paper were to study the pulsation characteristics of the incoming supersonic jet streamlining the body, as well as the pulsations of the bow shock wave in both axisymmetric and asymmetric flows. The recording frame rate in the experiments was up to 775 000 fps. Computer vision and various machine learning approaches were used to process a large number of shadowgraph images. Canny edge detection and Hough transform, as well as a convolutional neural network (CNN) based on YOLOv2 architecture, were used to automatically track the position of the bow shock relative to the body. Jet sound generation was studied - the amplitude dependence on the distance from the nozzle and the evolution in time. The position of the bow shock relative to the streamlined body and flow pulsations over time were analyzed. Bow shock pulsation spectra at zero angle of attack were compared to those at a small angle of attack (α = 2.5°–3°). The power spectral density was also estimated. The slope of the spectra for the zero angle of attack case was found to be horizontal and the spectra for the small angle of attack case had a slope close to −5/3. The strongest of the measured bow shock frequencies were found to be close to those of the jet mixing layer pulsations. The hardware signal spectrum was also measured and this signal can be considered as noise. The described approach makes it possible to obtain the physical characteristics of the flow at any point of interest. It was also shown that the automation of flow visualization experiments using computer vision techniques allows to increase the speed of data processing and obtaining new physical information, which may be important for engineers developing aircraft and spacecraft technologies.

Active Control of Jet–Wing Interaction Noise Using Plasma Actuators in a Narrow Frequency Band

For a jet located near the wing, instability waves present in jet mixing layer scatter on the wing trailing edge, leading to additional noise generation. In this article, the possibility of reducing this noise by active cancellation of natural instability waves has been experimentally studied. The active control system consists of near-field microphones from which the feedback signal is taken, a data processing module for signal filtering, and a high-frequency dielectric barrier discharge plasma actuator. Decrease in pressure perturbations in the near field of a turbulent jet associated with axisymmetric instability waves in a narrow frequency band was demonstrated. It was shown that this reduction leads to a corresponding noise decrease in the far field.

Study on the relationship between the length of mixing layer and the characteristic scale of mixing layer in multi-strut ejector

The flow structure of the multi-strut ejector based on jet segmentation has an obvious relationship with the transverse characteristic scale of the jet. Therefore, this paper studies the relationship between jet transverse characteristic size and mixing layer. Through the simulation of multi-strut ejectors with different number of struts, the development of mixing layer in the mixing process of jets with different transverse characteristic sizes is compared under the same boundary conditions. It is concluded that when there is no obvious coupling effect between the mixing layer and the shock & expansion wave, there is a linear relationship between the transverse characteristic scale of the jet and the mixing length of the flow direction, and the jet mixing with different transverse characteristic scales is consistent. When the shockwaves and expansion waves are strongly coupled with the mixing layer, the linear relationship between the jet transverse characteristic scale and the mixing layer length is not strong.

Computational Analysis of Thermal Striping in Primary Sodium System of Liquid Metal Fast Breeder Reactor Using Finite Volume Method

Thermal striping is associated with random fluctuations of temperature that occur at the nonisothermal jet stream interface or across thermally stratified fluid layers due to the high heat transfer coefficient of liquid sodium flow. The temperature fluctuations in the jet mixing or stratified layer regions are transmitted to the adjoining structures after minimal attenuation in a Liquid Metal Fast Breeder Reactor (LMFBR). In turn, the adjoining structure may experience high cycle fatigue and catastrophic failure caused by crack propagation. Investigations have been carried out in detail numerically, and frequency and amplitude of temperature fluctuations in 500-MW(electric) pool-type fast reactor [Prototype Fast Breeder Reactor (PFBR)] structures for practical applications have been observed. The investigations consist of numerical simulations at two levels. First, a published benchmark experiment is analyzed, and then, a suitable computational fluid dynamics (CFD) model is identified for simulating the thermal striping phenomenon numerically. After that, detailed flow and temperature fluctuations are predicted in the reactor structures by analysis carried out based on the CFD model. The values of the temperature fluctuations predicted are found to be within acceptable limits, as required by structural mechanics considerations in the study.

The Relationship between Contraction of the Ejector Mixing Chamber and Supersonic Jet Mixing Layer Development

Supersonic mixing layer development seriously impacts on the performance of an ejector, and the effect of mixing chamber contraction angle on supersonic jet mixing has been poorly studied. Numerical simulations are applied to investigate the effect of the mixing chamber contraction angle (φ) on the performance of a central ejector and supersonic mixing layer development pattern. The main findings of this study are as follows: the non-mixed length (l) is reduced by 22.12% when the mixing chamber contraction angle (φ) increases from 2° to 6°. Meanwhile, the secondary stream mass flow rate (ms) is reduced by 35.02%, and the total pressure loss is decreased by 18.37% at the outlet. l is positively correlated with ms and negatively correlated with the mixing layer thickness (σ). The mixing layer thickness (σ) grows highly linearly before the secondary flow is covered completely. The pressurization (P0δ/P0s) performance of the mixing layer will be progressively weaker than the total pressure loss because of the complex shock structure.

Generation of acoustic tones in round jets at a Mach number of 0.9 impinging on a plate with and without a hole

The generation of acoustic tones in four round jets at a Mach number of 0.9 impinging on a plate at a distance $L=6r_0$ from the nozzle exit, where $r_0$ is the nozzle radius, has been investigated by large-eddy simulation. Three plates are perforated by holes of diameters $h=2r_0$ , $3r_0$ and $4.4r_0$ , centred on the jet axis, whereas the fourth plate has no hole, in order to study the effects of the hole on the jet flow and acoustic fields. In all cases, acoustic tones emerge in the jet near field, upstream but also downstream of the plate for the perforated plates. Their frequencies are similar for all jets and close to those predicted for aeroacoustic feedback loops establishing between the nozzle and the plate, involving flow disturbances convected downstream and waves propagating upstream at the ambient speed of sound. Their levels, however, decrease with the hole diameter, by a few dB for $h\leqslant 3r_0$ but approximately by 40 dB for $h=4.4r_0$ . The features of the feedback loops are identified by computing two-dimensional space–time correlations and frequency–wavenumber spectra of the pressure fluctuations in the jet mixing layers. These loops are found to be closed by free-stream upstream-propagating guided jet waves, produced by the impingement of vortical structures on the plate for $h\leqslant 3r_0$ and by the scattering of the jet aerodynamic pressure fluctuations by the hole edges for $h=4.4r_0$ . Finally, an acoustic analogy is used to show that the contributions of the pressure fluctuations on the plate to the radiated noise are dominant.

Measurement of surface heat transfer caused by interaction of sonic jet and supersonic crossflow near injection hole

This paper investigates the surface heat transfer caused by interaction of a jet and a supersonic crossflow near the jet injection hole. A sonic jet with different momentum ratios (J=0.510, 1.018, 1.477) was injected perpendicularly into a crossflow with a Mach number of 3.0 in a supersonic wind tunnel. Surface temperature through time measured by infrared thermography was used to deduce surface heat flux. In addition, heat transfer coefficients and adiabatic wall temperatures were derived from time histories of surface heat flux and temperature. In order to consider an effect of conduction from the inner hole surface, a three-dimensional energy conservation is considered in the deduction process of the heat flux. As a result, the characteristics of the heat transfer near the hole and the change in the heat transfer with momentum ratios are presented. The separation vortex and recirculation vortex are found to be dominant flow features in terms of the augmentation of the heat transfer. The maximum heat transfer is observed at the immediate vicinity of the hole due to the flow oscillation from a jet-mixing layer. This oscillation resulted in a 390% of augmentation of the heat transfer near the hole compared to the freestream even at the lowest momentum ratio. Also, the augmentation near the hole is more susceptible to change of momentum ratio compared to the augmentation on the overall interaction area.

Experimental investigation of the fluctuating static pressure in a subsonic axisymmetric jet

Measuring the fluctuating static pressure within a jet has the potential to depict in-flow sources of the jet noise. In this work, the fluctuating static pressure of a subsonic axisymmetric jet was experimentally investigated using a 1/8” microphone with an aerodynamically shaped nose cone. The power spectra of the fluctuating pressure are found to follow the -7/3 scaling law at the jet centerline with the decay rate varying as the probe approaches the acoustic near field. Profiles of skewness and kurtosis reveal strong intermittency inside the jet shear layer. By applying a continuous wavelet transform (CWT), time-localized footprints of the acoustic sources were detected from the pressure fluctuations. To decompose the fluctuating pressure into the hydrodynamic component and its acoustic counterpart, two techniques based on the CWT are adopted. In the first method the hydrodynamic pressure is isolated by maximizing the correlation with the synchronously measured turbulent velocity, while the second method originates from the Gaussian nature of the acoustic pressure where the separation threshold is determined empirically. Similar results are obtained from both separation techniques, and each pressure component dominates a certain frequency band compared to the global spectrum. Furthermore, cross-spectra between the fluctuating pressure and the turbulent velocity were calculated, and spectral peaks appearing around Strouhal number of 0.4 are indicative of the footprint of the convecting coherent structures inside the jet mixing layer.

Numerical study on the mixing characteristics under transcritical and supercritical injection using large eddy simulation

Large eddy simulations of cryogenic nitrogen injection are performed on both transcritical and supercritical injection and mainly attentions are focus on the jet disintegration mechanism and mixing layer feature. The simulation results reveal that the thermal disintegration mechanical dominates the disintegration characteristic under supercritical conditions. The jet disintegration is delayed and longer dense core region is detected for the transcritical injection due to the large density gradient effects. Because of this disintegration mechanism, the Reynolds stresses in the transcritical case are significantly suppressed in the turbulent fluctuation. In addition, we define a mixing layer based on the density gradient and thicker mixing layer interfaces are formed in the supercritical case. The relationship between transport properties and the large density gradient are also investigated, results indicate that the large density gradients are influenced by the pseudo critical temperature and transport properties. Key words:

Intermittent event evaluation through a multifractal approach for variable density jets

Variable-density jets occur in many systems, including geophysical flows and industrial applications, exhibiting a large range of scales of Reynolds and Richardson numbers. A series of jets with varying densities was ejected vertically into a large ambient region. Using particle image velocimetry, the near-exit velocity fields were measured for three different gases exhausting into air: helium, air and argon. Experiments considered relatively low Reynolds numbers from approximately 1500 to 5500 with Richardson numbers near 0.001 in magnitude. These included a variety of flow responses, notably nearly laminar, turbulent and transitioning jet flows. Flows were examined through a multifractal framework, and the singularity spectrum showed the characteristics of the flow based on the evolution in the streamwise and wall-normal direction. The variation of the Hölder exponent displayed the asymmetry and intermittency of the flow. Similar to the Reynolds shear stress, the development of intermittent behavior is a function of downstream location with respect to changes in the Reynolds number. The density of the exiting jet also influences the location of high intermittency within the flow signal. Lower density jets provide increased variability of the signal within the ambient air and the shear layer close to the exit of the jet. Specifically, the highest degree of multifractality is observed within the mixing layer of the helium jet at a transitioning Reynolds number. Conditional averaging with respect to the fluctuating velocity components and the pointwise Hölder exponent reveals high velocity-intermittency interactions at the inside of the jet mixing layer when fluid is entrained and at the turbulent/non-turbulent interface when fluid is ejected. Finally, line integral convolution illustrates the impact of turbulent/non-turbulent interface on the jet dynamics.

Direct numerical simulation of an evaporating turbulent diluted jet-spray at moderate reynolds number

The evaporation of liquid droplets dispersed in jet-sprays is involved in many industrial applications and natural phenomena. Despite the relevance of the problem, a satisfactory comprehension of the mechanisms that regulate the process has still not been achieved. Indeed, a wide range of turbulent scales and an enormous number of droplets are typically involved, causing the numerical and theoretical tackling of the problem to be challenging. In this context, we address the Direct Numerical Simulation (DNS) of a turbulent jet-spray at a bulk Reynolds number Re=10,000. We focus on the effect of the jet Reynolds number on the evaporation process and the preferential segregation of droplets. The analysis is conducted by comparing the outcomes of the present DNS with that from a lower Reynolds number one in corresponding conditions, Re=6,000. The problem is addressed by employing the point-droplet approximation in the hybrid Eulerian-Lagrangian framework. We present detailed statistical analyses of both the gas and the dispersed phases. We found that the mean droplet vaporization length grows as the bulk Reynolds number is increased from Re=6,000 to Re=10,000, while keeping all the remaining conditions fixed. We attribute this result to the complex interaction between the inertia of the droplets and the dynamics of the turbulent gaseous phase. In particular, at the higher Reynolds number, the effect of the faster turbulent fluctuations of the jet mixing layer, which tend to fasten the process, is compensated by the slower mass transfer from the liquid droplets to the surrounding environment. We also observe intensive preferential segregation of the dispersed phase that originates from the entrainment of dry environmental air into the mixing layer and is intensified by small-scale clustering in the far-field region. We show how the preferential segregation is responsible for a strongly heterogeneous Lagrangian evolution of the dispersed droplets. All these aspects contribute to the Reynolds-number dependence of the overall droplet evaporation rate. Finally, we discuss the accuracy of the d-square law, often used in spray modelling, for present cases. We found that using this law based on environmental conditions leads to a relevant overestimation of the droplet evaporation rate.

Simulation of the Self-Ignition of a Cold Premixed Ethylene-Air Jet in Hot Vitiated Crossflow

The aim of this paper is to analyze the self-ignition of a jet flame in hot vitiated cross flow using Large Eddy Simulation with analytically reduced chemistry. A rich premixed ethylene-air mixture (phi = 1.2) at 300 K is injected into a hot vitiated crossflow at 1500 K. The simulated reacting flow steady-state was validated against experiments in previous publications and the focus of the present work is the transient self-ignition of the jet. It is shown that spontaneous ignition occurs at very lean mixture fractions in the form of reacting patches in the windward jet mixing layer. These patches grow, laterally wrap the jet and extend into the recirculation region. Chemical explosive mode analysis is performed to identify the chemically active regions that are precursors of the patches undergoing spontaneous ignition. It is shown that the self-ignition occurs at very lean fuel concentrations regions, which are leaner and hotter than the most reactive mixture fraction of the jet and crossflow. This is explained by the fact that the scalar dissipation is significantly lower in these very lean regions. Ultimately, the peak heat release moves toward the richer regions and an autoignition cascade governs the steady state flame anchoring.

Experimental Investigation of the Nozzle Roughness Effect on the Flow Parameters in the Mixing Layer of an Axisymmetric Subsonic High-Velocity Jet

The paper presents the results of an experimental investigation of the mixing layer parameters in the initial regions of transonic jets issuing from convergent nozzles with different internal roughness at the same Mach number in the jet core. It is shown that the mixing layer characteristics, namely, the radial profiles of the measured total pressure and the r.m.s. total pressure fluctuations, are self-similar in nature at distances greater than 1.5 of the nozzle exit diameter from the exit. The possibility of applicating nozzles with an increased degree of the internal surface roughness in investigating high-velocity jet flows is shown.

Investigation on the Source Locations of Axisymmetric Screech Tones Utilizing Data from Numerical Simulation

The source locations of axisymmetric modes of screech tones are numerically investigated. Fourth-order optimized compact scheme and fourth-order Runge–Kutta method are used to solve the 2-D axisymmetric Euler equations. The screech tone is successfully reproduced, and the change in wavelength with respect to jet Mach number shows good agreement with the experimental data. At various low supersonic jet Mach numbers, the time-averaged contours of Mach number and root-mean-square pressure are investigated to identify the location of maximum interaction between shock cell structures and vortices. The source locations of two axisymmetric modes, A1 and A2 modes, are distinctly visualized and identified; the screech tones of A1 mode are generated at the apex of fifth shock cell, and the screech tones of A2 mode are generated at the apex of fourth shock cell. Based on the observation, a simple formula for the prediction of axisymmetric modes of screech tones is proposed. The formula is derived based on a form of Rossiter equation, with the assumption of different convection speeds along the jet mixing layer. The proposed formula successfully estimates the frequency of two axisymmetric modes of screech tones, which verifies that the identified source locations of the axisymmetric screech tones are reasonable.

Laminar-turbulent transition in the near field of the jet at the instability regime of the jet source

The paper presents the results of an experimental study of the hydrodynamics of jets flowing from long tubes at low Reynolds numbers. The main attention in the research is given to the mechanism of interaction between pipe and jet instability, which results in a vortical motion in several spatial regions. In the experiments we used Hilbert-visualization, high-speed visualization, and measurements with a hot-wire anemometer. The subsonic gas jet flows into the air space from a long tube with a diameter of 2, 3.2, and 5 mm within the Reynolds number range of 200-6700. Air and Freon-22 were used as the working gases. The critical pipe Reynolds numbers are characterized by the mechanism of a two-stage instability caused by the formation of turbulent spots (puff) inside the tube and generation of vortex structures in the jet mixing layer. These organized structures (puff) exert a strong influence on the free jet flow destroying the laminar flow part. The obtained data make it possible to consider in detail the evolution of turbulent spots along the distance downstream, while the spots are generated in a cylindrical tube due to the laminar-turbulent transition.

Shock-Wave Structure of Supersonic Jet Flows

In the present paper, we give a brief overview of the studies of supersonic jet flows which were performed recently with the aim of gaining experimental data on the formation of the shock-wave structure and jet mixing layer in such flows. Considerable attention is paid to a detailed description of discharge conditions for supersonic jets, to enable the use of measured data for making a comparison with numerical calculations. Data on the 3D flow structure in the mixing layer of the initial length of a supersonic jet are reported. Scientific interest in this phenomenon is due to its practical significance in studying the possibility of intensifying the mixing process as well as in studying the sound-generation process.

Grid sensitivity of flow field and noise of high-Reynolds-number jets computed by large-eddy simulation

Three isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 105 are computed by large-eddy simulation using four different meshes in order to investigate the grid sensitivity of the jet flow field and noise. The jets correspond to two initially fully laminar jets and one initially strongly disturbed jet considered in previous numerical studies. At the exit of a pipe nozzle of radius r0, they exhibit laminar boundary-layer mean-velocity profiles of thickness [Formula: see text] and [Formula: see text], respectively. For the third jet, a peak turbulence intensity close to 9% is also imposed by forcing the boundary layer in the nozzle. The grids contain up to one billion points, and, compared to the grids used in previous simulations, they are finer in the axial direction downstream of the nozzle and in the radial direction on the jet axis and in the outer region of the mixing layers. The main flow field and noise characteristics given by the simulations, including the mixing-layer thickness, the centerline mean velocity, the turbulence intensities on the nozzle lip line and the jet axis, spectra of velocity and far-field pressure obtained from the jet near field by solving the isentropic linearized Euler equations, are presented. With respect to those from previous studies, the results are very similar for the initially laminar jet with thick boundary layers, but they differ significantly for the initially laminar jet with thin boundary layers and for the initially disturbed jet. For the latter two jets, using a finer grid leads to a faster flow development, to higher turbulence intensities in the shear layers and at the end of the potential core, to stronger large-scale structures, and to the generation of more low-frequency noise. Moreover, very small mesh spacings appear to be necessary all along the jet mixing layers, and in particular during their early stages of growth, to properly capture the formation and dynamics of the flow coherent structures and thus obtain results in good agreement with measurements available for high-Reynolds-number jets.