Flow Structure Of Jets Research Articles

Analysis of the unsteady flow characteristics of underwater supersonic gaseous jets

To examine the unsteady flow characteristics of underwater supersonic gaseous jets under different jet expansion conditions, a sophisticated numerical model is created. This model accurately predicts the intricate multiphase flow by considering the compressibility of the jet gas and energy exchange, which is then rigorously validated against experimental data. The development process of underwater supersonic gaseous jets displays notably unsteady features in terms of jet morphology, flow structure, and various flow field parameters when compared to atmospheric conditions. The unsteady phenomena, such as necking, breaking, bulging, and back-attack, are observed alongside significant pressure pulsations. These unsteady phenomena occur at a considerable distance from the nozzle exit under under-expanded conditions, while pressure pulsations do not impact the internal gas flow within the nozzle. However, under full-expanded and over-expanded conditions, unsteady phenomena near the nozzle exit lead to oscillatory pressure, causing shock waves to propagate inside the nozzle. This results in a notable increase in internal pressure pulsation and mass flow rate within the nozzle, ultimately affecting engine performance significantly.

A Numerical Approach and Study of the Shock-Wave Structure of Supersonic Jet Flow in a Nozzle

A Numerical Approach and Study of the Shock-Wave Structure of Supersonic Jet Flow in a Nozzle

Effect of Jet-to-Target Spacing on Flow and Heat Transfer Characteristics for Swirling Impinging Micro-Slot Jets

The influence of jet-to-target spacing on flow and heat transfer behaviors for swirling impinging micro-slot jets has been extensively studied. In this study, two complete revolutions of the generated helix under the slot nozzle thickness of 1.0 mm constrained the total spiraling length to 290 mm. The range of jet-to-target spacing varied from 1 to 6. The turbulent kinetic energy-epsilon model was employed to make predictions at Reynolds numbers from 5000 to 20000. The results showed that the heat transfer enhancement on the target wall depended on the pacing between the jet and the target. This is the jet flow structure exhibited a notable intensity of high-velocity vortices at the low jet-to-target spacing, whereas the jet flow impingement was abruptly reduced at the high jet-to-target spacing. The heat transfer rate also tended to increase as the Reynolds number increased. In addition, when the jet-to-target spacing was at 1, the average heat transfer value was between 32.9% and 54.2% higher than in other conditions due to high fluid acceleration.

A numerical study on the effect of rib size and location on thermo-fluid performance of synthetic jet

A two-dimensional, axisymmetric numerical model is used to study the effect of synthetic jet flow structure and thermal performance, validated with experimental data. The Reynolds number is set at 5473, and orifice-to-surface spacing (z/d) varies (2, 6, 10). Three square cross-section ribs at different radial positions are analyzed. Results show that ribs reduce heat transfer at the stagnation point, with a maximum decrease of 74.37% at z/d=10 for a 5mm×5mm rib at r/d=0.5 and a minimum of 47.70% at z/d=6 for the same rib. Heat transfer increases near the rib's leading edge but drops before and after the rib. No area-averaged heat transfer improvement is observed, with a maximum reduction of 69.24% at z/d=10 for a rib at r/d=2 and a minimum of 2.43% at z/d=6 for a rib at r/d=1. Stagnation pressure increases by 24.42% at z/d=2 and decreases by 15.66% at z/d=10 due to a rib at r/d=2.

Effect of U-shaped notches on nozzle performance and jet flow structures

In nacelle design, optimization of nozzle shape is crucial for flow control. To understand the effect of nozzle shape on its aerodynamic performance and jet flow structures, the numerical experiment is performed using the dual separate flow reference (DSFR) nozzle released by the Aero Systems Engineering (ASE) FluidDyne Laboratory. A pair of U-shaped notches are made on the nozzle. The results show that the discharge coefficient and thrust angle increase with the size of the notches in all the simulated operating conditions. The notches induce a pair of streamwise vortices, which enhance turbulent mixing and decrease turbulent kinetic energy in the mixing layer.

Experimental and numerical investigation to elucidate the fluid flow through packed beds with structured particle packings

The present paper presents an experimental and numerical investigation of the dispersion of the gaseous jet flow and co-flow for the simple unit cell (SUC) and body-centred cubic (BCC) configuration of particles in packed beds. The experimental setup is built in such a way that suitable and simplified boundary conditions are imposed for the corresponding numerical framework, so the simulations can be done under very similar conditions as the experiments. Accordingly, a porous plate is used for the co-flow to achieve the uniform velocity and the fully developed flow is ensured for the jet flow. The SUC and BCC particle beds consist of 3D-printed spheres, and the non-isotropy near the walls is mostly eliminated by placing half-spheres at the channel walls. The flow velocities are analysed directly at the exit of the particle bed for both beds over 36 pores for the SUC configuration and 60 pores for the BCC configuration, for particle Reynolds numbers of 200, 300, and 400. Stereo particle image velocimetry is experimentally arranged in such a way that the velocities over the entire region at the exit of the packed bed are obtained instantaneously. The numerical method consists of a state-of-the-art immersed boundary method with adaptive mesh refinement. The paper presents the pore jet structure and velocity field exiting from each pore for the SUC and BCC packed particle beds. The numerical and experimental studies show a good agreement for the SUC configuration for all flow velocities. For the BCC configuration, some differences can be observed in the pore jet flow structure between the simulations and the experiments, but the general flow velocity distribution shows a good overall agreement. The axial velocity is generally higher for the pores located near the centre of the packed bed than for the pores near the wall. In addition, the axial velocities are observed to increase near the peripheral pores of the packed bed. This behaviour is predominant for the BCC configuration as compared to the SUC configuration. The velocities near the peripheral pores can become even higher than those at the central pores for the BCC configuration. It is shown that both the experiments as well as the simulations can be used to study the complex fluid structures inside a packed bed reactor.

Kinematic analyses of wave–packet structures in non-isothermal jet flows: Effects of length scales

A kinematic wave–packet sound-source model is developed for non-isothermal jets based on large eddy simulation results of subsonic jets at temperature ratios 0.86, 1.0, and 2.7. To find the suitable variable for the sound-source model, coherent structures in these jets are extracted by leading modes of the proper orthogonal decomposition (POD), and they are classified according to spatial–temporal features. To extend the model, an approach is proposed to represent the growth and decay length scales separately by a single continuous function. Applying such function, the acoustic affections are discussed for the variable length scales of amplitude envelope, L, and coherence, Lc. The results show that the jet temperature desynchronizes the leading POD modes of radial velocity, pressure, and density, and the jet temperature changes the density mode from radial puffs into stripes or ridges. The axisymmetric component of the pressure clearly presents a train of radiant waves, as captured by its leading spectral-POD mode at the peak radiation frequency. Therefore, this pressure component is employed for modeling. In the wave-number domain, the elongation of L stretches the cross-spectral density (CSD) of the source signal, denoted by CSD(k1, k2), along the k1- and k2-axes; the decay of Lc stretches the CSD along the diagonal of the axes. Both of them tend to spread the CSD into a radiant region near the origin point, so as to enhance the radiation. The radiation seems insensitive to the variation of the L, as it only slightly distorts the CSD in the radiant region.

Examination of the Effect of Nozzle Geometry on Jet Flow Structure

Examination of the Effect of Nozzle Geometry on Jet Flow Structure

Numerical analysis and experimental validation of the jet impingement cooling of a turbine-blade leading edge at different rotation speeds

A detailed experimental and numerical validation studies are conducted on internal channel jet impingement cooling where seven jets impinging inside a rotating semi-cylindrical channel. The study objective is to mimic the cooling flow structure in gas turbine leading edge by using the rotating the semi-circular channel. These studies are conducted by considering a jet Reynolds number of 7500 at five different rotation speeds (ranging from 0 to 200 rpm). Numerical analysis is performed using the shear stress transport (SST) k−ω turbulence model with a properly analyzed fine mesh containing around eight million nodes. A test setup with required instrumentation is developed inhouse for this study. Two temperature measurement techniques, namely thermochromic liquid crystals (TLCs) and thermocouples, are adopted. Further, the target surface temperature contours are precisely analyzed by comparing the TLC temperature measurements with the numerical temperature results. The captured temperature contours indicated points of minimum-temperature regions, which corresponded to the jet impingement regions. By examining the temperature distribution along the axial centerline, a good agreement has been established between the numerical results and the experimental measurements. For a jet Reynolds number of 7500, increasing channel rotation speed from 0 to 250 rpm has reduced the variation in temperature between different jets. The size of inlet port used to guide flow to the feeding duct has a strong impact on jet formation and flow structure, and it has led to different mass flow rate across jets. Furthermore, a small deviation between numerical and experimental data can be observed near the end side of the channel owing to the radial and lateral heat transfer losses and outlet flow restriction.

Visualization investigation of jet ignition ammonia-methanol by an ignition chamber fueled H2

Ammonia is an ideal carbon-free fuel if the burning velocity problem can be successfully addressed. Blending ammonia with high-activity fuels and utilizing high-energy ignition are both suitable solutions for accelerating ammonia combustion. In this study, jet-controlled compound ignition (JCCI) by an ignition chamber fueled H2 was proposed to accelerate the premixed ammonia /methanol combustion. Visualization experiments were applied to evaluate the combustion performance. The effects of hydrogen energy substitution ratios (Ri) in the ignition chamber, methanol blend (Rm) in the main chamber, and orifice diameters (d) between the ignition chamber and main chamber on the combustion were investigated. By utilizing JCCI, it achieved a combustion duration of 85 ms, which is 47.7% shorter than spark ignition (SI) under an equivalent ratio of 1.0. Furthermore, JCCI produced excellent lean burn performance. At the equivalent ratio of 0.8 and 1.0 in the main chamber, JCCI model shortened the combustion duration by 20.9% and 52.2% respectively, compared with SI ignition. Additionally, it is necessary to adapt the appropriate hydrogen energy substitution ratio for different equivalent ratios of ammonia blends. For the equivalent ratio of 1.0 and 0.8 in the main chamber, the Ri = 1% and Ri = 2% achieved shorter jet delay and combustion duration, respectively. In addition, when the equivalent ratio was 1.0 and the methanol blend ratio Rm = 10%, Rm = 30% and Rm = 50%, the combustion duration reduced by 11.9%, 28.0%, and 42.1%, respectively, compared to Rm = 0%. This reduction resulted mainly from changes in the jet flow and flame structures due to the various hydrogen energy substitution ratios and methanol blend ratios. Furthermore, using a 3 mm orifice diameter, compared to a 6 mm orifice diameter, caused the combustion duration to decline by 63.4% and 54.3% for equivalent ratios of 1.0 and 0.8, respectively.

Search for mono-Higgs signals in bb¯ final states using deep neural networks

We study mono-Higgs signatures emerging in an illustrative new physics scenario involving Standard Model Higgs boson decays to bottom quark pairs using hybrid deep neural networks. We use a multilayer perceptron to analyze the kinematic observables and optimize the signal-to-background discrimination. The global color flow structure of hard jets emerging from the decay of heavy particles with different color charges is crucial to single out the mono-Higgs signature. Upon embedding the different color flow structures for signal and backgrounds into constructed images, we use a convolution neural network to analyze the latter. Specifically, the approach takes initially mono-type data as input, frittering away invaluable multisource and multiscale information. We then discuss a general architecture of hybrid deep neural networks that supports instead mixed input data. In comparison with single input deep neural networks, like multilayer perceptron or convolution neural networks, the hybrid deep neural networks provide higher capacity in feature extraction and thus in signal versus background classification performance. We provide reference results for the case of the high-luminosity Large Hadron Collider.

Synthetic jet flow driven by a standing-wave thermoacoustic heat engine

This paper presents an experimental investigation of the formation of synthetic jets into external quiescent environment for cooling purposes using a thermoacoustic engine which converts heat into acoustic oscillations. Thermoacoustic technology is proposed in this study as an option for enhancing the cooling effect in equipment normally relying on natural convection. For this purpose, a standing-wave thermoacoustic heat engine was modelled and built and its ability to generate synthetic jet flow structures as well as their propagation were examined. It was found that the engine is able to drive synthetic jets through 10 orifices at a temperature difference less than 150 °C. The working medium is CO2 at ambient pressure. In experiments, the performance of thermoacoustic engine was evaluated in terms of jet velocity and the required temperature difference across the stack by varying the heating power.

Study on Water Jet Characteristics of Square Nozzle Based on CFD and Particle Image Velocimetry

Water jet technology is widely used in various fields, in which the nozzle is an important element to form the jet. To solve the problem of low water jet operation efficiency of square nozzles, the internal flow channel structure of the nozzle of the key jet device is studied. Through the combination of computational fluid dynamics (CFD) and particle image velocimetry (PIV) experiments, the influence of main structural parameters such as the contraction angle and length-to-diameter ratio of the inner flow channel on the velocity and length of the constant-velocity core region is explored. Since the jet flow structure is a symmetrical structure along the axial direction, the model of the jet flow structure was built half of the model along the axial direction. The results show that a smaller length-to-diameter ratio and a smaller contraction angle of the nozzle result in better jet cohesion and lower dynamic pressure in the constant-velocity core area, which is more suitable for long-distance, low-pressure water jet operations.

Analysis of spark-generated waveforms coalescing in air using schlieren imaging

Mach waves generated by turbulent structures in supersonic jet flows have the potential to intersect and coalesce, a process proposed as a significant contributor to acoustic waveform steepening in the near field of a jet and to the noise referred to as crackle [Baars et al., AIAA (2013); Fiévet et al., AIAA (2016)]. Numerical simulations of intersecting waveforms have demonstrated that coalescence can lead to increased steepening that is dependent on intersection angle, waveform duration, and geometrical spreading [Willis et al., AIAA (2022)]. In this study, two simplified experiments involving a spark source in air are examined. The first involves grazing incidence on a rigid plane to model symmetric intersection between waveforms. The second involves intersecting waveforms emanating from side-by-side openings in a 3D-printed enclosure. Measurements of nonlinear evolution were made for the intersecting waveforms produced with each experimental setup and compared with measurements made for one waveform alone. Schlieren images of the coalescing waves allow for comparison of waveform steepening between the two cases without the limitations imposed by microphones. The measurements permit assessment of the extent to which coalescence enhances waveform steepening. [WAW is supported by the ARL:UT Chester M. McKinney Graduate Fellowship in Acoustics.]

Hydraulics of Wedge-shaped Flip Bucket to Investigate Flow Pattern with Retracted Bottom Plate

This study analyzed the flow pattern of a wedge-shaped flip bucket with a retracted bottom plate (WFB-R) that is quite different from conventional flip buckets. The flow pattern of the free jet release from WFB-R is determined by the relationship between water depth at the exit and wedge height. If the water depth is less than or equal to the wedge height, the free jet appears narrow and long, whereas if the water depth is greater, the free jet appears like a hammer. Hence, WFB-R offers the advantage of extending the free jet longitudinally, but its longitudinal length can be controlled, which allows it to avoid hitting the bank of the plunge pool. The experimental results indicate that, compared to constant-width flip buckets with retracted bottom plates (CFB-R), WFB-R reduces the maximum impact pressure on the plunge pool bottom to 70–86% of what it originally was. In addition, the flow structure of free jets issued from WFB-R was modeled using the k-ε turbulence model. The numerical results show that in the case of WFB-R, the shrinkage ratio should be less than or equal to a critical shrinkage ratio (0.63 for the present case) in order to stretch a free jet longitudinally. The length of the retracted bottom plate affects the longitudinal extension of the free jet. The longer the retracted bottom plate, the better the longitudinal stretching effect. The flow pattern is determined by the relationship between water depth at the exit of WFB-R and wedge height. Based on the projectile theory, a novel method was developed to predict the trajectory of free jets released from WFB-R by incorporating the average velocity, takeoff velocity coefficient, and takeoff angle coefficient. The Yangfanggou high dam project illustrates the application of WFB-R.

A Numerical Study of the Flow Field Driven by a Submerged, High-Speed, Gaseous Jet

Abstract The analysis of the numerical study of underwater high-speed gas jets is presented in this study. This work aims to understand the development of the flow structure of the gas jets submerged in water and assess the performance of the jet in terms of the thrust under varying operating conditions. The behavior of the submerged gas jet is studied under two operating parameters, namely, the pressure ratio (ratio of the pressure of the gas jet at the nozzle exit to ambient pressure) and the depth of water at which the propulsion takes place. The effort utilizes computational fluid dynamics using the finite volume method to solve the unsteady Reynolds-averaged Navier–Stokes equations in a two-dimensional axisymmetric domain combined with the mixture model for the multiphase flow. The unsteady behavior of different flow variables under varying operating parameters is discussed in detail. Further, the flow physics of a submerged supersonic gas jet is compared with a supersonic gas jet expanded in the air under a similar set of operating parameters. The effects of density difference between the gas and water have been studied from the comparative analysis.

Visualization of supersonic free jet flow structures subjected to various temperature and pressure ratio conditions

The standard Z-type schlieren visualization technique was used to experimentally investigate the effects of inlet boundary conditions (temperature and pressure) and nozzle geometry (convergent and convergent-divergent nozzles (C-D)) on a supersonic flow structure. The focus was on the Mach disk's length (Lm) and width (Wm). The inlet temperature (Tin) was 123 to 288 K, and the nozzle pressure ratio (NPR=Pin/Patm) was 3 to 20. When varying the NPR, over and under-expanded flow conditions were observed in the studied nozzle configurations. The results illustrate that when increasing NPR at Tin = 288 K, Lm increases for both over and under-expanded flow conditions. However, when increasing NPR in over-expanded conditions, Wm increases to reach the saturation point and then decreases for a C-D nozzle with a larger exit diameter, while it becomes constant for a small exit diameter. In contrast, Wm increased when increasing NPR for all nozzles in the under-expanded regime. At 123 K < Tin < 173 K, the shock cells were shrunken for C-D nozzles at NPR=5, and subsequently, Lm decreased. Nevertheless, Wm increased until Tin = 153 K and then decreased until Tin = 123 K, which disturbed the Mach disks and shock cells that governed the decrease in Wm.

Influence of sinusoidal shock generator on mixing performance of hydrogen and air co-flow jets in dual-combustor ramjet

This article presents a comprehensive study on the impacts of a wavy shock generator on the flow structure and mixing of air and hydrogen jets in dual combustion ramjet engine. Our focus is on flame and flow characteristics directly downstream of air and fuel injector where shock wave and shear layers are presented. Three different sinusoidal profiles are considered to disclose the impacts of shock strength on fuel penetration and diffusion. SST turbulence model is applied to obtain turbulence closure. Our studies show that the existence of the wavy shock producer amplifies the barrel shock near the fuel injector. Besides, the formation of expansion waves by the presence of a wavy shock generator increases the shock interactions and augments the mixing performance. Our findings indicate that the propagation and interaction of the expansion waves with boundary shear layer augment vortices downstream of splitter and expands hydrogen mixing region substantially.

A Numerical Study of a Submerged Water Jet Impinging on a Stationary Wall

The impinging jet is a classical flow model with relatively simple geometric boundary conditions, and it is widely used in marine engineering. In recent years, scholars have conducted more and more fundamental studies on impact jets, but most of the classical turbulence models are used in numerical simulations, and the accuracy of their calculation results is still a problem in regions with large changes in velocity gradients such as the impact zone. In order to study the complex flow characteristics of the water flow under the condition of a submerged jet impacting a stationary wall, the Wray–Agarwal turbulence model was chosen for the Computational Fluid Dynamics (CFD) numerical simulation study of the impacting jet. Continuous jets with different Reynolds numbers and different impact heights H/D were used to impact the stationary wall, and the results show that the jet flow structure depends on the impact height and is relatively independent of the Reynolds number. With the increase in the impact height, the diffusion of the jet reaching the impact area gradually increases, and its velocity gradually decreases. As the impact height increases, the maximum pressure coefficient decreases and the rate of decrease increases gradually, and the dimensionless pressure distribution is almost constant. In this paper, the flow field structure and pressure characteristics of a continuous submerged jet impacting a stationary wall are explored in depth, which is of great guidance to engineering practice.

Dynamics of wall jet flow under external pulsation

The present study aims to identify the dominant coherent structures in the wall jet flow subjected to external pulsation at Reynolds number 2600 (based on average jet exit velocity and nozzle diameter). The forcing frequency is varied between St = 0 and 0.99 (St is the Strouhal number). Quadrant analysis is employed to identify the relative contribution of different quadrant motions to the total Reynolds shear stress. Unlike boundary layer flows and channel flows, two distinct regions (inner shear region and outer shear region) are observed in the wall jet flows, and the characteristics of different quadrant motions change in these regions. About 70% of the total shear stress is contributed from the first and fourth quadrants in the outer shear region. We observe that ejection motion is more energetic than sweep motion in the downstream direction, although less frequent. The ejection motion is observed to be more violent for St = 0.44 than for the other frequencies. A proper orthogonal decomposition (POD) analysis reveals while the modal structures exist in different regions of the wall for different jet pulsation; there are no dominant modes (30 modes are required to recover about 75% of the total energy), and the energy is fairly distributed over a large number of modes. However, the POD analyses are capable of capturing the response of the wall jet to different jet pulsations. The most dominant and strongest modal structures are found nearer to the impingement region of the wall when St = 0.44 and the jet tends to laminarize for St &amp;gt; 0.9.