Rear Jet Research Articles

Optimization of collection efficiency for a dual-row jet mining machine based on CFD-DEM coupling

Deep-sea mineral resources possess vast reserves; yet, the depositional environment is extremely harsh, and the mining challenges are significant. Overcoming the technical hurdles of deep-sea mining and protecting the marine environment to achieve sustainable development and utilization of seabed resources are major challenges in the field of marine resource development. In this work, we employ a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach with the objective of enhancing the collection efficiency of deep-sea polymetallic nodules. The research focuses on adjusting and optimizing the collection parameters of a dual-jet collector. We examine the influence of the collector's jet angle and velocity on the collection efficiency. We conduct a numerical analysis of the movement characteristics of nodules, the distribution of the flow field, and the efficiency of nodule collection. The results indicate that when the rear jet angle is too small, the stripping and blocking effects on the nodules deteriorate. Conversely, when the jet angle is too large, the upwelling flow velocity decreases. The optimal collection effect is achieved with a rear jet angle of 55°. Lower jet velocities result in poorer nodule stripping, collection, and lifting. Higher jet velocities, however, increase seafloor disturbance and reduce the device's energy utilization efficiency. The collection efficiency is optimal at jet velocities ranging from 8 to 9 m/s. This research provides valuable insights for the design of high-efficiency dual-jet collector devices.

Effects of nozzle angles of a double-row jet collector on harvesting performance

Owing to its high collection efficiency, the double-row jet collector is an important application in industrial operations. Presently, few studies on its collection efficiency and impact from its front and rear nozzle angles are available. In this paper, the computational fluid dynamics–discrete element method is employed to numerically study the jet parameters of a typical double-row jet collector from three aspects: flow characteristics, particle movement and collection efficiency. Defining α as the front nozzle and β as the rear nozzle, five different combinations of jet angles were first selected and an optimal configuration of the front nozzle at 30°and the rear nozzle at 55° was last obtained, the simulation results were confirmed via the performance experiment. The influence of the change in the front and rear jet angles on the flow field of the collection is shown, and the results indicate that of the angle α and β, α plays a dominant role, while β has relatively little influence on the whole collecting process, which guides for the optimal design.

Fluid dynamic analysis and lift enhancement of material transport and mixing around airfoil through Lagrangian coherent structures

Flow separation around an airfoil can be effectively controlled by appropriately positioning a synthetic jet (SJ) on the airfoil surface, subsequently enhancing the airfoil's lift. This study conducts a fluid dynamics analysis of material transport and mixing related to momentum transport and mixing around an airfoil. The detailed analysis of the corresponding effects on lift improvement aids in understanding fluid dynamics through Lagrange Coherent Structures (LCSs) and their nonlinear dynamics. An additional SJ is introduced in the airfoil's rear section, combined with a high-speed, low-frequency SJ in the front section to form a combined SJ. The flow around the airfoil with this combined SJ is simulated. LCSs surrounding the SJs are identified and utilized to analyze the complex material transport and mixing processes. The impact of material transport and mixing on lift enhancement is explored through the evolution of LCSs and pressure distribution on the airfoil surface. Results demonstrate that the combined SJs further improve the airfoil's lift coefficient compared to single SJs, with the enhanced material transport and mixing of SJs playing a vital role in lift enhancement. The rear jet accumulates downstream of the rear slot, forming a tail vortex with high momentum and low pressure. The vortex generated by the front jet carries the tail vortex, continuing to move downstream when it reaches the rear slot. A pressure drop forms where the two vortices flow, enhancing the airfoil's lift. The rear jet can absorb fluid downstream and upstream of the jet slot, reducing pressure on the suction surface near the jet slot and significantly contributing to lift improvement.

Drag Reduction and Thermal Protection of the Combination of Aero Disk, Lateral Jet, and Rear Jet for Hypersonic Vehicle

This study proposes a combined scheme based on a spike-aero-disk, a lateral jet, and a rear jet to enhance hypersonic vehicles’ drag reduction and thermal protection performance. Numerical simulations were conducted using CFD methods to validate the scheme’s capabilities. The results demonstrate a significant improvement in drag reduction and thermal protection compared to the basic scheme with only a spike-aero disk. Furthermore, under the same mass flow rate conditions, the combined scheme with an extra rear jet is compared to a scheme with a spike-aero-disk-lateral jet, revealing a reduction of approximately 23.4% in the peak Stanton number, indicating a remarkable enhancement in drag reduction and thermal protection performance. The simulation results show that the use of lateral jet and rear jet improves the overall thermal protection ability and drag reduction ability of the vehicle.

Numerical Study of Multiple Momentum Jets in a Vegetated Crossflow

Vertically discharged multiple jets in crossflow is a common form of wastewater discharge. The presence of vegetation in the flow channel complicates the hydraulic characteristics of jets. The realizable k-ε turbulent model is used to simulate the flow, turbulence, and vortex characteristics of multiple jets with different spacing and jet-to-crossflow velocity ratios, to study the flow characteristics and vortex structure of multiple jets in a vegetated channel. The results reveal that vegetation inhibits the development of a counterrotating vortex pair. The jets with a low jet-to-crossflow velocity ratio are concentrated near the flow symmetry profile by the dual constraints of ambient flow and vegetation. The jets gradually spread outward and the counterrotating vortex pair become more obvious when the jet-to-crossflow velocity ratio increases. Vegetation reduces the shading effect of the front jet on the rear jet by accelerating the dissipation of shear layer vortices. The influence of the front jet on the rear jet decreases as the spacing increases.

Experimental and Numerical Research on the Formation Conditions and Entrainment Characteristics of Supercavity by Rear Jet Reflux

Aiming at better controlling the ventilated supercavity flow for drag reduction, the experimental and numerical researches of supercavity by rear gas reflux are proposed in this paper. Several experiments with different test bodies have been carried out to study the formation and collapse conditions of jet-reflux supercavity. An open-circulation water tunnel for ultra-high-speed jet experiment and air jet system is employed to form jet-reflux supercavity around the bodies installed in the forward strut. The experiment results show that the supercavity can be maintained by the reflux of tail jet flow when an initial supercavity covering the jet outlet is formed. However, the supercavity will be destroyed when the jet intensity is further enhanced. Under the same jet coefficient, the scale of jet-reflux supercavity extends as the increase of the body length, while the critical jet coefficient for the collapse of the supercavity decreases as the increase of the body length. The multiphase flow model coupling the VOF model and the level-set method is applied to capture the air-water interface. Then, the flow field characteristics of the jet-reflux supercavity are analyzed and compared with the ventilated supercavity. The streamline inside the cavity presents considerable three-dimensional asymmetry inflating flow characteristics. The variation of the gas reflux coefficient along the axial direction is obtained, which indicates that a handful of reflux gas are required to sustain the head cavity. Therefore, the jet-reflux supercavity can be formed within a certain range of the tail jet intensity. Although jet intensities are not equal to each other, the scale of cavity head is roughly maintained under the same reflux coefficient. When the supercavity gets closed to the nozzle outlet, the maximum scale of cavity is decreased, which leads to a weaker reflux at the outlet. The cavity interface will be impinged by the high-speed gas and mixture liquid, which obviously causes deformation and final collapse. In order to improve the stability of the jet-reflux supercavity, it is necessary to use the gas re-directed structure to control the reflux.

Experimental study on tilting behavior and blow out of dual tandem jet flames under cross wind

Fire hazards in outdoor environmental conditions cause huge damage to process safety in fuel transportation and pipeline leakage. Few literatures are reported about the combustion behaviors and instability characteristics of dual jet flames under cross wind, which are important to risk assessment and manufacturing design. In this work, a series of propane fire source with nozzle diameters of 3 mm, 4 mm and 5 mm are designed for different nozzle separation distances under cross wind. Results showed the tilt angle of rear jet flame was smaller than front jet flame due to the vitiated effective wind towards the rear jet flame by the blockage of the front jet flame. Furthermore, it’s found that the blow-out cross wind velocity of the rear jet flame is smaller than that of the corresponding single jet flame. The combustion of the front jet flame will restrict the rear jet flame to a certain extent. Finally, dimensional analysis and correlations are established, where the blow-out cross wind velocity of rear jet flame as well as that of the single jet flame can be well represented by Froude number. These new observations and correlation are helpful for further understanding combustion behaviors of dual tandem jet flames under cross wind.

Research on Penetration Performance of Tandem Damage Elements to Oblique Reactive Armor

In order to study the penetration performance of the tandem warhead against the oblique reactive armor target, ANSYS/LS-DYNA finite element simulation software was used to simulate the penetration process of the tandem warhead against the oblique reactive armor target when the front liner of the tandem warhead was made of different materials. The results show that when the front liner material is copper, the formed jet penetrates the reactive armor and will detonate it. The rear jet is greatly affected by the reactive armor, and the final penetration depth is 550 mm; when the front liner material is material is polytetrafluoroethylene (PTFE), the formed low-density jet penetrates the reactive armor and does not detonate it. The rear jet directly penetrates the main target plate along the low-density jet penetration hole, and the final penetration depth is 646 mm, compared with the tandem warhead As far as the front liner uses copper jet, the penetration depth is increased by 17.45%.

Recoil Reduction Method of Gun with Side to Rear Jet Controlled by Piston Motion

Excessive recoil severely restricts the loading of high-power traditional guns on modern vehicles. To reduce the recoil without breaking the continuous firing mode and reducing the projectile velocity, a recoil reduction method that controls the lateral ejecting of propellant gas by a piston was proposed. The recoil reduction device is symmetric about the barrel axis. First, a one-dimensional two-phase flow model of interior ballistic during the gun firing cycle was established. Next, the MacCormack scheme was used to simulate, and the piston motion was gained. Then the propagation of the rarefaction wave in the barrel was presented. Finally, the propulsion difference between the piston-controlled gun and the traditional gun was discussed. The results showed that the recoil momentum was reduced by 31.80%, and the muzzle velocity was decreased by just 1.30% under the reasonable matching of structural parameters.

Assessment on jet direction effect on drag and heat reduction efficiency for hypersonic vehicles

Introducing jet for drag reduction and thermal protection in hypersonic flows has attained worldwide attentions due to its giant potential, and this paper focused on the drag and heat reduction efficiency of unit mass flow rate for different jet directions in hypersonic flows. The developed code employed conjugate heat transfer approach to solve unsteady Reynolds-averaged Navier-Stokes equations and Fourier's heat conduction equation simultaneously, and then it was thoroughly validated by two classical experiments. Based on that, the flow field characteristics induced by different jet directions under the constant mass flow rate of jet were numerically investigated. Besides, two parameters were introduced to quantitatively evaluate the drag and heat reduction efficiency of different jet directions. The obtained results show that though the total mass flow rate of jet is constant, jet direction still has a decisive influence in manipulating the flow structures, and the rear jet would result in more complicated shock waves due to the interaction between the rear jet flow and spike. The force produced by the jet is the key role in determining the total drag, and the variation of pressure distribution along the blunt body is not the direct presentation of drag reduction performance. The opposing jet has the highest drag reduction efficiency, and lateral jet has the highest heat flux reduction. However, compared with the lateral and rear jets, the opposing jet has better thermal protection performance from the overall perspective as it can also protect the aerodome head from severe aerodynamic heating.

Numerical investigation of pitch motion induced unsteady effects on transverse jet interaction

Motion-induced unsteady effect is becoming one of the inevitable challenges for the improvement of high-speed and high-maneuvering projectiles performance under a transverse jet control. Therefore, three-dimensional, compressible, unsteady, Reynolds-averaged-Navier-Stokes numerical investigations are performed to illustrate motion-induced unsteady effects on transverse jet interaction. Two pitch motion types, including forced-oscillation and free-response-motion, were chosen to evaluate the potential effects. The current study found that time hysteresis and aerodynamic deviation appear comparing to steady counterparts. Forced-pitch-oscillation data indicate negative interaction-force-amplification-factors at a certain angle of attack while interaction-moment-amplification-factors show an obvious decrease at a relatively high angle of attack. Meanwhile, transverse-jet-controlled free-response-motion data, obtained through coupled CFD/RBD/JET method (a coupled method with CFD (Computation Fluid Dynamics) and RBD (Rigid Body Dynamics) under the control of a transverse jet), visualized positive enhancement of rear jet interaction, as the desired angle can be achieved in less time. Taken together, it is possible that motion-induced unsteady effects on transverse jet interaction may be underestimated and are worthy to be discussed deeply for energy-efficient and flight safety concerns.

Hypersonic drag reduction mechanism of a novel combinational spike and multi-opposing jets aerodynamic configuration

The shock wave drag significantly affects the aerodynamic performance of hypersonic vehicles, a novel combinational configuration is proposed to reduce the drag in this paper. The basic configurations are the spike, nose cone, front and rear jets, respectively. The numerical method is used to study the drag reduction performance. The combinational configuration has weaker reattachment shock wave than the other configurations, and both the peak pressure of nose cone and drag coefficient are decreased by more than 45%, which verifies the excellent drag reduction performance of combinational configuration. The influencing factors are analyzed in this paper. Lengthening the spike and increasing the total pressures of two jets can make the reattachment shock wave away from wall, weaken its intensity and increase the drag reduction performance. Furthermore, the change rates of wall pressure distribution of the nose cone and drag coefficient gradually decrease. When the sizes of two nozzles are small, the rear jet appears as the long penetration mode. When the sizes of two nozzles are larger than corresponding critical sizes, respectively, the rear jet will convert to the short penetration mode. Increasing the sizes of two nozzles makes the reattachment shock wave away from wall and increases drag reduction performance with the rear jet being SPM as well. The size and total pressure of front jet has greater influence on the drag coefficient. The influence of above factors on the flow characteristics and mass flow rate of nozzle is also studied in this paper.

Parameter study on drag and heat reduction of a novel combinational spiked blunt body and rear opposing jet concept in hypersonic flows

A novel combinational configuration is proposed for drag and heat reduction in this paper, the CFD numerical method is adopted to study its drag and heat reduction performance. The results show that the aerodisk enhances the compression of hypersonic free stream by the spike to reduce the intensity of reattachment shock wave. The rear jet pushes the reattachment shock wave away from the blunt body and reduce the intensity of reattachment shock wave as well. In addition, the low-temperature gas from rear jet can also cool the blunt body directly. Increasing the length-diameter ratio of the spike reduces the intensity of reattachment shock wave, the rear jet gas can be ejected farther to push the reattachment shock wave father away from the blunt body. Increasing the diameter of aerodisk enhances the compression of hypersonic free stream to reduce the intensity of reattachment shock wave, while the shock wave drag of spike gradually increases. Therefore, with the increase of diameter of aerodisk, the total drag coefficient of combinational configuration first decreases and then increases. The rear jet presents the long penetration mode and short penetration mode. Increasing the total pressure ratio and the size of nozzle can push the reattachment shock wave away from the blunt body and reduce the intensity of reattachment shock wave, more low-temperature gas can also be injected into the flow field to cool the blunt body effectively. Finally, the flat disk has better drag and heat reduction performance than conical disk and hemispherical disk.

An Experimental Characterization of the Interaction Between Two Tandem Planar Jets in a Crossflow

This study reports an experimental investigation of two planar jets in a crossflow in a tandem arrangement. Tests were conducted in an open-jet wind tunnel facility using two-dimensional (2D)-particle imaging velocimetry (PIV) measurement. Using the terminology in the dual jets in a quiescent ambient, the mean flow field of the crossflow arrangement was divided into a converging region, a merging region, and a combined region. An approach to determining the range of these three regions was proposed based on the mean characteristics of horizontal velocity profiles of the flow field, validated by the experimental data. The momentum-dominated near field (MDNF) for the rear jet in the dual-jet configuration was recognized using the horizontal offset of mean jet trajectory, which accordingly gives a quantitative definition of the MDNF range. Discussions were made on the effects of different parameters on the three regions and MDNF. Finally, snapshot proper orthogonal decomposition (POD) analysis was conducted, characterizing the coherent structures of the flow field, particularly the large-scale vortices. It was observed that the large-scale vortices mainly occur in the shear layers of the jets and their occurrence is affected by the parameters of the jets. In addition, compared with the single-jet configuration, the introduction of the front jet was found to contribute to the occurrence and development of the large-scale vortices.

Velocity distribution around a sphere descending in a linearly stratified fluid

The flow around a sphere descending at constant speed in a salt-stratified fluid is observed by particle image velocimetry. A unique characteristic of this flow is the appearance of a thin and high-speed rear jet whose maximum velocity can reach more than five times the sphere velocity. In this study we have investigated how the velocity distributions, especially those in the jet and in the boundary layer of the sphere, vary when the Froude number $Fr(=W^{\ast }/N^{\ast }a^{\ast })$ or the Reynolds number $Re(=W^{\ast }(2a^{\ast })/\unicode[STIX]{x1D708}^{\ast })$ ($W^{\ast }$: vertical velocity of the sphere, $N^{\ast }$: Brunt–Väisälä frequency, $a^{\ast }$: radius of the sphere, $\unicode[STIX]{x1D708}^{\ast }$: kinematic viscosity of the fluid) is changed. The results show that the radius of the jet and the thickness of the boundary layer are comparable, and they decrease for smaller Froude numbers and larger Reynolds numbers. Both of them are estimated at moderate Reynolds numbers by the primitive length scale of the stratified fluid ($l_{\unicode[STIX]{x1D708}}^{\ast }=\sqrt{\unicode[STIX]{x1D708}^{\ast }/N^{\ast }}$), or in non-dimensional form by $l_{\unicode[STIX]{x1D708}}^{\ast }/2a^{\ast }=(Fr/2Re)^{1/2}$. The overall velocity distribution in the lee of the sphere is measured to identify the internal wave patterns and their effect on the velocity variation along the jet. Corresponding numerical simulation results using the axisymmetry assumption are in agreement with the experimental results.

Reduction of fluid forces acting on a square prism using a planar jet

This experimental study focused on the concurrent minimization of the mean and fluctuating forces acting on a square prism in crossflow by creating a continuous jet through a narrow spanwise slot in it. For this purpose, three different predetermined injection surfaces (i.e., front, top, and rear) were individually studied for the injection ratios (IR) of 0, 1, 1.5, 2, 2.5, and 3 at a Reynolds number (Re) of 8000. The results showed that the rear jet is the optimum injection surface. For the rear jet configuration, the optimum IR is 1.5 in terms of mean total drag coefficient reduction, 〈CDT〉. In this case, the reductions in the 〈CDT〉 and the level of root mean square (RMS) of fluctuating pressure coefficient (CP RMS) on the side surfaces are about 29.7% and 68%, respectively. However, the maximum reduction in the CP RMS level on the side surfaces occurred at IR=2. In this case, the reductions in the 〈CDT〉 and the level of CP RMS on the side surfaces are about 27.5% and 88%, respectively. The underlying mechanism of force reduction was also demonstrated. Various flow patterns were identified with respect to IR at incidence angle (α) of 0°. It was also proved that, at relatively small angles of incidence, from α=0° to 20°, the active flow control method is still effective in reducing 〈CDT〉 and in suppressing the fluctuating side forces, which is indicative of periodic vortex shedding from either side of the square prism.

Modulating flow and aerodynamic characteristics of a square cylinder in crossflow using a rear jet injection

The temporally evolved flow behaviors around a square cylinder subject to modulation of a planar jet issued from the cylinder’s downstream surface into the wake were studied using the laser-assisted smoke flow visualization method and synchronized hot-wire anemometers. The drag force asserted on the square cylinder was obtained by measuring the surface pressures. Four characteristic flow modes (wake-dominated, transitional, critical, and jet-dominated) were observed in different regimes of freestream Reynolds number and jet injection ratio. In the wake-dominated mode, the jet swung periodically back and forth on the downstream surface due to the wake vortex shedding. In the transitional mode, the vortex shedding in the wake vanished so that the flow around the cylinder presented no periodic oscillations. In the critical mode, the wake width became smaller and therefore made the vortex shedding frequency larger than that observed in the wake dominated mode. In the jet-dominated mode, the jet had a large momentum that entrained wake fluids and therefore stabilized the instabilities of the wake, separated boundary layers on lateral surfaces, and stagnation point on the upstream surface. Two standing vortices appeared in the near wake beside the high-momentum jet. The width of the wake was decreased substantially by jet entrainment. The drag coefficient decreased with an increase in the jet injection ratio. The downstream surface jet injection caused the pressure coefficients to decrease at the upstream surface and to increase at the downstream surface. Therefore, the drag coefficients were decreased significantly by 26%, 33%, and 38% at the injection ratios of 0.5, 1.5, and 2.5, respectively.

Characteristics of Cascaded Fuel Injectors Within an Accelerating Scramjet Combustor

Performances of cascaded hydrogen injectors within an accelerating scramjet combustor were characterized via a three-dimensional numerical study. Two streamwise-aligned jets are employed, with the upstream injector half the diameter of the rear jet. Each are inclined at 45 deg to the freestream and achieve a jet-to-freestream momentum ratio of unity. Performance was evaluated over a range of freestream Mach numbers, modeling combustor entrance conditions on an accelerating access-to-space scramjet trajectory. Distance between injectors was varied to estimate the optimum jet-to-jet spacing to achieve robust performance across the Mach number range. It is shown that the downstream injector benefits jointly from shielding effects induced by the smaller upstream injector, as well as its increased diameter. Injector spacings greater than two total jet diameters displayed improved absolute jet penetration, spanwise spread, and entrainment rates over an equivalent single injector across all Mach numbers. Jet bow shock pressure recovery was improved at spacings above . Unique optimal injector spacings existed for each performance metric, at each freestream Mach number examined. Although no universal optimum spacing was determined, spacings between 4 and provided substantial performance enhancements over single injectors, throughout the accelerating scramjet flight conditions.

On the trajectory scaling of tandem twin jets in cross-flow in close proximity

An experimental study has been conducted on tandem twin jets in cross-flow (JICF) in close proximity to investigate the relationships between their trajectories, separation distances and velocity ratios. Results show that the front and rear jets, each with initially distinct jet trajectory, merge into a single trajectory shortly after they exhaust into the cross-flow. Furthermore, the merged tandem JICF attains deeper cross-flow penetration than that of a single JICF at the same velocity ratio. The front jet is also observed to provide ‘shielding’ for the rear jet such that the latter penetrates relatively deeper into the cross-flow, which corroborates observations made by earlier studies. In particular, the present study demonstrates that it is possible to collapse the tandem JICF merged trajectories by ‘rD’-scaling, where A and B coefficients show slight reductions and increments, respectively, with increasing separation distance. Collapsing the merged trajectories by using single JICF A and B coefficients leads to the notion of effective velocity ratio for tandem JICF, which enable the authors to propose a modification in the ‘rD’-scaling law for tandem JICF. Lastly, the modified ‘rD’-scaling law is applied to trajectory data from an earlier tandem JICF study, and its validity is demonstrated by the resulting good collapse.

Active control of flow around a square prism by slot jet injection

The main aim of the experimental study is to determine both the most effective injection surface and rate in order to ensure minimum drag and fluctuating forces on a square prism subjected to crossflow. All predetermined jet injection surfaces i.e. front, side, and rear, tested separately for injection ratios of IR = 0, 1, 1.5, 2 at Reynolds number of Re = 16,000. Surface pressures were measured by differential pressure transducer whereas instantaneous velocity measurements were performed by using multichannel Constant Temperature Anemometer (CTA). It was concluded that jet injection, especially from the rear surface, brought noticeable improvements to the flow characteristics of a square prism. For rear jet configuration with IR = 1.5, the mean drag coefficient () was reduced to 79.4% and CP RMS level on side surfaces was reduced to 20% of that of the single square prism. In addition, instantaneous flow visualization photographs and Strouhal number (St ) distribution across the injection ratio were also presented to identify the flow patterns and underlying mechanism of drag and fluctuating force reduction of square prism with rear jet configuration.