Forced Jet Research Articles

Instability Waves in High-Speed Jets: Near-and Far-Field DNS/LES Data Analysis

Solutions from inviscid linear stability theory (LST) and linear parabolized stability equations (PSE) were used to quantify the extent to which instability waves represent actual near-field dynamics in high-speed forced turbulent jets. The mean and unsteady flow data were taken from a well-validated large-eddy simulation (LES) database (Bodony and Lele, 2005). The jets and their operating conditions were heated high subsonic ( M j = 0.97, T j/ T∞ = 2.3, Re D = 84,000) and unheated supersonic ( M j = 1.95, T j/T∞ = 0.56, Re D = 336,000) jets with the same jet exit velocities. The coherent components of the turbulent jet flow fields were extracted using a filter based on proper orthogonal decomposition (POD). To examine the effects of nonlinear interactions and to test the decomposition method, forced laminar jets were also considered; the flow data were obtained from a direct numerical simulation (DNS) database for an unheated M j = 0.90 and Re D = 4,100 jet (Ryu, 2010). Two methods were used to extract instability waves from the nonlinear flow fields: first, bi-orthogonal relations between regular and adjoint instability waves were formulated from linearized Euler equations to determine the amplitude coefficients of the instability waves in nonlinear flows; then, a least-squares fit was applied to solutions from the PSE, POD, and LES data for the same purpose. The noise radiated by the deduced instability wave modes, POD, and LES were computed using the Kirchhoff surface integral method. Frequencies St = fD j/ U j = 0.1 ∼ 0.5 at azimuthal modes m = 0, 1, and 2 were compared. The adjoint decomposition method showed promising results only for the laminar jet. The eigenfunction shape in the radial direction and the growth and decay of energy in the streamwise direction were well captured. However, this method produced errors in analyses of turbulent jets due to sensitivity in regions where the mode had saturated and was decaying. The results of PSE and POD showed reasonably good agreement in the near field for the first helical mode ( m = 1). However, there was only limited agreement in the far field, and among other modes, in contrast to the good agreements described in recent studies for supersonic unforced jets. This is due to the strong nonlinear effects in forced turbulent jets considered in the present study. This finding confirms a limitation of linear stability theories when predicting the wavepacket dynamics of forced turbulent jets. The PSE solution for the subsonic jet showed very weak acoustic radiation, and the far-field noise displayed strong sensitivity to the location of the Kirchhoff surface. This suggests a limitation of direct propagation approaches for the PSE solutions of moderately heated subsonic jets.

Numerical simulation of lateral control in supersonic cross jet flow

To investigate the effect of angle of attack and jet position on the lateral control force in the cross jet flow,the paper firstly simulated the supersonic cross jet flow by the numerical methods. The calculated longitudinal wall pressure distribution agreed well with the experimental results. Jet force amplification factor in longitudinal direction and center of interaction force were used to evaluate the actual performance of lateral control force in jet interaction. Moreover,normal interaction force accumulation coefficient was introduced to analyze how four characteristic areas on the body surface affect the lateral control force in jet interaction.The numerical results indicate that the lateral control force in jet interaction,which is equivalent to a normal force,together with a nose-down moment around center of jet,are not equal to the jet force in design. The wake zone of low pressure is a determinant factor affecting the actual performance of lateral control force. As the angle of attack increases,the strength of the interaction pressure in the wake zone decreases,which will enhance the lateral control force. With the jet location moving backward,the scope of the wake zone diminishes,which will improve the actual performance of lateral control force.

Direct current-dielectrophoresis assisted microscale machining of metal by femtosecond pulsed laser

Accumulation of ablation debris particles interferes with femtosecond laser micromachining of high-aspect ratio grooves in metal. Debris removal from such grooves by an air or inert gas jet requires that the gas jet axis be aligned with the groove. This requirement constrains the motion system design and programming as well as the gas nozzle design for cutting of arbitrary shapes. Gas jet debris removal is also not feasible in vacuum environments. In this study, a novel technique based on DC-dielectrophoresis (DEP) for removing debris from the cut groove was investigated. Femtosecond laser ablation of linear and circular grooves from thick sections of aluminum was performed with no debris removal and with debris removal by air jet or DC-DEP force. Ablation depth and precision were compared for the three experimental conditions. Debris removal by DC-DEP force resulted in the most accurate, consistent, and orientation-independent machining of high-aspect ratio grooves. Moreover, larger electric fields exerted stronger DC-DEP force on debris particles and resulted in better ablation precision and machining rate.

Dynamic behavior of a spring-powered micronozzle needle-free injector

Conventional injection is still the leading method to deliver macromolecular therapeutics. Needle injection is considered a low compliance administration strategy, principally due to pain and needle phobia. This has fostered the research on the development of alternative strategies to circumvent the skin barrier. Among needle-free drug delivery methods, jet injection is an old strategy with great potential not yet completely disclosed. Here, the design, engineering and dynamic behavior of a novel spring-powered micronozzle needle-free injector is presented. Fluid mechanics was first studied in air to calculate jet force and speed as well as injection duration in different conditions. Polyacrylamide gel was used to simulate a soft tissue and to investigate the jet evolution over time of different injected doses. Finally, ex vivo characterization was carried out on pig skin. Results evidenced a direct dependence of the force, velocity, and duration with the injection volume. The model material allowed individuating the different steps of jet penetration and to attempt a mechanistic explanation. A different behavior has been recorded in the skin with interesting findings for subcutaneous and/or dermal delivery. Peculiar features with respect to existing jet injectors confers to this device good potentiality for a future clinical application.

Direct numerical simulation of free and forced square jets

Direct numerical simulation (DNS) of incompressible, spatially developing square jets in the Reynolds number range of 500–2000 is reported. The three-dimensional unsteady Navier–Stokes equations are solved using high order spatial and temporal discretization. The objective of the present work is to understand the evolution of free and forced square jets by examining the formation of large-scale structures. Coherent structures and related interactions of free jets suggest control strategies that can be used to achieve enhanced spreading and mixing of the jet with the surrounding fluid. The critical Reynolds number for the onset on unsteadiness in an unperturbed free square jet is found to be 875–900 while it reduces to the range 500–525 in the presence of small-scale perturbations. Disturbances applied at the flow inlet cause saturation of KH-instability and early transition to turbulence. Forced jet calculations have been carried out using varicose perturbation with amplitude of 15%, while frequency is independently varied. Simulations show that the initial development of the square jet is influenced by the four corners leading to the appearance hairpin structures along with the formation of vortex rings. Farther downstream, adjacent vortices strongly interact leading to their rapid breakup. Excitation frequencies in the range 0.4–0.6 cause axis-switching of the jet cross-section. Results show that square jets achieve greater spreading but are less controllable in comparison to the circular ones.

Effects of metal-vapor jet force on the physical behavior of melting wire transfer in electron beam additive manufacturing

The effects of metal-vapor jet force on the physical behavior of melting wire transfer in electron beam additive manufacturing were analyzed by both means of theoretical and experimental studies. The maximum size of the droplet in metal transfer was calculated by the static force balance model theoretically. Meanwhile, the transfer behavior was observed by using a charge-coupled device visual system. The calculated maximum size of the droplet matched well with the experimental data, indicating that the model could effectively predict the critical size of the droplet. Our results revealed three transfer modes induced by different transition heights: metal transfer, big-droplet transfer and molten-metal-bridge transfer. There were two stages in metal transfer at the high transition height of 10mm. In the first stage, the droplet with a radius of 1.5mm flew horizontally and dripped outside the molten pool because of the horizontal component of the metal-vapor jet force. In contrast, in the second stage, the droplet with a radius of 3.93mm flew vertically and dripped inside the molten pool with the reason of the vertical component of the metal-vapor jet force. When the transition height was reduced to 4mm, the big-droplet transfer occurred, which was similar to metal transfer. As the transition height decreased further, the molten-metal-bridge transfer mode appeared. This was the most stable transfer mode. In addition, the mechanism of each transfer mode was also proposed. The metal-vapor jet force was proved to play an important role in droplet growth and stability. Finally, it is essential to decrease the transition height to avoid the metal-vapor jet force from influencing the stability of process.

Numerical Study of Sub-Nozzle Flows for the Weft Transmission in an Air Jet Loom

The air jet loom is widely applied in the textile industry due to its high productivity, convenient controllability, high filling insertion rate, low noise and low vibration levels. High-Speed air jet weaves the weft yarn, and transports it through the weft passage. In the present study, a computational fluid dynamics method is applied to solve the incompressible Navier–Stokes equations with one-equation Spalart-Allmaras turbulence model. Which is used to solve the flow filed in a weft passage. The aim of this analysis is to determine the distribution of the flow velocity along the weft passage. Results revealed the strong relationship between air jet velocity and forces on the weft.).

Role of jet force technology in wound management

Role of jet force technology in wound management

Enhanced Lattice Boltzmann modelling of axisymmetric flows

An enhanced axisymmetric Lattice Boltzmann fluid simulation model developed by the second author involves no calculation for velocity gradients and has the feature that all external force terms can simply be added to the model. In this paper, the model is extended and applied to investigate the characteristics of two complex axisymmetric flows for further assessment of its capability. First of all, Taylor–Couette flow for Newtonian and non-Newtonian fluids is considered. The combined effects of the Reynolds number, the radius ratio and the power-law index n on the flow characteristics are analysed for an annular space of finite aspect ratio. Different forced axisymmetric laminar cold-flow jets are then taken into account with different values of forcing amplitudes in the jets. The numerical results are compared with existing numerical results and experimental data reported in the literature to demonstrate the accuracy of the model.

Research on a Test Method of Dynamic Force of Jet Elements

According to the characteristics of the dynamic force of jet elements, a kind of simple pendulum test system was presented. The dynamic jet force value was obtained indirectly by the acceleration value of the jet element under its force. The mathematic model of the test system was established and the test errors of the amplitude and delay time were analyzed. A processing method of dynamic force test data was presented based on the test curve and the parameters of amplitude and delay time were obtained. The result indicates that the test method can satisfy the testing requirements of dynamic force and it has advantages of simple structure, low demands of testing condition and simple method of data processing.

Behavior of the plasma characteristic and droplet transfer in CO2 laser–GMAW-P hybrid welding

The plasma behavior and metal transfer in CO2 laser + GMAW-P hybrid welding have been investigated. A 650 nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. The effect of the mutual distance and laser power on the metal transfer has been discussed. The results indicate that the laser-induced plasma plume have a significant impact to the arc shape, resistance, electrode melting, droplet formation, detachment, and impingement onto the workpiece. The laser-induced plasma changes the conductive path and forces affecting on the droplet. High laser power and short distance between laser beam and arc (DLA) reduce the pulse base time (PB) of the voltage of phase, increase the droplet detachment time (PD) and the pulse current time (PP) of the voltage of phase, and it also lead to an upward and inward force near the bottom of the droplet. As a consequence, the droplet formation time is increased, and eventually an off-axis droplet phenomenon is deduced. The vapor jet force induced by the the keyhole plasma acts on the droplet as a retention force; this force decreases when the DLA becomes larger and increases when the laser power becomes higher. The observation may help in understanding the weld characteristics with respect to variation in mutual distance and laser power which may be beneficial in using the main process parameters to produce desired weld quality.

Propulsion Mechanism of Catalytic Microjet Engines.

We describe the propulsion mechanism of the catalytic microjet engines that are fabricated using rolled-up nanotech. Microjets have recently shown numerous potential applications in nanorobotics but currently there is a lack of an accurate theoretical model that describes the origin of the motion as well as the mechanism of self-propulsion. The geometric asymmetry of a tubular microjet leads to the development of a capillary force, which tends to propel a bubble toward the larger opening of the tube. Because of this motion in an asymmetric tube, there emerges a momentum transfer to the fluid. In order to compensate this momentum transfer, a jet force acting on the tube occurs. This force, which is counterbalanced by the linear drag force, enables tube velocities of the order of 100 μm/s. This mechanism provides a fundamental explanation for the development of driving forces that are acting on bubbles in tubular microjets.

0516 長方形噴流のせん断層中に形成される渦構造の可視化

This study was focused on vortex structures in a forced rectangular jet. The periodic and alternative disturbance locally excited a pair of major shear layers. The flow visualization was performed by Laser Induced Fluorescence method to grasp 3-D image by stacking the time sequence of cross section images. As a result, the detail of tilting-link structure was observed, and it was revealed that the structure forms only in the excited jet. Mushroom shaped vortex enhances mixing and diffusion in the minor direction of the jet. On the other hand, the jet contraction in major direction is due to the self-induced velocity of the rectangular vortex ring.

An accurate quantification of the flow structure along the acoustic signal cycle in a forced two-phase jet

This paper provides an experimental study of an acoustically forced two-phase air jet generated by a convergent nozzle. The used particles are transparent glass spheres with diameters between 2 and 50 μm (which gives Stokes number of order 1) and the selected forcing frequency (f =400 Hz) induces a powerful nearly periodic flow pattern. Measurements were done by a two-colour Phase-Doppler Anemometer. The experimental setup is computer-controlled to provide an accurate control with a high long-term stability. Measurements cover the whole forcing signal cycle. Raw measurements were carefully post-processed to avoid bias induced by the forcing and the instrument setup, as well as obtain right mean values of the dispersed flow. The effect of the forcing and the particle load allows authors to establish the effect of the acoustic forcing and the particle load on the jet.

Investigation of Tank Pressurization and Pressure Control—Part I: Experimental Study

Self-pressurization and pressure control of cryogenic storage tanks have important design consequences for propellant and life support systems currently being planned for long duration space missions. During self-pressurization, the tank's liquid fill level and the heat load from the surroundings can have significant effects on the tank's thermal stratification and pressurization rate. When controlling pressure with a mixing jet, the velocity and temperature of the jet are important design parameters affecting the thermal destratification and pressure reduction time constants. In this work, a small-scale ground-based experiment was performed, as a precursor to a microgravity experiment, to investigate the effects of these variables on the pressurization and pressure control time constants in the tank and to assess the feasibility of using a forced jet mixer for reduced boil-off pressure control. Local pointwise temperature and pressure measurements, together with qualitative contours of the thermal field in the liquid, vapor, and wall region, were made to identify and characterize important self-pressurization and pressure control trends in 1 g.

Eddy Shape, Orientation, Propagation, and Mean Flow Feedback in Western Boundary Current Jets

AbstractThis manuscript revisits a study of eddy–mean flow interactions in an idealized model of a western boundary current extension jet using properties of the horizontal velocity correlation tensor to diagnose characteristics of average eddy shape, orientation, propagation, and mean flow feedback. These eddy characteristics are then used to provide a new description of the eddy–mean flow interactions observed in terms of different ingredients of the eddy motion. The diagnostics show patterns in average eddy shape, orientation, and propagation that are consistent with the signatures of jet instability in the upstream region and wave radiation in the downstream region. Together they give a feedback onto the mean flow that gives the downstream character of the jet and drives the jet's recirculation gyres. A breakdown of the eddy forcing into contributions from individual terms confirms the expected role of cross-jet gradients in meridional eddy tilt in stabilizing the jet to its barotropic instability; however, it also reveals important roles played by the along-jet evolution of eddy zonal–meridional elongation. It is the mean flow forcing derived from these patterns that acts to strengthen and extend the jet downstream and forces the time-mean recirculation gyres. This understanding of the dependence of mean flow forcing on eddy structural properties suggests that failure to adequately resolve eddy elongation could underlie the weakened jet strength, extent, and changed recirculation structure seen in this idealized model for reduced spatial resolutions. Further, it may suggest new ideas for the parameterization of this forcing.

Convective Heat Transfer in an Impinging Synthetic Jet: A Numerical Investigation of a Canonical Geometry

Synthetic jets are generated by an equivalent inflow and outflow of fluid into a system. Even though such a jet creates no net mass flux, net positive momentum can be produced because the outflow momentum during the first half of the cycle is contained primarily in a vigorous vortex pair created at the orifice edges; whereas in the backstroke, the backflow momentum is weaker, despite the fact that mass is conserved. As a consequence of this, the approach can be potentially utilized for the impingement of a cooling fluid onto a heated surface. In previous studies, little attention has been given to the influence of the jet's origins; hence it has been difficult to find reproducible results that are independent of the jet apparatus or actuators utilized to create the jet. Furthermore, because of restrictions of the resonators used in typical actuators, previous investigations have not been able to independently isolate effects of jet frequency, amplitude, and Reynolds number. In the present study, a canonical geometry is presented, in order to study the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. The unsteady Navier–Stokes equations and the convection–diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. A detailed analysis was performed on the correlation between the complex velocity field and the observed wall heat transfer. Scaling analysis of the governing equations was utilized to identify nondimensional groups and propose a correlation for the space-averaged and time-averaged Nusselt number. A fundamental frequency, in addition to the jet forcing frequency, was found, and was attributed to the coalescence of consecutive vortex pairs. In terms of time-averaged data, the merging of vortices led to lower heat transfer. Point to point correlations showed that the instantaneous local Nusselt number strongly correlates with the vertical velocity v although the spatial-temporal dependencies are not yet fully understood.

Large-eddy simulation of vortical structures in a forced plane impinging jet

Large-eddy simulations (LES) of two plane impinging jets have been conducted. Predictions were first conducted for a natural impinging jet and found to be in good agreement with the experimental data of Yoshida et al. The validated code was then used to study the vortical structures of a forced impinging jet which had been experimentally investigated by Sakakibara et al. The numerical results show that the predictions have clearly captured the spanwise rollers, successive ribs, cross ribs and wall ribs observed by Sakakibara et al. They also show the predicted average convection velocity to be in good agreement with the measured value. Overall, the present study demonstrates the potential of LES simulations as a reliable tool to optimize the performance of engineering systems involving the use of forced impinging jets by regulating cross ribs through the inlet perturbations.

Model Experiments with New Lance for Pneumatic Powder Injection

The paper presents the results of model experiments with new lance design. The lance was invented in Silesian University of Technology, Department of Foundry. It is dedicated for the pneumatic powder injection into liquid metal bath without lance submersion. The situation like that is common for small molten alloy volume when the problem of temperature decrease is significant. In such case the carrier gas introduction into metal as well as cooling with gas blowing onto metal bath surface should be avoided. The new lance was equipped with four parallel slots made on its outlet and a flange. This design causes the carrier gas evacuation before it reaches the molten metal surface. The article presents the model experiments made with use of the device for the diphase gas-powder jet forces recording. The comparison of two lances: normal steel pipe and this of new design was presented in the article, too.

Experimental investigation on arc characteristic and droplet transfer in CO2 laser–metal arc gas (MAG) hybrid welding

The present work reports results obtained using computer-based system techniques to investigate the metal transfer in laser–MAG hybrid welding. This arc characteristic information can be recorded by combining the sensing of the welding current and arc voltage with high speed imaging of the droplet transfer. An LD pumped green laser was used to overcome the strong interference from the welding arc. The arc energy determined mode of droplet transfer. When the arc energy is less than 4kW, the transfer mode is short circuit transfer, or globular transfer or transfer mode between short circuit and globular. When the arc energy exceeds the 4.68kW, the transfer mode is spray type. The laser energy has an important influence on the arc length, droplet size and the droplet velocity. When the arc energy is less than 4kW, the attraction and compression of laser on arc play an important role, which caused the average arc length increases with increasing laser power. When the arc power is beyond 4.68kW, the average arc length is decreasing with increasing laser power, which is attributed to expansion of the arc plasma, and enhancement of jet force of metal vapor on droplet. The droplet velocity decreases with increasing laser power, which is attributed to the attraction force of laser plasma to arc and the jet force of metal vapor produced by laser keyhole on droplet.