Water jet peening can effectively improve the fatigue strength of metal materials, and the outlet shape of nozzle greatly affects the effect of water jet peening. In this paper, the effects of nozzle outlet shape on water jet velocity and impact pressure is studied by numerical simulation, and the jet velocity and dynamic pressure for different standoff distances are also discussed. The results show that the water jets of square, circular and triangular nozzles are highly concentrated, and the water jet of elliptical nozzles is the most divergent. The axial velocity attenuation of the square nozzle along the axis is slower than that of the other three nozzles. The water axial velocity of the elliptical nozzle attenuates fastest and the length of the core segment of the water jet is the smallest. Within a certain axial distance, the dynamic pressure area in the central area of the elliptical water jet is obviously larger than that of the other three nozzles, and the effective treatment range is large, which is more suitable for the welding surface strengthening operation.

Experimental results of observing ethanol micro-jets expiring into a highly rarefied medium (vacuum) through a nozzle are presented. The study of the process was carried out both at the horizontal and vertical liquid stream from the source compared to the direction of gravity The residual background gas pressure in the vacuum chamber was maintained at a level much lower than the saturated vapor pressure of the working fluid at a given outlet temperature. The possibility of modeling complex processes of micro-fluids expiring into a medium with a given rarefied atmosphere on a compact vacuum gas-dynamic stand is shown. It is established that the long-term flow from a thin capillary or a small-diameter hole into a vacuum or a highly rarefied gas medium differs significantly from the well-studied flow modes into a dense gas medium, as well as from the pulsed flow modes into a vacuum. The paper describes the main features of the flow and the conditions for the occurrence of instability. It is shown that the long-term flow of a liquid micro-jet in a vacuum has a high degree of surface instability, with a large number of sudden changes in the direction, structure, and observed density. An explanation of the reasons for the destruction of the micro-jet is proposed. The formation of surface gas caverns causing explosive destruction of the micro-jet with the release of vapor-liquid droplets is established.

The effects of an active flapping jet actuator on the wake flow dynamics behind a circular cylinder in wind tunnel tests were investigated. An active flapping jet actuator was embedded in the cylinder in advance to invoke a spontaneous flapping jet into the cylinder's wake. The experiment, which was performed in a wind tunnel with a Reynolds number of Re = 1.99 × 10 4 , was based on the oncoming wind speed, cylinder diameter, and kinematic viscosity of the air at the laboratory's temperature. The flow-field structures behind the cylinder model with different dimensionless jet momentum coefficients, C u , were obtained using the high-speed particle image velocimetry technique. The proper orthogonal decomposition (POD) method was used to represent the variation of the POD mode energy, mode coefficients, and the reconstructed spreading vorticity. The dynamic temporal evolution and time-averaged results in the near wake region of the cylinder with and without active flapping-jet control were calculated and analyzed to illustrate the rich phenomena produced by, and the control effect of, the flapping jet. For C u values up to 0.0554, the periodic vortex shedding was pushed to farther wakes. Meanwhile, the time-averaged wake changed considerably, and the distributions of the turbulent kinetic energy and Reynolds shear stress decreased significantly. A data-driven dynamic mode decomposition method was used to extract the coherent structure of the wake of the cylinder embedded with the flapping jet actuator. The Strouhal number of the main mode of the C u = 0.0865 case was different from the natural case.

Impinging circular free-surface water jets are used in challenging cooling and cleaning tasks. In order to develop simulation models for process optimization, validation data are required, which are currently not available. Therefore, the flow field of these jets is studied for the first time with the novel laser Doppler velocity profile sensor. The mean velocity field and fluctuations are measured within the stagnation and adjacent redirection region for radial coordinates up to three times the nozzle diameter. In the examined parameter range with jet velocities up to 17 m/s and nozzle diameters up to 5.2 mm, i.e., Reynolds numbers up to 69 500, thin films of a few hundred micrometers are formed, which hinder the measurement with common optical measuring systems. Based on the measurement results, a comparatively low-cost volume of fluid simulation model is developed and validated that presumes a relaminarized film flow. The profiles measured and the simulated flow show very good agreement. In the future, the simulation model provides a basis for process optimization and the innovative measurement technology used will prospectively provide further detailed insights into other flows with high velocity gradients.

This work presents an analytical study of the flow and energy loss immediately downstream of rectangular sharp-crested weirs for free and submerged flows, using the theory of plane turbulent jets and the analysis of some relevant studies. The flow regimes downstream of the sharp-crested weir is characterized as the impinging jet and surface flow regimes. Based on the flow characteristics and the downstream tailwater depths, each flow regime is further classified, and the relative energy loss equation is developed. It is found that significant energy loss occurs for the regime of supercritical flow and the upper stage of impinging jet flow. The energy loss for the submerged flow regime is minimal.

Wiping jet impingement pressure is important in controlling the coating mass (thickness) and influencing the smoothness of the thin metallic coating produced in continuous galvanizing lines (CGLs). However, the fluctuation of the impingement pressure profile that directly impacts the coating smoothness has not been adequately understood. To study key features of the impingement pressure fluctuation, the instantaneous impingement pressure profiles obtained from large eddy simulations (LES) were analyzed using proper orthogonal decomposition (POD). Dominant fluctuation modes of pressure profiles can be differentiated based on the energy contents of the modes corresponding to different jet types, namely, mixing, nonmixing, and transitional mixing jet. The dominant modes of mixing jets in the wiping region contain comparable strength of all modes (flapping, pulsing, and out-of-phase multipulsing). Nonmixing jets do not show discernable fluctuation modes, and transitional mixing jets show pulsing and flapping modes only. Additionally, instantaneous maximum pressure gradients and their locations were determined from the reduced-order reconstructions of the pressure profiles. From the analysis, frequency spectra of the magnitude and location fluctuations of the maximum pressure gradients associated with each of the jet types can be clearly distinguished. This is the knowledge that may be helpful for CGL operators in the operation of wiping jets.

The spreading characteristics of particle-bearing jets down a gentle slope in a uniform ambient with confined side-walls are reported through a series of laboratory experiments. A round water jet is mixed with solid particles having particle volume fraction ranging between 0% and 0.4%. The jet Reynolds number is varied between Re = 3000 and 8000. We document that the jet front position varies with time t as x f ∝ t 2 / 3 . The jet front propagation in the presence of side-walls is found to be higher than the unconfined case and is attributed to the recirculation due to the confinement that increases the flow inertia and accelerates the flow. The jet propagation is found to be self-similar and is unaffected by the variations in the volume fraction and Re. The sediment pattern near the source, formed by the settling of the particles, exhibits a similar tear-drop shape, which is well-predicted using the unconfined jet theory that assumes a Gaussian velocity profile. The results have implication in engineering and environmental flows, where higher jet propagation rate for confined jets should be modeled accurately without modifying the sedimentation dynamics.

The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. This study carries out three-dimensional large eddy simulations (LESs) of the flow from a tubular radially lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. The flow is studied at two Reynolds numbers of around 6000 and 75,000, based on the effective jet diameter. The low Reynolds number jet is compared to that from a round nozzle and experimental data to demonstrate changes in mixing mechanisms. The present simulations confirmed the presence of Kelvin–Helmholtz (K–H)-like modes and their evolution. The analysis also confirms the evolution of three distinct types of structures—the large-scale streamwise modes at the lobe crests, corresponding K–H structures at the troughs, and an additional set of structures generated from the lobe walls. The higher Reynolds number simulations indicate changes in the mechanics with a subdued role of the lobe walls.

Here we consider velocity profiles of three dimensional attaching and attached jets emerging from circular nozzles. Like their well-studied two-dimensional counterparts, these wall jets lose momentum due to interactions with nearby surfaces. Unlike their two-dimensional counterparts, simple and quantitative expressions for the velocity profiles of three-dimensional wall jets remain elusive. Here we present a quantitative analytical model of the three-dimensional velocity profiles of attached jets inclusive of a local skin coefficient of friction. We compare these expressions to experimental velocity profiles at moderate and high nozzle Reynolds numbers to find reasonable quantitative agreement. This work has implications for a variety of industries including nuclear waste processing, where jet flows in mixing vessels suspend solids and gases trapped in radioactive waste tanks.

The effects of nozzle orientation on the mixing and turbulent characteristics of elliptical free twin jets were studied experimentally. The experiments were conducted using modified contoured nozzles with a sharp linear contraction. The centers of the nozzle pair had a separation ratio of 5.5. Two nozzle configurations were tested, twin nozzles oriented along the minor plane (Twin_Minor) and twin nozzles oriented along the major plane (Twin_Major) and the results were compared with a single jet. In each case, the Reynolds number based on the maximum jet velocity and the equivalent diameter was 10,000. A planar particle image velocimetry (PIV) system was used to measure the velocity field in the jet symmetry plane. It was observed that the velocity decay rate is not sensitive to nozzle orientation. However, close to the jet exit, the spread rate was highest in the minor plane. In addition, contour plots of swirling strength, Reynolds shear stress and turbulent intensities revealed significant differences between the minor and major planes. Velocity profiles showed little variation close to the jet exit, while further downstream the variations between the velocity profiles were more pronounced between the major and minor planes.

This study explores the structure of liquid/gas coaxial jets under forced and unforced conditions. The forcing is in the form of a transverse acoustic resonance within the confined space where the mixing occurs. The studied flows are relevant to combustion instabilities which involve an interaction between acoustic waves and reactant mixing. A variety of local and global signal processing methods were applied to digital flow visualization data to identify spatial and temporal features. The unforced case is in particular chaotic and influenced by a broad range of spatial and temporal phenomena. Proper orthogonal decomposition (POD) was able to extract flapping and convecting features, and spectral content of these behaviors is presented. The forced case results in organized structures that emerge above the background turbulence, including harmonics of the forcing frequency and nonlinear interactions between specific frequencies. The dynamic mode decomposition (DMD) performs the best in the forced case, clearly isolating all of these features. Wavelet analysis showed that forcing tended to reorganize energy from longer to shorter time scales. Bicoherence analysis of the data showed that the forcing causes a much different energy exchange in the outer and inner shear layers. The outer-to-inner jet coupling during forced conditions appears to be limited to an axial extent of about one to three inner jet diameters downstream of the jet exit. The recirculation zone between the inner and outer jet, extending about one inner jet diameter downstream, appears to disrupt the influence of forcing on the inner jet.

Detailed near- and far-field acoustic measurements were conducted for two circular stepped nozzles with 30 deg and 60 deg design inclinations at over- and perfectly-expanded supersonic jet flow conditions and compared to those for a circular nonstepped nozzle. Far-field acoustic results show that stepped nozzles play an insignificant role in altering noise emissions at perfectly expanded condition. At an over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. Noise spectra analysis and Schlieren visualizations show that noise reduction can be primarily attributed to mitigations in the broadband shock-associated noise (BSAN), due to the ability of the longer stepped nozzle in suppressing shock strengths at downstream region. Near-field acoustic measurements reveal that the source region, as well as the intensity of turbulent and shock noises, are highly sensitive to the stepped nozzle configuration. Furthermore, BSAN seems to be eliminated by the longer stepped nozzle in near-field region due to the shock structure modifications.

The goal of this study is to investigate the interactions between turbulent kinetic, internal, and magnetic energies in planar magnetohydrodynamic (MHD) jets at different regimes of Mach and Alfvén Mach numbers. Toward this end, temporal simulations of planar MHD jets are performed, using two types of initial fluctuating velocity field: (i) single velocity perturbation mode with a streamwise wavevector and (ii) random, isotropic perturbations over a band of wavevectors. At low Mach numbers, magnetic tension work results in a reversible exchange of energy between fluctuating velocity and magnetic fields. At high Alfvén Mach numbers, this exchange results in the equipartition of turbulent kinetic and magnetic energies. At higher Mach numbers, dilatational kinetic energy is (reversibly) exchanged with internal and magnetic energies, by means of pressure-dilatation and magnetic-pressure-dilatation, respectively. Therefore, at high Mach and Alfvén Mach numbers, dilatational kinetic energy is seen to be in equipartition with the sum of turbulent internal and magnetic energies. In each of the regimes, the consequent effect of the interactions on the background Kelvin–Helmholtz vortex evolution is also identified.

This work deals with the improvement in aerodynamic performance of a NACA0025 wing model using an array of slotted synthetic jets (SJs). A novel SJ actuator was designed and located at 30% of the chord from the leading edge (LE). Time-resolved particle image velocimetry (TR-PIV), force balance, static pressure distribution, and hotwire measurements were carried out in a subsonic wind tunnel to assess the performance enhancement due to the slotted SJ array. Initially, the SJ velocity was measured in quiescent flow condition at different actuation frequencies and amplifier voltages. Actuation at 1000 Hz and 200 V resulted in the highest blowing velocity of 10.5 m/s. Experiments were performed at various actuation frequencies, namely, 200, 600, and 1000 Hz. It was observed that actuation at 1000 Hz led to the highest increase in lift coefficient by 35.6% and reduction in average drag coefficient by 33%. TR-PIV measurements showed flow separation with flow reversal in the baseline case. After switching on the SJ array at 1000 Hz, the flow separation was completely eliminated. The momentum transfer from the highenergy primary flow to the retarding boundarylayer flow and actuation of SJ in a particular frequency range was observed to be the mechanisms for the flow separation control. Subsequently, fast Fourier transform (FFT) power spectra of hotwire data were computed from 40% to 80% of the chord. The FFT power spectra showed the successful stabilization of the flow field at the actuation of 1000 Hz.

The effect of viscosity on the unstable interface of the cylindrical jet is analyzed through viscous potential flow approach. The jet is moving radially and jet interface is experiencing Rayleigh–Taylor type instability. Previous studies have completely ignored the viscosity effect while considering the instability of a radially moving cylindrical jet. The fluids inside and outside jet are incompressible as well as viscous. The theoretical analysis provides us a second-order ordinary differential equation to establish the instability/stability criterion. The radial velocity and acceleration both have significant impact on the stability of the jet. We found that as viscosity enters to the analysis, perturbations grow rapidly. In addition, the acquired stability criterion is applied to the cylindrical jets in HYLIFE-II which is basically an inertial confinement fusion reactor.

We consider the problem of nonlinear rotating non-Newtonian jets in the presence of ambient flows. Using the original governing system of equations for such jet flows, we use scaling and perturbation techniques to reduce such system to a simplified one where the stress tensor is governed by Giesekus constitutive equations. We develop a method to take into account the effect of the ambient flow, which usually exists due to the externally imposed rotational constraint, on the formation of the non-Newtonian jet. We compute numerically the nonlinear steady solutions for the jet and determine expressions for the jet quantities like speed, stretching rate, radius, strain rate, and tensile force. These quantities are calculated for different values of the parameters such as those due to the ambient flow and viscoelasticity. We find that the ambient flow is stabilizing so that the strain rate, stretching rate, speed, and tensile force decrease with increase in the ambient flow effect, while the jet radius increases with such effect. The viscoelasticity of the fiber jet reduces notably the stabilizing effect of the ambient flow on the jet quantities, but the stabilizing effect of the ambient flow increases with the arc length of the jet centerline.

In this study, large eddy simulations (LES) of turbulent coflow jets are performed and designed to investigate the effects of the jet-to-coflow velocity ratio, V r , on jet characteristics. A fully developed turbulent pipe flow at Re = 10 , 000 , based on the bulk velocity and pipe diameter, is employed as the jet outlet in this work. A comparison between laminar and turbulent jets is performed against the experimental results of a jet produced by a fully developed turbulent pipe flow. For the coflow jet, simulations with different jet-to-coflow velocity ratios (V r = 3, 6, 12, and ∞ ) are performed to investigate the turbulence intensities and the decay of the centerline velocity of the jet. The results give two constant decay rates: K u ≈ 0.144 for single-phase jets and K u ≈ 0.133 for particle-laden jets. With a decrease in V r (i.e., a higher coflow velocity), the results show a higher peak value and a larger droop rate for turbulence intensities. This study is then extended to investigate particle distribution under a two-way coupling regime, using a Lagrangian framework. The particle velocity and distribution along the jet centerline, and the particle clustering and radial probability distribution in the jet downstream domain are analyzed with the same coflow jet parameters. The particles tend to move faster and distribute preferentially in the center region with a decrease in V r , which agrees with the increasing turbulence intensities along the jet centerline in the present work.

An analytical Bessel-sine series solution is presented for inviscid, incompressible, axisymmetric, annular impinging jets. The inflow velocity profile is relatable to an aircraft engine thrust reverser, and has a velocity deficit as well as nonzero vorticity function at the inner radius. As a consequence, inviscid recirculation appears at the impingement corner, the strength of which is made determinate by assuming a local patch of constant vorticity function. Results are first compared against prior round jet solutions, with rigorous convergence and accuracy assessment, before extension to annular impinging jets. Finally, while the Bessel-sine series are consistent with the governing equations and boundary conditions, computational issues arise with steep near wall gradients of a composite parameter that is inherent to the methodology, necessitating extremely fine mesh resolution. A hybrid of finite differences and Bessel series is proposed to address these numerical issues.

It has become apparent recently that the fluidic oscillators, also known as sweeping jets, can be used to create a combination of steady (streamwise vortices) and unsteady (spanwise vortices) forcing mechanisms which have the potential to fulfill many of the promises of active separation control. The fluidic oscillators contain no moving parts, but produce an unsteady component via a natural feedback loop inherent to their geometry. The oscillations are entirely self-induced and self-sustaining. Their simple and robust design and their effectiveness over a wide range of flow conditions make them more attractive than other flow control devices, such as synthetic jets and plasma actuators. Figure 1 shows the instantaneous jet generated in quiescent environment using the Improved Delayed Detached Eddy Simulation (IDDES) model, where the Large Eddy Simulation (LES) branch of the IDDES model is able to capture the turbulence structures properly. An instantaneous iso-surface of vorticity magnitude, colored by streamwise velocity for flow over a wall-mounted hump is depicted in Figure 2. As expected, a massive flow separation occurs behind the hump in the uncontrolled condition (Figure 2 (a)), with a nearly two-dimensional free shear layer at the edge of the separation line. Breakdown of the shear layer by an array of sweeping jets located slightly downstream of the separation line is seen in Figure 2 (b), which is followed by the elimination of the separation region behind hump. The three-dimensional structures generated by the sweeping jets are smaller and closer to the hump wall than those produced by the steady jets shown in Figure 2 (c). Presence of a large region of reversed flow near the hump wall in its aft section is also seen in the case of the steady jet. This study indicates a superior effectiveness of sweeping jets on separated flows.

An experimental investigation of nozzle orientation effects on turbulent characteristics of elliptic triple free jets was carried out for three nozzle configurations. The first configuration had each nozzle oriented along the minor plane (3_Minor), the next had two nozzles oriented along the minor plane and one along the major plane (Min_Maj_Min) and the last configuration had one nozzle oriented along the minor plane and two along the major plane (Maj_Min_Maj). The experiments were conducted using modified contoured nozzles with a sharp linear contraction for a nozzle spacing ratio of 4.1d, a nozzle equivalent diameter of 9 mm, and Reynolds number of 10,000. Nozzle orientation effects on the mean velocity, turbulent intensity, and Reynolds shear stress were discussed. The velocity decay, jet spread, merging point (MP), combined point (CP), and potential core length were used to characterize the effects of nozzle orientation on the mixing performance. The 3_Minor configuration had shorter potential core length and closer MP location which are indicative of a faster mixing in the converging region. The early merging of 3_Minor led to higher levels of streamwise turbulent intensity. One-dimensional plots revealed that jets approached self-similarity at a faster rate in the major axis. The orientation of the middle jet was found to be a key factor in determining transverse diffusion of the Reynolds shear stress in the plane of observation. Two-point correlations were used to provide insight into the effects of nozzle orientation on the spatial coherence of the large-scale turbulence structure and integral length scale.