An experimental study on jets issuing from elliptic inclined nozzles

This paper reports on an experimental flow visualisation and digital particle image velocimetry investigation on forced jets exhausting from aspect ratio equal to three elliptic nozzles with exits inclined at 30° and 60°. Flow images show that shear layer instabilities and subsequent vortex roll-ups are formed parallel to the inclined nozzle exits at 30° incline and that rapid re-orientation of the vortex roll-ups occurs at 60° incline. Flow observations also show that strong axis-switching occurs in a non-inclined elliptic nozzle. However, 30° and 60° elliptic inclined nozzles produce significant distortions to and suppression of the axis-switching behaviour, respectively. As a result, flow stresses and turbulent kinetic energy distributions become increasingly asymmetric. Their coherency and magnitudes along the shorter nozzle lengths also vary significantly. This can be attributed to the dissimilar formations of vortex roll-ups and rib structures, as well as unequal mutual interactions between them as the incline-angle increases. Lastly, results also show that unlike circular inclined nozzles, elliptic inclined nozzles do not produce serpentine-shaped jet columns nor lead to significant lateral jet-spread at large incline-angles.     

Vortex Behaviour and Velocity Characteristics of Jets Issuing from Hybrid Inclined Elliptic Nozzles

An experimental study on elliptic nozzles with hybrid flat- and inclined-sections is reported here. The hybrid flat and inclined sections are imposed along either the major- or minor-plane of the nozzles (HIN?A and B nozzles respectively). For HIN?A, results show prevalent pairings between adjacent vortex filaments and induced vortex-loops in the immediate vicinity of the flat-section to produce coalesced vortex roll-ups. Once they detach entirely from the nozzle, they proceed to undergo flow changes resembling that of conventional elliptic jets. HIN?B also leads to near-field vortex pairings but produce discrete inclined vortex roll-ups instead, with accompanying delay in rib structure formations. The roles of induced vortex-loops are significantly more limited in the present elliptic HIN than circular HIN investigated previously, due to the dominance of elliptic braid vortices. HIN?A produces significantly larger centerline velocity decay, as well as higher turbulence levels in the near vicinity of the nozzle exit. Vectoring of axial jet momentum is more apparent for HIN?B, where cross-stream entrainment is also relatively larger. Half-jet width results also demonstrate that both nozzle types eventually produce elliptic jets that undergo axis-switching. Lastly, momentum thickness results suggest that the present nozzle lip-modifications significantly enhance mixing characteristics along the plane upon which they are imposed in both HIN?A and B.     

UV-mediated coalescence and mixing of inkjet printed drops

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.     

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers (Re = 1 × 10⁴, 2 × 10⁴ and 3 × 10⁴) and four different inclination angles between the wall and the flow velocity at the nozzle (β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.     

Effectiveness distribution measurements for a row of heated circular jets impinging on a cylindrical convex surface at different inclinations

The spatially resolved effectiveness distributions for a single jet and row of circular jets impinging on a convex surface are reported in the present study. The impinging surface was inclined at 0°, 15°, 30° and 45° to the jet axis. Studies were conducted for a single curvature ratio equal to 0.05 at a constant Reynolds number equal to 40,000 for non-dimensional jet-to-target distances, L/d equal to 2, 4, 6, 8 and 10. Two non-dimensional jet-to-jet spacings, S/d, equal to 4 and 8 were studied. The effectiveness distribution for multiple jet impingement was noticed to be different from that for a single jet impingement. The entrainment from the surrounding was mitigated for the inner jets by the outer jets. The interaction of adjacent walljets forms a ‘barrier’ against the percolation of entrained ambient from the outer jet region towards the inner region. The zone of walljet interaction and region near to the inner jets were therefore observed to result in high effectiveness values. The inclined impingement of the jet reduces the strength of interaction of the walljets on up and downhill sides and thereby reduces the ‘barrier effect’ against the entrainment of ambient, which causes similar variation of effectiveness for all the jets in a row at high inclinations.     

Dependence of the self-oscillation period for a conical jet aerator cap on the jet width in the nozzle outlet

The penetration of free hollow thin-walled turbulent water jets into water is considered. These jets are generated in a conical jet aerator whose apex angle is 60°. The periods of steady regular self-oscillations appearing during the process of penetration are studied experimentally. A dependence of these periods on the annular nozzle gap width δ, 0.07 ≤ δ ≤ 0.12 cm, is analyzed for the jet discharge range 160 ≤ Q ≤ 550 cm³/s when the height H of the annular nozzle above the water surface belongs to the range 1 ≤ H ≤ 28 cm.     

Laboratory experiments of a jet impinging on a stratified interface

 The entrainment rates of vertical and inclined jets impinging on a stratified interface are measured in water tank experiments. At moderate Richardson number, the entrainment rate of the vertical jet is proportional to Ri ^-1/2, independent of Reynolds number. The inclined jets are tilted at 15° from the vertical. In one case, the jet nozzle is rotated about a vertical axis, so that the inclined jet precesses, while in the other, it is stationary. The inclined jets entrain at a rate proportional to Ri ^-3/2, whether precessing or not. This behavior is consistent with a new model of stratified entrainment which accounts for vortex persistence. Received: 15 October 1996/Accepted: 19 December 1996     

Experimental investigation of inclined liquid water jet flow onto vertically located superhydrophobic surfaces

In this study, the behaviour of an inclined water jet, which is impinged onto hydrophobic and superhydrophobic surfaces, has been investigated experimentally. Water jet was impinged with different inclination angles (15°–45°) onto five different hydrophobic surfaces made of rough polymer, which were held vertically. The water contact angles on these surfaces were measured as 102°, 112°, 123°, 145° and 167° showing that the last surface was superhydrophobic. Two different nozzles with 1.75 and 4 mm in diameters were used to create the water jet. Water jet velocity was within the range of 0.5–5 m/s, thus the Weber number varied from 5 to 650 and Reynolds number from 500 to 8,000 during the experiments. Hydrophobic surfaces reflected the liquid jet depending on the surface contact angle, jet inclination angle and the Weber number. The variation of the reflection angle with the Weber number showed a maximum value for a constant jet angle. The maximum value of the reflection angle was nearly equal to half of the jet angle. It was determined that the viscous drag decreases as the contact angle of the hydrophobic surface increases. The drag force on the wall is reduced dramatically with superhydrophobic surfaces. The amount of reduction of the average shear stress on the wall was about 40%, when the contact angle of the surface was increased from 145° to 167°. The area of the spreading water layer decreased as the contact angle of the surface increased and as the jet inclination angle, Weber number and Reynolds number decreased.     

Experimental study of slot jet impingement heat transfer on a wedge-shaped surface

An experimental investigation was conducted to study the convective heat transfer rate from a wedge-shaped surface to a rectangular subsonic air jet impinging onto the apex of the wedge. The jet Reynolds number, nozzle-to-surface distance and the wedge angle were considered as the main parameters. Jet Reynolds number was ranged from 5,000 to 20,000 and two dimensionless nozzle-to-surface distances h/w?=?4 and 10 were examined. The apex angle of the wedge ranged from 30° to 180° where the latter case corresponds with that of a flat surface. Velocity profile and turbulence intensity were provided for free jet flow using hot wire anemometer. Local and average Nusselt numbers on the impinged surface are presented for all the configurations. Based on the results presented, the local Nusselt number at the stagnation region increases as the wedge angle is decreased but, it then decreases over the remaining area of the impinged surface. Average Nusselt number over the whole surface is maximum when the wedge angle is 180° (i.e. plane surface) for any jet and nozzle-to-surface configuration.     

Self-oscillation regimes of the penetration of free conical thin-walled turbulent jets through a fluid surface

Certain results of an experimental investigation of hydrodynamic effects occurring, when free thin-walled turbulent jets issuing from a conical slot nozzle with a vertical axis penetrate through the surface of a fluid in a rectangular reservoir, are presented. The ranges of the jet flow rate and the spacing between the nozzle and the free surface, on which stable regular transverse self-oscillations of the boundaries of dome-shaped jets are observable, are determined. For fixed values of the conicity angle α = 60° and the nozzle slot width δ = 0.1 cm the characteristic form of the dependence of the self-oscillation period on the jet flow rate and the spacing between the slot nozzle and the free surface (dome height) is presented. The self-oscillation regime generation mechanism, together with the possible reasons for the revealed bifurcation changeover of the oscillation mode at certain values of the governing parameters and the hysteresis effects, are discussed. The salient features of the flows occurring on the surface and within the fluid beneath the dome are described.     

Dynamics of the flowfield generated by the interaction of twin inclined jets of variable temperatures with an oncoming crossflow

The present paper examines the common configuration of “twin inclined jets in crossflow” that is widely present in several industrial and academic, small and large-scale applications. It is particularly found in aerodynamic and engineering applications like VTOL aircrafts, the combustion mixing process and other chemical chambers. It can also be found in some domestic applications like chimney stacks or water discharge piping systems in rivers and seas. The twin jets considered in this work are elliptic as inclined with a 60° angle and arranged inline with the oncoming crossflow according to a jet spacing of three diameters. They are examined experimentally in a wind tunnel. The corresponding data is tracked by means of the particle image velocimetry technique in order to obtain the different instantaneous and mean dynamic features (different velocity components, vortices, etc.). The same case is numerically reproduced by the resolution of the Navier–Stokes equations by means of the finite volume method together with the Reynolds stress model second order turbulent closure model. A non-uniform mesh system tightened close to the emitting nozzles is also adopted. The comparison of the measured and calculated data gave a satisfying agreement. Further assumptions are adopted later in order to improve the examined configuration: a non-reactive fume is injected within the discharged jets and the jets’ temperature is varied with reference to a constant mainstream temperature. Our aim is to evaluate precisely the impact of this temperature difference on the flow field, particularly on the dynamics of the jets in a crossflow. This parameter, namely the temperature difference, proved mainly to accelerate the discharged jet plumes in the direction of the main flow, which enhanced the mixing, particularly in the longitudinal direction. The mixing in the other directions was also increased due to the weaker density of the jets, which enabled them to progress relatively unhindered before undergoing the impact of the crossflow.     

Control of two-degree-of-freedom vortex-induced vibrations of a circular cylinder using a pair of synthetic jets at low Reynolds number: Influence of position angle and momentum coefficient

In order to understand the effects of synthetic jets on the active control of two-degree-of-freedom (2DOF) vortex-induced vibrations (VIVs) of a circular cylinder, a series of numerical simulation were carried out at Reynolds number of 150. The synthetic jet excitation frequency was fixed at five times of the natural frequency of the cylinder in still water. The influence of two key parameters of synthetic jets, the position angle (α) and the momentum coefficient (Cu), on 2DOF VIVs was analyzed. Results indicated that both in-flow oscillation and cross-flow oscillation can be suppressed when the synthetic jets with sufficient momentum coefficient were positioned at the circular cylinder's leeward side (0° ≤ α ≤ 75°). When Cu = 4, 15° ≤ α ≤ 60°, the reductions of cross-flow and in-flow oscillation amplitudes were all larger than 99% and 70%, respectively. Besides, the in-flow oscillation frequency was locked-in to the excitation frequency of synthetic jets when the in-flow oscillation was effectively suppressed. A symmetric wake can be observed when the cross-flow oscillation was completely suppressed, and the 2P+2S vortex pattern can be observed when Cu = 4, α =165° and 180°.     

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

Temperature impact on the turbulence generated by the interaction of twin inline inclined jets in crossflow

Consideration is given to the interaction of twin tandem jets with an oncoming uniform crossflow. A variable temperature is assumed for the emitted jets while the crossflow is maintained constant, equivalent to the ambient temperature. Both jet nozzles are elliptic, as initially inclined with an angle of 60°, placed three diameters apart in line with the crossflow and discharge a nonreactive fume. The handled configuration is numerically simulated in the present work, by means of the finite volume method together with a non uniform grid system. The model is first validated with reference to available experimental data, in the simple isothermal case of air jets in air crossflow. It is then upgraded by considering a nonreactive fume discharged at a variable temperature. The upgraded model turbulence is described by means of the Reynolds Stress Model second order turbulent closure model. The present work is to our knowledge pioneering in the introduction of this particular model is such a configuration and its introduction proved to be highly valuable since is described satisfyingly the turbulent behavior of the resulting flowfield. This behavior is, precisely, specified in terms of shear stress components whose evolutions, explored along the different directions of the domain, showed a more pronounced vertical mixing, and gave rise to more significant vortices in most characterizing zones: near the injection plane as well as within the discharging nozzles.     

Breakup of liquid jets from non-circular orifices

The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.     

Modified flapping jet for increased jet spreading using synthetic jets

The present paper is an experimental investigation, using a PIV system, on modified rectangular jet flow co-flowing with a pair of synthetic jets placed symmetrically with respect to the geometric centerline of the main flow. The objective was to determine the optimal forcing conditions that would result in jet spreading beyond what would be obtained in a simple flapped jet. The main jet had an exit Re_h = 36,000, based on the slot height, h. The synthetic jets were operated in a periodic manner with a periodic momentum coefficient of about 3.3% and at a frequency of the main jet preferred mode. A short, wide angle diffuser of half angle of about 45° was attached to the main jet. Generally for the vectored jet, much of the flow features found here resembled those reported in the literature except that the deflection angle in this study increased with downstream distances inside the diffuser and then remained roughly unchanged thereafter. Larger jet spreading was achieved when the main jet was subjected to simultaneous actuation of the synthetic jets but the flow did not achieve the initial jet spreading that was observed in the vectored jet. Further jet spreading was achieved when the synthetic jets were alternately actuated in which each synthetic jet was actuated for a number of cycles before switching. This technique allowed the jet to flap across the flow between transverse positions larger than what would be obtained in a simple flip-flop jet. Under the present flow geometry and Reynolds number, it was found that when the ratio f_s/f_al, where f_s is the synthetic actuation frequency and f_al is the alternating frequency, was larger than 10, the mean streamwise velocity of the main jet had two peaks symmetrically placed with respect to the jet axis and the jet had the appearance of flowing into two streams each moving nearly parallel to the diffuser wall. For a value of f_s/f_al of about 10, the optimal value in this study, the desired flow properties were achieved in that, the mean velocity was nearly uniform with an increase in the jet width compared to the simultaneous actuations, and the jet flapping was more effective in redistributing and homogenizing the turbulent kinetic energy across the main jet.     

Opposed bubbly jets at different impact angles: Jet structure and bubble properties

The structure of two colliding water jets containing small gas bubbles is studied experimentally. The effects of the separation distance between jets, as well as the orientation angle, on the spatial distribution of bubbles have been considered. Results on the global structure of the final jet and bubble properties have been obtained using a high-speed video camera, and measurements of the positions of coalescence events are presented. Jets are introduced through inclined pipes (with a diameter of 0.7 mm) into a large water tank to avoid wall effects. Inclination angle has been changed from 0° to 45° with respect to the horizontal, resulting in a 0° up to 90° impact angle between jets. Generation of bubbles is controlled by a T-junction device where a regular slug-flow is created prior to injection. Bubble sizes have been measured, and a mean diameter of around 1 mm has been obtained using high values of the liquid flow rate. In the studied range of separation distances between the bubbly jets, a more homogeneous dispersion of bubbles is created as the distance between jets is decreased and the momentum flux of each jet is increased. Higher numbers of coalescences are observed when using smaller distance between jets, and the obtained measurements revealed that the number of bubble coalescence events is reduced significantly using high values of liquid flow rates.     

Integration of the boussinesq equations for steady channel and jet flows

The equations of long nonlinear waves in round jets and channels of arbitrary cross section are considered with account for the transverse acceleration of the fluid particles (Boussinesq approximation). In the general case of steady flows, the equations in the form of shallow water equations with the pressure expressed in terms of the variational derivative of the kinetic energy of a thin transverse fluid layer, have three first integrals with three arbitrary constants. Examples of solutions of the equations for solitary capillary-gravitation waves in rectangular and triangular channels are presented and compared with the higher approximations. The shape of the free boundary of the round jet is determined. In the case of outflow from a conical nozzle an analytical dependence of the jet contraction ratio on the conicity angle is obtained. The dependence is in agreement with the experimental data for angles of less than 45°.     

Numerical investigation of corner angle and wing number effects on fluid flow characteristics of a turbulent stellar jet

In this research the fluid dynamics characteristics of a stellar turbulent jet flow is studied numerically and the results of three dimensional jet issued from a stellar nozzle are presented. A numerical method based on control volume approach with collocated grid arrangement is employed. The turbulent stresses are approximated using k–ε and k–ω models with four different inlet conditions. The velocity field is presented and the rate of decay at jet centerline is noted. Special attention is drawn on the influence of corner angle and number of wings on mixing in stellar cross section jets. Stellar jets with three; four and five wings and 15–65° corner angles are studied. Also the effect of Reynolds number (based on hydraulic diameter) as well as the inflow conditions on the evolution of the stellar jet is studied. The Numerical results show that the jet entrains more with corner angle 65° and five wings number. The jet is close to a converged state for high Reynolds numbers. Also the influence of the inflow conditions on the jet characteristics is so strong.