Jet Oscillation Research Articles

Hot – wire study on the axisymmetric free jet dependence on the nozzle shape

The paper presents the results of experimental research in axisymmetric free jet performed in order to verify the previous numerical calculations. During the study the influence of the initial conditions at the outlet of the nozzle on the formation of self-sustained oscillations in free jet was determined. The variables affecting the initial conditions were: the shape of nozzle tip (free – slip and no – slip), contraction ratio (C = 128 and 144) and Reynold's number (Re = 5 000 – 40 000). Measurements were taken using the CTA probes. An analysis of the distributions of mean velocity at the nozzle exit shows that a laminar boundary layer was obtained, values of integral boundary layer parameters were not significantly affected by the type of nozzle tip. Some differences were observed in the turbulence intensity distributions in the exit plane and along the jet axis, where characteristic plateau could be noted for case with the free – slip nozzle tip, which indicates the presence of coherent structures in the flow. These results suggest that for almost the same initial conditions obtained for plane and profiled nozzle tip the modification of organized structures in the far flow field was obtained. For physical explanation of these phenomena it was necessary to perform spectral and spatio – temporal correlation analysis (not discussed in the present paper).

Aspect Ratio Effects on Fluid Flow Fluctuations in Rectangular Cavities

The flow from a submerged bifurcated nozzle into rectangular liquid-filled cavities with width-to-thickness ratios W/T = 6.5, 11, and 18 has been studied using free surface visualization and particle tracking. When W/T = 11 and when W/T = 18, self-sustained oscillations of the submerged jets and the free surface are present. When W/T = 6.5, the self-sustained oscillations are no longer present, but oscillations with the frequency of gravity waves occur. We propose a critical value of W/T above which self-sustained jet oscillations occur, based on the spreading angle of turbulent jets. When W/T is larger than this critical value, the shear layers of the jet reach the front and back wall of the cavity before the jet can impinge the side wall, resulting in semi two-dimensional flow in the plane between the front and the back wall. Two-dimensional recirculation zones form alongside the jet leading to the jet oscillations. When W/T is smaller than this critical value, the jet can develop like a free turbulent jet up to an impingement point at the narrow side wall. When the jet impinges the side wall, flow in the directions parallel and perpendicular to the front and back walls is possible, resulting in complex three-dimensional flow patterns. The critical value for W/T, based on the known 12 deg spreading angle of turbulent jets is W/T = 10, which is in good agreement with the experimental results.

Effects of electromagnetic forcing on self-sustained jet oscillations

The influence of electromagnetic forcing on self-sustained oscillations of a jet issuing from a submerged nozzle into a thin vertical cavity (width W much larger than thickness T) has been studied using particle image velocimetry. A permanent Lorentz force is produced by applying an electrical current across the width of the cavity in conjunction with a magnetic field from three permanent magnets across its thickness. As a working fluid a saline solution is used. The magnetic field is in the north-south-north configuration, such that the Lorentz force can be applied in an up-down-up configuration or in a down-up-down configuration by switching the direction of the electrical current. A critical Stuart number Nc was found. For N < Nc, the jet oscillates with a constant Strouhal number St, independent of the Reynolds number Re. For N > Nc and an oscillation enhancing up-down-up configuration of the Lorentz force, St grows with N as St \documentclass[12pt]{minimal}\begin{document}$\propto \sqrt{\mathrm{N}}$\end{document}∝N. In contrast, for N > Nc and an oscillation suppressing down-up-down configuration of the Lorentz force, all jet oscillations are suppressed.

Optimization of jet parameters to control the flow on a ramp

This study deals with the use of optimization algorithms to determine efficient parameters of flow control devices. To improve the performance of systems characterized by detached flows and vortex shedding, the use of flow control devices such as oscillatory jets are intensively studied. However, the determination of efficient control parameters is still a bottleneck for industrial problems. Therefore, we propose to couple a global optimization algorithm with an unsteady flow simulation to derive efficient flow control rules. We consider as a test case a backward-facing step with a slope of 25°, including a synthetic jet actuator. The aim is to reduce the time-averaged recirculation length behind the step by optimizing the jet blowing/suction amplitude and frequency.

Abrasive slurry jet micro-machining of holes in brittle and ductile materials

This paper investigated the effects of elasticity and viscosity, induced by a dilute high-molecular-weight polymer solution, on the shape, depth, and diameter of micro-holes drilled in borosilicate glass and in plates of 6061-T6 aluminum alloy, 110 copper, and 316 stainless steel using low-pressure abrasive slurry jet micro-machining (ASJM). Holes were machined using aqueous jets with 1wt% 10μm Al2O3 particles. The 180μm sapphire orifice produced a 140μm diameter jet at pressures of 4 and 7MPa. When the jet contained 50wppm of dissolved 8 million molecular weight polyethylene oxide (PEO), the blind holes in glass were approximately 20% narrower and 30% shallower than holes drilled without the polymer, using the same abrasive concentration and pressure. The addition of PEO led to hole cross-sectional profiles that had a sharper edge at the glass surface and were more V-shaped compared with the U-shape of the holes produced without PEO. Hole symmetry in glass was maintained over depths ranging from about 80–900μm by ensuring that the jets were aligned perpendicularly to within 0.2°. The changes in shape and size were brought about by normal stresses generated by the polymer. Jets containing this dissolved polymer were observed to oscillate laterally and non-periodically, with an amplitude reaching a value of 20μm. For the first time, symmetric ASJM through-holes were drilled in a 3-mm-thick borosilicate glass plate without chipping around the exit edge.The depth of symmetric blind holes in metals was restricted to approximately 150μm for jets with and without PEO. At greater depths, the holes became highly asymmetric, eroding in a specific direction to create a sub-surface slot. The asymmetry appeared to be caused by the extreme sensitivity of ductile materials to jet alignment. This sensitivity also caused the holes in metals to be less circular when PEO was included, apparently caused by the random jet oscillations induced by the polymer. Under identical conditions, hole depths increased in the order: borosilicate glass>6061-T6 aluminum>110 copper>316 stainless steel. The edges of the holes in glass could be made sharper by machining through a sacrificial layer of glass or epoxy.

Electromagnetic flow control of a bifurcated jet in a rectangular cavity

The effect of Lorentz forcing on self-sustained oscillations of turbulent jets (Re=3.1×103) issuing from a submerged bifurcated nozzle into a thin rectangular liquid filled cavity was investigated using free surface visualization and time-resolved particle image velocimetry (PIV). A Lorentz force is produced by applying an electrical current across the width of the cavity in conjunction with a magnetic field. As a working fluid a saline solution is used. The Lorentz force can be directed downward (FL<0) or upward (FL>0), to weaken or strengthen the self-sustained jet oscillations. The low frequency self-sustained jet oscillations induce a free surface oscillation. When FL<0 the amplitude of the free surface oscillation is reduced by a factor of 6 and when FL>0 the free surface oscillation amplitude is enhanced by a factor of 1.5.A large fraction of the turbulence kinetic energy k=12ui′ui′‾ is due to the self-sustained jet oscillations. A triple decomposition of the instantaneous velocity was used to divide the turbulence kinetic energy into a part originating from the self-sustained jet oscillation kosc and a part originating from the higher frequency turbulent fluctuations kturb. It follows that the Lorentz force does not influence kturb in the measurement plane, but the distribution of kosc can be altered significantly. The amount of energy contained in the self-sustained oscillation is three times lower when FL<0 compared to the situation with FL>0.

Oscillations in solar jets observed with the SOT of Hinode: viscous effects during reconnection

Transverse oscillatory motions and recurrence behavior in the chromospheric jets observed by Hinode/SOT are studied. A comparison is considered with the behavior that was noticed in coronal X-ray jets observed by Hinode/XRT. A jet like bundle observed at the limb in Ca II H line appears to show a magnetic topology that is similar to X-ray jets (i.e., the Eiffel tower shape). The appearance of such magnetic topology is usually assumed to be caused by magnetic reconnection near a null point. Transverse motions of the jet axis are recorded but no clear evidence of twist is appearing from the highly processed movie. The aim is to investigate the dynamical behavior of an incompressible magnetic X-point occurring during the magnetic reconnection in the jet formation region. The viscous effect is specially considered in the closed line-tied magnetic X-shape nulls. We perform the MHD numerical simulation in 2-D by solving the visco-resistive MHD equations with the tracing of velocity and magnetic field. A qualitative agreement with Hinode observations is found for the oscillatory and non-oscillatory behaviors of the observed solar jets in both the chromosphere and the corona. Our results suggest that the viscous effect contributes to the excitation of the magnetic reconnection by generating oscillations that we observed at least inside this Ca II H line cool solar jet bundle.

Numerical Study of Internal SEN Design Effects on Jet Oscillations in a Funnel Thin Slab Caster

Numerical Study of Internal SEN Design Effects on Jet Oscillations in a Funnel Thin Slab Caster

Behavior of shock waves formed in supersonic jet impinging on flat plate

This paper focuses on the behavior of shock waves formed in an underexpanded impinging jet. The jet becomes underexpanded when the pressure ratio exceeds the critical value across the convergent nozzle discharging it. When a flat plate is placed perpendicularly to the jet, the strong shock wave called ‘plate shock' appears in the flow field near the plate. The underexpanded jet discharged from a nozzle into the atmosphere is often used for industrial applications, e.g., an assist gas of laser cutting and a cooling jet in glass tempering process. The jet impinges on the work piece and spreads out on its surface. So this study concentrates on the relation between the jet oscillation and the location of the work piece and especially discusses behavior of shock formed in jet. The impinging jet was formed under conditions of different nozzle pressure ratios and nozzle-plate spacings and was visualized using the shadowgraph and schlieren methods. The instantaneous shape of jet was measured through analyzing the pictures taken at random under each condition and the oscillation pattern was examined. In addition, under the same conditions, a numerical study was carried out using TVD scheme. As a result, it was found that the oscillation pattern of plate shock depends on its location and the nozzle pressure ratio.

Effects of area-ratio on the near-field flow characteristics and deflection of circular inclined coaxial jets

An experimental study was carried out on 45° and 60° inclined coaxial jets, where secondary-to-primary jet area- and velocity-ratios were 4.0 and ranged from 0.5 to 2.0 respectively. Results reveal that the use of a relatively larger area-ratio here is able to suppress self-excited jet oscillations seen earlier in comparatively smaller area-ratio jets when velocity-ratio is 1.0. Flow visualization and PIV measurements demonstrate that this is due to the physically wider annular gap associated with a larger area-ratio. This reduces the extent to which primary and secondary ring-vortices can undergo vortex-pairing and merging seen in the previous study. Near-field centerline flow characteristics clarify the impact of area-ratio upon the flow fields, as well as its relationships with velocity-ratio and incline-angle. Unlike relatively smaller area-ratio jets, the effects of the velocity-ratio are found to be insignificant in the lower cases of 0.5 and 1 examined here. Correspondingly, primary jet deflections are found to be comparatively smaller for relatively larger area-ratio jets and significant only when velocity-ratio reaches 2.0. Lastly, jet velocity profile developments reveal that within the present measurement range, the two jet-streams in relatively larger area-ratio jets do not merge as rapidly as smaller area-ratio counterparts, particularly at a velocity-ratio of 2.0.

Effect of nozzle thickness on the self-excited impinging planar jet

The self-excited oscillation of a large aspect ratio planar jet impinging on a flat plate is investigated experimentally at a single transonic jet velocity to clarify the effect of varying the jet thickness on pattern of jet oscillation and frequency of resulting acoustic tone. The study has been performed for a series of jet thicknesses, 1mm to 4mm, each of which is tested for the complete range of plate position, i.e. impingement distance, over which acoustic tones are generated. The results reveal that the jet oscillation is controlled by a fluid-dynamic mechanism for small impingement distances, where the hydrodynamic flow instability controls the jet oscillation without any coupling with local acoustic resonances. At larger impingement distances, a fluid-resonant mechanism becomes dominant, in which one of the various hydrodynamic modes of the jet couples with one of the resonant acoustic modes occurring between the jet nozzle and the impingement plate. Within the fluid-resonant regime, the acoustic tones are found to be controlled by the impingement distance, which is the length scale of the acoustic mode, with the jet thickness having only minor effects on the tone frequency. Flow visualization images of the jet oscillation pattern at a constant impingement distance show that the oscillation occurs at the same hydrodynamic mode of the jet despite a four-fold increase in its thickness. Finally, a feedback model has been developed to predict the frequency of acoustic tones, and has been found to yield reasonable predictions over the tested range of impingement distance and nozzle thickness.

Experimental investigation on pressure oscillations caused by direct contact condensation of sonic steam jet

An experimental study has been carried out to investigate the pressure oscillation of the sonic steam jet in a pool. The exit diameter of the nozzle was 8mm and the steam mass flux was 298–865kg/(m2s), water temperature 20–70°C. The dominant frequency and amplitude of pressure oscillation have been analyzed. A theoretical model on pressure oscillation amplitude was set up and a semi-empirical correlation was given to predict the dimensionless R.M.S (root mean square) amplitude of pressure oscillation. The pressure oscillation is mainly caused by the variation of steam speed δu, heat transfer coefficient δh and net steam-water interface δS. The dominant frequency of the pressure oscillation decreased with the increase of the water temperature while increased in CO region and decreased in SC region with the increase of the steam mass flux. The amplitude of the pressure oscillation is inversely proportional to the dominant frequency. The dominant frequencies did not change with the variation of x/de and r/de. But the amplitudes decreased with the increase of x/de and r/de. An empirical correlation was suggested to predict the dimensionless R.M.S amplitude based on the experimental data. The predictions agreed well with the experiments, and the discrepancies were within ±30%.

Application of POD analysis to concentration field of a jet flow

This paper proposes a fan-shaped region of interest (ROI) for the analysis of the spreading flow of a round jet with proper orthogonal decomposition (POD). Time sequences of scalar concentration field of a jet in stagnant environment and in a counterflowing main stream is obtained with the time-resolved laser-induced fluorescence technique. The fan-shaped ROI re-samples the concentration field on a spatial array with resolutions varying with axial locations from the jet exit. This conforms to the increases of jet width and length scales of the flow structures as the jet spreads downstream. The use of this ROI is shown to achieve better reconstruction of the turbulent concentration fields using POD modes with smaller reconstruction errors. The enhanced POD analysis is applied to study the effect of counterflow strength on the flow features of the jet. It is found that the presence of a counterflow, with the exception of a very weak counterflow, causes concentration variations of very large scales in the form of lateral jet flapping and oscillations in jet penetration.

Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.

Progress of Pulsed Jet Technology in Study and Application

Jet pump was widely used in the realm of water conservancy and aerospace. The trouble of lower efficiency on transfer restricted its rapid development and further application. A large number of experimental studies showed that its efficiency could be improved by pulsed jet, and pulsed jet generated in the device had higher impact on object. Based on massive literatures, generation ways of pulsed jet and study marches of excited pulsed jet were introduced briefly. At the same time, the progress in study and application of self-excited pulsed oscillation jet were focused on. Furthermore, in view of the domestic present situation of reservoir dredging, the next researches of jet dredging combination with artificial density-flow were prospected

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.

Lower-Stratospheric Radiative Damping and Polar-Night Jet Oscillation Events

Abstract The effect of stratospheric radiative damping time scales on stratospheric variability and on stratosphere–troposphere coupling is investigated in a simplified global circulation model by modifying the vertical profile of radiative damping in the stratosphere while holding it fixed in the troposphere. Perpetual-January conditions are imposed, with sinusoidal topography of zonal wavenumber 1 or 2. The depth and duration of the simulated sudden stratospheric warmings closely track the lower-stratospheric radiative time scales. Simulations with the most realistic profiles of radiative damping exhibit extended time-scale recoveries analogous to polar-night jet oscillation (PJO) events, which are observed to follow sufficiently deep stratospheric warmings. These events are characterized by weak lower-stratospheric winds and enhanced stability near the tropopause, which persist for up to 3 months following the initial warming. They are obtained with both wave-1 and wave-2 topography. Planetary-scale Eliassen–Palm (EP) fluxes entering the vortex are also suppressed, which is in agreement with observed PJO events. Consistent with previous studies, the tropospheric jets shift equatorward in response to the warmings. The duration of the shift is closely correlated with the period of enhanced stability. The magnitude of the shift in these runs, however, is sensitive only to the zonal wavenumber of the topography. Although the shift is sustained primarily by synoptic-scale eddies, the net effect of the topographic form drag and the planetary-scale fluxes is not negligible; they damp the surface wind response but enhance the vertical shear. The tropospheric response may also reduce the generation of planetary waves, further extending the stratospheric dynamical time scales.

Self-sustained oscillations in a homogeneous-density round jet

The paper is devoted to a new phenomenon of self-sustained oscillations triggered in a round free homogeneous-density jet. It was shown by the experimental investigations supported by the numerical approach based on extensive-Large Eddy Simulations studies that such a self-sustained regime can be established in a homogeneous-density jet, provided that the boundary layer at the nozzle exit is sufficiently thin and the perturbation level sufficiently low. The growth rate of the naturally amplified unstable modes is high enough to induce backflow leading to self-excited oscillations.

Statistical Characterization of Arctic Polar-Night Jet Oscillation Events

AbstractA novel diagnostic tool is presented, based on polar-cap temperature anomalies, for visualizing daily variability of the Arctic stratospheric polar vortex over multiple decades. This visualization illustrates the ubiquity of extended-time-scale recoveries from stratospheric sudden warmings, termed here polar-night jet oscillation (PJO) events. These are characterized by an anomalously warm polar lower stratosphere that persists for several months. Following the initial warming, a cold anomaly forms in the middle stratosphere, as does an anomalously high stratopause, both of which descend while the lower-stratospheric anomaly persists. These events are characterized in four datasets: Microwave Limb Sounder (MLS) temperature observations; the 40-yr ECMWF Re-Analysis (ERA-40) and Modern Era Retrospective Analysis for Research and Applications (MERRA) reanalyses; and an ensemble of three 150-yr simulations from the Canadian Middle Atmosphere Model. The statistics of PJO events in the model are found to agree very closely with those of the observations and reanalyses.The time scale for the recovery of the polar vortex following sudden warmings correlates strongly with the depth to which the warming initially descends. PJO events occur following roughly half of all major sudden warmings and are associated with an extended period of suppressed wave-activity fluxes entering the polar vortex. They follow vortex splits more frequently than they do vortex displacements. They are also related to weak vortex events as identified by the northern annular mode; in particular, those weak vortex events followed by a PJO event show a stronger tropospheric response. The long time scales, predominantly radiative dynamics, and tropospheric influence of PJO events suggest that they represent an important source of conditional skill in seasonal forecasting.

Zonal-Mean Dynamics of Extended Recoveries from Stratospheric Sudden Warmings

AbstractThe recovery of the Arctic polar vortex following stratospheric sudden warmings is found to take upward of 3 months in a particular subset of cases, termed here polar-night jet oscillation (PJO) events. The anomalous zonal-mean circulation above the pole during this recovery is characterized by a persistently warm lower stratosphere, and above this a cold midstratosphere and anomalously high stratopause, which descends as the event unfolds. Composites of these events in the Canadian Middle Atmosphere Model show the persistence of the lower-stratospheric anomaly is a result of strongly suppressed wave driving and weak radiative cooling at these heights. The upper-stratospheric and lower-mesospheric anomalies are driven immediately following the warming by anomalous planetary-scale eddies, following which, anomalous parameterized nonorographic and orographic gravity waves play an important role. These details are found to be robust for PJO events (as opposed to sudden warmings in general) in that many details of individual PJO events match the composite mean.A zonal-mean quasigeostrophic model on the sphere is shown to reproduce the response to the thermal and mechanical forcings produced during a PJO event. The former is well approximated by Newtonian cooling. The response can thus be considered as a transient approach to the steady-state, downward control limit. In this context, the time scale of the lower-stratospheric anomaly is determined by the transient, radiative response to the extended absence of wave driving. The extent to which the dynamics of the wave-driven descent of the stratopause can be considered analogous to the descending phases of the quasi-biennial oscillation (QBO) is also discussed.