Jet Oscillation Research Articles

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.

Effects of Acoustic Excitation and Annular Swirl Strength on a Non-Premixed and Swirl-Stabilized Flame

The stabilization of non-premixed flame is often done by coupling an annular swirling flow. Acoustic excitation, however, can improve combustion through fuel jet oscillation but can sometimes blow out the flame. The writers aimed to experimentally investigate the effects of annular swirl strength on acoustically excited non-premixed flame. Visual observations coupled with flame length measurements showed that the flame can be particularly shortened and transformed to blue at a low swirl strength. Flame and flow images obtained through the Mie scattering technique showed that the recirculation bubble enlarged proportionally in accordance with the swirl strength. Jet oscillations were diagnosed by using a laser Doppler anemometer at the central jet. Turbulence and mixing properties obtained from the central jet velocity were statistically compared to identify the best combination of swirl strength and acoustic excitation level. Combustion performance was finely validated through flame length and temperature measurements.

Cusps, spouts and microfiber synthesis with microfluidics

We produced jets of two immiscible liquids in a standard microfluidic flow focusing geometry, using semi-dilute aqueous polymer solutions as the external phase and immiscible liquids, oil or photocurable polymers as the internal one. We map out the different flow regimes on a “phase diagram”. Two new flow regimes – oscillatory jet and spout – are observed, as a result of the non-Newtonian behavior of the external phase. In the spout regime, cusp-shaped interfaces form at the junction, emitting extremely small round jets (spouts), with diameters ranging between 1.2 and 7 μm, i.e. an order of magnitude smaller than the microchannel cross-sectional dimensions. These spouts are found to be stable over remarkably long distances. We analyze the properties of the cusps and the spouts in some detail. The system can be utilized to synthesize fibers of micrometric sizes: the fibers we obtained, under different flow conditions, have diameters ranging between 1.8 and 14 μm and lengths ranging between 0.5 mm and 2 centimeters.

0605 単純なフルイディック発振器の主要な形状効果についての研究(OS6-1 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

The authors investigate a self-excited oscillatory phenomenon of a two-dimensional confined jet with a cylinder as a downstream target, especially for both the effects of a streamwise target size a and an aspect ratio. As a result, the authors find that the jet's dominant frequency ?D can be approximately predicted by the proposed empirical formula (Hirata et al., 2011), whenever the jet stably oscillates at various values of the non-dimensional streamwise target size a/b where b denotes the length scale of the jet's breath. The effect of a/b upon the occurrence of the stable jet's oscillation is negligible at a/b≦10. Then, the occurrence can be predicted by the proposed empirical formula (Hirata et al., 2009). On the other hand, at a/b > 10, the a/b effect is not negligible. Furthermore, the authors find that the effects of the aspect ratio h/b upon both the dominant frequency and the occurrence of the stable jet's oscillation are negligible even at such low h/b as 3.

Structure and Oscillation of Underexpanded Jet

In this study, flow characteristics are experimentally and numerically discussed, especially concerning shock-cell structure and oscillatory phenomena of supersonic jet. The jet becomes underexpanded when the nozzle pressure ratio of convergent nozzle exceeds the critical value. The underexpanded jet is not uniform because of the presence of the expansion wave, the compression wave and the shock wave formed in it. Many vortices are induced in jet boundary region by shearing stress generated between supersonic jet flow and atmospheric air. They interact with shock waves composing cell node of jet, which is closely related to a noise radiating from the jet. In experiment, flow field is visualized and the shape of cell structure is examined. Furthermore, frequency of vortex generation is measured. The jet is simulated using TVD scheme and the results are compared with the experimental ones. Distortion of cell structure caused by three dimensional oscillation of jet and behavior of vortices are discussed.

S054021 せん断層励起による大気圧プラズマジェットの制御可能性([S054-02]噴流、後流およびはく離流れ現象の探求と技術革新(2))

This paper described on the controllability of jet shear layer excitation by sound wave to which apply an argon atmospheric-pressure plasma jet. Our previous study showed periodic swing and stretching motions of the plasma jet by the excitation, but the bound condition of the method is still not clear. We, therefore, investigate the limit of the control. A visualization of the plasma jet was conducted by use of a high speed video camera, and we analyzed spatial and temporal motions of the jet via image processing, and then its controllability was estimated. As a result, the asymmetric mode enhances the jet oscillations as increasing with the sound amplitude, but the symmetric mode only affects the jet stretching. The excitation conditions; amplitude and frequency show significant influence on the jet behavior. The detail phenomenon which focused on nature of the working gas and the surround fluid will be presented in the conference.

Probable change in the millennial-scale variation pattern of East Asian summer monsoon between the last glacial and Holocene and its association with westerly jet oscillations

Probable change in the millennial-scale variation pattern of East Asian summer monsoon between the last glacial and Holocene and its association with westerly jet oscillations

Jet Oscillation at Low-Altitude Operation Mode in Dual-Bell Nozzle

Jet Oscillation at Low-Altitude Operation Mode in Dual-Bell Nozzle

Jet Oscillation at Low-Altitude Operation Mode in Dual-Bell Nozzle

A flow oscillation phenomenon of the dual-bell nozzle at low-altitude operationmode is investigated with cold-flow experiments and numerical simulations. Results indicate the existence of three kinds of periodic flow oscillation modes: a symmetric lowest-frequency oscillation mode, which is an acoustic resonance in the axial direction in an extensionpart of thenozzle, and twohigh-frequencymodes lateral to thenozzle axis originatingdue to vortex shedding andacoustic resonance in the extension nozzle cross section.These periodicflowoscillations are characteristic to dualbell nozzles, and the extension part is the cause of these fluctuations, which are not observed in single-bell nozzles.

Exploring High Frequency Temporal Fluctuations in the Terminal Aneurysm of the Basilar Bifurcation

Cerebral aneurysms are a common cause of death and disability. Of all the cardiovascular diseases, aneurysms are perhaps the most strongly linked with the local fluid mechanic environment. Aside from early in vivo clinical work that hinted at the possibility of high-frequency intra-aneurysmal velocity oscillations, flow in cerebral aneurysms is most often assumed to be laminar. This work investigates, through the use of numerical simulations, the potential for disturbed flow to exist in the terminal aneurysm of the basilar bifurcation. The nature of the disturbed flow is explored using a series of four idealized basilar tip models, and the results supported by four patient specific terminal basilar tip aneurysms. All four idealized models demonstrated instability in the inflow jet through high frequency fluctuations in the velocity and the pressure at approximately 120 Hz. The instability arises through a breakdown of the inflow jet, which begins to oscillate upon entering the aneurysm. The wall shear stress undergoes similar high-frequency oscillations in both magnitude and direction. The neck and dome regions of the aneurysm present 180 deg changes in the direction of the wall shear stress, due to the formation of small recirculation zones near the shear layer of the jet (at the frequency of the inflow jet oscillation) and the oscillation of the impingement zone on the dome of the aneurysm, respectively. Similar results were observed in the patient-specific models, which showed high frequency fluctuations at approximately 112 Hz in two of the four models and oscillations in the magnitude and direction of the wall shear stress. These results demonstrate that there is potential for disturbed laminar unsteady flow in the terminal aneurysm of the basilar bifurcation. The instabilities appear similar to the first instability mode of a free round jet.

Analysis of time-resolved PIV measurements of a confined turbulent jet using POD and Koopman modes

We present a comparative analysis of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) computed from experimental data of a turbulent, quasi 2-D, confined jet with co-flow (Re = 11,500, co-flow ratio inner-to-outer flow ≈2:1). The experimental data come from high-speed 2-D particle image velocimetry. The flow is fully turbulent, and it contains geometry-dependent large-scale coherent structures; thus, it provides an interesting benchmark case for the comparison between POD and DMD. In this work, we address issues related to snapshot selections (1), convergence (2) and the physical interpretation (3) of both POD and DMD modes. We found that the convergence of POD modes follows the criteria of statistical convergence of the autocovariance matrix. For the computation of DMD modes, we suggest a methodology based on two criteria: the analysis of the residuals to optimize the sampling parameters of the snapshots, and a time-shifting procedure that allows us to identify the spurious modes and retain the modes that consistently appear in the spectrum. These modes are found to be the ones with nearly null growth rate. We then present the selected modes, and we discuss the way POD and DMD rank them. POD analysis reveals that the most energetic spatial structures are related to the large-scale oscillation of the inner jet (flapping); from the temporal analysis emerges that these modes are associated with a low-frequency peak at St = 0.02. At this frequency, DMD identifies a similar mode, where oblique structures from the walls appear together with the flapping mode. The second most energetic group of modes identified is associated with shear-layer oscillations, and to a recirculation zone near the inner jet. Temporal analysis of these modes shows that the flapping of the inner jet might be sustained by the recirculation. In the DMD, the shear-layer modes are separated from the recirculation modes. These have large amplitudes in the DMD. In conclusion, the DMD modes with eigenvalues on the unit circle are found to be similar to the most energetic POD modes, although differences appear due to the fact that DMD isolates structures associated with one frequency only.

Nonsteady dynamics of turbulent axisymmetric jets in stratified fluid: Part 1. Experimental study

The nonsteady dynamics of turbulent axially symmetrical jets in stratified fluid with a great density gradient (pycnocline) is studied in laboratory experiments in the large test reservoir with artificial temperature stratification at the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS) simulating wastewater flows from underwater collectors in coastal cities. An underwater survey of jets in the pycnocline region has been carried out along with synchronous measurements of internal waves. It is shown that a jet quasiperiodically oscillates in a vertical plane, thus effectively generating internal waves when interacting with the pycnocline. It is ascertained that the axially symmetrical mode of jet oscillations is a source of internal waves.

819 超音速不足膨張衝突噴流の自励振動による圧力変動の数値解析(0S.2 圧縮性流れの基礎と応用4)

819 超音速不足膨張衝突噴流の自励振動による圧力変動の数値解析(0S.2 圧縮性流れの基礎と応用4)

Review on fiber morphology obtained by bubble electrospinning and blown bubble spinning

Here we show an intriguing phenomenon in the bubble electrospinning process that the ruptured film might be stripped upwards by an electronic force to form a very thin and long plate-like strip, which might been received in the metal receiver as discontinuous backbone-like wrinkled materials, rather than smooth nano-fibers or microspheres. The processes are called the bubble electrospinning. The electronic force can be replaced by a blowing air, and the process is called as the blown bubble spinning. We demonstrate that the size and thickness of the ruptured film are the crucial parameters that are necessary to understand the various observations including beads and nanoporous materials. We identify the conditions required for a ruptured film to form discontinuous structure, and a critical width of the ruptured film to form a cylindrical fiber, above which a long and thin plate-like strip might be obtained, and a criterion for oscillatory jet diameter, which leads to bead morphology of the obtained fibers. The space of the adjacent beads depends on the fiber size. We anticipate our assay to be a starting point for more sophisticated study of the bubble electrospinning and the blown bubble spinning and for mass-production of both nanofibers and nanoscale discontinuous materials.

Two-Dimensional Complex FFT Analysis of Oscillations in a Thermal Plasma Jet

Multidirectional optical diagnostics combined with complex fast Fourier transform analysis have been used for investigations of oscillation phenomena in a thermal plasma jet. The results show possible coupling between the Helmholtz and boundary shear layer oscillations.

Numerical investigation of impinging gas jets onto deformable liquid layers

Impinging jets over liquid surfaces are a common practice in the metallurgy and chemical industries. This paper presents a numerical study of the fluid dynamics involved in this kind of processes. URANS simulations are performed using the volume of fluid (VOF) method to deal with the multiphase physics. This unsteady approach with the appropriate computational domain allows resolution of the big eddies responsible for the low frequency phenomena. The solver we used is based on the finite volume method and turbulence is modelled with the realisable k-ϵ model. Two different configurations belonging to the dimpling and splashing modes are under consideration. The results are compared with PIV and LeDaR experimental data previously obtained by the authors. Attention is focused on the surroundings of the impingement, where the interaction between jet and liquid film is much stronger. Finally, frequency analysis is carried out to study the flapping motion of the jet and cavity oscillations.

Resonant Excitation of Boundary Layer Instability of DC Arc Plasma Jet by Current Modulation

Instabilities of thermal plasma jets were studied on the basis of analysis of plasma radiation fluctuations recorded by an array of high frequency photodiodes. Characteristic frequencies of jet oscillations were found and spatial distribution of amplitude of plasma fluctuations was determined. The influence of arc current ripple on plasma instabilities was investigated for two types of power supply—classical thyristor controlled unit with the frequency of the current ripple 300 Hz and the rectifier with the high frequency converter and frequency of the current modulation 30 kHz. Generation of boundary layer instability with the current modulation frequency and its harmonics was proved using fast Fourier transform, contour plots and phase portraits. It was found that the character of fluctuations of plasma jet was substantially influenced by current ripple with the frequency or its harmonics close to the frequency of oscillations generated by boundary layer instability.

Quantitative analysis of the dripping and jetting regimes in co-flowing capillary jets

We study a liquid jet that breaks up into drops in an external co-flowing liquid inside a confining microfluidic geometry. The jet breakup can occur right after the nozzle in a phenomenon named dripping or through the generation of a liquid jet that breaks up a long distance from the nozzle, which is called jetting. Traditionally, these two regimes have been considered to reflect the existence of two kinds of spatiotemporal instabilities of a fluid jet, the dripping regime corresponding to an absolutely unstable jet and the jetting regime to a convectively unstable jet. Here, we present quantitative measurements of the dripping and jetting regimes, both in an unforced and a forced state, and compare these measurements with recent theoretical studies of spatiotemporal instability of a confined liquid jet in a co-flowing liquid. In the unforced state, the frequency of oscillation and breakup of the liquid jet is measured and compared to the theoretical predictions. The dominant frequency of the jet oscillations as a function of the inner flow rate agrees qualitatively with the theoretical predictions in the jetting regime but not in the dripping regime. In the forced state, achieved with periodic laser heating, the dripping regime is found to be insensitive to the perturbation and the frequency of drop formation remains unaltered. The jetting regime, on the contrary, amplifies the externally imposed frequency, which translates into the formation of drops at the frequency imposed by the external forcing. In conclusion, the dripping and jetting regimes are found to exhibit the main features of absolutely and convectively unstable flows, respectively, but the frequency selection in the dripping regime is not ruled by the absolute frequency predicted by the stability analysis.

Study on the Relation between Oscillation Frequency of Self-Excited Oscillation Pulsed Water Jet and Self-Excited Pulsed Water Jet Devices

Depending on different structure characteristic of nozzle (self-excited oscillation chamber and special boundary condition), the self-excited pulsed water jet can be produced,which can increase the jet pressure and having the cavitation phenomenon. So,it has been applied in oil driven well、mining and building etc.To study the relation among the frequency of the jetting peak pressure,the peak pressure of the plused-jetting and the self-excited pulsed water jet device,based on the theory of fluid network,the mathematical model of the self-excited oscillation pulsed water jet device was established and the dynamic characteristics parameters of the self-excited oscillation pulsed water jet were tested, which shows that the frequency of the self-excited oscillation nozzle depend on the structure parameter and the dynamic state characteristic of the fluid decisive .the frequency of the jetting peak pressure depends on natural frequency and damping ratio of the device.The experiment proves that the frequency of the jetting peak pressure decreases with the length of the self-excited oscillation chamber,increases with the rising of the pump pressure and there is an optimal length of the oscillation chamber corresponding to the jetting surge pressure. lastly, it clearly shows that the device has functions of oscillator and low pass filter and the jetting peak pressure is the highest when the jet dominant frequency close to the natural frequency of the device. The experimental results help to theory analyzing how to improve peak pressure of the self-excited oscillation jet and have some value in engineering.