Main Jet Research Articles

Maximum energy dissipation to explain velocity fields in shallow reservoirs

ABSTRACTShallow reservoirs are often used as sediment traps or storage basins, in which sedimentation depends on the flow pattern. Short rectangular reservoirs reveal a straight jet from inlet to outlet with identical recirculation zones on both sides. In longer reservoirs, the main jet reattaches to the side of the reservoir leading to small and large recirculation zones. Previous studies have found an empirical geometric relation describing the switch between these two flow patterns. In this study, we demonstrate, with a simple analytical model, that this switch coincides with a maximization of energy dissipation in the shear layer between the main jet and recirculation zones: short reservoirs dissipate more energy when the flow pattern is symmetric, while longer reservoirs dissipate more energy with an asymmetric pattern. This approach enables the prediction of the flow patterns without detailed knowledge of small scale processes, potentially useful in the early phase of reservoir design.

CONTROL OF SELF-SUSTAINED OSCILLATIONS OF A THREE-DIMENSIONAL WATER JET IN A SLENDER CHANNEL

This work focuses on control of an oscillatory main jet submerged in a slender channel of small thickness to width ratio, by two lateral opposite and normal injections arranged at the same height, on the end sidewalls of the channel right above the main jet exit. The objective of this work is about the control of the jet deviation and its oscillations frequency, according to the flow rate of the two lateral injections. The validation confirms that second-order turbulence models predict more accurately such flows than first-order models. Due to Coanda effect, jet oscillations have been highlighted for several Flow rate ratios (β). Two flow behaviours of time averaged flow fields corresponding to two ranges of β are detected. It appears that the key question of this flow is also the two oscillatory motions of the main and the lateral jets.

Numerical investigation the dynamics of vaporization at the flow of liquid methane in channel with variable section

Numerically transient dynamics of vaporization of the liquid bubbling up methane flow in the Laval nozzle are analyzed. The equation conservation of mass, momentum and energy with averaging of Favre are used. For numerical modeling of cavitation the additional equation for transport the volume fraction of vapour with special source terms for evaporation and condensations is used. Non-stationary process of evaporation is analyzed in various checkpoints in diffuser portion of the nozzle are investigated. By the study of the unsteady dynamics of changes the characteristic hydrodynamic quantities of two-phase gas-liquid flow, the main jet stream pattern of destruction of the process of liquid methane at the expiration of the nozzle throat are identified.

유체 진동기에 의해 생성된 제트의 유동가시화

A sweeping jet actuator (SJA) is an instrument generating pulsing jets with no moving elements. Because of its simple design and high durability to shock and vibration, SJA has recently drawn increasing attention for the application to flow control such as aerodynamic control of a wing and thrust vectoring of a jet engine. However, experimental and numerical studies on SJA have been limited to internal flow structure of SJA. In this study, we investigated the flow structure and its variation in the outlet of SJA. We carried out the experiment to understand the flow structures using PIV (Particle Image Velocimetry). The flow structure varies with a degree of the outlet and volume flow rate. There is leaking process during half jetting cycle. The process of the main jet can occur because the jet moving time increased from one side to the other side.

Control of Wall Pressure Flow Field with Micro Jets and Control Effectiveness

This paper presents an experimental study of airflow from convergent-divergentnozzles discharged into enlarged duct, focusing attention on the flow development in the duct. To investigate the influence of active control on the flow field developed in the duct, the tiny jets of 0.05 mmradiuslocated at 900interval along the base at 6.5 mm from the main jet were employed. The Mach number tested in the present study was 1.48. The area ratio tested was 2.56. The NPR tested are from 3, 5, 7, and 9 respectively. The L/D ratio of the duct was varied from 10 to 1. The level of expansion at the nozzle exit influences the wall pressure very strongly. When the micro jets were activated they found to influence the base region, taking the wall pressure to considerably higher values, for most of the cases. Flow field in the duct with and without control remains the same.

Active Control of Wall Pressure Flow Field at Low Supersonic Mach Numbers

This paper presents an experimental study of airflow from convergent-divergent nozzles discharged into enlarged duct, focusing attention on the flow development in the duct. To investigate the influence of active control on the flow field developed in the duct, the micro jets of 0.05 mm radius orifice placed at 900 interval along the base at 6.5 mm from the main jet were employed. The Mach number of this study was 1.3. The area ratio of the present study was 2.56. The NPR presented are from 3, 5, 7, 9, and 11 respectively. The L/D ratio of the duct was varied from 10 to 1. The level of expansion at the nozzle exit influences the wall pressure very strongly since in the present case all the NPRs of the tests are under expanded. For all the L/D the entire flow field in the duct is full of waves, the few shock wave is so powerful that the wall pressure is equal to the atmospheric pressure or more. At L/D = 3, it is seen that due to the influence of the back pressure the number of compression and expansion waves have reduced drastically. The flow field with and without control is identical.

Control of self-sustained jet oscillations in 3D thin rectangular cavity

The aim of this work is the control of an oscillatory jet submerged in a 3D thin rectangular cavity, with two opposite injections added on its thinnest sidewalls at the same height, perpendicular and above the jet exit. This type of control is required when the characteristics of the main jet (Reynolds number, nozzle size) are not modifiable. Due to Coanda effect, the oscillations of the main jet occur in the widest plane of the cavity, and depend on the mass flow of two injections. Unsteady, 3D problem is solved by finite volume method using URANS modeling. The validation confirms that second order models predict more accurately this flow configuration than first order models. Two behaviors of time average of flow fields corresponding for two ranges of β are detected. When 0≤β<25.51%, the main jet effect decreases to reach the case without lateral injections (β=0). Similarly for 25.51≤β≤50.67%, the main jet effect diminishes for higher value of β. Thus, β=25.51% is a threshold value for this type of flow configuration. Moreover, for β=25.51% the minimum values for the kinetic energy, main jet deflection angle and oscillations frequency are reached. Furthermore, the deflection angle reaches its maximum value for β=12.90% and the best deflections are recommended for β<25.51%.

Predicting concentrations of fine particles in enclosed vessels using a camera based system and CFD simulations

One of the main challenges in industries handling biomass is the consequence of the particle breakage of pelletised biomass in smaller fractions which can lead to fine particles smaller 500μm that can form dust clouds in the handling and storing equipment. These dust clouds present potential health and safety hazards as well as dust explosion hazards to plant operators because the airborne dust can occur in high concentrations close to the dust explosion limits of the biomass material, during the filling process of storage silos. Preventing dust explosions and the damage of plant infrastructures requires a profound understanding of the particle/air dynamics in the dust cloud circulating in the storage silo. The limited access to the storage facilities as well as the silo size requires a detailed study of the particle/air dynamics at different scales. Lab scale experiments were conducted as a first step to establishing a new optical method for measuring particle concentrations. A small scale experimental rig was fed centrally with different sized wood pellets and a single camera and a laser was utilised to capture the dust concentration in different areas of the silo. According to the experimental results, a higher mass concentration of dust was observed near the silo wall as well as near the main particle jet. However, the mass concentrations were below the explosive limits at the area in between main particle jet and silo wall. These experimental results were then feeding into a 2D CFD simulation representing the particle dynamics in the laser sheet (2D plane). Qualitative findings show a good agreement of the particle/air dynamics between experiments and simulations.

Effect of nozzle air supply temperature and volume flowrate on the jet flow from a typical ventilation nozzle in aircraft cabins

A ventilation nozzle’s airflow characteristics could have an influence on a passenger’s thermal comfort and dispersion of pollutants in an aircraft cabin, but there is little published research in this area. In this article, the temperature and air velocity from a nozzle jet were measured in a simulated three-row aircraft cabin at more than 220 points for both isothermal and non-isothermal jets. The non-isothermal jets were tested for air supply temperature differences of ± 5 and 10℃, and the isothermal jets for volume flowrates of 1.86 × 10−7, 2.67 × 10−7, and 4.03 × 10−7 m3/h. The results show that both the nozzle jet’s temperature and air velocity distributions are axially symmetrical and that the decay of the jet is rapid. The equation for the air velocity distribution was used to calculate the air velocity at any point in the main jet region for different volume flowrates. The temperature difference was shown to have little effect on the air velocity distribution for the main region of a nozzle jet, whereas the air flowrate was shown to significantly affect the air velocity distribution. This suggests that the effect of nozzle jet on passenger’s thermal comfort is mostly due to its air velocity distribution rather than temperature distribution.

Axisymmetric jet manipulated using two unsteady minijets

The manipulation of a turbulent axisymmetric jet is experimentally investigated based on two unsteady radial minijets. The Reynolds number is 8000. The mass flow rate ratio$C_{m}$of the two minijets to that of the main jet and the ratio$f_{e}/f_{0}^{\prime }$of the excitation frequency$f_{e}$to the preferred-mode frequency$f_{0}^{\prime }$in the natural jet are examined. The decay rate$K$of the jet centreline mean velocity exhibits a strong dependence on$C_{m}$and$f_{e}/f_{0}^{\prime }$and is classified into three distinct categories in terms of required$C_{m}$, achievable enhancement in$K$and flow physics involved. Great effort is made to understand the flow physics associated with the first category of the manipulated jet, under which$K$can be immensely improved with a very small$C_{m}$. Detailed measurements are conducted upstream and downstream of the nozzle exit using hot-wire, flow visualization and particle imaging velocimetry techniques. Whilst strong entrainment is predominant in the injection plane of the minijets, rapid spread occurs in the orthogonal non-injection plane. Three types of coherent structures are identified, i.e. the contorted ring vortex, two pairs of streamwise vortices and mushroom-like counter-rotating structures sequentially ‘tossed’ out radially in the non-injection plane. Their interactions account for the large rise in$K$. The unsteady disturbance of the minijets is found to play a key role in the formation and interaction of these vortices, which are distinct from those formed under the manipulation of steady minijets and other techniques. A conceptual model of the flow structure under manipulation is proposed.

Observations of the Structure and Dynamics of the Inner M87 Jet

M87 is the best source in which to study a jet at high resolution in gravitational units because it has a very high mass black hole and is nearby. The angular size of the black hole is second only to Sgr A*, which does not have a strong jet. The jet structure is edge brightened with a wide opening angle base and a weak counterjet. We have roughly annual observations for 17 years plus intensive monitoring at three week intervals for a year and five day intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43 GHz. The inner jet shows very complex dynamics, with apparent motions both along and across the jet. Speeds from zero to over 2c are seen, with acceleration observed over the first 3 milli-arcseconds. The counterjet decreases in brightness much more rapidly than the main jet, as is expected from relativistic beaming in an accelerating jet oriented near the line-of-sight. Details of the structure and dynamics are discussed. The roughly annual observations show side-to-side motion of the whole jet with a characteristic time scale of about 9 years.

Impacts of mesoscale activity on the water masses and circulation in the Coral Sea

AbstractThe climatological vision of the circulation within the Coral Sea is today well established with the westward circulation of two main jets, the North Caledonian Jet (NCJ) and the North Vanuatu Jet (NVJ) as a consequence of the separation of the South Equatorial Current (SEC) on the islands of New Caledonia, Vanuatu, and Fiji. Each jet has its own dynamic and transports different water masses across the Coral Sea. The influence of mesoscale activity on mean flow and on water mass exchanges is not yet fully explored in this region of intense activity. Our study relies on the analysis of in situ, satellite, and numerical data. Indeed, we first use in situ data from the Bifurcation cruise and from an Argo float, jointly with satellite‐derived velocities, to study the eddy influence on the Coral Sea dynamics. We identify an anticyclonic eddy as participating in the transport of NVJ‐like water masses into the theoretical pathway of NCJ waters. This transfer from the NVJ to the NCJ is confirmed over the long term by a Lagrangian analysis. In particular, this numerical analysis shows that anticyclonic eddies can contribute up to 70–90% of the overall eddy transfer between those seemingly independent jets. Finally, transports calculated using S‐ADCP measurements (0–500 m) show an eddy‐induced sensitivity that can reach up to 15 Sv, i.e., the order of the transport of the jets.

Upstream control of the frontal jet regulating plankton production in the Alboran Sea (Western Mediterranean)

AbstractUsing a coupled physical‐biological model, we document that a ∼30 km wide meandering jet constitutes a major source of biological enrichment in the Alboran Sea (Western Mediterranean) even in the absence of wind forcing and tidal dynamics. The level of enrichment is shown to vary markedly during the year depending on the upstream characteristics of the jet as it exits from the Gibraltar Strait. When its intensity is sufficiently low and characterized by weaker cross‐frontal density gradients during winter‐spring, the jet is weakly nonlinear and may not fulfill the necessary conditions for frontogenesis. It then remains weakly productive. In the case of stronger jet intensity (&gt;1.1 Sv) accompanied by stronger cross‐frontal density and velocity gradients within the Alboran Sea during summer‐autumn, the frontal jet becomes strongly nonlinear and ageostrophic with large cross‐frontal vorticity changes on the order of planetary vorticity. Under these conditions, upward vertical velocities in the range 10–50 m d−1 supply nutrients into the euphotic layer more effectively and support high‐level frontogenesis‐induced phytoplankton production on the anticyclonic side of the main jet axis. The strong eddy pumping mechanism also provides a comparable level of plankton production within strongly nonlinear elongated cyclonic eddies along the outer periphery of the frontal jet. The plankton biomass is advected partially by the jet along its trajectory and dispersed within the basin by mesoscale eddies and meanders.

Turbulent jet manipulation using two unsteady azimuthally separated radial minijets.

The active manipulation of a turbulent round jet is experimentally investigated based on the injection of two radial unsteady minijets, prior to the issue of the main jet. The parametric study is conducted for the mass flow ratio Cm of the minijets to the main jet, and the ratio fe/f0 of the minijet frequency to the preferred-mode frequency of the main jet. It is found that the decay rate of the jet centreline mean velocity could be greatly increased if the two minijets are separated azimuthally by an angle θ=60°, instead of by θ=180°. This increase is a consequence of the flapping motion of the jet column, and the formation process and generation mechanism of this flapping motion are unveiled by careful analysis of the experimental data.

Small meanders of the Kuroshio Extension and associated northward spreading of warm water: Three‐vessel simultaneous observations and an eddy‐resolving ocean model simulation

AbstractSmall meanders of the Kuroshio Extension (KE) were examined in the three vessel simultaneous observations carried out across the KE along 143°E between 2 and 7 July 2012 and the hindcast simulation by a high‐resolution ocean model for the Earth Simulator (OFES). The observations captured the crest of the small meander that passed 143°E. Behind the meander crest, the meander showed the northward separation of the front from the meandering jet. It was suggested that the meander spread the low‐density KE water to the north above the main pycnocline there. The OFES showed similar characteristics of small meanders. We focused on three major small meanders. In the upstream KE region west of 143°E, these meanders were subject to northeastward ageostrophic advection, by which the low‐density KE water was carried to the north and their fronts moved away from the KE jet above the main pycnocline. This resulted in the KE water and the front extending to the west behind the crest of the downstream‐propagating meanders. This feature was particularly obvious for the two cases of the meander and was consistent with the observations. The displaced front formed a shallow geostrophic jet to the north of the main KE jet, which induced geostrophic advection along the jet and advected the front further northward and downstream near the crest of the meandering front. The KE water spread by the small meanders then mostly evolved into warm core rings or warm streamers.

Closer view of the IGR J11014-6103 outflows

The complex X-ray system IGR J11014-6103 (a.k.a. the Lighthouse nebula) is composed of a bow-shock pulsar wind nebula (PWN) as well as large-scale jet-like features, all launched by IGR J11014-6103 which is moving supersonically in the interstellar medium. Previous observations suggested that the jet features stem from a ballistic jet of relativistic particles. In order to confirm the nature of the jet and the marginally detected counter-jet, we obtained a new deep 250 ks Chandra observation of the Lighthouse nebula. We performed detailed spatial and spectral analysis of all X-ray components of the system. The X-ray PWN is now better resolved and shows a clear bi-modal morphology. The overall helical pattern of the main jet is confirmed. However, there are large deviations from a simple helical model at small and large scales. Significant extended emission is now detected, encompassing the main jet all along its length. The brightness dip of the main jet at ~50" distance from the pulsar is confirmed, the extended emission however prevents conclusions about the coherence at this position of the jet. The counter-jet is now detected at high statistical significance. In addition, we found two small-scale "arcs" departing from the pulsar towards the jets. We also looked for possible bow-shock emission in front of the pulsar, with a short VLT/FORS2 H-alpha observation. No clear emission is found, most likely because of contamination from a diffuse surrounding nebulosity. The results of our X-ray analysis show that current expectations from a ballistic nature of the jets can explain satisfactorily some of the observational evidences but cannot fully reproduce the observations. The alternative scenario (diffusion of particles along preexisting magnetic field lines in the surrounding medium) however also continues to suffer from conflicts with the observations.

Closed-loop enhancement of jet mixing with extremum-seeking and physics-based strategies

The closed-loop control of a turbulent round air jet is experimentally investigated based on two unsteady minijets, with a view to enhancing jet mixing. The two minijets are placed at diametrically opposite locations upstream of the nozzle exit. The open-loop control experiments are first performed. Given the mass flow rate ratio C m of the minijets to that of the main jet, the decay rate $$\overline{K}$$ of jet centerline mean velocity exhibits a maximum at the frequency ratio f e/f 0 ≈ 1.0, where f e and f 0 are the excitation frequency of minijets and the preferred mode frequency of the natural main jet, respectively. An extremum-seeking feedback control has been developed to achieve autonomously the optimal control performance. It has been found that, given C m, this closed-loop control technique may obtain automatically and rapidly the optimal value of f e and the desired or maximum $$\overline{K}$$ , as achieved in the open-loop control. This control technique is robust and adaptable when the Reynolds number and initial excitation frequency are changed separately. A flow-physics-based feedback control strategy has also been investigated, which could achieve the optimal control performance automatically with a shorter convergence time than the extremum-seeking control, not robust though.

Modeling asymmetric cavity collapse with plasma equations of state.

We explore the effect that equation of state (EOS) thermodynamics has on shock-driven cavity-collapse processes. We account for full, multidimensional, unsteady hydrodynamics and incorporate a range of relevant EOSs (polytropic, QEOS-type, and SESAME). In doing so, we show that simplified analytic EOSs, like ideal gas, capture certain critical parameters of the collapse such as velocity of the main transverse jet and pressure at jet strike, while also providing a good representation of overall trends. However, more sophisticated EOSs yield different and more relevant estimates of temperature and density, especially for higher incident shock strengths. We model incident shocks ranging from 0.1 to 1000 GPa, the latter being of interest in investigating the warm dense matter regime for which experimental and theoretical EOS data are difficult to obtain. At certain shock strengths, there is a factor of two difference in predicted density between QEOS-type and SESAME EOS, indicating cavity collapse as an experimental method for exploring EOS in this range.

Variability, Instabilities, and Eddies in a Snowball Ocean

Abstract Oceanic variability and eddy dynamics during snowball Earth events, under a kilometer of ice and driven by a very weak geothermal heat flux, are studied using a high-resolution sector model centered at the equator, where previous studies have shown the ocean circulation to be most prominent. The solution is characterized by an energetic eddy field, equatorward-propagating zonal jets, and a strongly variable equatorial meridional overturning circulation (EMOC), on the order of tens of Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), restricted to be very close to the equator. The ocean is well mixed vertically by convective mixing, and horizontal mixing rates by currents and eddies are similar to present-day values. There are two main opposite-sign zonal jets near the equator that are not eddy driven, together with multiple secondary eddy-driven jets off the equator. Barotropic stability analyses, the Lorenz energy cycle (LEC), and barotropic-to-baroclinic energy conversion rates together indicate that both baroclinic and barotropic instabilities serve as eddy-generating mechanisms. The LEC shows a dominant input into the mean available potential energy (APE) by geothermal heat flux and by surface ice melting and then transformation to eddy APE, to eddy kinetic energy, and finally to mean kinetic energy via eddy–jet interaction, similarly to the present-day atmosphere and unlike the present-day ocean. The EMOC variability is due to the interaction of warm plumes driven by geothermal heating that reach the ocean surface, leading to ice-melt events that change the stratification and, therefore, the EMOC. The results presented here may be relevant to the ocean dynamics of planetary ice-covered moons such as Europa and Enceladus.

Raman microscopy investigation of beryllium materials

We report for the first time on the ability of Raman microscopy to give information on the structure and composition of Be related samples mimicking plasma facing materials that will be found in ITER. For that purpose, we investigate two types of material. First: Be, W, Be1W9, and Be5W5 deposits containing a few percents of D or N, and second: a Mo mirror exposed to plasma in the main JET chamber (in the framework of the first mirror test in JET with ITER-like wall). We performed atomic quantifications using ion beam analysis for the first samples. We also did atomic force microscopy. We found defect induced Raman bands in Be, Be1W9, and Be5W5 deposits. Molybdenum oxide has been identified showing an enhancement due to a resonance effect in the UV domain.