Jet Region Research Articles

Variations in global zonal wind from 18 to 100 km due to solar activity and the quasi-biennial oscillation and El Niño–Southern Oscillation during 2002–2019

Abstract. Variations of global wind are important in changing the atmospheric structure and circulation, in coupling of atmospheric layers, and in influencing the wave propagations. Due to the difficulty of directly measuring zonal wind from the stratosphere to the lower thermosphere, we derived a global balance wind (BU) dataset from 50∘ S to 50∘ N and during 2002–2019 using the gradient wind theory and SABER temperatures and modified by meteor radar observations at the Equator. The dataset captures the main feature of global monthly mean zonal wind and can be used to study the variations (i.e., annual, semi-annual, ter-annual, and linear) of zonal wind and the responses of zonal wind to quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), and solar activity (F10.7). The same procedure is performed on the MERRA-2 zonal wind (MerU) to validate BU and its responses below 70 km. The annual, semi-annual, and ter-annual oscillations of BU and MerU have similar amplitudes and phases. The semi-annual oscillation of BU has peaks around 80 km, which are stronger in the southern tropical region and coincide with previous satellite observations. As the increasing of the values representing QBO wind, both values of representing BU and MerU (short for BU and MerU) change from increasing to decreasing with the increasing height and extend from the Equator to higher latitudes. Both BU and MerU increase with the increasing of the values of multivariate ENSO index (MEI) and decrease with increasing F10.7​​​​​​​ in the southern stratospheric polar jet region below 70 km. The responses of winds to ENSO and F10.7 exhibit hemispheric asymmetry and are more significant in the southern polar jet region. While above 70 km, BU increases with the increasing of MEI and F10.7. The negative linear changes of BU at 50∘ N are absent in MerU during October–January. The discussions on the possible influences of the temporal intervals and sudden stratospheric warmings (SSWs) on the variations and responses of BU illustrate the following: (1) the seasonal variations and the responses to QBO are almost independent on the temporal intervals selected; (2) the responses to ENSO and F10.7 are robust but slightly depend on the temporal intervals; (3) the linear changes of both BU and MerU depend strongly on the temporal intervals; (4) SSWs affect the magnitudes but do not affect the hemispheric asymmetry of the variations and responses of BU at least in the monthly mean sense. The variations and responses of global zonal wind to various factors are based on BU, which is derived from observations, and thus provide a good complement to model studies and ground-based observations.

Prediction and Modelling of Plasma Jet Zone Temperature for DC Arc Graphite Purification

The random forest algorithm was used to analyze the degree of influence of four variables, namely, voltage, current, main gas flow rate, and powder feed, on the temperature of the plasma jet zone of the DC arc graphite purification. The BP algorithm was then used to build a network prediction model. After the training of the network, the validity of the model was verified by comparing the measured temperature values with the predicted results. From the results, it can be seen that the relative error of the prediction results is between -0.3 %and +0.3%, which proves that the built BP neural network temperature model has a satisfactory prediction effect. In addition, this paper also derives specific mathematical expressions based on the threshold and weight values of this temperature prediction model, which can be used to fit the temperature variation curve in the plasma jet region.

Optimization of the Classifying Efficiency of Spiral Jet Mills by Investigating the Flow Conditions and the Grinding Performance

AbstractThe achievable grinding fineness of spiral jet mills does not only depend on the comminution efficiency in the region of the nozzle jets but is also significantly affected by the separation efficiency in the region of the product outlet. Therefore, the aim of this study is to investigate the influences of the product outlet diameter and different particle trap rings on the flow conditions and the grinding performance, optimizing the classifying efficiency of spiral jet mills. The velocity fields analyzed by particle image velocimetry and the experimental grinding results show that the classification process can be optimized by both reducing the product outlet diameter and installing a particle trap ring.

Large eddy simulation of a premixed dual-fuel combustion: Effects of inhomogeneity level on auto-ignition of micro-pilot fuel

In a premixed dual-fuel (DF) methane-diesel engine, the ignition of the lean premixed methane/air mixture starts with the assistance of a pilot diesel injection. Auto-ignition of pilot fuel is important as it triggers the subsequent combustion processes. A delay in the auto-ignition process may lead to misfiring, incomplete combustion, and thus higher greenhouse emissions due to methane slip. Hence, a better understanding of the auto-ignition process for the pilot fuel can help to improve the overall engine performance, combustion efficiency, and to lower exhaust emission levels. In the present study, large eddy simulation (LES) is used to investigate the auto-ignition process of micro-pilot diesel in premixed DF combustion in a constant volume combustion chamber (CVCC). The entire DF combustion processes including methane gas injection, methane/air mixing, pilot diesel injection, and ignition are simulated. The numerical model is validated against experimental data. The present numerical model is able to capture the ignition delay time (IDT) within a maximum relative difference of 7% to the measurements. A higher relative difference of 38% is obtained when methane gas injection and mixing are omitted in the simulation and the methane/air is assumed homogeneous. This demonstrates the importance of inhomogeneity pockets. To study the effects of temperature and methane inhomogeneities separately, different idealized inhomogeneities in temperature and methane distribution are considered inside the CVCC. The inhomogeneity in the temperature is observed to have a more profound influence on the IDT than the methane inhomogeneity. The inhomogeneity pockets of temperature advance the first-stage ignition and, subsequently, the second-stage ignition. A sensitivity analysis on the effect of inhomogeneity wavelength reveals that the larger wavelengths enhance the combustion due to the presence of pilot diesel jets in the desirable regions for a longer time duration.

CLEAN imaging systematics of M87 radio jet

ABSTRACT The spectral index images of the jet in the nearby radio galaxy M87 have previously been shown with Very Long Baseline Interferometric arrays at 2–43 GHz. They exhibit flattening of the spectra at a location of inner (central) spine and towards outer ridges. This could imply optical depth effects, lower energy cut-off, or stratification of the emitting particles energy distribution. In this paper, we employ simulations of multifrequency VLBI observations of M87 radio jet with various model brightness distributions. CLEAN deconvolution errors produce significant features in the observed images. For intensity images, they result in the appearance of the inner ridge line in the intrinsically edge brightened jet models. For spectral index images, they flatten the spectra in a series of stripes along the jet. Another bias encountered in our simulations is steepening of the spectra in a low surface brightness jet regions. These types of the imaging artefacts do not depend on the model considered. We propose a method for the compensation of the systematics using only the observed data.

Flow characterization in the downhill region of a pulsed oblique round jet

Flow control can be achieved by inducing external pulsations on wall jet impingement. It can enhance the mixing ability and heat transfer characteristics. Evidence of streamwise vortices have been found in oblique jet impingement subjected to external pulsations.

Analysis of the quenching behavior in impinging flame: Flow and thermal characteristics

Considering the complex phenomenon of in-cylinder flame impingement occurring in internal combustion engines, flame-wall interaction in different impinging distance cases is presented for the non-premixed impinging flames. In this article, the quenching mechanism of impinging flame is focused on using experiments and numerical simulations. The quenching information on the flow and thermal field is obtained in the experiment. Large Eddy simulation is used to provide the details in the flow characteristics. In the wall jet region, multiple vortexes are formed accompanied by strong turbulence and large wall shear stress, which becomes more significant with the increase of the impinging distance. The low-temperature region corresponds to a large wall heat flux and Nusselt number, and this non-positive correspondence between temperature and heat transfer leads to excessive heat loss and weakens the flame stability. The quenching behavior in the impinging flame is analyzed in terms of the flow field and thermal characteristics.

Experimental investigation on time-averaged heat transfer characteristics of a row of pulsating jets

The heat transfer characteristics of a row of pulsating jets feeding by a cooling tube have been investigated for an active clearance control system. Experimental tests were performed to explore the effects of the frequency (rectangular signal, 5–30 Hz), Reynolds number (6000–20,000), jet-to-surface spacing ratio (6–10), and jet-to-jet spacing ratio (3–9) on the Nusselt number of the pulsating jets and the relative enhancement coefficient compared with the steady jets. Results show that the streamwise-averaged Nusselt number of the pulsating jets increases with the increase of frequency (5–25 Hz) and Re. Its distribution along the X/d is gentler with the increase of H/d and S/d. Heat transfer correlations are developed and two associated parameters SrRe0.911(H/S)0.348 and SrRe(H/d)0.540(S/d)-0.248 are obtained to effectively scale the enhancements. Maximum enhancements for the stagnation region and wall jet region are 29.3% and 26.8%, respectively. The heat transfer on the overall surface can be strengthened in a wider range of conditions compared with the stagnation region, which contributes to the heat transfer uniformity in certain conditions.

Prediction of gamma-ray emission from Cygnus X-1, SS 433, and GRS 1915+105 after absorption

Context. Stellar black hole X-ray binary stars (BHXRBs) are among the most luminous and powerful systems located in our Milky Way and in other galaxies of the Universe. Their jets are prominent sources of particles (e.g., neutrinos) and radiation emissions in energy ranges detectable by terrestrial and space telescopes, even from galaxies deep in the space. A significant factor, however, would be the photon absorption effect that occur due to scattering on the lower end of the energy radiation of the system’s surroundings. Aims. We aim to study in detail and extract predictions for the emitted gamma-ray intensities and integral fluxes of the jets emanating from BHXRB systems Cygnus X-1, GRS 1915+105, and SS 433. Toward this end, we also investigate the severe effects of gamma-ray absorption that eradicate part of the produced intensity spectra. Furthermore, we explore the jet regions that are most likely to emit unabsorbed gamma-rays capable of reaching detectors on Earth. Our goal is to calculate the integral fluxes before and after absorption for the abovementioned systems and compare the results with the very-high-energy gamma-ray observations of sensitive telescopes such as the MAGIC, H.E.S.S., Fermi-LAT, and so on. Methods. The implemented gamma-ray emission mechanisms initiate from the p − p scattering process inside the hadron-dominated jets following the well-known shock-wave particle acceleration. In addition, we estimate the optical depths of three absorption processes between gamma-ray photons and (i) accretion disk X-ray emission, (ii) black hole corona photons, and (iii) donor star thermal emission. We also examine the dependence of the absorption optical depths on various parameters, such as the disk’s temperature, coronal radius and, donor star luminosity. Results. We find that disk absorption is dominant for distances of z < 1010 cm from the black hole, while the donor star absorption dominates for 1010 < z < 1012 cm. Beyond that jet point, the absorption effects become significantly weaker. Cygnus X-1 presents the highest gamma-ray integral flux across the jet length, while GRS 1915+105 emits the least due to its weakly collimated jets. The jets of SS 433 emit gamma-rays only for z > 1010 cm due to severe disk absorption fueled by the system’s super-Eddington accretion limit.

Numerical study on the mixing and combustion characteristics of a liquid kerosene jet in a scramjet combustor

We numerically investigate the mixing and combustion process of a kerosene jet injected into a scramjet combustor with dual-cavity at different global fuel equivalence ratios using a compressible two-phase flow large-eddy simulation based on the Eulerian–Lagrangian approach. The results indicate that the calculated wall pressure distribution is basically consistent with the experimental result. The high-temperature zone and intermediate product CO distribution zone shown by the numerical simulation agree with the flame position observed by the experiment. When the global equivalent ratio is 0.149, the combustion is in the weak combustion mode. The flame only exists in the downstream cavity, which is mostly the premixed flame. The combustion efficiency is low, and the corresponding total pressure loss is small. When the global equivalent ratio is 0.187, the combustion is in the intense combustion mode, and the flame has a strong instability characteristic. The numerical simulation reproduces the flame flashback phenomenon that the flame backpropagation from the downstream region of cavity C1 to the upstream region of the liquid jet. During the flame flashback process, the flame gradually changes from premixed flame dominated to premixed flame and diffusion flame jointly dominated. In the weak combustion mode, the interaction between flame and droplet is weak, and droplet evaporation is less affected by combustion. While in the intense combustion mode, droplets evaporate more violently, the spray penetration depth increases, and some liquid droplets are transported upstream of the jet by the wall reflux zone.

Physics of moderately stretched electrified jets in electrohydrodynamic jet printing.

Electrohydrodynamic (EHD) jet printing involves the deposition of a liquid jet issuing from a needle stretched under the effect of a strong electric field between the needle and a collector plate. Unlike the geometrically independent classical cone-jet observed at low flow rates and high applied electric fields, at a relatively high flow rate and moderate electric field, EHD jets are moderately stretched. Jetting characteristics of such moderately stretched EHD jets differ from the typical cone-jet due to the nonlocalized cone-to-jet transition. Hence, we describe the physics of the moderately stretched EHD jet applicable to the EHD jet printing process through numerical solutions of a quasi-one-dimensional model of the EHD jet and experiments. Through comparison with experimental measurements, we show that our simulations correctly predict the jet shape for varying flow rates and applied potential difference. We present the physical mechanism of inertia-dominated slender EHD jets based on the dominant driving and resisting forces and relevant dimensionless numbers. We show that the slender EHD jet stretches and accelerates primarily due to the balance of driving tangential electric shear and resisting inertia forces in the developed jet region, whereas in the vicinity of the needle, driving charge repulsion and resisting surface tension forces govern the cone shape. The findings of this study can help in operational understanding and better control of the EHD jet printing process.

Heat transfer investigation on under-expanded supersonic impinging jets

In this study, wall-resolved large-eddy simulations are performed to investigate the underpinning physics of impingement heat transfer in under-expanded supersonic impinging jets. The jet is generated with an infinite-lipped nozzle of diameter D, has a nozzle-to-wall distance of 5D, a nozzle pressure ratio (NPR) of 3.4 and a Reynolds number of 6×104 based on the ideally-expanded jet velocity. Two thermal boundary conditions of a heated isothermal wall and an adiabatic wall for the impingement plate are considered.Both impinging jets in the current study feature a strong stand-off shock, which oscillates periodically in front of the impingement plate at two dominant frequencies. These oscillations result in two local peaks of the mean heat transfer coefficients with the first peak located in the stagnation region but shifted away from the stagnation point (i.e. jet axis), whilst the second peak is located in the wall jet region. The instantaneous fields of pressure, vorticity and Nusselt number are first discussed to study qualitatively the connection between these two peaks of the mean heat transfer coefficient and the near-wall flow structures. The characteristics of these two peaks are then analysed using the near-wall statistics and spectra of the pressure, axial velocity and Nusselt number distribution.It is found that the first peak of the Nusselt number forms as a consequence of the jet impingement of the non-periodic turbulent structures, whilst the second peak is linked to the reattachment of the recirculation bubble in the wall jet. It is shown that the unsteady separation and reattachment processes are strongly related to the oscillatory motion of the stand-off shock at the dominant frequencies. Last but not the least, the first local minimum close to the stagnation point is also found to be associated with the strong oscillatory behaviour of the stand-off shock.

Shocks and Photoionization of the Inner 650 au Jet of the Interacting Binary Star R Aquarii from Multiwavelength Hubble Space Telescope Observations

Astrophysical jets are present in a range of environments, including young stellar objects, X-ray binaries, and active galactic nuclei, but their formation is still not fully understood. As one of the nearest symbiotic binary stars, R Aquarii (D ∼ 220 pc) offers a unique opportunity to study the inner region within ∼600 au of the jet source, which is particularly crucial to our understanding of nonrelativistic jet formation and origin. We present high-angular resolution UV and optical imaging from the Hubble Space Telescope in six emission-line regions of the inner jet. Using these observations to obtain a range of representative line ratios for our system and kinematic data derived from a comparison with previous studies, we model the shocked gas in order to determine the relative roles of shock heating and photoionization in the R Aquarii system. We find that our shock models suggest that a nonzero magnetic field is needed to describe the measured line ratios. We also find that the Mg ii λλ2795,2802 intensities are overpredicted by our models for most of the jet regions, perhaps because of depletion onto grains or to opacity in these resonance lines.

Radio jet precession in M 81*

We report four novel position angle measurements of the core region M 81* at 5 GHz and 8 GHz, which confirm the presence of sinusoidal jet precession in the M 81 jet region, as suggested by Martí-Vidal et al. (2011, A&A, 533, A111). The model makes three testable predictions regarding the evolution of the jet precession, which we test in our data with observations from 2017, 2018, and 2019. Our data confirm a precession period of ∼7 yr on top of a small linear drift. We further show that two 8 GHz observation are consistent with a precession period of ∼7 yr but show a different time lag with respect to the 5 GHz and 1.7 GHz observations. We do not find a periodic modulation of the light curve with the jet precession and therefore rule out a Doppler nature for the historic 1998–2002 flare. Our observations are consistent with either a binary black hole origin for the precession or the Lense-Thirring effect.

Retrofitting Vertical Slot Fish Pass with Brush Blocks: Hydraulics Performance

The mean and turbulent flow characteristics of a vertical slot fish pass, with and without brush blocks, were investigated at the Cataloluk Small Hydropower Plant on the Tekir River, located in the Ceyhan River Basin of Turkey. Within the scope of the project, three-dimensional velocity measurements were performed at different hydraulic conditions. The prototype flow measurements showed that by placing brush blocks and the substrate in the vertical-slot pool: (i) the maximum velocity observed downstream of the slot was reduced by 39%; (ii) the maximum lateral component of the Reynolds shear stress observed in the slot region was reduced by a factor of 3; and (iii) the spatially averaged resultant velocity was reduced by 20%. With brush blocks, the turbulent jet region was reduced and recirculation regions disappeared. Furthermore, the spatially-averaged lateral component of the Reynolds shear stress was 3.3 times higher than the spatially-averaged streamwise component of the Reynolds shear stress because of the lateral velocity gradient and mixing in the pool. The present findings will contribute to potential improvements in the non and less efficiently-functioning vertical slot fish pass and other fish pass types by adding brush blocks.

Scalar interfaces in the near field of a unity velocity ratio coaxial jet

Planar laser-induced fluorescence is employed to investigate the concentration field in the developing region of a coaxial jet with an annular to inner jet velocity of one. Seeding the annular flow with a scalar shows that prior to the erosion of the inner scalar core, an inner interface exists, across which the scalar quantities exhibit sharp gradients akin to the traditional outer interface. Coarse grain filtering revealed that the outer interface establishes the universal fractal scaling before the onset of self-similarity, while the inner interface does not develop sufficient scale separation to do so due to the lack of shear.

Explaining Dynamics and Rapid Onset of the Somali Jet through Its Kinetic Energy Budget

Abstract The low-level Somali jet is the primary mechanism of moisture transport for the South Asian monsoon. It precedes monsoon onset over India and shares its key characteristic features such as rapid intensification and slower retreat during seasonal evolution. This study analyzes the kinetic energy (KE) budget of Somali jet region using high-spatiotemporal-resolution reanalysis (ERA5) to explain these key features. The KE budget reveals that in the Southern Hemisphere, the easterly flow that ultimately feeds the jet exhibits a conventional Ekman balance, with KE generation balanced by frictional dissipation. A unique “advective balance”—balance between KE generation in the northward flow and its advection emerges as the jet begins to form near the equatorial region. The fully formed Somali jet exhibits a three-way balance between KE generation, its advection, and dissipation. A nondimensional parameter–boundary layer local Rossby number (Ro) characterizes the transitions across these regimes. The large-Ro regime describes an advective balance under which KE-generating meridional winds become proportional to meridional pressure gradients yielding a nonlinear (quadratic) dependence of KE generation on pressure gradients. This nonlinear relation explains rapid onset of the jet as well as the asymmetry between rapid onset and slower retreat, leading us to propose a simple model for approximating the seasonal evolution of kinetic energy in the Somali jet given the evolution of pressure gradients. In summary, this work shows that Somali jet onset is closely tied to the seasonal evolution of Ro in the region where the advective boundary layer appears. Significance Statement The Somali jet is an important feature of the South Asian monsoon, contributing significantly to enhanced rainfall over the region. We study the dynamics of this jet by focusing on its kinetic energy (KE). Maintenance of the jet at different regions corresponds to different kinds of balances in the kinetic energy budget. A dimensionless parameter characterizes these different regimes of KE balance, which are captured at sufficiently high resolution. The rapid intensification of the jet can be explained as a nonlinear response of KE generation to the seasonal evolution of the north–south pressure gradient. These findings contribute to understanding of the jet and its nonlinear evolution, which is important for accurate representation and simulation of the Somali jet in climate models.

Numerical Study of Turbulent Swirling Diffusion Flame Under Lean and Rich Conditions Using Turbulence Realizable k-epsilon Model

ABSTRACT This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio.

Investigating the Linear Dynamics of the Near-Field of a Turbulent High-Speed Jet Using Dual-Particle Image Velocimetry (PIV) and Dynamic Mode Decomposition (DMD)

The quest for the physical mechanisms underlying turbulent high-speed jet flows is underpinned by the extraction of spatio-temporal coherent structures from their flow fields. Experimental measurements to enable data decomposition need to comprise time-resolved velocity fields with a high-spatial resolution—qualities which current particle image velocimetry hardware are incapable of providing. This paper demonstrates a novel approach that addresses this challenge through the implementation of an experimental high-spatial resolution dual-particle image velocimetry methodology coupled with dynamic mode decomposition. This new approach is exemplified by its application in studying the dynamics of the near-field region of a turbulent high-speed jet, enabling the spatio-temporal structure to be investigated by the identification of the spatial structure of the dominant dynamic modes and their temporal dynamics. The spatial amplification of these modes is compared with that predicted by classical linear stability theory, showing close agreement, which demonstrates the powerful capability of this technique to identify the dominant frequencies and their associated spatial structures in high-speed turbulent flows.

Experimental investigation on condensation regimes and transition boundary during bubble condensation in narrow rectangular channel

Direct steam contact condensation in subcooled water is widely used in various industries because of high efficiency in heat and mass transfer. A visual experiment is carried out to investigate the condensation process of vapor bubble in narrow rectangular channel. The steam injection mass flux, subcooled water temperature and water mass flow are 0.43–40 kg/m2·s, 50–93 °C, and 0–200 kg/m2·s, respectively. In order to obtain more accurate parameters of vapor bubble in narrow rectangular channel, a new bubble volume model was developed and compared with existed models. A three-dimensional condensation regime map is developed with the coordinates of steam injection flux, water mass flow, and water temperature. Five condensation patterns such as chug, oscillatory bubble, growth bubble, transition region and oscillatory jet are determined based on the dynamic behavior of steam-water interface. The bubble equivalent diameter and aspect ratio during evolution process are calculated, and these two parameters in each pattern exhibit a significant periodicity, except for the oscillatory Jet. The true area of heat transfer interface between bubble and subcooled water is much larger than the actual measured value under high steam injection mass flux. A transition criterion from oscillatory bubble regime to oscillatory jet regime is proposed by the analysis of the bubble critical diameter and the condensation heat transfer of steam-water interface.