Simulated Jet Research Articles

Blandford–Znajek jets in galaxy formation simulations: method and implementation

ABSTRACT Jets launched by active galactic nuclei (AGN) are believed to play a significant role in shaping the properties of galaxies and provide an energetically viable mechanism through which galaxies can become quenched. Here, we present a novel AGN feedback model, which we have incorporated into the arepo code, that evolves the black hole mass and spin as the accretion flow proceeds through a thin α-disc that we self-consistently couple to a Blandford–Znajek jet. We apply our model to the central region of a typical radio-loud Seyfert galaxy embedded in a hot circumgalactic medium (CGM). We find that jets launched into high-pressure environments thermalize efficiently due to the formation of recollimation shocks and the vigorous instabilities that these shocks excite increase the efficiency of the mixing of CGM and jet material. The beams of more overpressured jets, however, are not as readily disrupted by instabilities so the majority of the momentum flux at the jet base is retained out to the head, where the jet terminates in a reverse shock. All jets entrain a significant amount of cold circumnuclear disc material that, while energetically insignificant, dominates the lobe mass together with the hot, entrained CGM material. The jet power evolves significantly due to effective self-regulation by the black hole, fed by secularly driven, intermittent mass flows. The direction of jets launched directly into the circumnuclear disc changes considerably due to effective Bardeen–Petterson torquing. Interestingly, these jets obliterate the innermost regions of the disc and drive large-scale, multiphase, turbulent, bipolar outflows.

Magnetosheath jet evolution as a function of lifetime: global hybrid-Vlasov simulations compared to MMS observations

Abstract. Magnetosheath jets are regions of high dynamic pressure, which can traverse from the bow shock towards the magnetopause. Recent modelling efforts, limited to a single jet and a single set of upstream conditions, have provided the first estimations about how the jet parameters behave as a function of position within the magnetosheath. Here we expand the earlier results by doing the first statistical investigation of the jet dimensions and parameters as a function of their lifetime within the magnetosheath. To verify the simulation behaviour, we first identify jets from Magnetosphere Multiscale (MMS) spacecraft data (6142 in total) and confirm the Vlasiator jet general behaviour using statistics of 924 simulated individual jets. We find that the jets in the simulation are in quantitative agreement with the observations, confirming earlier findings related to jets using Vlasiator. The jet density, dynamic pressure, and magnetic field intensity show a sharp jump at the bow shock, which decreases towards the magnetopause. The jets appear compressive and cooler than the magnetosheath at the bow shock, while during their propagation towards the magnetopause they thermalise. Further, the shape of the jets flatten as they progress through the magnetosheath. They are able to maintain their flow velocity and direction within the magnetosheath flow, and they end up preferentially to the side of the magnetosheath behind the quasi-parallel shock. Finally, we find that Vlasiator jets during low solar wind Alfvén Mach number MA are shorter in duration, smaller in their extent, and weaker in terms of dynamic pressure and magnetic field intensity as compared to the jets during high MA.

An integrated approach to predict scalar fields of a simulated turbulent jet diffusion flame using multiple fully connected variational autoencoders and MLP networks

A novel integrated deep learning approach for data-driven surrogate modelling of combustion computational fluid dynamics (CFD) simulations is presented. It combines variational autoencoders (VAEs) with deep neural networks (DNNs) to predict detail cell-by-cell two-dimensional distributions of temperature, velocity and species mass fractions from high level inputs such as velocity and fuel and air mass fractions. The VAE model is used to generate low dimensional encodings of the CFD data and the DNN is used in turn to map boundary conditions to the encodings. The results show that regularization is required during all training phases. Sufficiently accurate results were achieved for the reproduced species mass fractions with mean average errors below 0.3 [%wt.]. The validation mean average percentage errors for the temperature and velocity fields are 1.7% and 7.1% respectively. It is therefore possible to predict detail two-dimensional contours of CFD solution data with adequate generalizability and accuracy.

Distributed training and scalability for the particle clustering method UCluster

In recent years, machine-learning methods have become increasingly important for the experiments at the Large Hadron Collider (LHC). They are utilised in everything from trigger systems to reconstruction and data analysis. The recent UCluster method is a general model providing unsupervised clustering of particle physics data, that can be easily modified to provide solutions for a variety of different decision problems. In the current paper, we improve on the UCluster method by adding the option of training the model in a scalable and distributed fashion, and thereby extending its utility to learn from arbitrarily large data sets. UCluster combines a graph-based neural network called ABCnet with a clustering step, using a combined loss function in the training phase. The original code is publicly available in TensorFlow v1.14 and has previously been trained on a single GPU. It shows a clustering accuracy of 81% when applied to the problem of multi-class classification of simulated jet events. Our implementation adds the distributed training functionality by utilising the Horovod distributed training framework, which necessitated a migration of the code to TensorFlow v2. Together with using parquet files for splitting data up between different compute nodes, the distributed training makes the model scalable to any amount of input data, something that will be essential for use with real LHC data sets. We find that the model is well suited for distributed training, with the training time decreasing in direct relation to the number of GPU’s used. However, further improvements by a more exhaustive and possibly distributed hyper-parameter search is required in order to achieve the reported accuracy of the original UCluster method.

Signatures of Cross-sectional Width Modulation in Solar Spicules due to Field-aligned Flows

Abstract We report the first observational detection of frequency modulation in the cross-sectional width of spicule structures due to field-aligned plasma flows. Cross-sectional width variations were estimated for the least superimposed off-limb spicules observed in high-resolution Hα imaging spectroscopy data. Analysis of estimated cross-sectional widths suggest periodic oscillations, concurrent with 2D numerical modeling for a jet structure in a stratified solar atmosphere. Spectral analysis for both observed and simulated cross-sectional widths indicate frequency modulation as noticeable shifts in estimated periodicities during rise and fall phases of field-aligned plasma flows in the jet structure. Furthermore, the presence of the first overtone in a dynamic/spicular waveguide is also evident in both the observed and the simulated jet structures. These harmonics can be an important tool for future chromospheric magnetoseismology investigations and applications to dynamic waveguides (like spicules).

Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study

The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.

Research on Automatic Cleaning Equipment with High Pressure Water Jet for Optical Pipelines

In order to solve the problems of high labor intensity, low efficiency, and unstable cleaning quality when cleaning optical pipelines with traditional manual high-pressure spray, automatic cleaning equipment with high pressure water jet for optical pipelines is designed. The working principle and the design points are introduced. The movement rule of the nozzle under self-propelled drive is analyzed by ANSYS, the results show that the self-propelled drive cannot meet the cleaning speed requirements. Therefore, the wire rope driving method is proposed to realize the control of the nozzle speed. Subsequently, the speed of the simulated water jet is analyzed by the STARCCM, it is inferred that the water jet is still in the basic section at the maximum target distance of the pipeline, which can be used for cleaning. For verifying the cleaning effect, the prototype is applied to the cleaning of the actual optical pipelines. The verification results show that the cleaning equipment can meet the requirements of 100 / class clean, and the efficiency is increased by 70% compared with the traditional manual high-pressure spray.

Numerical predictions of near field behavior of variable density non-reacting turbulent round jets

Non-reacting turbulent round jets with density variations are computationally investigated in the near field region. Helium, Air and Carbone dioxide are the considered gases emitted from the nozzle exit and discharged in a weakly co-flow air stream, leading to three different density ratios; 0.14 [Helium/Air], 1.0 [Air/Air] and 1.52 [CO2/Air]. Two closure turbulence models are tested; the first order standard k-ε model and the second order Reynolds Stress Model (RSM) and confronted to experimental data. Predicted results elaborated by the RSM model are found to be in good agreement with experiments. Mean and turbulent flow properties, obtained for the velocity and scalar concentration field, are presented for the three simulated jets. It is found that the lighter gas (Helium) tends to mix more rapidly with the co-flow ambient air than heavier gases (Air and CO2). The entropy generation growth is shown to be greater for the Helium jet.

Data-driven three-dimensional super-resolution imaging of a turbulent jet flame using a generative adversarial network.

Three-dimensional (3D) computed tomography (CT) is becoming a well-established tool for turbulent combustion diagnostics. However, the 3D CT technique suffers from contradictory demands of spatial resolution and domain size. This work therefore reports a data-driven 3D super-resolution approach to enhance the spatial resolution by two times along each spatial direction. The approach, named 3D super-resolution generative adversarial network (3D-SR-GAN), builds a generator and a discriminator network to learn the topographic information and infer high-resolution 3D turbulent flame structure with a given low-resolution counterpart. This work uses numerically simulated 3D turbulent jet flame structures as training data to update model parameters of the GAN network. Extensive performance evaluations are then conducted to show the superiority of the proposed 3D-SR-GAN network, compared with other direct interpolation methods. The results show that a convincing super-resolution (SR) operation with the overall error of ∼4% and the peak signal-to-noise ratio of 37 dB can be reached with an upscaling factor of 2, representing an eight times enhancement of the total voxel number. Moreover, the trained network can predict the SR structure of the jet flame with a different Reynolds number without retraining the network parameters.

Interplay of various particle acceleration processes in astrophysical environment

AbstractAstrophysical systems possess various sites of particle acceleration, which gives rise to the observed non-thermal spectra. Diffusive shock acceleration (DSA) and stochastic turbulent acceleration (STA) are the candidates for producing very high energy particles in weakly magnetized regions. While DSA is a systematic acceleration process, STA is a random energization process, usually modelled as a biased random walk in energy space with a Fokker-Planck equation. In astrophysical systems, different acceleration processes work in an integrated manner along with various energy losses.Here we study the interplay of both STA and DSA in addition to various energy losses, in a simulated RMHD jet cocoon. Further, we consider a phenomenologically motivated STA timescale and discuss its effect on the emission profile of the RMHD jet. A parametric study on the turbulent acceleration timescale is also conducted to showcase the effect of turbulence damping on the emission structure of the simulated jet.

A New Code for Relativistic Hydrodynamics and its Application to FR II Radio Jets

AbstractTo study the dynamics of relativistic flows in astrophysical objects such as radio jets, we have developed a new special relativistic hydrodynamic (RHD) code based on the weighted essentially non-oscillatory (WENO) scheme, a high-order finite difference scheme. The code includes different WENO versions, and high-order time integration methods such as the 4th-order accurate Runge-Kutta (RK4) and strong stability preserving RK (SSPRK), as well as the equations of state (EOSs) that closely approximate the EOS of the single-component perfect gas in relativistic regime. Additionally, it is optimized for the reproduction of complex structures in multi-dimensional flows, and implements a modification of eigenvalues for the acoustic modes to effectively control carbuncle instability. As the first application of the code, we have simulated ultra-relativistic jets of FR II radio galaxies, and studied the nonlinear flow structures, such as shocks, velocity shear, and turbulence, through large-scale.

Large-scale poloidal magnetic field dynamo leads to powerful jets in GRMHD simulations of black hole accretion with toroidal field

ABSTRACT Accreting black holes (BHs) launch relativistic collimated jets, across many decades in luminosity and mass, suggesting the jet launching mechanism is universal, robust, and scale-free. Theoretical models and general relativistic magnetohydrodynamic (GRMHD) simulations indicate that the key jet-making ingredient is large-scale poloidal magnetic flux. However, its origin is uncertain, and it is unknown if it can be generated in situ or dragged inward from the ambient medium. Here, we use the GPU-accelerated GRMHD code h-amr to study global 3D BH accretion at unusually high resolutions more typical of local shearing box simulations. We demonstrate that turbulence in a radially extended accretion disc can generate large-scale poloidal magnetic flux in situ, even when starting from a purely toroidal magnetic field. The flux accumulates around the BH till it becomes dynamically important, leads to a magnetically arrested disc (MAD), and launches relativistic jets that are more powerful than the accretion flow. The jet power exceeds that of previous GRMHD toroidal field simulations by a factor of 10 000. The jets do not show significant kink or pinch instabilities, accelerate to γ ∼ 10 over three decades in distance, and follow a collimation profile similar to the observed M87 jet.

A pressure decomposition framework for aeroacoustic analysis of turbulent jets

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e, not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise.

The role of magnetic field geometry in the evolution of neutron star merger accretion discs

ABSTRACTNeutron star mergers are unique laboratories of accretion, ejection, and r-process nucleosynthesis. We used 3D general relativistic magnetohydrodynamic simulations to study the role of the post-merger magnetic geometry in the evolution of merger remnant discs around stationary Kerr black holes. Our simulations fully capture mass accretion, ejection, and jet production, owing to their exceptionally long duration exceeding 4 s. Poloidal post-merger magnetic field configurations produce jets with energies Ejet ∼ (4–30) × 1050 erg, isotropic equivalent energies Eiso ∼ (4–20) × 1052 erg, opening angles θjet ∼ 6–13°, and durations tj ≲ 1 s. Accompanying the production of jets is the ejection of $f_\mathrm{ej}\sim 30\!-\!40{{\ \rm per\ cent}}$ of the post-merger disc mass, continuing out to times >1 s. We discover that a more natural, purely toroidal post-merger magnetic field geometry generates large-scale poloidal magnetic flux of alternating polarity and striped jets. The first stripe, of $E_\mathrm{jet}\simeq 2\times 10^{48}\, \mathrm{erg}$, Eiso ∼ 1051 erg, θjet ∼ 3.5–5°, and tj ∼ 0.1 s, is followed by ≳4 s of striped jet activity with $f_\mathrm{ej}\simeq 27{{\ \rm per\ cent}}$. The dissipation of such stripes could power the short-duration gamma-ray burst (sGRB) prompt emission. Our simulated jet energies and durations span the range of sGRBs. We find that although the blue kilonova component is initially hidden from view by the red component, it expands faster, outruns the red component, and becomes visible to off-axis observers. In comparison to GW 170817/GRB 170817A, our simulations underpredict the mass of the blue relative to red component by a factor of few. Including the dynamical ejecta and neutrino absorption may reduce this tension.

Influence of high humidity and salt-rich spray environment on ventilation effect in urban undersea road tunnel

The urban undersea road tunnels are featured with high frequent traffic congestions and a strict environmental control standard. The internal environment in urban undersea road tunnels is generally controlled by jet fans. The high-humidity, salt-rich spray and acid gas environment in urban undersea road tunnels accelerate the jet fans corrosion, which deteriorates the internal environment and makes the ventilation effect unable to meet the control standards. Detailed examples of pressure drop caused by jet fan corrosion are sparsely available in the publicly available literature. Therefore, this study is to investigate the influence of jet fan corrosion on ventilation effect in urban undersea road tunnel. Based on the Xiamen Xiang’an undersea tunnel, the field measurement of lift produced by jet fan in the tunnel was carried out. Combined with theoretical analysis and Fluent dynamic grid numerical model, the mechanism of reducing lift by jet fan corrosion and the attenuation law of jet fan lift with corrosion time were studied. The results show that salt spray corrosion environment in undersea road tunnel increases surface roughness of the jet fan blades and flow-through components through the effects of accumulation and sedimentation, erosion and corrosion, thereby reducing the lift. After 20 years of operation, the numerical simulated jet fan lift will be reduced by 20.3%, and the ventilation effect will be deteriorate by 20.3% simultaneously. Combined with the field measurement results and the service life of jet fan, the safety factor of ventilation system configuration in undersea tunnel is proposed. Results evidenced that the mechanism of reducing lift by jet fan corrosion in urban undersea road tunnel, which can provide basic data for further research and provide reference for the design of the same type of tunnel ventilation system.

Simulating schlieren and shadowgraph images from LES data

Geometrical optics ray-tracing is used to derive schlieren and shadowgraph images from large-eddy simulation (LES) data of a jet in supersonic crossflow and to compare with experimental data. Including the components of the optical system that forms the image in the simulation is found to be important. The technique produces images that replicate flow physics more faithfully than straight-line path integration and other techniques, and more efficiently than physical-optics techniques. Applications of these simulated images are demonstrated in supersonic flows. Time-correlated pairs of shadowgraph images taken from the LES using this technique are used in conjunction with an image-correlation velocimetry technique to compare the estimated convection velocity field in the LES to that of experiments of the same flow. Agreement between the two is good with a maximum variance of 5% by some metrics. This technique can aid in the validation of LES results, allowing quantitative comparison between experiment and simulation, and to extract information unattainable by experiment alone. Comparisons of simulated and experimental jet penetration into the supersonic freestream are also made.Graphic abstract

High resolution volumetric dual-camera light-field PIV

Light-field particle image velocimetry (LF-PIV) was recently introduced to measure three-dimensional, three-component velocity field with just a single light-field camera. One of the major challenges lies in the small viewing aperture affecting the depth resolution of such a single-camera based LF-PIV approach. In the present study, we show that this limitation may be mitigated by a dual light-field camera framework, one which includes a novel volumetric calibration model derived from Gaussian optics, a particle intensity reconstruction algorithm based on the multiplicative algebraic reconstruction technique and a post-processing technique for the reconstructed particle intensity field. The proposed approach was firstly validated with synthetic light-field particle images as well as experimental light-field images of five tiny glass beads imitating tracer particles. Secondly, parametric studies were conducted to analyze the influence of the viewing angle and seeding particle density on the reconstruction quality and spatial resolution. In particular, synthetic light-field particle images of a direct numerical simulation jet data set were utilized to compare the performance of single- and dual-camera LF-PIV techniques. Finally, experimental volumetric flow field results of a circular vortex-ring were also measured by single- and dual-camera LF-PIV techniques and compared. It is determined here that an additional light-field camera can mitigate the elongation effects of reconstructed particles and improve the measurement resolution in the depth direction.

Differences in post-chronic jet lag parameters in male and female mice

ABSTRACT Chronic jet lag has deleterious effects on many physiological parameters. Most simulations of chronic jet lag consist of repeated advances only, while real-life conditions may differ. We compared number of transients, activity (α) and rest (ρ), and robustness of rhythm by autocorrelation in male and female AKR mice; before, during, and after simulated chronic jet lag (CJL); and phase angle of entrainment and accuracy before and after CJL, using alternate 9-h advances and delays of the light-dark schedule. Females exhibited no scalloping. Compared to pre-CJL values, α was significantly more in females post-CJL, indicating α decompression. The levels of activity increased during and after CJL, particularly in females, indicating behavioural feedback on the circadian rhythm. Robustness of rhythm (autocorrelation coefficient, and amplitude) was significantly suppressed during CJL in both males and females. Females took more transients than males to re-entrain to the final LD shift at the end of CJL. More transients were needed to re-entrain at the end of CJL, compared to a single shift. The study suggests that animals undergoing CJL incorporating alternating advances and delays are able to avoid α compression associated with jet lag, and also that females are detrimentally affected more than males. Abbreviations: SCN: Suprachiasmatic Nuclei; CJL: Chronic Jet Lag; LD: Light-dark

Investigating the topology dependence of quark and gluon jets

As most target final states for searches and measurements at the Large Hadron Collider have a particular quark/gluon composition, tools for distinguishing quark- from gluon-initiated jets can be very powerful. In addition to the difficulty of the classification task, quark-versus-gluon jet tagging is challenging to calibrate. The difficulty arises from the topology dependence of quark-versus-gluon jet tagging: since quarks and gluons have net quantum chromodynamic color charge while only colorless hadrons are measured, the radiation pattern inside a jet of a particular type depends on the rest of its environment. Given a definition of a quark or gluon jet, this paper studies the topology dependence of such jets in simulation. A set of phase space regions and jet substructure observables are identified for further comparative studies between generators and eventually in data.

Brief communication: Recent changes in summer Greenland blocking captured by none of the CMIP5 models

Abstract. Recent studies note a significant increase in high-pressure blocking over the Greenland region (Greenland Blocking Index, GBI) in summer since the 1990s. Such a general circulation change, indicated by a negative trend in the North Atlantic Oscillation (NAO) index, is generally highlighted as a major driver of recent surface melt records observed on the Greenland Ice Sheet (GrIS). Here we compare reanalysis-based GBI records with those from the Coupled Model Intercomparison Project 5 (CMIP5) suite of global climate models over 1950–2100. We find that the recent summer GBI increase lies well outside the range of modelled past reconstructions and future GBI projections (RCP4.5 and RCP8.5). The models consistently project a future decrease in GBI (linked to an increase in NAO), which highlights a likely key deficiency of current climate models if the recently observed circulation changes continue to persist. Given well-established connections between atmospheric pressure over the Greenland region and air temperature and precipitation extremes downstream, e.g. over northwest Europe, this brings into question the accuracy of simulated North Atlantic jet stream changes and resulting climatological anomalies over densely populated regions of northern Europe as well as of future projections of GrIS mass balance produced using global and regional climate models.