Radial Velocity Distribution Research Articles

FSR 1776: A new globular cluster in the Galactic bulge?

Context. Recent near-IR surveys have uncovered a plethora of new globular cluster (GC) candidates towards the Milky Way bulge. These new candidates need to be confirmed as real GCs and properly characterised. Aims. We investigate the physical nature of FSR 1776, a very interesting star cluster projected towards the Galactic bulge. This object was originally classified as an intermediate-age open cluster, and has recently been re-discovered independently and classified as a GC candidate (Minni 23). Firstly, our aim is to confirm its GC nature; secondly, we determine its physical parameters. Methods. The confirmation of the cluster existence is checked using the radial velocity (RV) distribution of a MUSE data cube centred at FSR 1776. The cluster parameters are derived from isochrone fitting to the RV-cleaned colour-magnitude diagrams (CMDs) from visible and near-IR photometry taken from VVV, 2MASS, DECAPS, and Gaia all together. Results. The predicted RV distribution for the FSR 1776 coordinates, considering only contributions from the bulge and disc field stars, is not enough to explain the observed MUSE RV distribution. The extra population (12% of the sample) is FSR 1776 with an average RV of −103.7 ± 0.4 km s−1. The CMDs reveal that it is 10 ± 1 Gyr metal-rich population with [Fe/H]phot ≈ +0.2 ± 0.2, [Fe/H]spec = +0.02 ± 0.01(σ = 0.14 dex), located at the bulge distance of 7.24 ± 0.5 kpc with AV ≈ 1.1 mag. The mean cluster proper motions are (⟨μα⟩,⟨μδ⟩) = (−2.3 ± 1.1, −2.6 ± 0.8) mas yr−1. Conclusions. FSR 1776 is an old GC located in the Galactic bulge with a super-solar metallicity, among the highest for a Galactic GC. This is consistent with predictions for the age–metallicity relation of the bulge, being FSR 1776 the probable missing link between typical GCs and the metal-rich bulge field. High-resolution spectroscopy of a larger field of view and deeper CMDs are now required for a full characterisation.

Numerical investigation of hydrodynamics and cluster characteristics in a chemical looping combustion system

Chemical looping combustion (CLC) technology is promising for high-efficient carbon dioxide capture with low energy penalty. Most existing CLC systems were constructed based on two fluidized bed reactors. In this work, a full-loop numerical model was established for a novel CLC system integrated with a circulating fluidized bed riser fuel reactor and a moving bed air reactor. For the fuel reactor, the radial and axial distributions of gas-solid concentration and velocities are fully investigated. It is found single-sided particle return affects radial distribution symmetry of gas-solid flows, but the influence gradually decreases with increasing reactor height. The cluster characteristics including time fraction, frequency, duration time and concentration are also investigated, and results demonstrate the particle clusters are more difficult to form and last long in the center of fuel reactor. For the air reactor, the operation performance of a tower-type reactor is compared with a common cylindrical reactor, and the cavity is not formed in the top of the tower-type reactor. For the whole system, effects of the particle diameter on pressure profiles, solid circulation stability and other flow dynamics are evaluated. Overall, the inner flow characteristics will provide fundamental guidance for the design, optimization and scale-up of the system.

Collision statistics and settling velocity of inertial particles in homogeneous turbulence from high-resolution DNS under two-way momentum coupling

Statistical quantities related to the motion of low inertia particles in homogeneous isotropic turbulence are investigated by means of direct numerical simulations applying the Eulerian–Lagrangian approach and the point-particle approximation. The modelled systems imitate, among others, the microphysical processes relevant for cloud droplets in typical atmospheric flows. To complement former studies, the turbulent flow is simulated using high-resolution grids, covering a wide range of Reynolds numbers, up to Rλ≈500. The effect of mutual interaction of the carrier and dispersed phases, known as two-way momentum coupling, is considered, and the new results are compared with those obtained under one-way coupling. The droplet kinematic statistics, including the radial relative velocity and radial distribution functions, and dynamic properties, such as the collision kernel and settling velocity, are reported over a wide range of mass loadings. The results show that the radial distribution function of “nearly touching” droplets decreases as the mass loading increases when gravity is not considered. In the presence of gravity there is an enhancement in clustering for dilute systems while a reduction occurs when the two-way coupling effect becomes considerable. An opposite trend is observed for relative velocities due to transfer of momentum from droplets to the fluid, decorrelating the motion of neighbouring droplets. By increasing Rλ, or equivalently enlarging the domain size at fixed energy dissipation rate, the clustering weakens if gravity is considered, while relative velocities augment, ultimately leading to a higher collision kernel. The average settling velocity increases with Rλ. It also increases with mass loading, especially for low-inertia droplets. When performing simulations of small particles at considerable mass loadings a parameterisation is unavoidable to reduce the numerical cost. We use the super-droplet approach and demonstrate its impact on physical fidelity of the modelled systems. The effect of the large-scale forcing scheme, deterministic or stochastic, on the droplet dynamic statistics is also investigated.

Gas velocimetry based on infrared laser-induced fluorescence

A novel method for gas velocity field measurements by means of infrared molecular tagging velocimetry is reported with proof-of-principle demonstration in a carbon dioxide (CO2) axisymmetric turbulent jet. Infrared laser-induced fluorescence utilizes the resonant vibrational energy level transitions of small gas molecules, such as CO2, to “tag” and trace the flow of the molecules by taking subsequent images of the infrared emission. Spectroscopic model of the molecular vibrational energy transfer processes is taken into account to design and optimize the measurement scheme. The infrared images are then analyzed, with detailed consideration of molecular diffusion, lateral velocity, and fluorescence lifetime, to yield quantitative velocity field distribution. The radial velocity distributions in the jet main region, with velocities ranging from 7 to 50 m/s, are obtained and shown to be in excellent agreement with theoretical predication and previous experimental works. Velocity uncertainties are discussed and estimated to be 7.7%, 6.7%, 6.1% for Re = 104, 2×104, 3×104 (maximum velocity uc=18.3,34.6,50.5 m/s), respectively. Spatial resolution along the laser beam is estimated to be 107 μm. To the best of the authors' knowledge, this is the first work of infrared molecular tagging velocimetry. With powerful excitation lasers targeting strong infrared molecular absorption transitions, this technique presents great potential for simultaneous flow-scalar field measurements at much-improved accuracy, spatial and temporal resolution, that can be used for the study of low-speed micro-flows, or instantaneous snapshots of turbulent flows.

Modeling the Evolution of a cluster of gravitating bodies taking into account their absolutely inelastic collisions

AbstractNumerical simulation of evolution of a cluster of a finite number of gravitating bodies interacting only by their intrinsic gravity has been carried out. The goal of the study was to reveal the main characteristic phases of the spatial distribution of material bodies constituting the cluster. In solving the problem, the possibility of interbody collisions was taken into account, the collisions being assumed to be absolutely inelastic. Forces external to the body cluster under consideration were ignored. Among all the internal force factors acting within the cluster, only the gravitational interaction was taken into account. The total mass of all the gravitating bodies of the cluster was assumed to remain constant during the entire evolution. The Cauchy problem with natural initial conditions was considered. To check the process of solution, the so‐called rotation curve was used which represents the current radial distribution of orbital velocities of the cluster bodies. The numerical analysis showed time variations of the model cluster rotation curve and, particularly, the fact that the rotation curve horizontal section is only a short moment in evolution of the gravitating bodies cluster. The results obtained within the scope of classical mechanics show that it is possible to represent all the rotation curve variations for the observed galaxies without appealing to the hypothesis of non‐observable gravitating “dark matter”.

Comparison of Different Impellers for Gas Dispersion in Power-Law Fluids

The 3D multiphase computational fluid dynamic (CFD) models have been developed for gas–liquid systems in a standard baffled stirred tank reactor (diameter 0.3 m) for non-Newtonian power-law fluids. The standard k-ε model has been used to model the turbulence, and drag has been modeled with the Grace drag model with Brucato modifications. CFD simulations have been carried out for three impellers, namely, the Rushton turbine, pitched blade turbine, and hydrofoil impeller. The effects of impeller design, power input per unit volume of the fluid (P/V) (1200–5200 W/m3), fluid rheology (K = 0–0.1561 Pa·sn, n = 0.687–1.000), and superficial gas velocity (7.1–28.3 mm/s) have been analyzed. The CFD model has been validated with the local distribution of axial, radial, and tangential velocities and overall gas holdup data reported in the literature. The CFD model has been used to predict the regime transition from flooding to loading and complete dispersion. A good agreement was observed for the prediction of regime transition for the Rushton turbine when compared with the experimental data. The average value of absolute error for the prediction of the gas holdup is 20%, which indicates that the developed CFD model can be successfully used to predict the gas holdup and regime transition in stirred tank reactors. Among the impellers studied, the hydrofoil impeller shows the highest gas holdup and uniform gas distribution in the vessel at low power consumption.

Numerical investigation of transient thermo-fluid processes in a Ranque-Hilsch vortex tube

A 2D numerical investigation is performed to better understand the transient thermo-fluid processes in a vortex tube for a cold mass fraction equal to0.44. The results along the Ranque-Hilsch vortex tube reveal a close agreement with past numerical and experimental data. The distribution of axial, radial, and tangential velocities as well as the stagnation pressure and temperature are examined at different positions for different time steps. The results indicate that the tangential velocity is the most significant velocity component and dominates the heat transfer and energy conversion processes. In addition, it is evident that the core of the cold end experiences the highest pressure gradient along with almost zero tangential and radial velocities. The maximum axial velocity appears close to the inlet and its value increases with higher cold mass fractions due to higher pressure gradient which may lead to the enlargement of the inner vortex. The impact of different cold mass fractions in the range of 0.22−0.82 at tube length ratios x/L equal to 0.05, 0.5, and 0.9 for different time steps are examined. The results indicate that there is relatively no vortex decomposition near the cold outlet.

Experimental study on the combustion characteristics of rectangular fire plumes

This paper presents a comprehensive experimental investigation on the combustion characteristics of rectangular fire plumes. A series of propane fire tests with different aspect ratios, initial heat release rates, and scales are performed. The axial and radial distributions of velocity and temperature as well as the flame length are measured and discussed. The radial distribution and half-width of the axial velocity and temperature all indicate that the fire flow evolves from three- to quasi-two-dimensional as the aspect ratio increases from unity to higher values. In this process, the asymmetric entrainment in the length and width directions of the rectangular fire is verified to play an important role in the flow development. A flame length model is derived, which reasonably well correlates with the data from the current and previous experiments under different aspect ratios, initial heat release rates, and scales. A shape factor extracted from the model can be used as a measure of the aspect ratio effect separately. A dimensionless height coupled with the shape factor well correlates the axial temperature and velocity data under different aspect ratios and initial heat release rates.

Presence of red giant population in the foreground stellar substructure of the Small Magellanic Cloud

ABSTRACT The eastern region of the Small Magellanic Cloud (SMC) is found to have a foreground stellar substructure, which is identified as a distance bimodality (∼12 kpc apart) in the previous studies using red clump (RC) stars. Interestingly, studies of red giant branch (RGB) stars in the eastern SMC indicate a bimodal radial velocity (RV) distribution. In this study, we investigate the connection between these two bimodal distributions to better understand the nature and origin of the foreground stellar substructure in the eastern SMC. We use the Gaia Early Data Release 3 astrometric data and archival RV data of RGB stars for this study. We find a bimodal RV distribution of RGB stars (separated by ∼35–45 km s−1) in the eastern and south-western (SW) outer regions. The observed proper motion values of the lower and higher RV RGB components in the eastern regions are similar to those of the foreground and main-body RC stars, respectively. This suggests that the two RGB populations in the eastern region are separated by a similar distance to those of the RC stars, and the RGB stars in the lower RV component are part of the foreground substructure. Based on the differences in the distance and RV of the two components, we estimate an approximate time of formation of this substructure as 307 ± 65 Myr ago. This is comparable with the values predicted by simulations for the recent epoch of tidal interaction between the Magellanic Clouds. Comparison of the observed properties of RGB stars, in the outer SW region, with N-body simulations shows that the higher RV component in the SW region is at a farther distance than the main body, indicating the presence of a stellar counter-bridge in the SW region of the SMC.

Taylor-Aris dispersion for N-zone and continuous systems with variable sorption strength – extending Aris’s approach

It is shown Aris’ general solution method for the asymptotic axial dispersion in nested two-layer systems with a radial distribution of the axial velocity and with different diffusion and sorption properties in each layer can be generalized to nested and distributed N-zone systems and systems with continuously varying sorption strength by introducing a generalized flux expression J = −D.K.∇(C/K). In this expression, the local diffusion coefficient D and the local sorption constant K can be either piecewise constant (N-zone system) or continuous functions of the radial space coordinates (continuous system). After a mathematically sound derivation of the general solution, it is validated by comparing the resulting expressions for Dax with either existing literature expressions or with the dispersion coefficient found by numerically solving the complete, time-dependent advection–diffusion mass balance for a number of selected examples. Applications can be found in continuous flow membrane separations, chromatography, and flows through extruded mesoporous ceramic beds.

Steady flow of high-paraffin bituminous binder in cylindrical tube in terms of Herschel – Bulkley fluid

This paper deals with the flow of bitumen binder in a cylindrical tube described by the Hershel-Bulkley fluid. The dependence is suggested for the fluid flow rate and the pressure drop. Dependences are also determined for the radial velocity distribution and effective viscosity. It is found that at a low pressure drop, the non-Newtonian properties of the medium lead to a significant hydraulic resistance due to the internal structure. With an increase in the pressure drop, the hydraulic resistance of the flow decreases.

Analysis and prediction of heat transfer deterioration of supercritical pressure cryogenic methane in a vertical tube

Developing a compact vaporizer with high thermal efficiency is of great significance to the storage and usage of Liquid Natural Gas (LNG). Numerical simulations were performed to investigate the flow and heat transfer characteristics of supercritical pressure methane in a vertical tube, and primary focus was to reveal the mechanism of two-peak wall temperatures occurrence. Six turbulent models have been confronted with experimental data, and the Renormalization Group (RNG) k-ε model is adopted. When Heat Transfer Deterioration (HTD) occurs, two-peak wall temperatures appear in the upward flow with high mass flux. The phenomenon of two-peak wall temperatures were explained in detail based on radial distributions of velocity and turbulent kinetic energy at different axial positions. The “M”-shape velocity distribution promotes the generation of turbulence in the core region. The reversal of radial flow direction enhances the mixing between hot and cold fluids. A new criterion was proposed for predicting the critical heat flux which causes the onset of HTD in supercritical pressure methane.

A Corotating Group of Dwarf Galaxies around NGC 2750 as a Centaurus A Analog

Abstract We study a low-mass spiral galaxy NGC 2750 (∼5 times less massive than the Milky Way), located ∼40 Mpc away in a sparse region that hosts seven satellite galaxies within a projected distance of 150 Kpc. Among them six are star-forming dwarfs with stellar masses of >≈107 M ⊙ and one is an early-type dwarf with a stellar mass of 2.6 × 106 M ⊙. The star-forming dwarfs are gas rich, with gas mass fractions as high as Log(M H i /M *) = 1.2 and their star formation rates vary between 0.03 and 0.35 M ⊙ yr−1. The projected distances and measured radial velocities of the six star-forming satellites provide a high probability that they are members of the group. The radial velocity distribution of the six satellites exhibits evidence of a systematic corotation. We devise a method to quantify such a corotation signal by calculating the correlation coefficient between satellites’ relative line-of-sight velocities (ΔV r) and their sky-projected distances from the host (R p). For the NGC 2750 system, we show a clear correlation between ΔV r and R p, with a Pearson’s R correlation coefficient c = 0.90 and p-value = 0.005. We compare the NGC 2750 system with the Centaurus A (NGC 5128) system, a previously known corotating group, and conclude that the former could be a small version of the latter but is located in an isolated field, away from the influence of the large-scale structure. The results open up opportunities to explore the galactic anisotropy phenomenon on a low-mass scale and in a rarified environment.

Study on multiphase flow field in Falcon separator by CFD simulation

ABSTRACT Flow field characteristics of Falcon separator are studied by CFD simulation based on Euler-Euler model as multiphase flow model and Reynolds stress model as turbulence model, including particle movement state in delamination area and separation area, three phase (Tangential, radial and axial directions) velocity distribution and turbulence field distribution. Simulation results show that particle movement trajectory is a three-dimensional helical structure with circular motion in groove, which can be regarded as “a circular harmonic motion of a roller coaster”; velocity distribution in the delamination area is almost same as other centrifugal separators but it is quite different in the groove; with feeding of recoil water, the turbulence intensity in the separation area is increased obviously, but it has a lower influence on the disturbance at upper layer of convective membrane, which enhanced the separation effect.

Investigation of a GL-EMMS gradual drag model by comparative simulations of bubble columns

The mesoscale model for gas–liquid flow based on the energy minimization multi-scale method, which is denoted by the GL-EMMS model, demonstrates its capability of predicting the regime transition in bubble columns. Since the dominant mechanisms and stability conditions are important for this mesoscale model, it is desirable to reveal more details through the computational fluid dynamics (CFD) work. This work proposes a gradual drag correlation based on the GL-EMMS model, and then incorporates it into the Euler–Euler (E–E) and Euler–Lagrange (E–L) frameworks respectively. Cases with different gas distributors are simulated via the opensource platform of OpenFOAM. A simplified meshing treatment of the inlet boundary is adopted and has proven effective to reducing the computing load. The E–E simulation predicts the total gas holdup and the radial distribution of liquid velocity well, but obvious deviation is observed in the radial distribution of gas holdup, especially at lower superficial gas velocities for the single-orifice aeration. Nonetheless, this shortage can be overcome saliently by the E–L method. These verified simulations enable further possible iterative investigation between the GL-EMMS model and the CFD work, and also inspire some meaningful simulating strategy for bubble columns.

Deposition Process and Equivalent Markov Motion of High-Inertia Particles in a Long Straight Pipeline

Abstract Two methods are used to study the process of particle deposition in a turbulent pipe flow. The Monte Carlo method tracks 10,000 particles in the turbulent pipe flow to reproduce the deposition process of the particles. The deposition velocity of the particles is determined by calculating the proportion of particles passing through the test section. The simplified deposition model uses an equivalent Markov motion instead of the radial movement of the particle in the turbulent core. The probability that a particle leaves the turbulent core depends on the radial particle position and the probability density distribution of the random vortex. The probability that a particle penetrates the boundary layer can be solved by integrating the probability density distribution of radial particle velocity. The deposition velocity of particles can be obtained by calculating the probability of an individual particle leaving the turbulent core and penetrating the boundary layer. Five experimental data series from the literature are applied to examine the predictive abilities of the two methods. The results demonstrate that the Monte Carlo method can be properly used to track the particle deposition process in the diffusion–impaction and inertia-moderated regimes. The simplified model is suitable for high-inertia particles.

Flow Rate along the Length of the Swirling Vortex Axis at an Intake

In this study, the characteristics of the flow in the region of swirling vortex are examined. The potential flow model based on the summing infinite number of spherical sinks along the vortex core is introduced to predict the flow field and the flow rate along the vortex axis. The flow towards the swirling vortex core has considerable effects on the radial velocity distribution within the ambient fluid flow region near the intake. The agreement between available test data relating to the radial velocity and the method introduced in this study is found to be satisfactory.

Analysis of Unsteady Flow Characteristics of Centrifugal Pump under Part Load Based on DDES Turbulence Model

To better reveal the mechanism of the rotor-stator interference between the impeller and the guide vane and the evolution process of the stall vortex under the part-load conditions, numerical simulation is carried out based on the DDES turbulence model, which can better capture vortex structure. And the pressure pulsation and the radial velocity distribution of the centrifugal pump are studied. The vortex structure and pressure fluctuation of pump internal flow field under part-load condition of Q = 0.4 Qdes are mainly analyzed. The analysis results show that the stall vortex is formed at the inlet of the impeller and evolves to the outlet of the impeller, the front cover to the rear cover according to the fluid flow direction, and then disappears. Besides, under the part-load condition, the vorticity of the impeller outlet is always obviously less than that of the impeller inlet as the flow rate increases. Due to the asymmetric action of the volute, the radial velocity distribution law of flow channel C1 is different from other flow channels at different blade heights. By analyzing the radial velocity, the phenomenon that the jet-wake flow impacts the guide vane with the rotation of the impeller is the main reason for the rotor-stator interference. And large radial velocity gradients appear at the front and rear cover plates, which will cause high energy loss and reduce pump efficiency. Besides, the conclusion can be drawn that the region with the strongest rotor-stator interference is the inlet region of the guide vane suction surface. It also occurs near the volute tongue but is lower due to the effect of the guide vane. This research may serve as a reference for the safe operation of centrifugal pumps under part-load conditions.

Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy

Abstract We present the serendipitous discovery of an extremely broad (ΔV LSR ∼ 150 km s−1), faint (T mb < 10 mK), and ubiquitous 1667 and 1665 MHz ground-state thermal OH emission toward the second quadrant of the outer Galaxy (R gal > 8 kpc) with the Green Bank Telescope. Originally discovered in 2015, we describe the redundant experimental, observational, and data quality tests of this result over the last five years. The longitude–velocity distribution of the emission unambiguously suggests large-scale Galactic structure. We observe a smooth distribution of OH in radial velocity that is morphologically similar to the H i radial velocity distribution in the outer Galaxy, showing that molecular gas is significantly more extended in the outer Galaxy than previously expected. Our results imply the existence of a thick (−200 pc < z < 200 pc) disk of diffuse ( n H 2 ∼ 5 × 10−3 cm−3) molecular gas in the outer Galaxy previously undetected in all-sky 12CO surveys.

Radial velocity map of solar wind transients in the field of view of STEREO/HI1 on 3 and 4 April 2010

Context. The solar wind transients propagating out in the inner heliosphere can be observed in white-light images from Heliospheric Imager-1 (HI1), an instrument of the Sun Earth Connection Coronal and Heliospheric Investigation on board the Solar Terrestrial Relations Observatory (STEREO), from two perspectives. The spatial velocity distribution inside solar wind transients is key to understanding their dynamic evolution processes. Aims. We generated a velocity map of transients in 3D space based on 2D white-light images and used it to estimate the expansion rate as well as some kinematic properties of solar wind transients. Methods. Based on the recently developed correlation-aided reconstruction method in our previous work, which can recognize and locate 3D solar wind transients from STEREO/HI1 image data, we further developped a new technique for deriving the spatial distribution of the radial velocities of the most pronounced features inside solar wind transients. Results. The technique was applied to events including a coronal mass ejection (CME) and three small-scale transients, so-called blobs, observed by HI1 on 3–4 April 2010 to reconstruct their radial velocity maps. The results match the forward-modeling results, simulations, and in situ observations at 1 AU fairly well. According to the obtained spatial distributions of height and radial velocity of the CME, we analyzed the self-similarity of the radial expansion of the CME ejecta. The dimensionless radial expansion rate of the northern and middle parts of the CME ejecta varies in the range of 0.7−1.0 at heliocentric distance between 25 R⊙ and 55 R⊙ and the rate of the southern part in the range of 0.3−0.5, suggesting that the CME structure was distorted and shaped by the ambient solar wind. The technique we developed is expected to be applied to more events.