Velocity Clouds Research Articles

Dusty Cloud Acceleration by Radiation Pressure in Rapidly Star-forming Galaxies

Abstract We perform two-dimensional and three-dimensional radiation hydrodynamic simulations to study cold clouds accelerated by radiation pressure on dust in the environment of rapidly star-forming galaxies dominated by infrared flux. We utilize the reduced speed of light approximation to solve the frequency-averaged, time-dependent radiative transfer equation. We find that radiation pressure is capable of accelerating the clouds to hundreds of kilometers per second while remaining dense and cold, consistent with observations. We compare these results to simulations where acceleration is provided by entrainment in a hot wind, where the momentum injection of the hot flow is comparable to the momentum in the radiation field. We find that the survival time of the cloud accelerated by the radiation field is significantly longer than that of a cloud entrained in a hot outflow. We show that the dynamics of the irradiated cloud depends on the initial optical depth, temperature of the cloud, and intensity of the flux. Additionally, gas pressure from the background may limit cloud acceleration if the density ratio between the cloud and background is . In general, a 10 pc-scale optically thin cloud forms a pancake structure elongated perpendicular to the direction of motion, while optically thick clouds form a filamentary structure elongated parallel to the direction of motion. The details of accelerated cloud morphology and geometry can also be affected by other factors, such as the cloud lengthscale, reduced speed of light approximation, spatial resolution, initial cloud structure, and dimensionality of the run, but these have relatively little affect on the cloud velocity or survival time.

The Effects of Ram Pressure on the Cold Clouds in the Centers of Galaxy Clusters

Abstract We discuss the effect of ram pressure on the cold clouds in the centers of cool-core galaxy clusters, and in particular, how it reduces cloud velocity and sometimes causes an offset between the cold gas and young stars. The velocities of the molecular gas in both observations and our simulations fall in the range of 100–400 km s−1, which is much lower than expected if they fall from a few tens of kiloparsecs ballistically. If the intracluster medium (ICM) is at rest, the ram pressure of the ICM only slightly reduces the velocity of the clouds. When we assume that the clouds are actually “fluffier” because they are co-moving with a warm-hot layer, the velocity becomes smaller. If we also consider the active galactic nucleus wind in the cluster center by adding a wind profile measured from the simulation, the clouds are further slowed down at small radii, and the resulting velocities are in general agreement with the observations and simulations. Because ram pressure only affects gas but not stars, it can cause a separation between a filament and young stars that formed in the filament as they move through the ICM together. This separation has been observed in Perseus and also exists in our simulations. We show that the star-filament offset, combined with line-of-sight velocity measurements, can help determine the true motion of the cold gas, and thus distinguish between inflows and outflows.

All-sky photogrammetry techniques to georeference a cloud field

Abstract. In this study, we present a novel method of identifying and geolocalizing cloud field elements from a portable all-sky camera stereo network based on the ground and oriented towards zenith. The methodology is mainly based on stereophotogrammetry which is a 3-D reconstruction technique based on triangulation from corresponding stereo pixels in rectified images. In cases where clouds are horizontally separated, identifying individual positions is performed with segmentation techniques based on hue filtering and contour detection algorithms. Macroscopic cloud field characteristics such as cloud layer base heights and velocity fields are also deduced. In addition, the methodology is fitted to the context of measurement campaigns which impose simplicity of implementation, auto-calibration, and portability. Camera internal geometry models are achieved a priori in the laboratory and validated to ensure a certain accuracy in the peripheral parts of the all-sky image. Then, stereophotogrammetry with dense 3-D reconstruction is applied with cameras spaced 150 m apart for two validation cases. The first validation case is carried out with cumulus clouds having a cloud base height at 1500 m a.g.l. The second validation case is carried out with two cloud layers: a cumulus fractus layer with a base height at 1000 m a.g.l. and an altocumulus stratiformis layer with a base height of 2300 m a.g.l. Velocity fields at cloud base are computed by tracking image rectangular patterns through successive shots. The height uncertainty is estimated by comparison with a Vaisala CL31 ceilometer located on the site. The uncertainty on the horizontal coordinates and on the velocity field are theoretically quantified by using the experimental uncertainties of the cloud base height and camera orientation. In the first cumulus case, segmentation of the image is performed to identify individuals clouds in the cloud field and determine the horizontal positions of the cloud centers.

Relativistic jet feedback – II. Relationship to gigahertz peak spectrum and compact steep spectrum radio galaxies

We propose that Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources are the signposts of relativistic jet feedback in evolving galaxies. Our simulations of relativistic jets interacting with a warm, inhomogeneous medium, utilize cloud densities and velocity dispersions in the range derived from optical observations, show that free-free absorption can account for the $\sim \rm GHz$ peak frequencies and low frequency power laws inferred from the radio observations. These new computational models replace the power-law model for the free-free optical depth in the \citep{bicknell97a} model by a more fundamental model involving disrupted log-normal distributions of warm gas. One feature of our new models is that at early stages, the low frequency spectrum is steep but progressively flattens as a result of a broader distribution of optical depths, suggesting that the steep low frequency spectra discovered by \citet{callingham17a} may possibly be attributed to young sources. We also investigate the inverse correlation between peak frequency and size and find that the initial location on this correlation is determined by the average density of the warm ISM. The simulated sources track this correlation initially but eventually fall below it, indicating the need for a more extended ISM than presently modelled. GPS and CSS sources can potentially provide new insights into the phenomenon of AGN feedback since their peak frequencies and spectra are indicative of the density, turbulent structure and distribution of gas in the host galaxy.

Numerical simulation analysis of valve spool double-nozzle with abrasive flow

In order to study the numerical simulation of abrasive flow polishing small hole parts, the servo valve spool double nozzle parts were used as the research object, and the solid-liquid two-phase abrasive flow were used for numerical simulation, getting the dynamic pressure, velocity cloud and turbulent kinetic energy of the abrasive flow are obtained at different inlet velocities. Through the comparative analysis, the effect of the process parameters on the effect of the abrasive flow polishing small hole parts was obtained, which can provide a theoretical basis for the continuous improvement of the ultra-precision machining technology to promote the abrasive flow, so that the fatigue strength of the workpiece is improved, the reliability of the workpiece is enhanced and its service life is prolonged.

Intra-hour direct normal irradiance forecasting through adaptive clear-sky modelling and cloud tracking

Hybrid fossil-solar power cycles possess high solar-to-electric conversion efficiency and the potential to provide stable power output without the need for costly storage systems. However, the power output of the concentrating solar power (CSP) component of the plant can fluctuate in sync with intermittent direct normal irradiance (DNI). For hybrid plants, the controllable fossil-fuel unit can be used to compensate for shortfalls in CSP output during periods of light DNI intermittency or provide the entire plant output during periods of high DNI intermittency. This fossil-fuel unit requires 5–10 min to ramp its power output up or down to perform these balancing functions. Thus, by accurately predicting future DNI, and hence CSP output, the fossil-fuel unit can guarantee stable plant-wide power output during periods of intermittency.This paper develops an intra-hour DNI prediction system using a ground-based cloud motion vector (CMV) framework and real-time DNI measurements. This system comprises a clear-sky DNI model and a cloud fraction prediction algorithm. The presented CSM is based on the Ineichen model (Ineichen, 2008), where the model parameters are adaptively estimated from identified clear-sky DNI measurements over a moving window. For the cloud fraction prediction model, this paper presents an enhanced “sector-ladder” method (Quesada-Ruiz et al., 2014) that uses the weighted mean of circular quantities (Fisher, 1995) and autoregressive filtering to improve cloud flow predictions. Furthermore, a method to forewarn against periods of high DNI intermittency using the generated DNI predictions is presented.The proposed DNI prediction system is evaluated using 37 days of sky-camera images and DNI data collected over the summer of 2014/2015 at the University of Queensland. Over all test days, the adaptive CSM has an average root mean square error of 3.06%, which represents a 19% improvement over a CSM that uses the optimal model parameters from the previous day’s data. Additionally, the modifications to the cloud flow prediction algorithm (the sector-ladder method) are shown to improve the cloud velocity prediction accuracy by a factor of seven over a period of visually determined constant cloud velocity. We find the overall prediction accuracy of the DNI prediction system to be statistically similar to the accepted short-term benchmark of persistence; however, it performs more consistently over a range of weather conditions and is able to forewarn against periods of impending intermittency with 93% accuracy. The latency from data collection to prediction is less than 30 s, making the method eminently suitable for real-time applications.

Laminar burning velocity and structure of coal dust flames using a unity Lewis number CFD model

Despite decades of research, predictive methods remain unavailable to estimate flame propagation in dust clouds under industrial scenarios. The complexity of scaling the fundamental processes occurring in multiphase flames to industrial geometries, and a lack of tools to explore and extend knowledge in this area, may be key factors missing in the research literature. The main objective of this work is to verify the ability of a CFD model based on a unity Lewis number assumption to explore laminar burning velocity in coal dust clouds. A second objective is to perform parametric analysis including the role of surface reactions, particle diameter, and initial system temperature. The third and final objective is to explore the impact of discrete particle combustion on flame structure and burning velocity. Despite a simplified treatment of gas phase transport properties, single-step devolatilization, and single-step surface reaction, the current model correctly captures the effects of particle diameter and initial temperature on burning velocity and demonstrates good agreement with previous investigations once preheating in the experimental results is accounted for. Furthermore, the reduced model complexity may allow future investigation by the current authors and other research groups into different combustible dusts, more detailed system geometry, and turbulent flow conditions. Lastly, the results of the current study provide a baseline that more comprehensive modeling methods may be compared to, which is currently missing in the literature.

Robust cloud motion estimation by spatio-temporal correlation analysis of irradiance data

The main contributor to spatio-temporal variability in the solar resource is clouds passing over photovoltaic (PV) modules. Cloud velocity is a principal input to many short-term forecast and variability models. In this paper spatio-temporal correlations of irradiance data are analyzed to estimate cloud motion. The analysis is performed using two spatially and temporally resolved simulated irradiance datasets generated from large eddy simulation. Cloud motion is estimated using two different methods; the cross-correlation method (CCM) applied to two or a few consecutive time steps and cross-spectral analysis (CSA) where the cloud speed and direction are estimated by cross-spectral analysis of a longer time series. CSA is modified to estimate the cloud motion direction as the case with least variation for all the velocities in the cloud motion direction. To ensure reliable cloud motion estimation, quality control (QC) is added to the CSA and CCM analyses. The results show 33% (52°) and 21% (6°) improvement in the cloud motion speed (direction) estimation using the modified CSA and CCM over the original methods (without QC), respectively. In general, CCM results are accurate for all the different cloud cover fractions with average relative mean bias error (rMBE) of cloud speed and mean absolute error of cloud direction equal to 3% and 3°, respectively. For low cloud cover fractions, CSA estimates the cloud motion speed and direction with rMBE and mean absolute error equal to 10% and 11°, respectively. However, for high cloud cover fractions and unsteady cloud speed, CSA results are not reliable for 3–4 h time series; however, splitting the whole time series into shorter time intervals reduces the rMBE and mean absolute error to 15% and 16° respectively.

On mechanisms separating stars into normal and chemically peculiar

The paper argues in favor of the assumption that magnetic and non-magnetic protostars, from which CP stars were formed, are the objects that had rotation velocities of the parent cloud V smaller than a critical value V c . At V greater than the critical value, differential rotation emerges in the collapsing protostellar cloud, which twists magnetic lines of force into an’ invisible’ toroidal shape and disturbs the stability of the atmosphere. In magnetic protostars, the loss of angular momentum is due to magnetic braking, while in metallic protostars, the loss of rotation momentum occurs due to tidal interactions with a close component. HgMn stars are most likely not affected by some braking mechanism, but originated from the slowest protostellar rotators. The boundary of V c where the differential rotation occurs is not sharp. The slower the protostar rotates, the greater the probability of suppressing the differential rotation and the more likely the possibility of CP star birth.

Investigation of diocotron modes in toroidally trapped electron plasmas using non-destructive method

Experiments with trapped electron plasmas in a SMall Aspect Ratio Toroidal device (SMARTEX-C) have demonstrated a flute-like mode represented by oscillations on capacitive (wall) probes. Although analogous to diocotron mode observed in linear electron traps, the mode evolution in toroids can have interesting consequences due to the presence of in-homogeneous magnetic field. In SMARTEX-C, the probe signals are observed to undergo transition from small, near-sinusoidal oscillations to large amplitude, non-linear “double-peaked” oscillations. To interpret the wall probe signal and bring forth the dynamics, an expression for the induced current on the probe for an oscillating charge is derived, utilizing Green's Reciprocation Theorem. Equilibrium position, poloidal velocity of the charge cloud, and charge content of the cloud, required to compute the induced current, are estimated from the experiments. Signal through capacitive probes is thereby computed numerically for possible charge cloud trajectories. In order to correlate with experiments, starting with an intuitive guess of the trajectory, the model is evolved and tweaked to arrive at a signal consistent with experimentally observed probe signals. A possible vortex like dynamics is predicted, hitherto unexplored in toroidal geometries, for a limited set of experimental observations from SMARTEX-C. Though heuristic, a useful interpretation of capacitive probe data in terms of charge cloud dynamics is obtained.

Flame propagation behaviours in nano-metal dust explosions

Flame propagation behaviours in 40nm titanium, aluminium and iron dust clouds were investigated by high-speed photography. 40nm titanium, aluminium and iron dust flames were all characterized by discrete single glowing burning particles with smoothly spherical front. “Micro explosions” caused by the fragment ejection occurred in both 40nm titanium and aluminium particles burning processes. At nano scales, the heat and mass transfer regimes were absolutely different with micro particles, which resulted in different propagation velocities. The average pulsating flame propagation velocities of 40nm titanium, aluminium and iron dust clouds were 0.565m/s, 0.189m/s and 0.035m/s, respectively. From SEM analysis, it was inferred that 40nm titanium, aluminium and iron particles were burnt in liquid, gas, and solid-phase, respectively. From XPS analysis, it was found that N2 played a significant role in titanium particles combustion. 40nm titanium combustion products contained 43% TiO2 (Ti4+), 27% Ti2O3 (Ti3+), 21% TiO (Ti2+) and 9% TiN (Ti3+). While 40nm aluminium combustion products just contained 100% Al2O3 (Al3+), and 40nm iron combustion products contained 49% Fe2O3 (Fe3+), 26% Fe3O4 (Fe3+/Fe2+), 15% FeO (Fe2+) and 10% iron nonoxides. Inerted by nitrogen for preventing nano‑titanium dust explosions was inadvisable.

Instant velocity and consistency of emitted cloud change by the different levels of canister filling with Metered Dose Inhalers (MDIs), but not with Soft Mist Inhalers (SMIs): a bench study

Background: Inhalation is the preferred route for respiratory drug delivery, but several factors contribute to the variability of the respirable dose fraction. Instant velocity and the dynamic characteristics of the droplet cloud represent crucial factors. Aim was to measure and compare the instant velocity and the consistency of emitted cloud from five different MDIs (A - Salbutamol sulphate 100mcg, GSK; B - Salbutamol sulphate 100mcg, Valeas; C - Salmeterol xinafoate/Fluticasone propionate 25/125mcg, GSK; D - Formoterol fumarate/Bechlomethasone propionate 6/100mcg, Chiesi; E - Formoterol fumarate/Fluticasone dipropionate 5/125mcg, Mundipharma) and one SMI (Tiotropium bromide 5mcg, Boehringer Ingelheim), at different distance from the nozzle and canister filling. Methods: Measurements were made at 90, 50, and 10% of canister filling, and at 5, 10, and 20 cm from the nozzle, for a total of 972 puffs. A high speed video photography protocol was adopted and high speed cameras (1.200 frames/ sec.) were used. Data were acquired by means of specialized softwares. Temperature, humidity, and vibrations occurrence were strictly controlled during measurements. Statistics: Anova and p < 0.05 were accepted as the minimum significance level. Results: MDIs generated different Instant velocities: MDI B generated the highest, while MDI A the lowest. As expected, velocity decreased in proportion to the distance from the nozzle. Except with MDI C, instant velocity decreased significantly over the first 50% of canister emptying, but dropped by >33% at 90% of emptying with all other MDIs (p < 0–037; p < 0.001; p < 0.005, and p < 0.001, respectively). Instant velocity was extremely lower (p < 0.001) and constant for all levels of canister filling (p = ns) with SMI. All MDIs had a very fast jet phase, ranging 0.01–0.03 s at 10 cm, and 0.03–0.05 s at 20 cm from the nozzle, without any significant difference from each other (p= ns). MDIs generated a cloud similarly tight (p = ns) at 10 and 20 cm from the nozzle, while it was extremely wider and constant with the SMI (p = 0–001). Also the cloud turbulence was minimized during the SMI emission. Discussion and Conclusions: MDIs are characterized by a substantial variability in both their instant velocity and consistency of the emitted cloud at different levels of canister filling. SMI generates a much slower soft mist cloud which is constantly homogeneous and independent of canister emptying. These peculiarities assessed at bench are suggesting a higher dose consistency and a much more effective therapeutic performance also in real life.

ALMA Images of the Orion Hot Core at 349 GHz

Abstract We present ALMA images of the dust and molecular line emission in the Orion Hot Core at 349 GHz. At 0.″2 angular resolution the images reveal multiple clumps in an arc ∼1″ east of Orion Source I, the protostar at the center of the Kleinmann–Low Nebula, and another chain of peaks from IRc7 toward the southwest. The molecular line images show narrow filamentary structures at velocities >10 km s−1 away from the heavily resolved ambient cloud velocity ∼5 km s−1. Many of these filaments trace the SiO outflow from Source I, and lie along the edges of the dust emission. Molecular line emission at excitation temperatures 300–2000 K, and velocities >10 km s−1 from the ambient cloud, suggest that the Hot Core may be heated in shocks by the outflow from Source I or from the Becklin–Neugebauer (BN)/SrcI explosion. The spectral line observations also reveal a remarkable molecular ring, ∼2″ south of SrcI, with a diameter ∼600 au. The ring is seen in high-excitation transitions of HC3N, HCN v2 = 1, and SO2. An impact of ejecta from the BN/SrcI explosion with a dense dust clump could result in the observed ring of shocked material.

The determination of virtual cloud density with the application of inverse transformation method

In the presented paper data on solar power stations power in the world and Ukraine are given. Also data on the installed peak power of solar power stations in Ukraine in 2016 are given. It is shown that the efficiency of such power stations is provided by maximum energy selection from solar panels at each specific moment in time and efficient operation of the storage on some basic interval. The necessity of implementation of two-channel control in distributed generation systems due to Heisenberg uncertainty principle is shown. It should control the basic interval to ensure the necessary level of energy to charge the storage duration and the minimal duration of observation interval to ensure the maximum level of energy, taken from solar panels. The maximum value of the energy generated by solar panels depends on the existence of clouds and their density, because when the cloud passes over solar panels, part of them is shaded and the level of output energy decreases. Knowing the average level of energy that can be obtained from solar power station and energy levels that excess and is less than average, allows implementing the predictive control on the next basic interval. However, such control is inaccurate. One of the approaches to improve the accuracy is a prediction on a certain small interval with subsequent correction and step-by-step displacement until the end of the basic interval. Considering the solar panels output current as a final result of solar radiation passage through the external environment it is advisable to assess the impact of this environment on the magnitude of generated energy. One of the steps of this identification is to determine the clouds density, on the magnitude of which the amount of energy that falls on the solar panels surface depends. The Beer law is presented. It describes the reduction of the total irradiation intensity, calculated per unit of surface area perpendicular to the direction of irradiation propagation. The method for determination of the virtual cloud density, which takes into account both the values of direct and reflected solar radiation, with reverse transformation is given. It enhances the value of the average level of energy, generated by solar power station on some basic interval. It is shown that in order to find the cloud density it is necessary to have a set of projections for all possible positions of the cloud. It is proposed to approximate the form of the cloud projection with a circle of defined radius to simplify the calculations. Three cases of the ratio between the linear velocity of the clouds and the velocity of the sun, which is defined by its angular displacement, are considered: first is when the linear velocity of the clouds is much greater than the velocity of the sun, which is defined by its angular displacement, then at some observation interval the position of the sun is fixed; second is when the velocity of the clouds is much less than the velocity of the sun; and third is when the velocity of clouds and the velocity of the sun are the values of the same order. This case combines the features of the previous two cases. For each case the scheme for virtual cloud density calculation, formulas for calculation of solar irradiation intensity, cloud projections on the solar panels and the linear absorption coefficient, which values are correlated with the density of the cloud are given.Ref. 13, fig. 4.

Development of an advection model for solar forecasting based on ground data first report: Development and verification of a fundamental model

A new spatiotemporal forecasting model of photovoltaic output power is proposed. Irradiance measurements are first interpolated onto a grid. By repeating this process over several time steps, cloud motion is visualized and the velocity of the clouds is estimated. Using these estimations of cloud velocity, the cloud motion in a future time interval is simulated using advection diffusion equations and the average irradiation over a target region is estimated. The proposed model is applied to two distinctly sized areas, a utility-scale photovoltaic plant of 1.32km2 and a 6400-km2 region in Kyushu, Japan. The predictions of the transients in the average irradiation for the utility-scale photovoltaic system and regional irradiance are in good agreement with measurements; root-mean-square errors of data on the time scale of a minute are 1.14% and 1.36%, respectively. The simulation of the distribution of irradiation within the photovoltaic system also agrees well with measurements.

Instant velocity and consistency of emitted cloud change by the different levels of canister filling with Metered Dose Inhalers (MDIs), but not with Soft Mist Inhalers (SMIs): a\xa0bench study

BackgroundInhalation is the preferred route for respiratory drug delivery, but several factors contribute to the variability of the respirable dose fraction. Instant velocity and the dynamic characteristics of the droplet cloud represent crucial factors. Aim was to measure and compare the instant velocity and the consistency of emitted cloud from five different MDIs (A - Salbutamol sulphate 100mcg, GSK; B - Salbutamol sulphate 100mcg, Valeas; C - Salmeterol xinafoate/Fluticasone propionate 25/125mcg, GSK; D - Formoterol fumarate/Bechlomethasone propionate 6/100mcg, Chiesi; E - Formoterol fumarate/Fluticasone dipropionate 5/125mcg, Mundipharma) and one SMI (Tiotropium bromide 5mcg, Boehringer Ingelheim), at different distance from the nozzle and canister filling.MethodsMeasurements were made at 90, 50, and 10% of canister filling, and at 5, 10, and 20 cm from the nozzle, for a total of 972 puffs. A high speed video photography protocol was adopted and high speed cameras (1.200 frames/sec.) were used. Data were acquired by means of specialized softwares. Temperature, humidity, and vibrations occurrence were strictly controlled during measurements. Statistics: Anova and p < 0.05 were accepted as the minimum significance level.ResultsMDIs generated different Instant velocities: MDI B generated the highest, while MDI A the lowest. As expected, velocity decreased in proportion to the distance from the nozzle. Except with MDI C, instant velocity decreased significantly over the first 50% of canister emptying, but dropped by >33% at 90% of emptying with all other MDIs (p < 0–037; p < 0.001; p < 0.005, and p < 0.001, respectively). Instant velocity was extremely lower (p < 0.001) and constant for all levels of canister filling (p = ns) with SMI. All MDIs had a very fast jet phase, ranging 0.01–0.03 s at 10 cm, and 0.03–0.05 s at 20 cm from the nozzle, without any significant difference from each other (p = ns). MDIs generated a cloud similarly tight (p = ns) at 10 and 20 cm from the nozzle, while it was extremely wider and constant with the SMI (p = 0–001). Also the cloud turbulence was minimized during the SMI emission.Discussion and ConclusionsMDIs are characterized by a substantial variability in both their instant velocity and consistency of the emitted cloud at different levels of canister filling. SMI generates a much slower soft mist cloud which is constantly homogeneous and independent of canister emptying. These peculiarities assessed at bench are suggesting a higher dose consistency and a much more effective therapeutic performance also in real life.

Registration of Multiple Spalls and Structures of Ejected Particle Clouds

Experimental results on the study of spallation and cavitation from solid (Cu and Pb) and liquefied Pb are presented. Pb ejecta transporting in air is also studied with a multi-probe laser interferometry method that is applied to detect and quantify the particle cloud velocity and surface fragmentation. The laser diagnostic is designed to probe ejecta velocity- and dispersion-profiles within ejecta fragment clouds, and spallation structures.

Entrainment in trouble: cool cloud acceleration and destruction in hot supernova-driven galactic winds

Efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds. For fiducial assumptions about the timescale for cloud shredding from high-resolution simulations (which neglect magnetic fields) we show that cool clouds with temperature from $T_{c}\sim 10^{2}-10^{4}$ K seen in emission and absorption in galactic winds cannot be accelerated to observed velocities by the ram pressure of a hot wind. Taking into account both the radial structure of the hot flow and gravity, we show that this conclusion holds over a wide range of galaxy, cloud, and hot wind properties. This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow. Given these difficulties with ram pressure acceleration, we discuss alternative models for the origin of high velocity cool gas outflows. Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud's life. For $T_{c}=10^{3}$\,K and $10^{4}$\,K clouds, we show that if conductive evaporation can be neglected, the cloud shredding timescale must be $\sim15$ and 5 times longer, respectively, than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations.

Gas emission and dynamics in the infrared dark cloud G31.23+0.05

We performed a multiwavelength study towards the infrared dark cloud (IRDC) G31.23+0.05 with new CO observations from Purple Mountain Observatory and archival data (the GLIMPSE, MIPSGAL, HERSCHEL, ATLASGAL, BGPS and NVSS surveys). From these observations, we identified three IRDCs with systemic velocities of (cloud A), (cloud B) and km s (cloud C) in the line of sight towards IRDC G31.23. Analyses of the molecular and dust emission suggest that cloud A is a filamentary structure containing a young stellar object; clouds B and C both include a starless core. Clouds A and B are gravitationally bound and have a chance to form stars. In addition, the velocity information and the position-velocity diagram suggest that clouds A and B are adjacent in space and provide a clue hinting at a possible cloud-cloud collision. Additionally, the distribution of dust temperature shows a temperature bubble. The compact core in cloud A is associated with an UCHII region, an IRAS source, H2O masers, CH3OH masers and OH masers, suggesting that massive star formation is active there. We estimate the age of the HII region to be (0.03–0.09) Myr, indicating that the star inside is young.

Micro-structure response and fracture mechanisms of C/SiC composites subjected to low-velocity ballistic penetration

Dynamic response and fracture mechanisms of Carbon Fiber Reinforced Silicon Carbide Composites (C/SiC) especially during low-velocity ballistic penetration are studied both experimentally and numerically. The gas-gun facility is used to fire spherical metallic projectile for striking velocity of 150ms−1 on the target panels, and the impact phenomenon is captured through high-speed photography. A micro-structure based approach is employed to model C/SiC target in this paper. This proposed numerical technique captured the mechanical response (residual energy, expansion process and velocity of debris cloud, fracture morphology and mode) of target, with adequate accuracy. The fracture modes involve void collapse, delamination, fiber bundle splitting and breakage. The debris cloud possesses two types of constituents, classified by fragments' volume and high-energy powdering column at the front. The experimental and calculated results emphasize that the impact velocity, projectile shape and hardness have significant influence on the mechanical behavior of C/SiC composites, including fragment size, fracture surface morphology, fracture mode and mechanism.