Radial Spreading Research Articles

Flow field characteristics of a confined, underexpanded transient round jet

The flow field of an impulsively started round, confined nitrogen jet was investigated using combined high speed schlieren imaging and particle velocimetry (PIV) measurements. PIV measurements were carried out at five different, normalized times (55 ≤t*≤ 392) relative to jet intrusion into a constant volume chamber. Between 100 <t*< 250, the NPR linearly increased to that for a moderately underexpanded jet (NPR ≈ 3.5). Distributions of the mean flow and Reynolds normal and shear stresses revealed two different stages in jet development. In stage I (t* = 55–103), prior to clear shock cell appearance, the jet was characterized by a leading, toroidal vortex whose induced recirculatory motion inhibited the growth of the trailing jet's shear layer instabilities and radial spreading. In stage II (t* = 196 and 392), the jet became moderately underexpanded (NPR ≥ 2) and close to the nozzle exit, flow characteristics resembled those of a “co-annular” jet. The co-annular region did not extend beyond 15 D. An analysis of instantaneous vortex numbers and strengths further supported the two identified stages in jet development and their connection to shear layer instability growth. Based on the distributions of mean flow and Reynolds stresses, it was shown that the static pressure gradient along the jet's centerline is mainly governed by the dynamic pressure gradient. Gradients of the Reynolds normal and shear stresses play a minor role. Important for gaseous fuel injection at high injection pressures, results point at limited mixing during stage I and enhanced mixing during stage II.

Effect of liquid–air interface on particle cloud dynamics in viscous liquids

Three-dimensional numerical investigations have been performed to study the effect of liquid–air interface on particle cloud evolution in the stagnant pool of viscous liquid. Computations have been carried out using open-source computational fluid dynamics package open-source field operation and manipulation under different operating conditions. A hybrid multifluid–volume-of-fluid-based solver is used to identify the influence of liquid–air interface on particle cloud dynamics. Mainly, two different operating conditions have been considered in the present study, viz., falling particle cloud within the liquid and falling from the liquid–air interface. The effect of particle Reynolds number (Re) on cloud evolution has also been considered in the present study. The effect of the liquid–air interface on particle cloud dynamics has been qualitatively explained with the help of particle volume fraction iso-surface, liquid velocity vectors, and iso-Q-surface, and quantitatively explained with the help of average particle cloud velocity, penetration depth, plume half-width, and particle cloud mass. Proper orthogonal decomposition-based analysis has been used to explain the vortex structures generated in the viscous fluid for different cases. Releasing particle cloud from liquid–air interface decreases radial spreading as compared to the case of particle release within the liquid. Particle cloud evolution pattern is found to depend on particle Reynolds number only for the case with the liquid–air interface. The liquid–air interface's downward movement enhances and upward movement suppresses the spreading of the particle cloud, which subsequently alters the particle dispersion mechanisms in the later stages.

Viscous fingering of miscible annular ring

Abstract

Insights into near nozzle spray evolution, ignition and air/flame entrainment in high pressure spray flames

Experimental investigations were performed to study the dynamic processes related to early development of high pressure sprays and their effects on ignition, local soot formation and its entrainment as well as spray to spray interactions in a six-hole common rail diesel injector. These spray-flame measurements were performed using ultra-high speed imaging technique in an optically accessible constant volume chamber maintained under reactive high pressure environment. The acquired back scattered light from fuel droplets and the broadband natural soot luminosity images and their analysis have revealed new insights on how the radial dispersions of fuel sprays during its early development at high injection pressures affect the ignition, flame stabilisation and soot formation in diesel spray flames.Analysis of the high speed data have shown that radial spreading also called as bulge in this paper occurs during the very early stages of fuel spray development closer to nozzle. The cloud of finely dispersed fuel droplets trapped within these bulges loses its moment and evaporates quickly relative to liquid core of the spray to form a locally ignitable mixture and this initiates ignition from these locations. Local flame kernels formed from these local ignition sites tend to develop into small pockets of sooting flame, which subsequently moves counter to the main spray towards the nozzle and gets entrained into the liquid core of spray. At times, these sooting pockets are also transported to into the next spray when the bulges are large. The sooting flames that are transported towards the nozzle and into the liquid spray core are quenched upon its interaction with the core of liquid spray. Quenched flames are rich in unburnt hydrocarbon and soot particles and they have the potential to significantly impact the pollutant formation through altering the equivalence ratio within the core of fuel spray. Wide variations in the sizes were observed on the formation of bulges, and it also varied between sprays from different orifices of a multi-hole injector. These anomalies are mainly related to the complexity of nozzle sac flow affecting the early spray development.

Experimental study on the rock erosion performance of a pulsed abrasive supercritical CO2 jet

An experimental study on the erosion behaviors of pulsed abrasive supercritical CO2 jets (PASJ) was performed using sandstone as the target specimen. The erosion depth, erosion area, and mass loss of the crater caused by PASJ were discussed in detail. The optimal configuration of the Helmholtz nozzle was first determined, and then influences of erosion-related working parameters including the jet pressure, abrasive particle size, ambient fluid and jet impingement angle on the rock erosion performance were analyzed with this nozzle. The results indicate that increasing jet pressure helps improve the erosion depth and area, which is mainly the result of increased jet kinetic energy. The erosion capability of PASJ is highly dependent on the size of abrasive particles. Coarser particles shows greater effectiveness in producing deeper erosion craters and the jets are more likely to retain rock-erosion ability as the standoff distance increases. At a given standoff distance, the crater eroded in air is deeper than those eroded in gaseous CO2 and in supercritical CO2, because the particles are slowed much more slowly in air under decreased drag. Moreover, the area of the crater eroded by PASJ in air is also largest due to the highest radial spreading rate. Finally, it was found that mass loss and erosion depth first increase and then decrease with the increase of jet impingement angle (θ) and the maximum values are reached at θ = 70°, whilst the erosion area increases monotonically with decreasing jet impingement angle.

Disentangling the role of surface topography and intrinsic wettability in the prey capture mechanism of Nepenthes pitcher plants

Nepenthes pitcher plants capture prey with leaves specialised as pitfall traps. Insects are trapped when they ‘aquaplane’ on the pitcher rim (peristome), a surface structured with macroscopic and microscopic radial ridges. What is the functional significance of this hierarchical surface topography? Here, we use insect pad friction measurements, photolithography, wetting experiments and physical modelling to demonstrate that the ridges enhance the trap's efficacy by satisfying two functional demands on prey capture: Macroscopic ridges restrict lateral but enhance radial spreading of water, thereby creating continuous slippery tracks which facilitate prey capture when little water is present. Microscopic ridges, in turn, ensure that the water film between insect pad and peristome remains stable, causing insects to aquaplane. In combination, the hierarchical ridge structure hence renders the peristome wettable, and water films continuous, so avoiding the need for a strongly hydrophilic surface chemistry, which would compromise resistance to desiccation and attract detrimental contamination.

Dynamic processes of the Dora Kamiyama rockslide in the Tibetan Plateau, China: geomorphic implication

The mechanisms of high-speed and long-runout landslides are reflected in the geomorphology of their deposits. Comprehensive field investigation using unmanned aerial vehicles photography was undertaken to observe special landforms of the Dora Kamiyama rockslide in the Tibetan Plateau. The dynamic processes of the rockslide were analysed based on the complex topography that remained. The age of sediments sampled from the landslide lake was tested through optically stimulated luminescence (OSL). In translation zone II, the landslide material near both edges underwent sinistral or dextral shearing motions, forming levees. In accumulation subzones III-1 and III-2, a rapid extensional sliding process contributed to toreva block formation. Transverse and longitudinal ridges developed due to compression-dominated processes in subzone III-2. Meanwhile, high-speed material was overturned in subzone III-3 due to the original topographical resistance, and carapace facies featuring abundant megablocks were clearly visible on the surface. Ridges confined by troughs were well-developed across the area experiencing rapid radial spreading motion in subzone III-4. Finally, dynamic parameters of the rockslide were calculated. The equivalent friction coefficient and the angle of reach were 0.40 and 21.80°, respectively. The maximum sliding velocity was estimated between 37.42 and 54.73 m/s. The OSL ages showed that the occurrence time of the rockslide can be estimated to be 4.24 ± 0.35 ka, defining it as an ancient landslide in the Holocene.

СОПРЯЖЕНИЕ ДВУХ РАВНОМЕРНЫХ ПОТОКОВ

The authors consider the problem of coupling two two-dimensional in terms of turbulent potential uniform flows. The flow movement is considered in a smooth horizontal channel with relatively short length of watercourses, when the flow resistance forces can be ignored. The main task is to determine the most rational form of the side walls of the flow interface.
 Radial spreading of a turbulent flow can be one of the main types of General flows for coupling various forms of flow. First, the flow is transferred through a simple expansion wave to a radial one, then through a simple compression wave it is transferred to a uniform flow. The solution of the problem with this formulation allowed us to improve and update the solution of problems on the coupling of flows.

Helicity effects on inviscid instability in Batchelor vortices

In this paper we investigate the instability properties of Batchelor vortices with a large swirl number and a fixed axial velocity deficit. In particular, it elucidates the effect of the helicity profile on the instability of the vortices as swirling wakes. In a linear stability analysis, a negative helicity profile destabilised a vortex with a large swirl number; the name given to this instability is ‘helicity instability’. Note that helicity instability is qualified for the case of axial flow with wake. In contrast, a conventional Batchelor vortex was stable at swirl numbers above a value of circulation, which is determined by the axial velocity deficit. The instability was related to a parameter proportional to the square of the inverse azimuthal vorticity thickness. Decreasing this helicity-profile parameter increased the growth property of the vortex. Such unstable features (helicity effects) were also studied in direct numerical simulations of vortices subjected to small random disturbances at Mach numbers 2.5 and 5.0. The instability based on the vorticity thickness originally grew at the outer edge of the vortex, whereas the instability waves in a conventional Batchelor vortex originate inside the vortex core. The simulation results support the results of the linear stability analysis on the helicity profile when the parameter is small. Because of the helicity instability, the nonlinear developments yielded a large fluctuation field with many small scales and high radial spreading rates. Even at the Mach number of 5.0, negative helicity exerted a much greater destabilisation effect than a zero entropy gradient. Therefore, the investigated novel effect established a reasonably powerful instability in compressible fluids, which is favourable for supersonic mixing.

Late Pliocene onset of the Cona rift, eastern Himalaya, confirms eastward propagation of extension in Himalayan-Tibetan orogen

Several competing hypotheses have been proposed to explain east-west extension observed across the Himalayan orogen, based primarily on observations from the western and central Himalaya. They make predictions for the temporal and spatial patterns of deformation in the eastern Himalaya. These tectonic models include radial spreading or oroclinal bending of the Himalayan arc, oblique convergence of the Indian continent, tearing or lateral detachment of the Indian slab and eastward flow of lithosphere. Here, for the first time we constrain the activity history of the Cona rift, the only north-trending rift currently recognized in the eastern Himalaya, based on biotite and K-feldspar 40Ar/39Ar and zircon and apatite (U–Th)/He thermochronology, to evaluate these proposed rifting mechanisms. Low-temperature thermochronological results suggest that the Cona rift is the youngest rift system in the Himalayas: after a slow phase of exhumation since ∼14 Ma (∼0.2–0.13 mm/yr), normal faulting initiated here at ∼3.0–2.3 Ma with a fault slip rate of ∼3.8–1.6 mm/yr and a horizontal extension magnitude of ∼2–5 km. Analysis of rifting across the Himalayas shows that rift initiation ages young eastward, which is matched by eastward decreasing rift-extension magnitudes. Monotonically eastward younging rift development is consistent with the tectonic model involving eastward lithospheric flow driven by Indian slab dynamics and coupled asynchronous gravitational potential energy gradients.

Comparison of experimental and model liquid distribution in large packed bed of Raflux rings 50-5

This work presents a continuation of our investigations on the radial liquid distribution in packed beds with open-structure random packings, by means of a dispersion model, in a scale, close to industrial one. Using experimental data for uniform initial irrigation, the optimal parameters’ values of the three parameters of the dispersion model are obtained by two-parameter identification. One of the parameters (the radial spreading coefficient) is calculated independently by using experimental data for a point source initial irrigation. The dispersion model solution at optimal parameters’ values is compared with both experimental and TUM WelChemCell model literature data for the liquid radial distribution in a packed column with a diameter of 1.2 m and random packing Raflux rings 50-5. The maldistribution factor of the liquid distribution is also calculated and compared. The comparison shows very good agreement between our results and the literature data and confirms the dispersion model capability to predict the liquid distribution in large columns with open-structure packings.

Dynamic behaviors of nanoscale binary water droplets simultaneously impacting on flat surface

Dynamic wetting of nanoscale binary water droplets simultaneously impacting on flat copper surface is explored using molecular dynamics simulations (MDS). The spreading performance is described through the images of shape evolution and the temporal variations of radial spreading factor and dynamic contact angle. The corresponding spreading characteristic is separately assessed by the following five key aspects: impact velocity, impact direction, the potential depth between oxygen and surface atoms, the center distance of binary nanodroplets and nanodroplet diameter, respectively. Here, the simulated results show that the binary nanodroplets with a low impact velocity exhibit the advancing, receding and coalescing motion states and the fluid with the high impact speed spreading on the solid surface may develop into jet flow along the radial direction. For impact direction, the radial momentum will cause larger axial wetting length with the increase of impact angle. In addition, a wide motion regions of binary water droplets impingement from partial wetting to non-wetting are observed, which helps us better understand the feature of nanodroplet equilibrium. Besides, the influence of coupling strength on dynamics behavior is also considered. We find that the center distance of binary droplets is relatively small, which will be conducive to the jetting occurrence. The enhancement of momentum and mass transport of the colliding two equal-sized nanodroplets also boosts the jetting as the droplet diameter increases.

Complex eikonal methods applied to geodesic acoustic mode dynamics

Techniques developed in the domain of optical theory are applied to investigate the behavior of Geodesic Acoustic Modes (GAMs). In this context, we show that this approach represents a powerful basis for the description of many characteristics of radial propagation and spreading of GAMs. The most attractive feature of these techniques is represented by their universality and intuitive applicability. We present and apply two different complex-eikonal methods able to describe the spreading of GAMs in terms of local plane waves. The methods are “inhomogeneous wave tracking” and “paraxial WKB” theory. We demonstrate their applicability and efficacy to the GAM dynamics problem by means of a comparison with gyrokinetic simulations.

An experimental investigation of interacting swirling multiple jets

This article deals with the experimental investigation of multiple interacting jets, which may be interested in many engineering applications such as design of a ventilation supply device. The main objective of this study is to achieve the best configuration for use in ventilation applications. To achieve this, several parameters have been considered and discussed such as the imbalance in temperature and diffuser orifices position with relative imbalance in flow rate between central and peripheral jets. Flow rate has been adjusted at Reynolds numbers, ranging from 104 to 3?104. The present study is carried out under uniform heat flux condition for each diffuser, and air is used as a working fluid. Experiences concerning the fusion of several jets show that the resulting jet is clearly more homogenized under the influence of the central swirling jet. Highlights of such an investigation show that, if the relative position of the central jet is higher, the radial spreading of the resultant jet is more important when all jets are in the same plane. This spreading is also improved compared to the case where the relative position of the peripheral jets is higher, thereby allowing to process a large volume of air. In addition, it becomes attractive to operate, especially when we aim premises homogenization.

Artificial bird strike on Hopkinson tube device: Experimental and numerical analysis

This work shows a combined experimental-numerical research in bird impact. In order to perform the experimental tests, a artificial bird has been prepared and impacted against a Hopkinson tube in a wide range of impact velocities (70–200 m/s). The Hopkinson tube was designed in order to measure the induced force transmitted in the tube by the impact. This force could be used to compare different experimental tests and also to validate the numerical models proposed. In addition, the whole process of impact was recorded by means of high speed video cameras. The images captured allow to perform the analysis of the bird kinematics during the impact. Numerically, in order to reproduce the high deformations experienced by the artificial bird in the impact process, the Smooth Particle Hydrodynamics (SPH) technique has been used. Concerning the artificial bird material behaviour, four different models were employed, combining the two material models and two equations of state most used in the literature. The four cases have been compared with the experimental measurements and benchmarked. After the analysis of the results, it can be concluded that the combined experimental-numerical methodology proposed successfully can be used to study and validate the numerical models for simulating the behaviour of soft impactor when subjected to high velocity impacts. It can be seen that the normal impact forces induced by the impact are reproduced adequately for all the numerical models. However the radial spreading of the soft impactor is not reproduced as adequately as the other cases, especially in low velocity impacts. This effect can be important to reproduce the radial distribution of pressures and the secondary impacts produced by this radial expansion.

Axisymmetric squeezing dynamics of resin droplet with surface tension and its numerical solution

We investigate the 2D axisymmetric quasi-static dynamics of a UV-curable optically clear resin droplet squeezed between two parallel glass walls using a static force. The radial spreading speed and resulting radial spread are examined using an analytical derivation and a numerical discretized approximation including the effect of the surface tension. Under a boundary condition, the vertical downward velocity of the glass wall is determined by the static force and capillary pressure, and a pressure discontinuity occurs across the air-liquid interface. The capillary pressure is estimated from the Young-Laplace equation. A numerical approximate solution is then derived from the governing equations to obtain a discretized set of linear equations to serve as an alternative theoretical solution. A 2D axisymmetric section with a staggered grid is adopted for a pressure-based segregated velocity-pressure coupled solution process using the SIMPLE algorithm. The change in the rheological properties due to the UV-curing effect is then added to the squeezing dynamics solution with an arbitrary initiation time and the power amplitude of the UV curing. According to a few rheological test results for different UV powers, the relationship between the UV energy (calculated using the UV power) and the resin viscosity is confirmed. The numerical solution is validated, and both theoretical solutions are compared with the experimental results of several squeezing tests with and without UV curing.

Modeling ballistic phonon transport from a cylindrical electron beam heat source

Recent electron microscopy experiments have used focused electron beams as nanoscale heat sources or thermometers to enable high spatial resolution studies of heat transfer in nanostructures. When the electron beam radius is smaller than the heat carrier mean free path, Fourier’s law will underpredict the temperature rise due to electron beam-induced heating, motivating the development of subcontinuum models to interpret thermal electron microscopy measurements. Here, electron beam-induced heating of nonmetallic samples is modeled by applying a recently developed general solution of the governing Boltzmann transport equation (BTE) under the relaxation time approximation. The analytical BTE solution describes thermal phonon transport from a time-periodically heated cylindrical region in a homogeneous infinite medium. The BTE results show that ballistic phonon effects in this radial heat spreading scenario are more conveniently represented using a ballistic thermal resistance rather than an effective thermal conductivity. Calculations of this ballistic resistance for three semiconductors (Si, GaAs, and 3C-SiC) show that ballistic effects dominate the total thermal resistance to radial heat flow for typical STEM or SEM beam radii (<10 nm), indicating that the ballistic resistance could potentially be measured using thin-film electron beam heating experiments. However, combining the BTE solution with recent calorimetric measurements shows that the magnitude of the temperature rise remains negligibly small (<1 K) under typical electron microscopy conditions, even when considering these ballistic effects. These BTE modeling results can be used to quantify electron beam-induced heating or to design experiments probing ballistic phonon transport using electron beam heat sources.

Internal Tide Attenuation in the North Pacific

AbstractMultisatellite altimetry and an eddy‐resolving model with tides are used to quantify the attenuation of the mode‐1 M2 internal tide as it propagates from three major sources in the North Pacific. The model is used to correct the altimetric fluxes for the nonstationary signal that altimeters cannot detect. Because internal tides in the North Pacific are highly stationary, these corrections do not materially impact the decay rate estimates. Fluxes are integrated in wedges extending from the sources to account for interference and radial spreading. Observed attenuation rates are consistent with e‐folding scales between 750 and 3,000 km, suggesting weak dissipation rates (≤10−10 W/kg or 0.75×10−3 W/m2) compared to typical open‐ocean turbulence levels, implicating near‐inertial waves and higher‐mode internal tides in providing the balance of the dissipation in the ocean interior.

An Analytic Model for an Evolving Protoplanetary Disk with a Disk Wind

Abstract We describe an analytic model for an evolving protoplanetary disk driven by viscosity and a disk wind. The disk is heated by stellar irradiation and energy generated by viscosity. The evolution is controlled by three parameters: (i) the inflow velocity toward the central star at a reference distance and temperature, (ii) the fraction of this inflow caused by the disk wind, and (iii) the mass-loss rate via the wind relative to the inward flux in the disk. The model gives the disk midplane temperature and surface density as a function of time and distance from the star. It is intended to provide an efficient way to calculate conditions in a protoplanetary disk for use in simulations of planet formation. In the model, disks dominated by viscosity spread radially while losing mass onto the star. Radial spreading is the main factor reducing the surface density in the inner disk. The disk mass remains substantial at late times. Temperatures in the inner region are high at early times due to strong viscous heating. Disks dominated by a wind undergo much less radial spreading and weaker viscous heating. These disks have a much lower mass at late times than purely viscous disks. When mass loss via a wind is significant, the surface density gradient in the inner disk becomes shallower, and the slope can become positive in extreme cases.

Characteristics and transport mechanism of the Nyixoi Chongco rock avalanche on the Tibetan Plateau, China

For studying rock avalanches, complex surficial and internal characteristics have been regarded as important indicators for understanding avalanche transport mechanism. In the present study, deposits of a massive Holocene rock avalanche, termed the Nyixoi Chongco rock avalanche, located on the Lhasa Terrane of Tibetan Plateau in China are investigated, based on a combination of remote sensing data and in-situ observations. It is revealed that abundant surface landforms, including Toreva blocks, longitudinal and transverse ridges, en echelon ridges, and hummocks, are commonly developed. Corresponding to these surface morphologies, a series of internal sedimentary structures, including jigsaw structures, inner shear zones, aligned clasts, diapiric structures, convoluted laminations, and basal décollements, are also well developed. Through an analysis of these surficial and internal characteristics, we propose that a rapid motion, similar to a laminar flow, most likely occurred after the avalanche mass detaching from the source area. In this process, the detached avalanche mass was mainly emplaced by a frictional, simple shear process characterized by a bulldozing effect that occurred between the sliding mass and the substrate. From the proximal to the distal sections of the avalanche, the transport of the rock mass can be classified into four major processes: an extension-dominated sliding process in the transition zone, a compression-dominated sliding process in subzones III-1 and III-2 of the accumulation zone (III), a sliding process co-dominated by compression and lateral spreading in subzone III-3, and a rapid radial spreading process in subzone III-4. The results of our study provided an improved understanding of the transport mechanisms of this rock avalanche.