Showing papers on "Critical ionization velocity published in 2018"
TL;DR: In this article, the authors argue that the large interfacial strain needed to achieve bonding does not necessarily require adiabatic shear instability to trigger, and instead, they suggest that the interaction of strong pressure waves with the free surface at the particle edges can cause hydrodynamic plasticity that effects bonding, without requiring Shear instability, and they proceed on this basis to postulate and confirm a proportionality between critical velocity and the bulk speed of sound.
195 citations
TL;DR: In this paper, the dynamics of back-layering flow induced by a continuous buoyant release in a naturally ventilated inclined tunnel were investigated using brine water experiments, showing that the back-layer length is independent of buoyancy flux of the releasing source.
62 citations
TL;DR: In a thin cell whose thickness is smaller than that of the initial droplet size, the droplets show more complex trajectories, including "figure-8s" and zigzags, which are attributed to autochemotaxis of the droplet.
Abstract: The self-propelled motions of micron-sized nematic liquid crystal droplets in an aqueous surfactant solution have been studied by tracking individual droplets over long time periods. Switching between self-propelled modes is observed as the droplet size decreases at a nearly constant dissolution rate: from random to helical and then straight motion. The velocity of the droplet decreases with its size for straight and helical motions but is independent of size for random motion. The switching between helical and straight motions is found to be governed by the self-propelled velocity, and is confirmed by experiments at various surfactant concentrations. The helical motion appears along with a shifting of a point defect from the self-propelled direction of the droplet. The critical velocity for this shift of the defect position is found to be related with the Ericksen number, which is defined by the ratio of the viscous and elastic stresses. In a thin cell whose thickness is smaller than that of the initial droplet size, the droplets show more complex trajectories, including "figure-8s" and zigzags. The appearance of those characteristic motions is attributed to autochemotaxis of the droplet.
60 citations
TL;DR: In this article, the authors investigated the vibration characteristics of pipes conveying fluid in the supercritical range by using Timoshenko beam theory for the first time and derived the nonlinear transverse vibration governing equation.
50 citations
TL;DR: In this paper, a set of tunnel fire experiments using a 1:20 scale model tunnel was conducted taking into account the scaling of heat conduction through the tunnel walls, and new correlations for estimating the critical velocity and backlayering distance were presented, and discussed the difference between their correlations and the previous ones from the viewpoint of the thermal properties of the tunnel wall materials.
50 citations
TL;DR: In this article, the steady-state response of a uniform infinite Euler-Bernoulli elastic beam resting on a Pasternak elastic foundation and subjected to a concentrated load moving at a constant velocity along the beam is analytically investigated.
46 citations
TL;DR: In this article, a composite thin-walled cantilever pipe conveying fluid supported at free end by linear translational and rotational springs is considered and the governing equations of the system are developed by extended Hamilton's principle for open systems.
43 citations
TL;DR: In this article, the effect of blockage ratio on critical velocity was investigated in a model tunnel of (1:20 of a tunnel section) to study the influence of heat loss in the experimental model.
41 citations
TL;DR: In this article, the evolution characteristics of the critical velocity in a tunnel with the coupling effect between ceiling centralized mechanical smoke exhaust and longitudinal ventilation have been studied, and a new empirical model was proposed to predict the critical ventilation velocity.
39 citations
TL;DR: In this paper, an analytical method is presented to calculate ground vibrations from a tunnel in a multi-layered half-space using the transfer matrix method, the dynamic system matrix for the multilayered soil overlying a halfspace or bedrock is obtained.
39 citations
TL;DR: In this paper, the authors carried out theoretical analyses and numerical simulations to investigate the driving force necessary for achieving the critical velocity in downhill tunnels and found that buoyant source location has a considerable effect on the condition for achieving critical velocity.
TL;DR: In this article, the evolution of the properties of cold-sprayed Ti6Al4V (Ti64) coatings with particle velocities ranging from 730 to 855 m/s using pure N2 and N2-He mixture as the propellant gases.
Abstract: Cold-spraying is a relatively new low-temperature coating technology which produces coatings by the deposition of metallic micro-particles at supersonic speed onto target substrate surfaces This technology has the potential to enhance or restore damaged parts made of light metal alloys, such as Ti6Al4V (Ti64) Particle deposition velocity is one of the most crucial parameters for achieving high-quality coatings because it is the main driving force for particle bonding and coating formation In this work, studies were conducted on the evolution of the properties of cold-sprayed Ti64 coatings deposited on Ti64 substrates with particle velocities ranging from 730 to 855 m/s using pure N2 and N2-He mixture as the propellant gases It was observed that the increase in particle velocity significantly reduced the porosity level from about 11 to 16% due to greater densification The coatings’ hardness was also improved with increased particle velocity due to the intensified grain refinement within the particles Interestingly, despite the significant differences in the coating porosities, all the coatings deposited within the velocity range (below and above critical velocity) achieved a high adhesion strength exceeding 60 MPa The fractography also showed changes in the degree of dimple fractures on the particles across the deposition velocities Finite element modelling was carried out to understand the deformation behaviour of the impacting particles and the evolutions of strain and temperature in the formed coatings during the spraying process This work also showed that the N2-He gas mixture was a cost-effective propellant gas (up to 3-times cheaper than pure He) to deliver the high-quality Ti64 coatings
TL;DR: In this article, the square honeycomb core is compressed quasi-statically and dynamically in order to investigate the mechanical properties of it, and the plastic collapse stress under static compression is mathematically discussed by investigating the collapse mechanisms of cells from numerical simulations.
TL;DR: In this article, the formation and propagation of a compaction wave in a low density polymeric foam under intermediate velocity projectile impact loading is investigated experimentally, and the results are discussed in terms of the compaction-wave characteristic parameters such as compactionwave velocity, axial strain, particle velocity, etc.
TL;DR: In this paper, the authors explored the validity of using single critical Froude number of 4.5 by investigating the original sources and comparing it to recent research results and found that using this value produces a significantly lower critical velocity for a wide range of fire sizes and therefore it is not conservative.
TL;DR: In this article, the authors considered the excitation of ICWs by ion and electron beams in a compensated current system and investigated the competition between reactive and kinetic instabilities, showing that the reactive instabilities can well agree quantitatively with the numerical results by the fluid model.
Abstract: Ion cyclotron waves (ICWs) can play important roles in the energization of plasma particles. Charged particle beams are ubiquitous in space, and astrophysical plasmas and can effectively lead to the generation of ICWs. Based on linear kinetic theory, we consider the excitation of ICWs by ion and electron beams in a compensated-current system. We also investigate the competition between reactive and kinetic instabilities. The results show that ion and electron beams both are capable of generating ICWs. For ICWs driven by ion beams, there is a critical beam velocity, v bi c , and critical wavenumber, k z c , for a fixed beam density; the reactive instability dominates the growth of ICWs when the ion-beam velocity and the wavenumber , and the maximal growth rate is reached at for a given . For the slow ion beams with , the kinetic instability can provide important growth rates of ICWs. On the other hand, ICWs driven by electron beams are excited only by the reactive instability, but require a critical velocity, (the Alfven velocity). In addition, the comparison between the approximate analytical results based on the kinetic theory and the exact numerical calculation based on the fluid model demonstrates that the reactive instabilities can well agree quantitatively with the numerical results by the fluid model. Finally, some possible applications of the present results to ICWs observed in the solar wind are briefly discussed.
TL;DR: In this paper, a quasi-unsteady model with a theoretical model of the memory function was used to predict the critical velocity with the static fluid force coefficients obtained from steady RANS simulations.
TL;DR: In this paper, the influence of non-uniform velocity profile attributable to slip boundary condition and viscosity of fluid on the dynamic instability of carbon nanotubes (CNTs) conveying fluid is investigated.
Abstract: In this article, the influences of non-uniform velocity profile attributable to slip boundary condition and viscosity of fluid on the dynamic instability of carbon nanotubes (CNTs) conveying fluid are investigated. The nonlocal elasticity theory and the Euler–Bernoulli beam theory are employed to derive partial differential equation of nanotubes conveying fluid. Furthermore, a dimensionless momentum correction factor (MCF) is obtained as a function of Knudsen number (Kn) so as to insert the effects of non-uniform velocity profile into the equation of motion. In continuation, complex eigen-frequencies of the system are attained with respect to different boundary conditions, the momentum correction factor, slip boundary condition and nonlocal parameter. The results delineate that considering the effects of non-uniform velocity profile could diminish predicted critical velocity of flow. Therefore, the divergence instability occurs in the lower values of flow velocity. In addition, the MCF decreases through enhancement of Kn; hence, the effects of non-uniform velocity profile are more noticeable for liquid fluid than gas fluid.
TL;DR: In this paper, the authors formulated and solved the problem of spherically symmetric, steady state, adiabatic accretion onto a Schwarzschild-like black hole obtained recently.
Abstract: We formulate and solve the problem of spherically symmetric, steady state, adiabatic accretion onto a Schwarzschild-like black hole obtained recently. We derive the general analytic expressions for the critical points, the critical velocity, the critical speed of sound, and subsequently the mass accretion rate. The case for polytropic gas is discussed in detail. We find the parameter characterizing the breaking of Lorentz symmetry will slow down the mass accretion rate, while has no effect on the gas compression and the temperature profile below the critical radius and at the event horizon.
TL;DR: In this article, a multilayer 1050 H14 aluminum corrugated core was investigated both experimentally and numerically using the perfect and imperfect models between 0.0048 and 90m/s−1.
Abstract: The crushing behavior of a multilayer 1050 H14 aluminum corrugated core was investigated both experimentally and numerically (LS-Dyna) using the perfect and imperfect models between 0.0048 and 90 m s−1. The dynamic compression and direct impact tests were performed in a compression type and a modified Split Hopkinson Pressure Bar set-up, respectively. The investigated fully imperfect model of the corrugated core sample represented the homogenous distribution of imperfection, while the two-layer imperfect model the localized imperfection. The corrugated core experimentally deformed by a quasi-static homogenous mode between 0.0048 and 22 m s−1, a transition mode between 22 and 60 m s−1 and a shock mode at 90 m s−1. Numerical results have shown that the stress-time profile and the layer crushing mode of the homogeneous and transition mode were well predicted by the two-layer imperfect model, while the stress-time profile and the layer crushing mode were well approximated by the fully imperfect model. The fully imperfect model resulted in complete sequential layer crushing at 75 and 90 m s−1, respectively. The imperfect layers in the shock mode only affected the distal end stresses, while all models implemented resulted in similar impact end stresses. The distal end initial crushing stress increased with increasing velocity until about 22 m s−1; thereafter, it saturated at ~2 MPa, which was ascribed to the micro inertial effect. Both the stress-time and velocity-time history of the rigid-perfectly-plastic-locking model and the critical velocity for the shock deformation were well predicted when a dynamic plateau stress determined from the distal end stresses in the shock mode was used in the calculations.
TL;DR: In this article, the authors focus on the supercritical velocity in longitudinally ventilated tunnels and identify the reason for the appearance of the so-called super-critical velocity, a ventilation velocity that becomes independent of the heat release rate as this latter becomes large.
TL;DR: In this article, the effects of thermo-fluidic parameters on the nonlinear dynamic behaviors of single-walled carbon nanotube conveying fluid with slip boundary conditions and resting on linear and nonlinear elastic foundations under external applied tension and global pressure is studied using homotopy perturbation method.
Abstract: In this paper, effects of thermo-fluidic parameters on the nonlinear dynamic behaviours of single-walled carbon nanotube conveying fluid with slip boundary conditions and resting on linear and nonlinear elastic foundations under external applied tension and global pressure is studied using homotopy perturbation method. From the result, it is observed that increase in the Knudsen number, the slip parameter, leads to decrease in the frequency of vibration and the critical velocity while natural frequency and the critical fluid velocity increase as the in stretching effect increases. Also, as the Knudsen number increases, the bending stiffness of the nanotube decreases and in consequent, the critical continuum flow velocity decreases as the curves shift to the lowest frequency zone. As the change in temperature increases, the natural frequencies and the critical flow velocity of the structure increase for the low or room temperature while at high temperature, increase in temperature change, decreases the natural frequencies and the critical flow velocity of the structure. Further, it is established that the alteration of nonlinear flow-induced frequency from linear frequency is significant as the amplitude, flow velocity and axial tension increase. The developed analytical solutions can be used as starting points for better understanding of the relationship between the physical quantities of the problem.
TL;DR: In this article, the coexistence and interplay of spin and mass superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate was investigated, and it was shown that the mass and spin supercurrents are suppressed by energetic barriers up to the upper critical velocity (gradient) exceeding the Landau critical velocity.
Abstract: The paper investigates the coexistence and interplay of spin and mass superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate. Superfluidity is possible only in the presence of uniaxial anisotropy (linear and quadratic Zeeman effect). This follows from the topology of the order-parameter space (vacuum manifold). According to the Landau criterion, the critical phase gradients, both for mass and spin supercurrents, vanish at the phase transition from the easy-plane to the easy-axis anisotropy. However, mass superfluidity is still possible at the phase transition. This is because the Landau criterion signals instability only with respect to nonsingular vortices with special ratio between circulations of mass and spin currents. Phase slips produced by these vortices are not sufficient for complete decay of supercurrents. Full decay of supercurrents requires phase slips with vortices of another topological class and larger energy. These phase slips are suppressed by energetic barriers up to the upper critical velocity (gradient) exceeding the Landau critical velocity. The upper critical velocity does not vanish nor has any anomaly in the critical point at the phase transition from the easy-plane to the easy-axis anisotropy.
TL;DR: In this article, the velocity at the top surface that is barely enough to dislodge the trapped droplet was determined using computational fluid dynamic analysis (CFD) and the proportionality factor was derived.
TL;DR: In this paper, the computational fluid dynamics (CFD) analysis of cold spray process including the de Laval nozzle was done with the different type of turbulence models such as the standard k-ɛ model, Realizable k-α model and Reynolds stress model.
TL;DR: In this paper, the effect of increase in pitch ratio on instability threshold for plain tube arrays and increase in fin density on the instability threshold of finned tube arrays is investigated. But, the authors only considered finned arrays with two different pitch ratios; 2.6 and 2.1.
Abstract: Failure of tubes in shell and tube exchangers is attributed to flow induced vibrations of such tubes. There are different excitations mechanisms due to which flow induced vibration occurs and among such mechanisms, fluid elastic instability is the most prominent one as it causes the most violent vibrations and may lead to rapid tube failures within short time. Fluid elastic instability is the fluid–structure interaction phenomenon which occurs when energy input by the fluid force exceeds energy expended in damping. This point is referred as instability threshold and corresponding velocity is referred as critical velocity. Once flow velocity exceeds critical flow velocity, the vibration amplitude increases very rapidly with flow velocity. An experimental program is carried out to determine the critical velocity at instability for plain and finned tube arrays subjected to cross flow of water. The tube array geometry is parallel triangular with cantilever end condition and pitch ratios considered are 2.6 and 2.1. The objective of research is to determine the effect of increase in pitch ratio on instability threshold for plain tube arrays and to assess the effect of addition of fins as well as increase in fin density on instability threshold for finned tube arrays. Plain tube array with two different pitch ratios; 2.1 and 2.6 and finned tube arrays with same pitch ratio; 2.6 but with two different fin pitches; such as fine (10 fpi) and coarse (4 fpi) are considered for the experimentation. Connors’ equation that relates critical velocity at instability to different parameters, on which instability depends, has been used as the basis for analysis and the concept of effective diameter is used for the present investigation. The modal parameters are first suitably modified using natural frequency reduction setup that is already designed and developed to reduce natural frequency and hence to achieve experimental simulation of fluid elastic instability within the limited flow capacity of the pump. The tests are carried out first on plain tube arrays to establish the same as the datum case and results are compared to known results of plain tube arrays and hence the quality of the test rig is also assessed. The fluid elastic vibration tests are then carried out on finned tube arrays with coarse and fine fin pitches and effects of fins and fin pitch on instability threshold are shown. The vibration response of the tube is recorded for each gradually increasing flow rates of water till instability point is reached. The parameters at the instability are then presented in terms of dimensionless parameters to compare them with published results. It is concluded that, arrays with higher pitch ratios are unstable at comparatively higher flow velocities and instability threshold for finned tube arrays is delayed due to addition of the fins. Further, it is concluded that, instability threshold for finned tube arrays with fine fin pitch is delayed compared to coarse fin pitch and hence for increased fin density, instability threshold is delayed. The experimental results in terms of critical velocities obtained for different tube arrays subjected to water cross flow will serve as the base flow rates for air–water cross flow experiments to be conducted in the next phase.
TL;DR: In this article, the Eulerian method was used to simulate the relationship between the preheating temperature and the critical velocity, and the relationship appeared to be a linear relationship when the jet formed.
Abstract: In this research, the Eulerian method was used to simulate the relationship between the preheating temperature and the critical velocity. The size and shape of the void (no-material-existence zone) and the thickness of the model were investigated. Various preheating temperatures were used throughout the whole model. The critical velocity was determined with jet’s appearance. The simulation results demonstrated the void size and shape influenced the results, although the thickness did not. The critical velocity decreased as the preheating temperature rose. The relationship between critical velocity and minimum preheating temperature when the jet formed appeared to be a linear relationship. The experimental results confirmed the simulation results. The linear equation could be used to predict the preheating temperature at a certain velocity below the critical value. The corresponding steady maximum PEEQ (equivalent plastic strain) value at the minimum preheating temperature could be divided into a s...
TL;DR: In this article, a semi-analytical approach is applied to investigate three-dimensional (3D) vibration in a coupled pavement and ground system subjected to a rectangular moving load, where the pavement is simplified as an infinitely long orthogonal anisotropic elastic plate, and the ground materials are assumed to be multi-layered and transversely isotropic soils obeying Biot's dynamic poroelastic theory.
TL;DR: In this paper, a dynamic simulation approach to investigate liquid loading in gas wells for a fluid flowing in the mist flow regime is presented, using coupled thermodynamic and hydrodynamic models as well as constitutive equations that incorporate the Peng-Robinson equation of state and convex hull algorithm.
TL;DR: In this paper, the filling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and filling stage by gate velocity, and the gate velocity continues to increase with the increase of pressurizing speed or expansion angle.
Abstract: The mold filling behavior of gradual expansion structure in low pressure casting was studied by two phase flow model using the Volume of Fluid method, and was verified by water simulation with a Plexiglas mold. To get smooth mold filling process and provide a guide for the pressurizing speed design in the producing practice, the mathematical model with the pressurizing speed, expansion angle and height of the gradual expansion structure was established. For validation experiments, ZL205A alloy castings were produced under two different pressurizing speeds. Weibull probability plots were used to assess the fracture mechanisms under different pressurizing speeds. Mechanical properties of ZL205A alloy were applied to assess the entrainment of oxide film. The results show that the filling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and filling stage by gate velocity. The gate velocity continues to increase in the gradual expansion structure, and increases with the increase of pressurizing speed or expansion angle. Under the effect of the falling fluid raised by the jet flow along the sidewall, the fluid velocity decreases in the jet zone from ingate to free surface. As such, oxide film entrainment does not occur when the gate velocity is greater than the critical velocity, andthe gate velocity no longer reflects the real state of the free surface. The scatter of the mechanical properties is strongly affected by the entrainment of oxide films.