Average Initial Velocity Research Articles

Test and mechanism analysis for crack propagation by blasting in rock under the condition of unidirectional load

In deep rock blasting, there are different constraint conditions such as unidirectional loading, bidirectional loading and tridirectional loading due to in-situ stress. The similar model test of rock blasting under unidirectional load was carry out to study the law of crack propagation in rock by blasting in this study. A transparent material which conformed to the mechanical properties of hard rock is used to make specimens. A high-speed camera was used in some model blasts. After blasting the surface cracks on the specimen were measured, including the near zone and middle zone of blasting. The results showed that: (1) the radial main cracks in the specimens under initial static load propagated along the principal stress direction, which is different from that without initial stress, and there is almost no radial main crack in the direction perpendicular to the principal stress. (2) The average length of radial crack and diameter of circumferential fracture circle gradually decrease with the increase of unidirectional initial stress, but the diameter of compression crushing circle increase. (3) The maximum and average initial velocity of radial crack growth decrease with the increase of unidirectional initial stress.

Towards understanding mechanistic subgroups of osteoarthritis: 8-year cartilage thickness trajectory analysis.

Many studies have validated cartilage thickness as a biomarker for knee osteoarthritis (OA); however, few studies investigate beyond cross-sectional observations or comparisons across two timepoints. By characterizing the trajectory of cartilage thickness changes over 8 years in healthy individuals from the OA initiative data set, this study discovers associations between the dynamics of cartilage changes and OA incidence. A fully automated cartilage segmentation and thickness measurement method were developed and validated against manual measurements: mean absolute error = 0.11-0.14 mm (n = 4129 knees) and automatic reproducibility = 0.04-0.07 mm (n = 316 knees). The mean thickness for the medial and lateral tibia (MT, LT), central weight-bearing medial and lateral femur (cMF, cLF), and patella (P) cartilage compartments were quantified for 1453 knees at seven timepoints. Trajectory subgroups were defined per cartilage compartment such as stable, thinning to thickening, accelerated thickening, plateaued thickening, thickening to thinning, accelerated thinning, or plateaued thinning. For tibiofemoral compartments, the stable (22%-36%) and plateaued thinning (22%-37%) trajectories were the most common, with an average initial velocity (μm/month), acceleration (μm/month2 ) for the plateaued thinning trajectories LT:-2.66, 0.0326; MT:-2.49, 0.0365; cMF:-3.51, 0.0509; and cLF:-2.68, 0.041. In the patella compartment, the plateaued thinning (35%) and thickening to thinning (24%) trajectories were the most common, with an average initial velocity, acceleration for each -4.17, 0.0424; 1.95, -0.0835. Knees with nonstable trajectories had higher adjusted odds of OA incidence than stable trajectories: accelerated thickening, accelerated thinning, and plateaued thinning trajectories of the MT had adjusted odds ratio (OR) of 18.9, 5.48, and 1.47, respectively; in the cMF, adjusted OR of 8.55, 10.1, and 2.61, respectively.

Experimental study on rock cross-cut coal uncovering with ultra-high pressure hydraulic slotting

In view of the coal seam with low permeability, high gas content, high gas pressure and high outburst risk, the rock cross-cut coal uncovering is carried out, and the Xinji No.1 Coal Mine in Anhui Province is selected as the engineering background. The experimental study on the rock cross-cut coal uncovering by the ultra-high pressure hydraulic slotted crosscut is carried out, and the amount of slag discharged by the hydraulic slotting and the initial velocity of gas emission are investigated. The results show that after the ultra-high pressure hydraulic slotting measures are adopted, the equivalent radius of the uncovering area of the crosscut of coal seam group is increased by 31.9 times, the average initial velocity of gas emission is increased by 3.5 times, the amount of drilling work is reduced by 47%, and the coal is discharged by the slotting. The chip rate is more than 3 ‰ and the coal uncovering time is shortened by 50%. Based on this, the paper tests the effect of the measures in the area of rock cross-cut coal uncovering. The maximum residual gas content is 4.4m3/t, and the maximum residual gas pressure is 0.16MPa. The practice shows that the hydraulic slotting measures can greatly reduce the residual gas pressure (content) of the coal seam. Therefore, the ultra-high pressure hydraulic slotting technology can be used as an effective means for rock cross-cut coal uncovering in high gas and low permeability coal seam, and it can provide important reference and reference for similar conditions.

Optimization of the specific kinetic energy of the pre-grooved stun grenade fragments based on HyperMesh, LS-DYNA and Matlab

Large-scale fragments formed by packaging shell of stun grenade are easy to cause irreversible damage to human body in close range. In order to reduce the specific kinetic energy of shell fragments as much as possible, the pre-grooving method is proposed to optimize the safety of packaging shell. According to the process of fragment forming, flying and impacting, the shell breaking model, fragment dispersion model and average specific kinetic energy calculation model are established respectively. The non-grooved shell and inner grooved shell are simulated by the joint simulation method of HyperMesh, LS-DYNA and Matlab. The fragment mass distribution, initial velocity and average specific kinetic energy are obtained and compared. The results show that under the same main charge parameters, the mass distribution of fragments is more concentrated and the large mass fragments are less in the inner grooved shell than in the non-grooved shell; the average initial velocity of fragments in the inner grooved shell is lower than that in the non-grooved shell; the attenuation of average specific kinetic energy in the inner grooved shell is faster than that in the non-grooved shell, which is safer than that in the non-grooved structure.

Effects of robotic intervention associated with conventional therapy on gait speed and resistance and trunk control in stroke patients

Objective: To verify the effects of gait and robotic stair training with G-EO System, associated with conventional rehabilitation, on gait speed and endurance and trunk control of stroke participants. Methods: Retrospective study with 28 participants in the chronic phase of the disease. G-EO System was used for gait and stair robotic intervention. 20-session protocol of 20 minutes associated with conventional multidisciplinary therapy. The 10-meter Walk Test (10mWT), 6-minute Walk Test (6MWT) and Trunk Impairment Scale (TIS) tools were used. P values <0.05 were considered statistically significant with Wilcoxon test before and after intervention. Results: Significant differences found in the tests. TIS presented initial mean value of 14.29 (± 5.30) and final value of 17.04 (± 4.49), with p = 0.00044. 10mWT presented average initial velocity of 0.498 m/s (± 0.27) and final velocity of 0.597 m/s (± 0.32), p = 0.00008. 6mWT presented mean initial value of 155.89m (± 85.96) and final value of 195.39m (± 109.78), p = 0.00152. Conclusion: Gait and stair robotic therapy, associated with conventional therapy, was effective in promoting increased speed, endurance aptitude for greater gait distances and trunk control in individuals with chronic stroke after stroke.

ON THE CALCULATION OF CONVECTIVE HEAT TRANSFER UNDER MUTUAL-ACTION OF A JET WITH LIMITING SURFACE

The paper proposes a method for calculating convective heat transfer in the interaction of a single circular jet with a flat surface. The differences of the proposed method from the existing ones are given. The concepts “energodynamic potential of the flow” and “energodynamic power of the flow” are introduced, allowing to determine the intensity of convective heat transfer at “gas-solid” boundary. Differences of the proposed definitions from the existing ones are given: heat flux and heat flux density. The principal difference between the heat flux density q and the energy dynamic potential qэ is as follows: the heat flux density q for convective heat transfer problems means the amount of heat that is transferred from a liquid to a solid surface (or vice versa) per unit of time through a unit of heat exchange surface area. Thus, quantity q characterizes the intensity of convective heat transfer process at the interface. The energy dynamic potential qэ characterizes the flow property as a source or carrier of heat. Value of qэ characterizes the specific energy power of the fluid flow. When calculating the heat transfer, it was proposed to divide the jet when interacting with the flat surface into two parts: before the interaction – the jet part, after – the fan flow. The method for calculating convective heat transfer under jet heating, in which the Reynolds criterion calculated by characteristics of the gas at the nozzle exit is decisive, is not entirely correct. It is proposed to use criteria specific to the fan flow. Characteristic values for the fan flow are its initial average velocity Uвп, distance from the critical point of the jet (point of intersection of vertical axis of the jet with the surface) to the current coordinate of radius downstream. To assess the changes in basic characteristics of a free jet at different distances from the nozzle exit to limiting surface, dependences of the following criteria are presented: jet expansion coefficient; jet injection coefficient; velocity coefficient for any jet section; velocity coefficient for any jet section, except h/d0 = 0; relation of the Reynolds criteria, confirming the need to carry out calculations of heat transfer on the values characteristic separately for the fan flow.

Physical application: Cannon case

The study of physical phenomena by means of guided experimentation and experimental thinking, allow students to infer and understand the reason for the different variations that evidence. Parabolic motion of a projectile powered by a cannon under the spring mechanism, generates discussion regarding the choice of the proper angle, according to a certain distance, a known average initial velocity, and a given height. Give the blank is a great encouragement, however, being able to explain which conditions of the environment influenced the failed launches, generates a space of dialogue and a durable concrete learning.

Stabilizing effect of large average initial velocity in forced dissipative PDEs invariant with respect to Galilean transformations

We describe a topological method to study the dynamics of dissipative PDEs on a torus with rapidly oscillating forcing terms. We show that a dissipative PDE, which is invariant with respect to the Galilean transformations, with a large average initial velocity can be reduced to a problem with rapidly oscillating forcing terms. We apply the technique to the viscous Burgers' equation, and the incompressible 2D Navier–Stokes equations with a time-dependent forcing. We prove that for a large initial average speed the equation admits a bounded eternal solution, which attracts all other solutions forward in time. For the incompressible 3D Navier–Stokes equations we establish the existence of a locally attracting solution.

Dynamic process analysis for the initiation and movement of the Donghekou landslide-debris flow triggered by the Wenchuan earthquake

The Donghekou landslide-debris flow was a remarkable geological disaster triggered by the Wenchuan earthquake in 2008. The dynamic process of a rapid landslide-debris flow is very complicated and can be divided into two aspects: the slope dynamic response of the earthquake and the mass movement and accumulation process. A numerical method combined with a finite difference method (FDM) and discrete element method (DEM) for simulation of landslide-debris flow under seismic loading is presented. The FDM and DEM are coupled through the critical sliding surface, initiation time and velocity. The dynamic response of the slope is simulated by the finite difference method, and critical sliding surface is determined using the earthquake response spectrum method. The landslide initiation time and the velocity are determined by time–history analysis. The mass movement and accumulation process is simulated using the discrete element method. Simulation results demonstrate that the maximum amplification coefficient of dynamic acceleration for the Donghekou slope is approximately 3.909, the initiation time of landslide is approximately 6.0s, and the average initial velocity of the sliding mass is approximately 0.85m/s. The failure of the slope is the result of elevation-orientated amplification effect and the sliding mass triggered with a small initial velocity. The numerical simulated result of the maximum sliding velocity is approximately 66.35m/s, and the mass is disintegrated rapidly because of collision and free fall. The landslide velocity decreases when the flowing mass reaches a lower slope angle and gradually comes to a stop, and the total travel distance is approximately 2400m.

Study on the initial velocity distribution of exhaled air from coughing and speaking

Increasing concerns about the spread of airborne pathogens such as severe acute respiratory syndrome (SARS) and novel swine-origin influenza A (H1N1) have attracted public attention to bioaerosols and protection against them. The airborne pathogens are likely to be expelled from coughing or speaking, so the physical data of the exhaled particles plays a key role in analyzing the pathway of airborne viruses. The objective of this study was to analyze the initial velocity and the angle of the exhaled airflow from coughing and speaking of 17 males and 9 females using Particle Image Velocimetry (PIV) and acrylic indoor chamber. The results showed that the average initial coughing velocity was 15.3m/s for the males and 10.6m/s for the females, while the average initial speaking velocity was 4.07m/s and 2.31m/s respectively. The angle of the exhaled air from coughing was around 38° for the males and 32° for the females, while that of the exhaled air from speaking was around 49° and 78° respectively. Also, the linear relation between the tested subject’s height and their coughing and speaking velocity was shown in this study.

Comparison of modes of feedback on glide performance in swimming

The software product “GlideCoach” was recently developed to give quantitative and qualitative feedback on the glide performance of a swimmer (Naemi & Sanders, 2008). This study compared the effect of feedback on glide performance from GlideCoach with video and verbal feedback. Nineteen elite swimmers were randomly assigned to one of three groups: Group 1 and 2 included six swimmers and Group 3 included seven swimmers. All participants performed ten dives in each of five sessions. Each group received one of three forms of feedback (video, video and verbal, and GlideCoach and verbal) for four sessions. In the fifth, retest session, performed 4 weeks after the fourth session, all groups received GlideCoach and verbal feedback only. This enabled the analysis of GlideCoach and verbal feedback on performance of the groups that had not yet received this feedback and assessment of the retention ability for the group that had. Feedback resulted in all groups recording an improvement, as indicated by effect sizes, for average velocity, glide factor (related to resistive drag), and initial velocity (P < 0.05). The improvement following the GlideCoach and verbal feedback was greater than that of the two other feedback methods for all variables of interest (P < 0.05), with effect sizes ranging from 1.0 to 2.5, compared with values less than 0.6 for the other feedback methods. We conclude that GlideCoach feedback is effective in improving glide performance.

Reorganization of the Poiseuille profile in nonisothermal flows in the reactor

The change in the temperature and velocity profile of the gas flow in a cylindrical reactor with variation in the reactor-wall temperature has numerically been investigated. It has been shown that the use of the Poiseuille profile in calculating the transient temperature profile of such a flow is quite justified. The appearance of two extrema of the radial velocity of the indicated flow on its heating (cooling) has been predicted. The maximum radial flow velocity reaches several percent of its initial average velocity.

Influence of geometry on liquid oxygen magnetohydrodynamics

Magnetic fluid actuators have performed well in industrial applications, but have a limited temperature range due to the freezing point of the carrier fluid. Liquid oxygen (LOX) presents a pure, paramagnetic fluid suitable for use in a cryogenic magnetic fluid system; therefore, it is a potential solution to increasing the thermal range of magnetic fluid technology without the need for magnetic particles. The current study presents experimental work regarding the influence of geometry on the dynamics of a LOX slug in a 1.9 mm quartz tube when pulsed by a solenoid in a closed volume. A numerical analysis calculated the optimal solenoid geometry and balanced the magnetic, damping, and pressure forces to determine optimal slug lengths. Three configurations comprised the experiment: (1) a 24-gauge wire solenoid with an optimized 2.7 cm length slug, (2) a 30-gauge wire solenoid with an optimized 1.3 cm length slug, and (3) a 30-gauge wire solenoid with a nonoptimized 2.5 cm length slug. Typically, the hydrodynamic breakdown limit is calculated and used to determine the system range; however the experiment showed that the hydrodynamic breakdown limit was never reached by the slug. This implied that, instead, the system range should factor in a probabilistic risk of failure calculated as a function of the induced pressure change from its oscillations. The experimental data were also used to establish a nondimensional relationship between the maximum displacement and initial magnetic pressure on the slug. The average initial velocity of the slug was found to be proportional to the initial magnetic pressure, Mason number, and slug length. The results of this study can be used in the design and optimization of a LOX fluid system for space or low-temperature applications.

Heating of ions by low-frequency Alfven waves

This paper reports a new physical mechanism that enables the heating of ions by a low-frequency parallel propagating Alfven wave of finite amplitude in a low beta plasma. The heating does not rely on ion cyclotron resonance. The process has two stages: First, ions, whose initial average velocity is zero, are picked up in the transverse direction by the Alfven wave and obtain an average transverse velocity. Second, at a given location the parallel thermal motions of ions produce phase differences (randomization) between ions leading to the heating of ions. The randomization (or heating) process saturates when phase differences are sufficiently large. The time scale over which ions are significantly heated is ∼π∕(kvth) (vth is the initial ion thermal speed and k is the wave number). The heating is dominant in the perpendicular (to the background magnetic field) direction. Subsequently, a large ion temperature anisotropy is produced. During the heating process, ions are also accelerated in the parallel direction and obtain a bulk flow speed along the background magnetic field.

Experimental and Numerical Analysis of Turbulent Opposed Impinging Jets

A fundamental study of two turbulent, directly opposed impinging jets in a stagnant ambient fluid, unconfined or uninfluenced by far-field walls, is presented. By experimental investigation and numerical modeling the fundamental characteristics of direct impingement of two turbulent axisymmetric round jets under seven different geometrical and flow-rate configurations (L * = L/d = {5, 10, 20}, where L is nozzle to nozzle separation distance and d is nozzle diameter, and Re = ρU 0 d/μ = {1500, 4500, 7500, 11000}, where ρ is fluid density, μ is dynamic viscosity of fluid, and U 0 is average initial velocity of fluid) are discussed. Flow visualization and velocity measurements performed using various laser-based techniques have revealed the effects of Reynolds number Re and dimensionless nozzle to nozzle separation L * on the complex flow structure. Similarity analysis of the initial freejet development and developing radial jet found Re = 11 x 10 3 and L * = 20 as the only case where the freejets exhibit a self-preserving development

Translational and rotational excitation of the CO2(000) vibrationless state in the collisional quenching of highly vibrationally excited 2-methylpyrazine: Kinetics and dynamics of large energy transfers

The relaxation of highly vibrationally excited methylpyrazine (C5N2H6) by collisions with CO2 molecules has been investigated over the temperature range 243–364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot methylpyrazine (E′=40 987 cm−1) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=58–80) of the 0000 ground state of CO2 resulting from collisions with hot methylpyrazine were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 0000 state. In addition, the temperature dependence of the state resolved, absolute rate constants for collisions populating high J states of CO2 was determined. The rotational population distributions, distributions of recoil velocities, and quenching rates for production of CO2 high J states (J=58–80) exhibit a very weak temperature dependence. The slight temperature dependence indicates that CO2 molecules which scatter into high J states of the ground vibrationless level originate from rotational levels near the mean of the precollision thermal rotational distribution. A gap law model is used to estimate the average initial rotational state and velocity of the CO2 bath, which allows for the calculation of the energy transfer magnitudes, ΔE. The measured energy transfer probabilities which are indexed by final bath state are resorted as a function of ΔE to create the energy transfer distribution function, P(E,E′) from E′−E∼1500–6000 cm−1. P(E,E′) is fit to both single exponential and biexponential functions to extract a value for the average energy transferred in a single collision of methylpyrazine and CO2. This average energy transfer value is compared to donor loss energy transfer studies as well as previous bath energy gain studies on the pyrazine/CO2 and C6F6/CO2 systems. On average, methylpyrazine donates more energy per collision to CO2 than pyrazine but not as much as C6F6; however, methylpyrazine has the lowest probability for single collision energy transfers larger than 2000 cm−1 of the three molecules studied using this technique.

Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

. Dynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation threshold, the ablated mass, and the particle size were investigated. The ablation plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm2 the average initial velocity of the ejected particles was deduced from the extension of the plume to range from 120–400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopically determined absorption coefficient of cornea αcornea. Where absorption due to proteins is high (at λ=6.2 and 6.5 μm), ablated mass, velocity and recoil momentum behave according to αcornea. For the first time, variations of the ablation plume from pulse to pulse were observed. Those, as well as the particle size, not only depend on the absorption coefficient, but also on the predominant absorber.

A theory of 1/f noise

Universal 1/f noise is shown to originate from the variations in the initial velocities of conduction electrons after collisions with defects. Defect atoms, after being struck by conduction electrons, recoil and form an impacted mass of lattice atoms. Conduction electrons emitted after a collision have an average initial velocity which steadily decreases as the impacted mass builds up. It is shown that this process yields a noise spectrum of the form f−n where n is in the vicinity of one and has an amplitude given by the Hooge empirical formula. Variations from the Hooge result are discussed. The model described applies to all semiconductors and metals.

Numerical simulation of dynamical production of vortices by critical and subcritical bubbles.

We investigate the dynamics of collisions of bubbles formed in a first order phase transition in 2+1 dimensions and study the production of vortices. A numerical simulation is carried out by prescribing bubble configurations as part of the initial data and then evolving by equations of motion. We study nonuniformities in the vortex configuration caused by asymmetries in bubble collisions and find that it can lead to large vortex velocities (0.3c--0.6c). We also simulate the vortex production in a phase transition by randomly nucleating bubbles at various time steps and study the distribution of vortices produced. Our results show that initially a relatively large number of vortices form. Some vortex-antivortex pairs annihilate with the final number of vortices being consistent with the usual estimates based on the Kibble mechanism. The average initial velocity of the vortices is found to be \ensuremath{\simeq}0.4c. We have investigated the dynamics of subcritical bubbles which shows many novel features. We find that a collapsing subcritical bubble bounces back after each collapse and can lead to the formation of a vortex if two (or more) critical bubbles collide with it with appropriate values of Higgs phases. This vortex has a large velocity due to the large momentum of the critical bubble walls and eventually escapes into the false vacuum. Our results have implications for cosmic strings as well as for certain condensed matter systems (such as liquid crystals) which may provide a testing ground for these results (when viscosity, etc., are properly accounted for).

Mechanical effects of erbium:YAG laser bone ablation.

When intending to use pulsed erbium:YAG laser radiation for the ablation of small bones, e.g., the ossicular chain or footplate, not only temperature and thermal damage, but also mechanical side effects become important. In studies on desiccated bone the total recoil momentum caused by laser ablation was measured using a sensitive pendulum. The momentum depends linearly on the radiant energy of the laser pulse with a slope of 10(-4) kg m s-1/J. The temporal course of the recoil force during ablation observed by means of a piezoelectric transducer is nearly proportional to the laser pulse intensity. By evaluating recoil momentum and mass loss data, the average initial velocity of the plume was calculated to be 230 to 280 m s-1. The kinetic energy is approximately 1% of the input energy. The measurements support to the hypothesis of a thermally induced explosive process.