جستجوی مقالات مرتبط با کلیدواژه « برخورد » در نشریات گروه « مکانیک »

The interaction of thruster plumes with satellite components can have undesirable effects, such as disturbance force/torque, thermal loading, and species deposition in the surfaces. The purpose of this paper is to use the Direct Simulation Monte Carlo (DSMC) method to analyze the 3D plume impingement flows and investigate its effects. Two impingement problems are computed. The impact of a jet of nitrogen on an inclined flat plate is considered. Good agreement is found between surface quantities calculated by DSMC and experimental data. The plume of a hydrazine control thruster firing in a model satellite configuration is simulated. Surface quantities and net impingement effects are calculated. The effects of partial displacement of the thruster locations on the results have also been investigated. The results show that a 20% displacement of the thruster location can change the disturbance force/torque by up to 15% of the initial values.

One of the very important cooling processes in industries is cooling via droplet spraying. In this research, the impact of Water-Silica nanofluid drop on a hot surface in the film boiling regime is studied. Governing equations are incompressible continuity, momentum and energy equations. The level set method is used for interface tracking and the ghost fluid method is used to impose discontinuities at the interface. The effect of adding nanoparticles and also impact velocity on the droplet contact time, droplet spreading and heat transfer from surface is investigated. Droplet spreading and heat transfer rate increase by an increase in impact velocity. But, the effect of impact velocity on the droplet contact time is negligible. Droplet contact time and spreading increase by an increase in nanoparticle’s volume fraction. Nanoparticles addition has more effect at higher impact velocities. Volume fraction enhancement, increases heat transfer rate and total heat dissipated from the surface.

Sandwich panels are used due to the ability of the core to absorb energy efficiently to Damped the kinetic energy from collision and exposure to the explosive wave. In the present study, the effect of the orientation of the core of honeycomb sandwich panel to the lateral plates and the loading direction on the absorption and dissipation of energy due to the cabin lift with a sandwich panel embedded in the elevator hole has been investigated. Six angles of 0, 5, 10, 20, 30 and 45 degrees were used for this purpose. It was observed that by increasing the angle of orientation of the panel core, the stiffness of the panel will fall, and this decrease will occur with a greater increase in the angle of the more orientation of the core strongly. By increasing the amount of honeycomb core angle from the optimal limit, the structure will experience compression and due to large plastic strains, which causes the stiffness of the panel to rise and the panel acts as a rigid body due to its incompressibility and its energy depreciation is severely reduced.

In this study, the effects of impact of a projectile on a fuel tank are studied using the finite element method and compared with experimental method. Due to penetration of the bullet into the tank, large internal pressures from the fluid are imposed on the tank's walls which can damage it. The considered fluid structure interaction (FSI) problem is solved in an Eulerian-Lagrangian reference frame by using the LS-Dyna software. By comparing of the results obtained from the simulations and the experimental data, it can be seen that the LS-Dyna software is able to model the different phases of event accurately. In previous researches mostly the penetration and cavitation phases are investigated numerically. In this paper all phases namely penetration, cavitation, stresses applied to tank’s walls and bullet exit are investigated. The comparison between the Von Mises stress of walls in the fluid-filled tank and the empty one signifies 30 percent growth of the maximum Von Mises stress in the wall of the fluid-filled tank compared to the walls of the empty tank. Also in addition to what has been done in previous numerical works, the failure mode of fluid-filled tanks are determined numerically. The numerical results show that because fluid-filled tank walls are pre-stress due to the fluid shock waves, the failure mode of fluid-filled tank is quite different with the failure mode of the empty one.

Analysis of multibody mechanical systems using computational dynamics has been greatly developed recently. When these systems operate at higher speeds and under impact conditions, the assumption that the links behave as rigid bodies does not lead to exact analysis and the energy dissipation due to the internal damping of flexible bodies can make considerable effect on the behavior of the system. This issue becomes more important under impact conditions. In this paper, considering the internal damping of the flexible coupler of a high-speed crank-slider mechanism under impact conditions, the effect of different parameters on the reduction of vibration – which is due to contact forces - is investigated using multibody system dynamics.
The internal damping of the coupler is described by Rayleigh’s proportional damping model. Having derived the constrained equations of motion by Euler-Lagrange augmented method, the effect of flexibility and internal damping is presented for the crank-slider mechanism with the flexible coupler under impact conditions. In addition, the effect of the material type on the internal damping and vibration reduction is carefully investigated.

In this paper, elasto-plastic time dependent impact of high-speed projectile on water surface is simulated numerically using Arbitrary Lagrangian- Eulerian (ALE) method. The projectile is considered as elasto-plastic solid and its mesh is generated by Lagrangian approach. The water is also assumed as compressible fluid so its mesh is produced by Eulerian method. Three steps simulation are performed in this research; static, dynamic stress analysis and also impact analysis of full degrees of freedom (DOF) projectile on water surface using ALE method. The effects of fluid compressibility and cavitation are considered in last analysis. In order to validate results, the stress wave propagation produced in the projectile due to water impact is compared with exact ones. The results show that the maximum error compare with exact ones is 5%. Also the magnitude of maximum stress and location/path of fracture in the projectile are compared with experimental data. The good agreement between the predicted and analytical/experimental values shows the accuracy of this numerical algorithm. The impact of projectile on water surface is simulated with different angles. The results show that the safe zone of impact angle for present projectile is ±0.5°.

In this paper, simultaneous impact of two parallel drops on a thin liquid film is investigated using the lattice Boltzmann method. The purpose of this study is to investigate the effects of surface tension (characterized by Weber number), distance between two drops, and gas kinematic viscosity on the impact. The developed numerical model in this paper which is based on the Shan and Chen pseudo-potential two-phase model makes it possible to access large density ratios, low viscosities, and tunable values of surface tension independent of the density ratio. The model is validated by comparing the coexistence densities with those of Maxwell analytical solution, evaluating the Laplace law for a droplet, and simulating single droplet impact on a thin liquid film. Simulation results of two drops simultaneous impact show that after impact, two jets raised between the drops join each other and form a central jet. Height of this jet increases with time leading to separation of secondary droplets from its tip. When the surface tension value is decreased, the central jet height is increased, but size of the separated droplets is reduced. The crown shape observed in single drop impact is also seen in simultaneous impact of two drops. Increasing distance between two drops leads to a smaller central jet height and an increase in the crown radius. The crown height, however, was found to be independent of the distance. Finally, increasing gas kinematic viscosity reduces the central jet rising speed and delays separation of secondary droplets from the jet.

The energy absorption characteristics of honeycomb sandwich cylindrical columns under axial crushing process depend greatly on the amount of material which participates in the plastic deformation. The interaction effects between the honeycomb core and the column skins greatly improve the energy absorption efficiency, which in turns depend on the shape of honeycomb core. Therefore, in this paper, the energy absorption characteristics of tapered honeycomb sandwich cylindrical columns with square, triangle, kagome and diamond cores under axial crushing loads are investigated by nonlinear finite element analysis. In this paper energy absorption, crushing force and interaction effect between skin and core for 4 models are investigated. Also energy absorption efficiency for tapered tube and cylindrical tube with similar core is investigated and the results are compared. In order to reduce the computational cost of crush simulations, finite element method is employed to formulate specific energy absorption and peak crushing force.

Nowadays, a great number of researches are done by scientists to provide some models that can predict the passenger injuries in crashes. In this paper, a hybrid model of vehicle and passenger is proposed to predict the head acceleration in the front crash. A lumped mass model with 12-degree-of-freedom (DOF) is firstly used to predict the behavior of vehicle in front crash. In this model, any member of vehicle is modeled as a lumped mass and connected to the other members through some springs and dampers. The unknown coefficients of such model are obtained using genetic algorithm to minimize the deviation between the results of experimental and suggested model. The parameters of model are established by experimental results of a real world car, namely the HONDA ACORD2011, in an accident velocity of 48 km/h. Also, the validity of the proposed model is checked by experimental results of mentioned vehicle at two other crash velocities of 40 km/h, and 56 km/h. The results show that the proposed model is an efficient framework for preliminary designing of both structure and parameter design of vehicle to improve its crash worthiness. Moreover, a multi-body dynamic model of driver is proposed to predict the head injury in front crash. The seat acceleration which has been calculated using vehicle’s model is considered as input of this model.

In this paper based on the plastic stress wave propagation theory, a new analytical model is presented for impact of the cylindrical projectiles on the rigid surface. In this analytical model, Johnson-Cook material behavior equation,Mie–Grueneisen equation of state, Johnson-Cook strain at fracture equation and kinematics equations of motion, are used. Also, the conservation equations of mass, momentum and energy in front of plastic stress wave are determined.For solving the governing equations, a computer program is developed. The values of stress, strain, strain rate,velocity and length and radius of rigid and deformed region of projectile were determined. Using new model proposed in this paper, the compressive stress at different plastic strain and strain rates were determined. The results of the new model have been compared with ABAQUS finite element simulations and available experimental results.It shows that the length and radius of deformed region of projectile predicted by the new model have good agreements with experimental results and ABAQUS FE simulations. Furthermore, the compressive dynamic stress is determined as a function of the final projectile dimensions and material behavior coefficients.