Obstacle Interaction Research Articles

Inert nanoparticle suppression of gas explosion in the presence of obstacles.

The suppressing effects of inert nanoparticles on methane–air explosion, in an obstructed chamber with internal dimensions of 150 mm × 150 mm × 500 mm, were experimentally investigated. To this end, the flame behaviors in the presence of obstacles as well as overpressure transients during the explosions with and without nanoparticles were compared. Additionally, the effects of density, diameter, and material of nanoparticles on the suppressing behaviors were analyzed as well. The results showed that the methane–air deflagrating flame remains generally light blue if the nanoparticles are added. In particular, the flame obstacle interaction may enhance the suppression effect of the nanoparticles, and the flame acceleration rate and the peak overpressure decrease significantly. Increasing explosion suppression is seen up to about 100 g m−3 particle density, but further increase in particle density, up to 150 g m−3, yields no further increase in the explosion suppression ability. And as the particle size decreases, the suppressing effect is more evident. The experiments also showed that Al(OH)3, Mg(OH)2, and SiO2 all can be used to suppress the flame propagation and overpressure. However, the metal hydroxides suppress the methane explosions even more efficiently than SiO2 particles; Al(OH)3 particles have a slightly better inhibiting effect than Mg(OH)2. Mechanisms for the observed phenomena were discussed.

An IB-LBM investigation into the aerodynamic coefficients in relation to the rotation intensity of a tornado-like wind

A tornadic wind is essentially considered as an airflow that simultaneously translates and rotates. Numerical simulations of this kind of hybrid flow remain inadequate due to many numerical difficulties, one of the major challenges consisting in the establishment of a set of boundary conditions that are, for the tornado–obstacle interaction scenario, both rational in physics and simple in numerical implementation. Inspired by the success of immersed-boundary (IB) lattice Boltzmann method (LBM) for simulations of fluid–structure interaction problems, this study proposes a new outlet of the IB-LBM framework for investigation of tornadic wind effects, featuring a reformed interpretation of the Rankine-Combined Vortex Model (RCVM) that considerably facilitates the boundary condition setup. Moreover, the main purpose of this study is to examine the tornadic wind loadings in relation to the rotation intensity of a tornado, and presents a practical Newton’s bi-section-like method for determining the critical rotation intensity beyond which the aerodynamic coefficients no longer increase when Reynolds number rises. This critical rotation intensity serves to characterize tornadic winds, such that the tornado with a rotation density below its critical value can be considered as mainly dominated by the translation part and, otherwise, the dominance no longer belongs only to the translational component of the tornado. Since it has been rather conventional that, when studying tornadoes, Reynolds number is determined using only the translation velocity as characteristic velocity, the present tornado study intends to suggest, through a number of numerical test series, that more attention be paid to the insufficiently explored rotational component, which physically tends to play a more dominant role when an intensive rotation is present in a tornado scenario.

On the propagation mechanism of a detonation wave in a round tube with orifice plates

This study deals with the investigation of the detonation propagation mechanism in a circular tube with orifice plates. Experiments were performed with hydrogen air in a 10-cm-inner-diameter tube with the second half of the tube filled with equally spaced orifice plates. A self-sustained Chapman–Jouguet (CJ) detonation wave was initiated in the smooth first half of the tube and transmitted into the orifice-plate-laden second half of the tube. The details of the propagation were obtained using the soot-foil technique. Two types of foils were used between obstacles, a wall-foil placed on the tube wall, and a flat-foil (sooted on both sides) placed horizontally across the diameter of the tube. When placed after the first orifice plate, the flat foil shows symmetric detonation wave diffraction and failure, while the wall foil shows re-initiation via multiple local hot spots created when the decoupled shock wave interacts with the tube wall. At the end of the tube, where the detonation propagated at an average velocity much lower than the theoretical CJ value, the detonation propagation is much more asymmetric with only a few hot spots on the tube wall leading to local detonation initiation. Consecutive foils also show that the detonation structure changes after each obstacle interaction. For a mixture near the detonation propagation limit, detonation re-initiation occurs at a single wall hot spot producing a patch of small detonation cells. The local overdriven detonation wave is short lived, but is sufficient to keep the global explosion front propagating. Results associated with the effect of orifice plate blockage and spacing on the detonation propagation mechanism are also presented.

Harnessing atomistic simulations to predict the rate at which dislocations overcome obstacles

Predicting the rate at which dislocations overcome obstacles is key to understanding the microscopic features that govern the plastic flow of modern alloys. In this spirit, the current manuscript examines the rate at which an edge dislocation overcomes an obstacle in aluminum. Predictions were made using different popular variants of Harmonic Transition State Theory (HTST) and compared to those of direct Molecular Dynamics (MD) simulations. The HTST predictions were found to be grossly inaccurate due to the large entropy barrier associated with the dislocation–obstacle interaction. Considering the importance of finite temperature effects, the utility of the Finite Temperature String (FTS) method was then explored. While this approach was found capable of identifying a prominent reaction tube, it was not capable of computing the free energy profile along the tube. Lastly, the utility of the Transition Interface Sampling (TIS) approach was explored, which does not need a free energy profile and is known to be less reliant on the choice of reaction coordinate. The TIS approach was found capable of accurately predicting the rate, relative to direct MD simulations. This finding was utilized to examine the temperature and load dependence of the dislocation–obstacle interaction in a simple periodic cell configuration. An attractive rate prediction approach combining TST and simple continuum models is identified, and the strain rate sensitivity of individual dislocation obstacle interactions is predicted.

Predicting the effects of microstructural features on strain localization of a two-phase titanium alloy

Abstract Strain localization influenced by microstructural features has an important effect on mechanical properties of α + β titanium alloy. To address the effect, a microstructure-based finite element model is established. In this model, regions for primary α (αp) and transformed β matrix (βt) are generated from real microstructures of a two-phase titanium alloy (TA15 alloy); the plastic flow behaviors of these two features are determined directly rather than from single phase alloy. A constitutive equation of αp is developed with consideration of dislocation–obstacle interaction, whereas the constitutive equation of βt is determined by nanoindentation tests. Finally, the calculated stress–strain responses of the alloy are verified by experiments. The simulated results show that strain localization bands (SLBs) have two morphologies: short and long-continuous SLBs. Lots of short SLBs appear mainly in βt and αp when the volume fraction of αp is small and moderate, respectively. Long-continuous SLBs appear mainly in αp when the volume fraction of αp is large. With the increase of αp in SLBs, the strength of the alloy decreases while the ductility increases. By decreasing the disparity of strength between αp and βt, the strain gradient in SLBs reduces and the ductility of the alloy increases.

Flow structure formed due to interaction of a supersonic jet with a porous obstacle

Results of studying impingement of a supersonic underexpanded air jet onto a finite-thickness porous metal obstacle whose frontal plane is normal to the jet axis and whose side surface is impermeable for the gas flow are reported. The case of a non-porous obstacle of the same diameter is considered for determining the effect of porosity on gas-dynamic characteristics of jet–obstacle interaction.

Numerical modeling of flow evolution for an internal solitary wave propagating over a submerged ridge

Numerical simulations are performed to investigate the flow evolution of a depression ISW propagating over a submerged ridge. A finite volume based Cartesian grid method is adopted to solve the Reynolds averaged Navier–Stokes equations using a k−ε model for the turbulent closure. Results reveal a significant transient clockwise vortex and an internal hydraulic jump emerging on the front slope of the obstacle, during the wave–obstacle interaction. This interaction generates an asymmetrical interface and pycnocline thickness on both sides of the obstacle. During this process, the amplitude and velocity of the leading waveform increase transiently, but both quickly decrease once the wave passes the obstacle. As the ridge slope decreases, the wave–ridge interaction weakens, as well as the strength of vorticity and turbulent kinetic energy. In addition, the wave may affect the nutrient transport on the front slope to a maximum depth about 50% height of the obstacle or about 1.78 times of the incident wave amplitude when a large ISW breaks on the slope.

The interaction of a Prandtl–Meyer wave and a quasi-one-dimensional flow region

A mathematical model for the interaction between a Prandtl–Meyer flow and a quasi-one-dimensional subsonic or supersonic stream is developed. The analytical description of the shape of the slipstream dividing two interacting streams is achieved, and its parametrical analysis is performed. The results of the study may be of practical use in supersonic jet flow–obstacle interaction analysis, which is important for the safety of space vehicle launches.

Simulation of Transmission Electron Microscope Images of Dislocations Pinned by Obstacles

From a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron microscope (TEM), the obstacle strength factor could be evaluated from pinning angles of dislocation cusps. We simulated this process: we produced a dislocation cusp by molecular dynamics simulation of interaction between an edge dislocation and a void or a hard precipitate in copper, and calculated the TEM image by multislice method. In two-beam conditions, cusp images showed inside-outside contrast depending on the sign of the diffracting vector and other variations with the specimen geometry. The pinning angles measured on TEM images ranged up to a few tens of degrees and were between the true angles for the two partial dislocations. Characteristics and contrast mechanisms of cusp images were discussed based on those of dislocation dipoles. (doi:10.2320/matertrans.MD201312) (Received September 3, 2013; Accepted October 2, 2013; Published November 15, 2013) Various crystal lattice defects induced by neutron irradi- ation, such as precipitates, voids, dislocation lines and loops are responsible for degradation in mechanical properties: increase in yield stress, loss of ductility, and increase in ductile-brittle transition temperature. A number of researches have been devoted for defect structural evolution and mechanical property changes under neutron irradiation for nuclear materials development. Understanding the mecha- nisms involved is necessary to construct models for estimating the lifetime of components of nuclear power plant. In an elementary process of dislocation-obstacle interac- tion, a gliding dislocation is pinned by obstacles and bows out to form arcs between the neighboring pinning points, which induces cusps on the dislocation at obstacles. The apex angle of the dislocation cusp is referred to as the pinning angle o. The dislocation breaks away by bypassing or cutting through the obstacle when the pinning angle reaches a critical value oc. Stronger obstacles have smaller critical angles. The obstacle strength factor i ¼ cosðoc=2Þ and the distance between the neighboring pinning points are the key parame- ters that relate the defect microstructure to the change in macroscopic mechanical properties. There have been re- ported a few attempts to evaluate the factor from a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron micro- scope (TEM). 1­3) The method has not been widely applied so far, due to high technical levels required for in situ experiments. Another difficulty comes from TEM images with a limited resolution both in time and space for measuring pinning angles of radiation-induced obstacles, typically less than a few nanometers in size, at a moment of the breakaway. Alternatively, molecular dynamics (MD) simulations have been applied extensively for the dislocation- obstacle interaction. 4­7) In the present study, we performed TEM image simulation of dislocation cusps to examine whether TEM images reveal the cusp structure in the scale suitable for evaluating the obstacle strength factor. For this purpose, we stopped MD simulation of interaction between an obstacle and an edge dislocation just before the breakaway. We then calculated TEM images of dislocation cusps under various conditions using the multislice method. We compared apex angles between the cusp structure and on the calculated images, which would support the experimental evaluation of the obstacle strength factor.

On the ground-vehicle induced flows and obstacle interaction for energy harvesting purposes

CFD simulations were conducted with the aim of studying the flow field induced by a road vehicle moving at high speed for energy harvesting purposes. 2D and 3D approaches were used and focus was put on the interaction between the vehicle and an obstacle placed upon the road simulating the shape of a generic harvesting device. 2D RANS simulations have proved that a separated region forms and that a trailed flow exists upon the vehicle which thickness do not exceed 1.1 times the vehicle height. The presence of trailing vehicles was analyzed and the raising of the separation region evaluated. The 3D steady approach showed that 2D modeling did not adequately predict velocity and pressure values even if general trends were captured. Different panel shapes were then compared to assess the effect of placing an obstacle near the moving vehicle. 2D URANS simulations suggested that to overcome an obstacle placed 1m above the vehicle roof, an energy boost is required, while 3D unsteady calculations showed negligible variations in the vehicle's drag with approaching the obstacle. Furthermore, simulation of the full 3D domain allowed to identify forces time-histories and loading directions acting on a circular cylinder, with definition of an equivalent uniform velocity to characterize the flow impinging on the obstacle. Results showed that force coefficients and equivalent velocity maximums locate in the vehicle nose region, where inviscid-like effects dominates.

Analysis of strengthening in AA6111 during the early stages of aging: Atom probe tomography and yield stress modelling

In this work, a series of aging treatments has been conducted on AA6111 alloy samples for various times at ambient temperature (so-called natural aging) and at temperatures between 60 and 180°C (artificially aged). The time at artificial ageing was chosen such that samples with approximately the same yield stress were produced. The microstructures of these alloy samples have been carefully characterized using atom probe tomography together with advanced cluster-finding techniques in order to obtain quantitative information about the changes in distribution of both the solute clusters and early-stage precipitates that are formed. The size distribution of clusters has been mapped onto the glide plane and then the stress necessary for a dislocation to pass through the range of obstacles has been estimated using an areal glide model where the dislocation–obstacle interaction strength has been assumed to be related to the obstacle size on the glide plane. It is demonstrated that the contribution of cluster strengthening during artificial aging at higher temperatures is dominated by the high number density of small clusters (Guinier radius <1nm), whereas the situation during room temperature natural aging is more complex.

Indentation-based assessment of the dependence of geometrically necessary dislocations upon depth and strain rate in FCC materials

In the present paper, an assessment is presented of the indentation depth and strain rate dependency of “geometrically necessary” dislocations. Pyramidal indentation tests were performed at various loading rates from 1 to 1000mN/s on annealed samples of pure copper, 70/30 brass, and 5052 aluminum alloy to study the effect of indentation strain rate on the indentation depth dependence of the average indentation stress. The model of Nix and Gao was applied to calculate the density of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) as a function of indentation depth. The GND density displayed the characteristic decrease with increasing h. The average indentation stress, σind was observed to decrease with increasing h and, for any given h increase with increasing loading rate. This observed dependence of σind upon ε̇ind was analyzed for the data obtained from small indentation depths, up to 800nm, to assess the operative mechanism of time-dependent deformation associated with the GNDs. It was observed that for the high and medium stacking fault energy (SFE) 5052 aluminum and pure copper, the thermal activation energy ΔGThermal of the deformation rate followed essentially the same dependence upon σind regardless of ε̇ind, however in the 70/30 brass test material, which possess a lower value of SFE, the ΔGThermal showed a dependence upon σind that was highly strain rate dependent. In the case of the high SFE material, the apparent activation volume, V⁎ of the deformation process was found to decrease with increasing ρGNDs in a way indicative of deformation occurring by a process that is listed by dislocation/dislocation interactions. Our data indicate that in fcc materials of low SFE (i.e. 70/30 brass) the deformation during nano/micro-indentation occurs by a more complex mechanism than simple time dependent dislocation glide limited by dislocation–obstacle interaction. The operative deformation mechanism most likely involves micro-twinning and geometrically necessary twins (GNTs).

Lagrangian vortex sheets for animating fluids

Buoyant turbulent smoke plumes with a sharp smoke-air interface, such as volcanic plumes, are notoriously hard to simulate. The surface clearly shows small-scale turbulent structures which are costly to resolve. In addition, the turbulence onset is directly visible at the interface, and is not captured by commonly used turbulence models. We present a novel approach that employs a triangle mesh as a high-resolution surface representation combined with a coarse Eulerian solver. On the mesh, we solve the interfacial vortex sheet equations, which allows us to accurately simulate buoyancy induced turbulence. For complex boundary conditions we propose an orthogonal turbulence model that handles vortices caused by obstacle interaction. In addition, we demonstrate a re-sampling scheme to remove surfaces that are hidden inside the bulk volume. In this way we are able to achieve highly detailed simulations of turbulent plumes efficiently.

Passive and active acoustic properties of a diaphragm at low Mach number

In this paper a low noise flow duct facility that performs a multimodal characterization of the passive and active acoustic properties of obstacles in the aero-acoustic conditions of an automotive air conditioning system is presented. The experimental procedure is made of two steps. In the first one the passive data, i.e. the multimodal scattering matrix, is measured with a multi-sources method. In the second step the outgoing modal pressure spectra radiated upstream and downstream by the flow obstacle interaction source is achieved. A numerical simulation of the experiment based on a 3D-Finite Element Method is developed. A very good agreement between the theoretical and experimental results of the no flow scattering matrix of a diaphragm is found. The effect of the flow on the scattering matrix is shown before performing the measurement of the aero-acoustic source characteristics of the diaphragm.

Amplitude decay and energy dissipation due to the interaction of internal solitary waves with a triangular obstacle in a two-layer fluid system: the blockage parameter

Experiments were carried out in a wave flume to examine the propagation of depression-type internal solitary waves (ISWs) over a submerged triangular-shaped obstacle. We observed the distortion and breaking of the ISW depression in a two-layer stratified fluid system as induced by the obstacle on the bottom. Physical factors, such as the amplitude of the wave, the thickness of the two layers, and the height of the submerged obstacle were utilized in our analysis of the wave–obstacle interaction. Wave breaking was determined in relation to input parameters arranged before the experimental runs. A classification scheme based on the blockage parameter ζ was proposed. The various degrees of ISW-obstacle interaction were visually classified with three schematic forms (ζ < 0.55 for a weak encounter; 0.55 < ζ < 0.7 for a moderate encounter; and 0.7 < ζ for wave breaking). Furthermore, the reflection and transmission coefficients were utilized to explore the resultant influence on changes in amplitude and energy of an ISW. The wave characteristics (e.g., amplitude-based wave reflection, energy-based reflection, amplitude-based transmission, and energy-based transmission) during the wave–obstacle interaction showed an approximately linear relation with the blockage parameter. Influenced by the submarine obstacle, the transmitted waves were found to always consist of a leading pulse (solitary wave) followed by a dispersive wave train. Moreover, due to bottom friction the energy budget of the leading pulse was reduced by a factor of at least 5% which could approach 50% in cases with strong breaking.

Flow–obstacle interaction in rapid granular avalanches: DEM simulation and comparison with experiment

This paper investigates the interaction between rapid granular flow and an obstacle. The distinct element method (DEM) is used to simulate the flow regimes observed in laboratory experiments. The relationship between the particle properties and the overall flow behaviour is obtained by using the DEM with a simple linear contact model. The flow regime is primarily controlled by the particle friction, viscous normal damping and particle rotation rather than the contact stiffness. Rolling constriction is introduced to account for dispersive flow. The velocity depth-profiles around the obstacles are not uniform but varying over the depth. The numerical results are compared with laboratory experiments of chute flow with dry granular material. Some important model parameters are obtained, which can be used to optimize defense structures in alpine regions.

Fluid flow interaction with an obstacle near free surface

The two-dimensional problem of thin obstacle interaction with compressible fluid flow near free surface under microgravity was regarded. The fluid was assumed occupying infinite semi-space, gravity was neglected as compared with fluid inertia. The obtained exact solution shows that for relatively thin layer the resistance force does not depend on the layer thickness, but depend on body length and inclination angle, and surpasses twice the respective values for the infinite medium, which could be explained by additional wave resistance arising in the presence of free surface.

Blast response of a lined cavity in a porous saturated soil

The paper proposes a comprehensive approach to simulate the blast response of a lined cavity in a porous soil. To calculate the soil–structure contact pressure, the coupled Godunov-variational-difference approach was developed. The lining is modeled by a Timoshenko elastic–plastic shell with kinematic linear hardening. To solve the problem in the lining domain, the variational-difference method is applied. The soil is modeled by the Lyakhov three-phase model that takes into account both bulk and shear elastic–plastic behavior, including the effect of soil pressure on the yield strength for the stress tensor deviator. The problem of blast wave propagation within the soil medium is solved by the Godunov method. The coupled approach to calculate the soil–lining contact pressure is based on the relationships on the shock and rarefaction waves with finite-difference equations of the shell motion using a simple iteration method. It allows the reduction of the contact problem to the self-similar symmetric Riemann problem. Solution of the problem of an explosion in a porous medium, and analysis of the soil–obstacle interaction under the blast action using the proposed method show good correspondence with available experimental results. Also, the plane problem of blast response of the circular cavity lined by a thin steel lining was solved. The effect of the gas volumetric content in the soil on the incident shock wave pressure as well as on the contact pressure and lining meridian strain was studied.

Rapid phase transformation kinetics on a nanoscale: Studies of the α → β transformation in pure, nanocrystalline Ti using the nanosecond dynamic transmission electron microscope

Using the unique capabilities and high time resolution of dynamic transmission electron microscopy (DTEM), the fast kinetics of α → β-phase transformation in nanocrystalline Ti films were investigated using single-shot electron diffraction and bright-field TEM images. From quantitative analysis of the diffraction patterns, the transformation rates were determined for temperatures between transition start (1155 K) and melt temperature (1943 K). The experimental data were summarized in a time–temperature-transformation (TTT) curve with nanosecond time resolution. Theoretical TTT curves were calculated using analytical models for isothermal martensite and available thermodynamic data. Above 1300 K, there is excellent agreement between the experiment and the discrete–obstacle interaction model, suggesting that the nucleation rate and thermally assisted motion of the martensite interface are controlled by interface–solute atom interactions. However, theory predicts much slower transformation rates near the transition temperature than experiment. Experimental data fits using the Pati–Cohen model suggests that an increase in autocatalytic nucleation may partially account for the fast transformation rates at lower temperatures.

On the features of dislocation–obstacle interaction in thin films: large-scale atomistic simulation

Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in thin films have a number of remarkable features. A particular example is the interaction between a screw dislocation and a stacking fault tetrahedron (SFT) in Cu, which can be directly compared with in situ observations of quenched or irradiated fcc metals. If the specimen is thin, the dislocation velocity is slow, and the temperature is high enough, a segment of the original SFT can be transported towards the surface via a double cross-slip mechanism and fast glide of an edge dislocation segment formed during the interaction. The mechanisms observed in the simulations provide an explanation for the results of in situ straining experiments and the differences between bulk and thin film experiments.