Free-surface granular flows down heaps

The present paper extends the granular-flow constitutive model of Savage (1998 J Fluid Mech 377:1–26) to treat spherical particles. Savage accounted for both quasi-static and collisional stresses by considering: (i) strain-rate fluctuations embodied in a critical state plasticity model, as well as, (ii) individual particle velocity fluctuations modelled by granular-flow kinetic theory. In the present work, the governing equations of the kinetic theory of Jenkins (1998 In: Hermann HJ, Luding S (eds) Physics of Dry Granular Media. Kluwer Academic pp. 353–370) for identical spherical, smooth, inelastic particles are supplemented with additional quasi-static terms that have forms patterned after the corresponding terms in the equations of Savage for two-dimensional disk-like particles. The resulting equations along with side-wall and free-surface boundary conditions are applied to examine free-surface granular flow down a heap contained between two frictional vertical side walls. Width-averaged equations of motion are integrated to obtain depth profiles of mean velocity, granular temperature, solids fraction and the Savage–Jeffrey parameter. Detailed comparisons are made with particle-tracking experiments. When the gap between the vertical side walls is fairly narrow, good agreement is found between the predicted and the measured profiles of mean velocity and granular temperature.     

Effect of impact energy on the shape of granular heaps

We study experimentally the shape of a granular heap formed pouring a granular material into a vertical Hele-Shaw cell and analyze the effect of the grain impact energy. We propose a continuous model for the steady profile of the heap that explicitly considers energy dissipation of flowing grains through inelastic collisions. We solve the model analytically and analyze the resulting height profile as a function of several parameters, such as the restitution coefficient of the grains and their impact energies. We find good agreement between theory and experiments. Received: 9 June 1999     

Influence of pressure distribution on flow field temperature reconstruction

This research proposes an issue that has previously been omitted in flow field temperature reconstruction by optical computerized tomography (OCT). To prove that it is not reasonable to always assume an isobaric process occurs when OCT is adopted to obtain the temperature distributions of flow fields, a propane-air flame and an argon arc plasma are chosen as two practical examples for experiment. In addition, the measurement of the refractive index is achieved by moiré deflection tomography. The results indicate that the influence of pressure distribution on temperature reconstruction is a universal phenomenon for various flow fields. Hence, the condition that can be introduced to estimate when an isobaric process can no longer be assumed is presented. In addition, an equation is offered to describe the temperature reconstruction imprecision that is caused by using the supposed pressure instead of the practical pressure.     

Influence of obstacles on rapid granular flows

Summary. One means of preventing areas from being hit by avalanches is to divert the flow by appropriately constituted obstacles. Thus, there arises the question how a given avalanche flow is modified by obstructions and how the diverted flow depth and direction emerge. In this paper rapid gravity-driven dense granular flows, partly blocked by obstacles with different shapes, sizes and positions, are numerically investigated by solving the hyperbolic Savage-Hutter equations with an appropriate integration technique. The influences of the obstructions on the granular flows are graphically demonstrated and discussed for a finite mass and a steady inflow of granular material down an inclined plane, respectively. These flows are accompanied by shocks induced by both the presence of the obstacles and the transition of granular flows from an inclined surface into a horizontal run-out zone when the velocity transits from its supercritical to its subcritical state. The numerical results show that the theory is capable of capturing key qualitative features, such as shocks wave and particle-free regions.     

Effects of wall roughness on pressure loss and velocity distribution for air in fixed and moving granular layers

Increase in friction between the material and the wall under counterflow conditions is accompanied by a reduction in the pressure loss and an increase in the nonuniformity of the gas flow in a horizontal cross section of the bed.Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 1, pp. 107–116, January, 1972.     

Influence of geometry on polychromatic speckle contrast

Understanding speckle behavior is very important in speckle metrology application. The contrast of a polychromatic speckle depends not only on surface roughness and the coherence length of a light source, as shown in previous works, but also on optical geometry. We applied the Fresnel approach of diffraction theory for the free-space geometry and derived a simple analytical relationship between contrast, coherence length, size of illuminated spot, and distances between source, object, and observation plane. The effect of contrast reduction is found to be significant for low-coherence light sources.     

The influence of particle geometry and the intermediate stress ratio on the shear behavior of granular materials

The behavior of granular materials is very complex in nature and depends on particle shape, stress path, fabric, density, particle size distribution, amongst others. This paper presents a study of the effect of particle geometry (aspect ratio) on the mechanical behaviour of granular materials using the discrete element method (DEM). This study discusses 3D DEM simulations of conventional triaxial and true triaxial tests. The numerical experiments employ samples with different particle aspect ratios and a unique particle size distribution (PSD). Test results show that both particle aspect ratio (AR) and intermediate stress ratio \((b=({\upsigma }_{2}'-{\upsigma }_{3}')/({\upsigma }_{1}'-{\upsigma }_{3}'))\) affect the macro- and micro-scale responses. At the macro-scale, the shear strength decreases with an increase in both aspect ratio and intermediate stress ratio b values. At the micro-scale level, the fabric evolution is also affected by both AR and b. The results from DEM analyses qualitatively agree with available experimental data. The critical state behaviour and failure states are also discussed. It is observed that the position of the critical state loci in the compression \((e-p')\) space is only slightly affected by aspect ratio (AR) while the critical stress ratio is dependent on both AR and b. It is also demonstrated that the influence of the aspect ratio and the intermediate stress can be captured by micro-scale fabric evolutions that can be well understood within the framework of existing critical state theories. It is also found that for a given stress path, a unique critical state fabric norm is dependent on the particle shape but is independent of critical state void ratio.     

Influence of stack geometry on the performance of thermoacoustic refrigerator

The work reported in this paper is focused on the performance of a thermoacoustic refrigerator under various operating conditions. The experiments were conducted with various stack geometries fabricated with epoxy glass and Mylar material. Four stacks with different pore sizes are used to evaluate the performance of the refrigerator. Stack 1 has parallel plates of Mylar material 0.12 mm thick spaced 0.36 mm apart. Stacks 2, 3 and 4 are made of epoxy glass with pores of circular cross-section having 1, 2 and 3 mm diameter, respectively. The entire resonator system was constructed from aluminium material coated with polyurethane material from inside to reduce conduction heat losses. Helium gas was used as a working fluid. The experiments were conducted with different drive ratios ranging from 1.6% to 2% with varying cooling load from 2 to 10 W. For the experiments, operating frequencies from 200 to 600 Hz with mean pressure varying from 2 to 10 bar in steps of 2 bar each were considered. The temperatures of the hot end and cold end of the heat exchangers were recorded using RTDs and a data acquisition system under various operating conditions. The coefficient of performance (COP) and relative COP (COPR) are evaluated. Results show that COP of the refrigerator rises with increase of cooling load and decreases at higher drive ratio. It was also observed that the temperature difference between the hot end and cold end of the stack is higher at 2 W cooling load for 400 Hz operating frequency. The temperature difference between the hot end and cold end of the stack was observed to be 19.4, 17.2, 14 and 12.4°C for stacks 1, 2, 3 and 4, respectively, for 10 bar mean pressure and 2 W cooling load. The temperature difference and COP of the parallel plate stack are better compared with other stack geometries.     

几何参数对线性压缩机板弹簧性能的影响

在设计了11款不同基圆半径、厚度及涡旋槽偏心距的圆渐开线型板弹簧的基础上,利用ANSYS有限元分析软件,对上述板弹簧的刚度及应力特性进行了分析与比较,总结出了主要几何参数对板弹簧性能的影响.     

Improvement of pressure distribution to arbitrary geometry with boundary condition represented by polygons in particle method

The boundary condition represented by polygons in the moving particle semi‐implicit method can accurately represent geometries and treat complex geometry with high efficiency. However, inaccurate wall contribution to the Poisson's equation leads to drastic numerical oscillation. To address this issue, in this research, we analyzed the problems of the Poisson's equation used in the boundary condition represented by polygons. The new Poisson's equation is proposed based on the improved source term (Tanaka and Masunaga, Trans Jpn Soc Comput Eng Sci, 2008). The asymmetric gradient model (Khayyer and Gotoh, Coastal Engineering Journal, 2008) is also adopted to further suppress the numerical oscillation of fluid particles. The proposed method can dramatically improve the pressure distribution to arbitrary geometry in three dimensions and keep the efficiency. Four examples including the hydrostatic simulation, dam break simulation, and two complex geometries are verified to show the general applicability of the proposed method. Copyright © 2017 John Wiley & Sons, Ltd.     

Influence of particle shape on sheared dense granular media

We study by means of molecular dynamics simulations of periodic shear cells, the influence of particle shape on the global mechanical behavior of dense granular media. At large shear deformation samples with elongated particles, independent of their initial orientation, reach the same stationary value for both shear force and void ratio. At the micro-mechanical level the stress, the fabric and the inertia tensors of the particles are used to study the evolution of the media. In the case of isotropic particles the direction of the principal axis of the fabric tensor is aligned with the one of the principal stress, while for elongated particles the fabric orientation is strongly dependent on the orientation of the particles. The shear band width is shown to depend on the particle shape due to the tendency of elongated particles to preferential orientations and less rotation.     

Influence of pore geometry on the effective response of porous media

The generalized method of cells (GMC) is used to study the influence of pore geometry on the effective elastic properties and inelastic response of porous materials. Periodic microstructures with four distinct pore geometries are studied and the results for effective elastic properties are compared with several other available models and experimental results. Predictions for the inelastic response of porous alumina are presented for tensile loading, as a function of pore geometry and pore volume fraction, with the inelastic behavior of the bulk material modeled using a unified visco-plasticity theory. All results are presented for discrete pore shape and discrete porosity. It is shown that pore geometry can have a significant influence on both elastic and inelastic response, that pore geometry can be associated with parameters from other models, and that the generalized method of cells is an efficient, flexible and reliable method of analysis for such problems.     

Influence of target geometry on the parameters of a laser-emission discharge

The relationship between the geometry of a plasma-forming target and the parameters of a laser-emission discharge is investigated. It is shown that by selecting the corresponding geometry of an irradiated sample it is possible to change the coefficient of laser radiation energy conversion into the energy of unidirectional quasistationary electric current. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 2, pp. 240–242, March–April, 2008.     

Influence of porosity on the dynamic properties of a loose granular layer

Processes accompanying the sudden retardation of a layer with its different porosity are analyzed by using a perfected dynamic model of a loose granular medium. The results are compared with systematic experiments.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 6, pp. 965–974, June, 1987.     

Features of static pressure in dense granular media

It is shown that the partial pressure, i.e. the contribution of contacts with a given force to the total average pressure, in a granular packing in quasistatic flow increases linearly from zero with the force level both in two and three dimensions. It reaches its maximum for the average force and decays for larger forces. We found that a well-defined sub-texture, composed of the contacts carrying a force below the average force, does not contribute to the shear stress, so that its contribution to the average pressure is mechanically similar to a hydrostatic pressure.     

Influence of contact geometry on hardness behavior in nano-indentation

In this paper the influence of contact geometry, including the round tip of the indenter and the roughness of the specimen, on hardness behavior for elastic-plastic materials is studied by means of finite element simulation. We idealize the actual indenter by an equivalent rigid conic indenter fitted smoothly with a spherical tip and examine the interaction of this indenter with both a flat surface and a rough surface. In the latter case the rough surface is represented by either a single spherical asperity or a dent (cavity). Indented solids include elastic perfectly plastic materials and strain hardening elastic–plastic materials, and the effects of the yield stress and strain hardening index are explored. Our results show that due to the finite curvature of the indenter tip the hardness versus indentation depth curve rises or drops (depending on the material properties of the indented solids) as the indentation depth decreases, in qualitative agreement with experimental results. Surface asperities and dents of curvature comparable to that of the indenter tip can appreciably modify the hardness value at small indentation depth. Their effects would appear as random variation in hardness.