The influence of fuel injection and heat release on bulk flow structures in a direct-injection, swirl-supported diesel engine

Particle image velocimetry is applied to measure the vertical (r–z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume.     

Interaction of a high-speed combustion front with a closely packed bed of spheres

The head-on collision of a combustion front with a closely packed bed of ceramic-oxide spheres was investigated in a vertical 76.2 mm diameter tube containing a nitrogen diluted stoichiometric ethylene–oxygen mixture. A layer of spherical beads in the diameter range of 3–12.7 mm was placed at the bottom of the tube and a flame was ignited at the top endplate. Four orifice plates spaced at one tube diameter were placed at the ignition end of the tube in order to accelerate the flame to either a “fast-flame” or a detonation wave before the bead layer face. The mixture reactivity was adjusted by varying the initial mixture pressure between 10 and 100 kPa absolute. The pressure before and within the bead layer was measured by flush wall-mounted pressure transducers. For initial pressures where a fast-flame interacts with the bead layer peak pressures recorded at the bead layer face were as high as five times the reflected Chapman–Jouget detonation pressure. The explosion resulting from the interaction developed by two distinct mechanisms; one due to the shock reflection off the bead layer face, and the other due to shock transmission and mixing of burned and unburned gas inside the bead layer. The measured explosion delay time (time after shock reflection from the bead layer face) was found to be independent of the incident shock velocity. As a result, the explosion initiation is not the direct result of the shock reflection process but instead is more likely due to the interaction of the reflected shock wave and the trailing flame. The bead layer was found to be very effective in attenuating the explosion front transmitted through the bead layer and thus isolating the tube endplate. This paper is based on work that was presented at the 21th International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

A Study on Flame Forming of Bowl Shaped Surface with Various Spiral Irradiating Schemes

In this paper, various strategies of spiral irradiating scheme for the flame forming of a bowl shaped surface are investigated experimentally and numerically. Experimental work is performed using a flame torch integrated with a 2-axis CNC workstation. The ABAQUS implicit solver is used in the numerical simulation. Three different strategies of the spiral irradiating scheme are investigated for the flame forming of a bowl shaped surface. The first strategy is the Simple spiral irradiating scheme, the second is the Rotational spiral irradiating scheme, and the third is the Symmetrical-Rotational spiral irradiating scheme. The results show that using the SymmetricalRotational spiral irradiating scheme, a bowl shaped surface with the maximum deformation can be produced, followed by using the Rotational scheme, and the Simple spiral scheme. It is also concluded from the results that the spiral irradiating scheme with Symmetrical-Rotational, Rotational and Simple spiral schemes lead to the maximum symmetries in the produced bowl shaped surface, respectively. All the numerical results are in good agreement with the experimental observations.     

Combustion in a horizontal channel partially filled with a porous media

Experiments were carried out to investigate the combustion propagation phenomenon in a horizontal channel partially filled with ceramic-oxide spherical beads. A 1.22 m long, 43 mm nominally thick layer of spherical beads is located at the ignition end of a 2.44 m long, 76 mm square channel. Tests were performed with 6.4 and 12.7 mm diameter beads. A flame is ignited at the bead end wall by an automotive spark ignition system. Flame propagation and pressure measurements are obtained via ionization probes and piezoelectric pressure transducers mounted on the top and bottom surfaces of the channel. High-speed schlieren video was used to visualize the structure of the explosion front. Experiments were performed with a 31% nitrogen diluted stoichiometric methane–oxygen mixture at room temperature and at an initial pressure in the range of 15–50 kPa. For initial pressures of 15 and 20 kPa the flame accelerates to a velocity close to the speed of sound in the combustion products. For initial pressure of 30 kPa and higher DDT occurs in the gap above the bead layer. An explosion front propagating at a velocity just under the CJ detonation velocity is detected in the bead layer even though the bead layer pore size is much smaller than the detonation cell size. It is demonstrated that flame propagation within the bead layer is the driving force behind the very rapid flame acceleration observed, however the DDT event occurring in the gap above the bead layer is not affected by the bead layer porosity. Schlieren video indicates that the structure of the explosion front varies across the channel height and with propagation distance down the channel.     

Computer-extended series for a source/sink driven gas centrifuge

We have reformulated the general problem of internal flow in a modern, high speed gas centrifuge with sources and sinks in such a way as to obtain new, simple, rigorous closed form analytical solutions. Both symmetric and antisymmetric drives lead us to an ordinary differential equation in place of the usual inhomogeneous Onsager partial differential equation. Owing to the difficulties of exactly solving this sixth order, inhomogeneous, variable coefficient ordinary differential equation we appeal to the power of perturbation theory and techniques. Two extreme parameter regimes are identified, the so-called semi-long bowl approximation and a new short bowl approximation. Only the former class of problems is treated here. The long bowl solution for axial drive is the correct leading order term, just as for pure thermal drive. New O(1) results are derived for radial, drag and heat drives in two dimensions. Then regular asymptotic, even ordered power series expansions for the flow field are carried out on the computer to O(ε⁴) using MACSYMA. These approximations are valid for values of ε near unity. In the spirit of Van Dyke, one can carry out this expansion process, in theory, to apparently arbitrary order for arbitrary but finite decay length ratio. Curiously, the flows induced by axial and radial forces are proportional for asymptotically large source scale heights, x^*. Corresponding isotope separation integral parameters will be given in a companion paper.     

Hydrodynamic interactions of dilute polymer solutions under shear flow in a narrow channel

Hydrodynamic interactions on dilute solutions of spherical beads under shear flow are calculated with the method of induced forces. The Navier-Stokes equation is considered in the Stokes approximation. Hydrodynamic interactions cause the drag to be anisotropic in space.Numerical solutions are obtained for the added stress, caused by polymeric molecules in solution in a narrow channel under shear flow. The polymeric molecules are considered as Hookean spring-dumbbells.Slip velocity and the effective viscosity are obtained taking different dumbbells' bead radii. Transversal migration in the channel is obtained for different bead radii.     

Propagation of nitromethane detonations in porous media

The characteristics of the propagation of a detonation in chemically sensitized nitromethane in a dense porous medium are investigated. By introducing liquid NM+15% (by weight) DETA into densely packed beds of solid spherical glass beads 66μm to 2.4 mm in diameter, a highly heterogeneous explosive mixture is obtained. The critical (i.e., failure) charge diameter of this mixture is systematically measured in unconfined charges over a wide range of bead sizes. Velocity measurements are also made for the various charges. It is found that there exists a critical bead size above which the critical diameter decreases with increasing bead size and below which it decreases with decreasing bead size. This result indicates an abrupt change in the mechanism of propagation at the critical bead size. Velocity measurements further support this by emphasizing the different behavior above and below the critical point.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method （DEM） of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3-6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated.The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

Operability windows in viscoelastic slot coating flows using a simplified viscoelastic-capillary model

The viscoelastic-capillary model to predict approximately coating windows for the stable operations of viscoelastic coating liquids is derived using a lubrication approximation in slot coating processes. Pressure distributions and velocity profiles for viscoelastic liquids based on the Oldroyd-B and Phan-Thien and Tanner (PTT) models are solved in the coating bead region considering the Couette-Poiseuille flow feature and the pressure jumps at upstream and downstream menisci. Practical operating limits for the uniform coating of rheologically different liquids that are free from leaking and bead break-up defects are constructed under various conditions, incorporating the position of the upstream meniscus as an important indicator while determining limits. The shift of the uniform operating range shows different patterns for the Oldroyd-B liquid with a constant shear viscosity and the PTT liquid with a shear-thinning nature in comparison with the Newtonian case. The windows predicted by the simplified model are corroborated with experimental observations for one Newtonian and two viscoelastic liquids.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method (DEM) of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3–6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated. The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

Finite element analysis of flow in a gas-filled rotating annulus

Linearized multidimensional flow in a gas centrifuge can be described away from the ends by Onsager's pancake equation. However a rotating annulus results in a slightly different set of boundary conditions from the usual symmetry at the axis of rotation. The problem on an annulus becomes ill-posed and requires some special attention. Herein we treat axially linear inner and outer rotor temperature distributions and velocity slip. An existence condition for a class of non-trivial, one-dimensional solutions is given. New exact solutions in the infinite bowl approximation have been derived containing terms that are important at finite gap width and non-vanishing velocity slip. The usual one-dimensional, axially symmetric solution is obtained as a limit. Our previously reported finite element algorithm has been extended to treat this new class of problems. Effects of gap width, temperature and slip conditions are illustrated. Lastly, we report on the compressible, finite length, circular Couette flow for the first time.     

The transient Stokes flow induced by a point source of electric current

Summary The development of the magnetohydrodynamic Stokes flow field due to the discharge of an electric current from a point into a viscous incompressible fluid bounded by a hemispherical bowl is investigated. We also discussed, as a special case, the flow induced for the case when the radius of the hemispherical bowl becomes infinitely large and showed that the time taken to establish the steady state flow in the interior of the hemisphere is reduced by a factor of ten compared with the situation where the flow takes place in a semi-infinite fluid. Our solution is analytic as compared with the existing numerical solution.

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Sommario Si studia lo sviluppo del campo di moto magnetoidrodinamico alla Stokes prodotto dall'immissione di una corrente elettrica in un punto di un fluido viscoso incomprimibile limitato dalla superficie di un emisfero. Si discute anche, come caso particolare, il moto prodotto nel caso in cui il raggio dell'emisfero diviene infinito e si dimostra che il tempo richiesto a stabilire un moto stazionario nell'emisfero si riduce di un fattore dieci rispetto alla situazione in cui il moto ha luogo in un fluido semi-infinito. La soluzione presentata è analitica, non numerica come in lavori precedenti.

     

Dynamics of kinks in biological membranes

kinks created in a biological membrane by the interaction with a movable bead. We arrive at the evolution equations for both the bead and the membrane, whence we conclude that the force exerted on the bead by a fixed membrane points in the direction along which the curvature of the membrane is more concentrated. This is the first step towards understanding the basic mechanism behind the dynamics of protein aggregation which takes place on biological membranes. Received November 6, 2001 / Published online February 4, 2002     

应力调整对石英玻璃珠低速冲击破碎行为的影响

结合高速摄影技术,应用SHPB加载装置,分别使用钢制、铝制和有机玻璃制3种透射杆,对直径约7.90、11.80、15.61 mm 3种尺寸的石英玻璃珠进行了低速冲击实验。根据不同透射杆条件下的玻璃珠破碎过程中的载荷-位移曲线,结合有限元软件计算玻璃珠在冲击作用下载荷的变化情况以及实验过程中玻璃珠的应变,探讨了应力调整对玻璃珠破碎过程的影响。结果表明:相同冲击条件作用下,改变透射杆的材料,会改变玻璃珠破碎过程中的载荷分布,即透射端边界波阻抗的改变会导致反射波发生改变,从而导致玻璃珠内部载荷发生变化;透射杆为铝材和有机玻璃材质时,玻璃珠在破碎过程中的载荷明显下降,在加载过程中伴随着垫块的变形,玻璃珠内部的应力调整时间变长;透射杆为钢杆时,玻璃珠的应变主要表现为两端最大,越靠近中间应变越小,对于透射杆为铝杆和有机玻璃杆的玻璃珠,透射端局部出现了卸载行为。采用有机玻璃透射杆之后,局部应力和变形降低的结果使得玻璃珠在经受较大的变形之后发生破碎,表明玻璃珠的破碎行为由局部变形和局部变形梯度共同控制。     

Computer extended series for a thermally driven gas centrifuge

Linearized, multidimensional, thermally driven flow in a gas centrifuge can be approximately described in regions away from the ends by Onsager's homogeneous pancake equation.¹ Upon reformulation of the general problem, we find a new, simple and rigorous closed form, analytical solution by assuming a special separable solution and replacing the usual Ekman end cap boundary conditions with idealized impermeable, free slip boundary conditions. Then the flow may be described by an ordinary differential equation with solutions in terms of simple, classical functions. By identifying a small parameter, say ?, defining the semi-long bowl approximation, and assuming a power series expansion in ?, a sequence of asymptotic approximations to the master potential is obtained. Not surprisingly, the leading order term involves the well known ‘long bowl’ solution. Using the so-called ‘solving’ property of the 1-D pancake Green's function,² we determine the next higher order solution. This recursive process is carried out on the computer to find all the terms up to O(?⁴). Consequently, the solution of some complex rotating, viscous, heat conducting flow problems that normally require large mainframe computers can be better understood.     

Finitely extensible bead spring chains in time-dependent shear flows

The configurational and rheological properties of bead spring chains in time-dependent shear flows are calculated. The finite extensibility is incorporated through the constraint of constant contour length of the chain. Start-up of shear flow yields a stress overshoot, whereas oscillatory shear flow yields the same frequency dependence of the dynamic moduli as the simple bead spring model. The results show that finite extensibility can lead to non-linear rheological behavior of dilute polymer solutions. The influence of preaveraged hydrodynamic interaction on the obtained results is discussed.     

Subsidence above volumetric and waterdrive gas reservoirs

A fully coupled consolidation model has been developed for the simulation of the surface subsidence above gas reservoirs. The model is based on the Biot Theory and the material balance equation for hydrocarbon reservoirs. The model is extremely versatile and can handle such complex situations as vertical cross-sections where several gas reservoirs and aquifers are exploited at different levels. Computer runs were used to generate several reservoir formation profiles and the surface subsidence bowl for a variety of conditions. These results indicate the importance of various parameters which are disregarded in proelasticity models.     

Experimental Study of Strongly Nonlinear Resonances and Anti-Resonances in Granular Dimer Chains

Previous theoretical works considered the intrinsic dynamics of one-dimensional uncompressed granular dimer (diatomic) chains composed of pairs of dissimilar spherical elastic beads in Hertzian interaction. Such ordered granular media exhibit essentially nonlinear acoustics and have been characterized as ‘sonic vacua’ due to the fact that the speed of sound in these media (as defined in classical acoustics) is zero. Yet, depending on the mass ratios of the pairs of dissimilar beads of these dimers, it was proven that they may possess countable infinities of anti-resonances leading to solitary waves (this in spite of their high inhomogeneity), or countable infinities of strongly nonlinear resonances leading to passive strong attenuation of propagating pulses through energy radiation by means of excitation of traveling waves. The aim of this work is to experimentally verify the existence of these strongly nonlinear dynamics through a series of experiments involving granular dimer chains supported by flexures. By carefully designing the supporting flexures so that their dynamics is sufficiently ‘soft’ and thus separate from the ‘stiff’ dynamics governing the bead to bead interactions, we overcome a basic limitation for the experimental realization of such dimer systems, namely the construction of one-dimensional dimer chains with beads of different radii. Our results confirm experimentally the occurrence of nonlinear resonances and anti-resonances in dimer chains, and conclusively prove the capacity of appropriately designed granular dimers for passive strong attenuation of propagating pulses due to nonlinear resonance. Moreover, we validate the theoretical prediction that within the elastic range of bead to bead dynamical interactions the results are fully re-scalable with respect to energy. This work provides the first experimental evidence of strongly nonlinear resonances and anti-resonances in essentially nonlinear ordered granular media.     

Modelling of a single drop impact onto liquid film using particle method

The process of single liquid drop impact on thin liquid surface is numerically simulated with moving particle semi‐implicit method. The mathematical model involves gravity, viscosity and surface tension. The model is validated by the simulation of the experimental cases. It is found that the dynamic processes after impact are sensitive to the liquid pool depth and the initial drop velocity. In the cases that the initial drop velocity is low, the drop will be merged with the liquid pool and no big splash is seen. If the initial drop velocity is high enough, the dynamic process depends on the liquid depth. If the liquid film is very thin, a bowl‐shaped thin crown is formed immediately after the impact. The total crown subsequently expands outward and breaks into many tiny droplets. When the thickness of the liquid film increases, the direction of the liquid crown becomes normal to the surface and the crown propagates outward. It is also found that the radius of the crown is described by a square function of time: r_C = [c(t ? t₀)]^0.5. When the liquid film is thick enough, a crown and a deep cavity inside it are formed shortly after the impact. The bottom of the cavity is initially oblate and then the base grows downward to form a sharp corner and subsequently the corner moves downward. Copyright © 2004 John Wiley & Sons, Ltd.