Mechanism of amplification of convergent shock waves in porous media

The effect of amplification of moderate-intensity converging shock waves in porous media with decreasing initial density, revealed by numerically solving the hydrodynamics equations, was demonstrated for ID converging waves and for a 2D problem of the compression of porous material in conical solid targets. The latter problem was also treated within the framework of the simplest model of dynamic deformation of solids, with consideration given to shear stresses. The calculation results for porous graphite, aluminum, and Teflon samples are presented. Both closed targets and targets with an outlet orifice were considered. When modeling the intense shock loading of graphite, its transformation into diamond was taken into account.     

Semianalytical solution of the problem of converging shock waves

An approach to the determination of the self-similarity parameter in the problem of converging strong shock waves is suggested. This approach allows one to obtain analytical expressions that approximate the numerical solution. For adiabatic constants gamma = 6/5-7, the values of the obtained self-similarity parameter differ by <1% from the values determined by the numerical procedure. In addition, accurate analytical characteristics of the reflected shock wave are obtained.     

Intense shock waves in bubble media

Shock waves in bubble media are studied experimentally. Data are obtained on the structure, velocity of propagation, and pressure of shock waves incident on a solid boundary and reflected from it. The experimental data are compared with the results of calculation of shock wave parameters.     

Numerical study on permeability characteristics of fractal porous media

The fractal Brownian motion is utilized to describe pore structures in porous media. A numerical model of laminar flow in porous media is developed, and the flow characteristics are comprehensively analyzed and compared with those of homogeneous porous media. Moreover, the roles of the fractal dimension and porosity in permeability are quantitatively described. The results indicate that the pore structures of porous media significantly affect their seepage behaviors. The distributions of pressure and velocity in fractal porous media are both non-uniform; the streamline is no longer straight but tortuous. When Reynolds number Re 1, the dimensionless permeability is independent of Reynolds number, but its further increase will lead to a smaller permeability. Moreover, due to the higher connectivity and enlarged equivalent aperture of internal channel network, the augment in porosity leads to the permeability enhancement, while it is small and insensitive to porosity variation when ε 0.6. Fractal dimension also plays a significant role in the permeability of porous media. The increase in fractal dimension leads to the enhancement in pore connectivity and a decrease in channel tortuosity,which reduces the flow resistance and improves the transport capacity of porous media.     

Velocity and timing of multiple spherically converging shock waves in liquid deuterium

The fuel entropy and required drive energy for an inertial confinement fusion implosion are set by a sequence of shocks that must be precisely timed to achieve ignition. This Letter reports measurements of multiple spherical shock waves in liquid deuterium that facilitate timing inertial confinement fusion shocks to the required precision. These experiments produced the highest shock velocity observed in liquid deuterium (U(s) = 135 km/s at ～2500 GPa) and also the first observation of convergence effects on the shock velocity. Simulations model the shock-timing results well when a nonlocal transport model is used in the coronal plasma.     

Weak shock waves in negative-dispersion nonequilibrium media

The “frozen” and equilibrium shock adiabats for a gas with sustained steady-state nonequilibrium are constructed accurate to the second order of smallness. With these adiabats, the pattern of and stability conditions for weak shock waves in negative-dispersion nonequilibrium media, where the speed of low-frequency (equilibrium) sound exceeds that of high-frequency (frozen) sound, are considered. On the basis of a model nonlinear equation describing the evolution of gasdynamic perturbations in low-dispersion media, the nonstationary evolution of shock waves at a negative low-frequency nonlinearity coefficient is analyzed. This situation corresponds to a low-frequency adiabat convex upwards. It is shown that a step autowave may arise in this case whose amplitude is entirely specified by the nonequilibrium parameters of the medium and correlates with the point where the low-frequency and high-frequency adiabats intersect. In addition, it is found that the initial unsteady shock wave may split into two steady ones: a step autowave followed by a steady smooth-front expansion shock.     

Effects of the localization of deformations and mass transfer in converging shock waves

Effects of shock wave interaction and the related phenomena of the localization of plastic deformation, destruction, and mass transfer in metal ball samples subjected to explosive loading at pressures of 36 to 150 GPa are studied. A correlation between the macro- and microstructural changes and the geometrical conditions of loading according to various schemes is found. It is shown that the mass transfer effects are of hydrodynamic origin. The depth penetration of the material was 3.2 mm in narrow channels and 0.3 mm in solid material.     

Observation of shock transverse waves in elastic media

We report the first experimental observation of a shock transverse wave propagating in an elastic medium. This observation was possible because the propagation medium, a soft solid, allows one to reach a very high Mach number. In this extreme configuration, the shock formation is observed over a distance of less than a few wavelengths, thanks to a prototype of an ultrafast scanner (that acquires 5000 frames per second). A comparison of these new experimental data with theoretical predictions, based on a modified Burger's equation, shows good agreement.     

Phase and structure state of titanium loaded by spherically converging shock waves

The phase and structural states of titanium spheres loaded by spherical converging shock waves of various intensities were studied layer by layer by means of X-ray diffraction, optical, and transmission electron microscopy. It was established that defects of different types (twins, dislocations, and adiabatic shear bands) are produced during high-rate deformation occurring in materials under such method of pulsed loading. The amount and distribution of the defects depend on the loading intensity. The presence of the ω-phase is revealed only in the layers near the external surface of the titanium sphere after low-intensity loading.     

Magnetic resonance imaging of chemical waves in porous media

Magnetic resonance imaging (MRI) provides a powerful tool for the investigation of chemical structures in optically opaque porous media, in which chemical concentration gradients can be visualized, and diffusion and flow properties are simultaneously determined. In this paper we give an overview of the MRI technique and review theory and experiments on the formation of chemical waves in a tubular packed bed reactor upon the addition of a nonlinear chemical reaction. MR images are presented of reaction-diffusion waves propagating in the three-dimensional (3D) network of channels in the reactor, and the 3D structure of stationary concentration patterns formed via the flow-distributed oscillation mechanism is demonstrated to reflect the local hydrodynamics in the packed bed. Possible future directions regarding the influence of heterogeneities on transport and reaction are discussed.     

Effective tortuosity and different acoustic waves in porous media

Summary A simple method has been suggested to estimate the acoustic characteristics of porous structure from a hybrid model—a hybridisation of Biot's phenomenological model and the microscopic multiple-scattering theory which introduces the idea of an effective tortuosity. Without using any adjustable parameter this model may be used to provide rough estimates of the tortuosity, the fast, the shear and the slow sound speeds. The predictions are compared with observation on water-saturated glass bead samples. The author of this paper has agreed to not receive the proofs for correction.     

Nonlinear waves in porous media saturated with live oil

A nonlinear equation is obtained for waves propagating in porous media of arbitrary consolidation (relative rigidity) saturated with live (i.e., air-bearing) oil. The equation describes the evolution of fast and slow Biot-Frenkel longitudinal acoustic waves propagating in both directions and allows one to analyze the reflected waves and their interaction. For a wave of the second kind, the diffusion coefficient is determined. The dependences of the dispersion and dissipation parameters on the rigidity of the oil pool structure and on the depth of the oil pool occurrence are analyzed.     

On the second-type nonlinear elastic waves in porous media

Nonlinear second-type (matrix) waves are studied with special emphasis on the formation of saw-tooth shock waves. Configurations of the elastic waves in specific cases of porous gas-saturated sedimentary are calculated.     

多孔介质中盐指现象的数值模拟

运用基于杂交网格的高精度数值方法研究了多孔介质中的盐指现象.该算法将基于边界拟合坐标下的高精度有限差分法和高精度的泊松方程快速求解器有效地结合在一起,从而达到提高整体的计算精度、计算效率和稳定性的目的.通过比较不同孔隙率的多孔介质对盐指对流的传热传质效应的影响,发现在标准孔隙率较低的多孔介质中,盐度扩散的速度明显比热扩散的速度快,盐指很快触及上下壁面,使得上下层的盐度梯度迅速减小,这是与非多孔介质具有明显差异之处. 关键词： 多孔介质 双扩散对流 盐指     

Elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media

Abstract

This paper studies the elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media with the nonzero boundary slip velocity for pore size distribution. The coefficient bF _m (ω) that measures the deviation from Poiseuille flow friction in such media is presented. Based on this coefficient, we investigate the properties of elastic waves by calculating their phase velocities and attenuation coefficients as functions of frequency and the behaviour of the dynamic permeability. The study shows that the pore size distribution removes oscillations in all physical quantities in the non-Newtonian regime. Consideration of the nonzero boundary slip effect in non-Newtonian (Maxwell) fluid-saturated porous media results in (a) an overall increase of the dynamic permeability, (b) an increase of phase velocities of fast Biot waves and shear waves except in the low frequency domain and an overall increase of phase velocity of slow Biot waves and (c) an overall increase of the attenuation of three Biot waves in the intermediate frequency domain except in the deeply non-Newtonian regime. The study also shows that the attenuation coefficient of slow Biot waves is small in the deeply non-Newtonian regime at higher frequency, which encourages us to detect slow Biot waves in oil-saturated porous rock.     

Elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media

This paper studies the elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media with the nonzero boundary slip velocity for pore size distribution. The coefficient bF_m(ω) that measures the deviation from Poiseuille flow friction in such media is presented. Based on this coefficient, we investigate the properties of elastic waves by calculating their phase velocities and attenuation coefficients as functions of frequency and the behaviour of the dynamic permeability. The study shows that the pore size distribution removes oscillations in all physical quantities in the non-Newtonian regime. Consideration of the nonzero boundary slip effect in non-Newtonian (Maxwell) fluid-saturated porous media results in (a) an overall increase of the dynamic permeability, (b) an increase of phase velocities of fast Biot waves and shear waves except in the low frequency domain and an overall increase of phase velocity of slow Biot waves and (c) an overall increase of the attenuation of three Biot waves in the intermediate frequency domain except in the deeply non-Newtonian regime. The study also shows that the attenuation coefficient of slow Biot waves is small in the deeply non-Newtonian regime at higher frequency, which encourages us to detect slow Biot waves in oil-saturated porous rock.