A. M. Oparin

Expansion of matter heated by an ultrashort laser pulse

Recent experiments have utilizied high-power subpicosecond laser pulses to effect the ultrafast heating of a condensed material to temperatures far above the critical temperature. Using optical diagnostics it was established that a complicated density profile with sharp gradients, differing substantially from an ordinary rarefaction wave, forms in the expanding heated matter. The present letter is devoted to the analysis of the expansion of matter under the conditions of the experiments reported by D. von der Linde, K. Sokolowski-Tinten, and J. Bialkowski, Appl. Surf. Science 109/110, 1 (1996); K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Proc. Soc. Photo-Opt. Instum. Eng. 3343, 46 (1998); and, K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Phys. Rev. Lett. 81, 224 (1998). It is shown that if the unloading adiabat passes through the two-phase region, a thin liquid shell filled with low-density two-phase matter forms in the expanding material. The shell moves with a constant velocity. The velocity in the two-phase material is a linear function of the coordinate (flow with uniform deformation), and the density is independent of the coordinate and decreases with time as t−1.

Destruction of a solid film under the action of ultrashort laser pulse

Molecular-dynamics (MD) simulation of the destruction of a crystal film heated by a femtosecond laser pulse was carried out. Heating is assumed to be instantaneous, because there is no time for the material to be displaced during the pulse. Film destruction is caused by the interaction of unloading waves. It can be considered as a model of a more complex process of splitting out of a thin surface layer from a massive target in the case where the layer remains solid after heating. It was found that the crystal order is broken due to the stretching strains and to the strong anisotropy of residual stress, resulting in a bipartition of the layer separating from the target. The lattice stretching and the formation of anisotropic stresses are due to the expansion of a heated lattice.

Generalization of the hybrid monotone second-order finite difference scheme for gas dynamics equations to the case of unstructured 3D grid

A generalization of the explicit hybrid monotone second-order finite difference scheme for the use on unstructured 3D grids is proposed. In this scheme, the components of the momentum density in the Cartesian coordinates are used as the working variables; the scheme is conservative. Numerical results obtained using an implementation of the proposed solution procedure on an unstructured 3D grid in a spherical layer in the model of the global circulation of the Titan’s (a Saturn’s moon) atmosphere are presented.

Numerical modeling of shock-wave instability in thermodynamically nonideal media

A numerical analysis of the nonlinear instability of shock waves is presented for solid deuterium and for a model medium described by a properly constructed equation of state. The splitting of an unstable shock wave into an absolutely stable shock and a shock that emits acoustic waves is simulated for the first time.

On stochastic mixing caused by the Rayleigh-Taylor instability

The mixing of contacting substances is considered. The evolution of the mixing layer over a long time period from multimode initial perturbations is investigated numerically in the short-scale and wide-range cases. In the case of a short-scale initiation, the flow is stochastic in the sense that the time of the considered evolution exceeds the period of correlation. The effect of the amplitude of wide-range perturbations on the dynamics of mixing is analyzed. The scale-invariant properties of the spectral and statistical parameters of turbulent mixing are investigated for the first time. The universal spectra characterizing the turbulence mixing in the entire self-similar interval on a unified basis are obtained. The simulation is based on the effective algorithms with high approximating qualities, which have been tested earlier.

Structurization of chaos

Vortex cascades of instabilities forming a core are studied. Large-scale linear waves in a fluctuating medium are described.

Physical processes underlying the development of shear turbulence

A physical model of the development of turbulence in free shear flows is proposed. The model is based on the results of numerical simulations of turbulent flow development. The main ideas of the proposed theory of turbulence are stated as follows: the onset of turbulence begins with the formation of large vortices; spectral energy transfer involves both direct and inverse cascades; and the inertial range of the energy spectrum develops as a result of concurrent direct and inverse cascades. The dominant physical factors that determine the spectrum include Joukowski forces.

Three-dimensional array structures associated with Richtmyer-Meshkov and Rayleigh-Taylor instability

A boundary separating adjacent gas or liquid media is frequently unstable. Richtmyer-Meshkov and Rayleigh-Taylor instability cause the growth of intricate structures on such boundaries. All the lattice symmetries [rectangular (pmm2), square (p4mm), hexagonal (p6mm), and triangular (p3m1) lattices] which are of interest in connection with the instability of the surface of a fluid are studied for the first time. They are obtained from initial disturbances consisting of one (planar case, two-dimensional flow), two (rectangular cells), or three (hexagons and triangles) harmonic waves. It is shown that the dynamic system undergoes a transition during development from an initial, weakly disturbed state to a limiting or asymptotic stationary state (stationary point). The stability of these points (stationary states) is investigated. It is shown that the stationary states are stable toward large-scale disturbances both in the case of Richtmyer-Meshkov instability and in the case of Rayleigh-Taylor instability. It is discovered that the symmetry increases as the system evolves in certain cases. In one example the initial Richtmyer-Meshkov or Rayleigh-Taylor disturbance is a sum of two waves perpendicular to one another with equal wave numbers, but unequal amplitudes: a1(t=0)≠a2(t=0). Then, during evolution, the flow has p2 symmetry (rotation relative to the vertical axis by 180°), which goes over to p4 symmetry (rotation by 90°) at t→∞, since the amplitudes equalize in the stationary state: a1(t=∞)=a2(t=∞). It is shown that the hexagonal and triangular arrays are complementary. Upon time inversion (t→−t), “rephasing” occurs, and the bubbles of a hexagonal array transform into jets of a triangular array and vice versa.

On the Neutral Stability of a Shock Wave in Real Media

The results of the theoretical analysis and computer simulation of the behavior of neutrally stable shock waves with real (van der Waals gas, magnesium) equations of state are presented. An approach is developed in which the region of the neutral stability of a shock wave for each pressure value in front of the wave is determined from the analysis of the equation of state. A simple algorithm is developed to determine the cause of acoustic perturbations (a shock front or an external source) immediately from the flow pattern. In contrast to the predictions of the linear theory, the amplitude of the perturbations of the neutrally stable shock wave decreases with time, although this process is noticeably slower than in the case of an absolutely stable shock wave.

Stability and Ambiguous Representation of Shock Wave Discontinuity in Thermodynamically Nonideal Media

The nonlinear analysis of the behavior of a shock wave on a Hugoniot curve fragment that allows for the ambiguous representation of shock wave discontinuity has been performed. The fragment under consideration includes a section where the condition L > 1 + 2M is satisfied, which is a linear criterion of the instability of the shock wave in media with an arbitrary equation of state. The calculations in the model of a viscous heat-conductive gas show that solutions with an instable shock wave are not implemented. In the one-dimensional model, the shock wave decays into two shock waves or a shock wave and a rarefaction wave, which propagate in opposite directions, or can remain in the initial state. The choice of the solution depends on the parameters of the shock wave (position on the Hugoniot curve), as well as on the form and intensity of its perturbation. In the two-dimensional and three-dimensional calculations with a periodic perturbation of the shock wave, a “cellular” structure is formed on the shock front with a finite amplitude of perturbations that does not decrease and increase in time. Such behavior of the shock wave is attributed to the appearance of the triple configurations in the inclined sections of the perturbed shock wave, which interact with each other in the process of propagation along its front.