A family of conservative schemes for the axisymmetric contact smoothed particle hydrodynamics (CSPH) method, which ensure the accuracy and stability in modeling of complex multi-material flows of compressible media, is introduced. Among these schemes, the most convenient ones are considered. Simulations with the proposed schemes may be also improved by embedding of MUSCL reconstruction into a numerical scheme, or by correcting the kernel gradient as was proposed earlier for the Cartesian case. Verification of the proposed method is performed on several test problems: Sod's cylindrical test, Taylor bar test, and Sedov's point explosion. The conservative properties of the scheme are demonstrated. Finally, a set of simulations on air shock wave weakening by a breakaway sand barrier is performed and compared to experimental results.

The ideal laser source for the emerging research field of nonlinear Terahertz (THz) spectroscopy should offer radiation with a large versatility and deliver both ultra-intense multi-octave spanning single-cycle pulses and user-selectable multi-cycle pulses at narrow linewidth. The absence of such a table-top source has hampered advances in numerous THz disciplines including imaging, nonlinear photonics and spectroscopy, selective out-of-equilibrium excitation of condensed matter and quantum systems. Here we introduce a highly versatile table-top THz laser platform providing single-cycle GV/m transients as well as spectrally narrow pulses tunable in bandwidth and central frequency across 5 octaves with hundreds of MV/m field strength. The compact scheme is based on optical rectification of a temporally modulated laser beam in organic crystals. It allows for the selection of THz oscillation cycles from 1 to >50 and central frequency tuning range from 0.5 to 7 THz by directly changing the modulation period of the driving laser. The versatility of the THz source is demonstrated by providing a broadband 5-octave spanning spectrum as well as a spectrally narrow line tunable across the full optical rectification phase-matching band with a minimum width of dv=30 GHz, corresponding to dE=0.13 meV and lambda^-1=1.1 cm-1. The presented table-top source shows performances similar or even beyond to that of a large-scale THz electron accelerator facility but offering in addition versatile multi-color and advanced femtosecond pump-probe opportunities at ultralow timing jitter.

The paper presents a two-phase hydrodynamic model for the numerical simulation of collective motion in a thin layer of active colloids containing spherical microswimmers. The model accounts for three fundamental mechanisms governing the dynamics of the active colloid: the random motion of the microswimmers, their mutual collisions, and their interaction with the surrounding fluid phase. The accurate resolution of the characteristic time scales associated with each mechanism is crucial for reproducing the different dynamic modes. The model reproduces two primary modes of motion: Brownian and collective, as well as the transition between them. It is demonstrated that hydrodynamic interactions begin to play a significant role when the microswimmer velocity exceeds a critical threshold. At this point, the kinetic energy transferred to the fluid phase is sufficient to generate a noticeable feedback effect on the swimmers' motion. Conversely, a further increase in microswimmers' velocity enhances the role of collisions, causing the system to revert from a collective mode back to a Brownian-like state. A similar transition occurs at higher volume fractions of microswimmers within the colloid.

This paper introduces a novel methodology for modeling stationary shock waves in porous materials, which employs the recently developed moving window technique. The core of this method is the iterative adjustment of the reference frame to the boundary conditions that regulate the entry and exit of Lagrangian particles from a fixed computational domain, which are used to model the flow of a compressible medium. A Godunov-type smoothed particle hydrodynamics (SPH) method with reconstruction of values at the contact is employed for the purposes of modeling. Kernel gradient correction for this method is proposed to enhance the precision of the approximation. The shock Hugoniot of porous copper is calculated, and the structure of the compacting wave and elastic precursor in porous copper at shock amplitude near the yield strength of solid copper is studied.

In this work, we compare various models for describing the phase transition of the fluid hydrogen into a conducting state, including both chemical models of plasma and first-principle simulations within the framework of the density functional theory (DFT). The comparison of the results indicates the plasma nature of the phase transition in warm dense hydrogen. We propose a concept of a new class of first-order phase transitions: ionization or dissociation-driven phase transitions, with which the plasma phase transition in fluid hydrogen can be associated.