Vinayak Mishra

Tensile Instability and Artificial Stresses in Impact Problems in SPH

The smooth particle hydrodynamics (SPH) is a meshless computational technique that is popular in the modeling of impact and penetration problems. However, SPH is liable to a tensile instability that manifests itself as a bunching of nodes and formation of artificial voids and no generally accepted formulation exists to counter this instability. We examine the performance of two methods that have been proposed to deal with the tensile instability— the Monaghan artificial stresses and the Godunov-type SPH. The impact and penetration of 0.5 cm radii steel spheres on 2 mm thick aluminium plate at 3.1 km/s is chosen for comparison. We show that the artificial void formation in St-Al impact is suppressed but not eliminated by using Monaghan stresses while the void formation is entirely eliminated by using Godunov-type formulation of SPH that was proposed by Parshikov and Medin.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

A comparison of quotidian equation of state of aluminium with ab-initio calculations

We present results of theoretical calculation of the equation of state of Al covering several orders of magnitude of density and temperature. The data is generated using a QEOS model. The QEOS data have been used to calculate Us - Up and P - V hugoniots. The calculated hugoniots show good agreement with the experimental hugoniots. The cold curve generated by QEOS model has been compared with FP-LAPW results for compressed states and good agreement has been found. The QEOS data has also been compared with AIMD simulation results for expanded states - agreement is not good. These comparisons confirm that QEOS results are accurate for compressed states and less accurate for low temperature expanded states of Al.

A VOF based multi-material method to study impact and penetration problems

A 2D axisymmetric Eulerian method is developed for studying multi-material elastic-plastic problems involving large material deformations. It consists of a Lagrangian plus remap strategy using a volume-of-fluid (VOF) based material interface tracking. The multi-material formulation used in this method allows one to update the energy and stress components of each material in a mixed cell independently. This assumes common strain-rates to all materials present in a mixed cell. The equivalent pressure and stress in a mixed cell are determined using volume weighted average. The present scheme, therefore, eliminates the commonly used pressure relaxation method, mixed EOS evaluation and energy partition schemes. The stress components of each material are transported using a second-order monotonic upwind scheme (MUSCL) due to van Leer. The capability of the proposed method is demonstrated by applying to various impact and penetration problems involving large material deformations. Reasonable agreement with experimental results are observed.

Time-Resolved Vibrational Spectroscopy of Polytetrafluoroethylene Under Laser-Shock Compression

Shock-wave-induced high pressure and nanosecond time-resolved Raman spectroscopic experiments were performed to examine the dynamic response of polytetrafluoroethylene (PTFE) in confinement geometry targets. Time-resolved Raman spectroscopy was used to observe the pressure-induced molecular and chemical changes on nanosecond time scale. Raman spectra were measured as a function of shock pressure in the 1.2–2.4 GPa range. Furthermore, the symmetric stretching mode at 729 cm–1 of CF2 was compared to corresponding static high-pressure measurements carried out in a diamond anvil cell, to see if any general trend can be established. The symmetric stretching mode of CF2 at 729 cm–1 is the most intense Raman transition in PTFE and is very sensitive to change in pressure. Therefore, it can also be utilized as a pressure gauge for large amplitude shock wave compression experiments. A maximum blueshift of 12 cm–1 for the 729 cm–1 vibrational mode has been observed for the present experimental pressure range. A comparative study on the similarities and differences from the earlier work has been done in detail. One-dimensional radiation hydrodynamic simulations were performed to validate our shock compression results and are in very good agreement.

First-principles study of high temperature and high-pressure behavior of carbides and nitrides of group IVB elements

Full potential linearized augmented plane wave method combined with quasi-harmonic approximation, has been used to perform the calculations of thermophysical properties of carbides and nitrides of the group IVB elements at high temperature and pressure. Relative accuracy of linear density approximation (LDA) and generalized gradient approximation (GGA) exchange correlation potentials have been tested. Specific heat () obtained through LDA and GGA agrees with experimental data up to 1500 K. Above 1500 K, GGA gives better agreement whereas LDA under-estimates the specific heat. LDA overestimates the bulk modulus, GGA gives better agreement with the experimental data. High-temperature bulk modulus follows the Wachtman formula. Calculated isotherms agree with published experimental results. The transformation pressures () from NaCl-type structure (B phase) to CsCl-type structure (B phase), and collapsed volumes () at () have been predicted. The stability and hardness of these compounds are related with the calculated density of states.

Comparisons between Fast Shock Tube Simulations and Tests

The experiments of Menikoff et al on a projectile hypervelocity launcher using a fast shock tube (FST) are modelled using smooth particle hydrodynamics (SPH) technique. In a FST, the progressive detonation of a co-axial HE cylinder induces a cumulative shock in the liquid-filled core. This shock hits a thin flyer and accelerates it to hypervelocity. The comparisons are made on flyer velocity profile, peak pressure and shock speed in liquid core. The SPH reproduces the qualitative and quantitative aspects of the FST and is well-suited to the high strain-rate feature of this experiment.

Generation of Electrical Conductivity Data of Al using Ab‐initio Molecular Dynamics simulations

Electrical conductivity data of Al for the densities from normal to a fraction of normal density and temperatures up to a few thousand Kelvin are important inputs for the hydrodynamic simulations of exploding aluminium foils. We have generated conductivity data of aluminium, by using ab‐initio molecular dynamics simulations and the Kubo Greenwood formula. ABINIT code has been used for performing AIMD simulations. Our results are in good agreement with published results.