A method is proposed for the calculation of bulk viscosity, ${\mu }_b$, of dilute gases using nonequilibrium molecular dynamics (NEMD) simulations. The method uses measurement of mechanical ($P_{\mathrm{mech}}$) and thermodynamic pressure ($P_{\mathrm{thermo}}$) in a NEMD simulation of an expanding fluid and then relates the bulk viscosity to them by the relation ${\mu }_b=(P_{\mathrm{mech}}-P_{\mathrm{thermo}})/\mathbf{\nabla }·\vec{u}$, where $\mathbf{\nabla }·\vec{u}$ is the controlled rate of expansion of the fluid per unit volume. A special emphasis is given to the fundamental physical understanding of bulk viscosity in this work. The proposed method is demonstrated for the estimation of bulk viscosity of dilute nitrogen gas in the temperature range of 200–800 K. Variation of bulk viscosity with temperature is reported in the above-mentioned temperature range and is found to be in a reasonably good agreement with the data available in the literature. Furthermore, variation of bulk viscosity with pressure and volumetric expansion rate is obtained. A weak linear dependency of bulk viscosity with pressure is observed, which increases with temperature. However, no significant effect of the rate of volumetric expansion is observed until ${10}^8{\mathrm{s}}^{-1}$. Moreover, the effect of the direction of volumetric change (expansion vs. compression) is also demonstrated in this work.

A novel, two-way coupled, dusty-gas flow model has been developed in the direct simulation Monte Carlo (DSMC) framework and employed for the dust-dispersion study on lunar surface. In this model, the gas–gas collisions are modeled probabilistically, whereas, grain–grain interactions are computed deterministically. Most importantly, the gas–grain interactions are modeled in a two-way coupled manner through the consideration of momentum and energy exchange between the two phases. The proposed model is validated against the two-phase theoretical relations for a zero-dimensional simulation. The computational model is used to study the dust dispersion problem due to plume impingement on lunar surface. The influence of particle diameter and hovering altitudes on gas and grain phases, and dust transportation are analyzed in the modified DSMC framework. Furthermore, the sensitivity of the two-way coupled gas–grain interaction model is discussed in relation to the one-way coupled model.

A conjugate thermal analysis is performed for a charring ablative material by coupling an in-house direct simulation Monte Carlo flow solver with an in-house material thermal response solver at the fluid–solid interface. The coupling is achieved by exchanging and updating the interface properties (convective heat flux and wall temperature) between the flow and thermal solvers at certain anchor points along the reentry trajectory. Iterative and non-iterative coupling techniques are studied in this work and discussed. In-house flow-thermal code is validated by comparing surface heat flux and temperature variation with those obtained from an open-source code, Stochastic PArallel Rarefied-gas Time-accurate Analyzer, from Sandia National Laboratories, USA. The effect of introducing pyrolysis gas at the interface in the flow-thermal analysis is studied by applying a blowing correction function in an iterative manner. The effect of surface recession is also studied by enabling material degradation due to thermal ablation.

In this work, we present a study for the estimation of bulk viscosity using the equilibrium molecular dynamics-based Green–Kubo method. We have performed a parametric study to find optimal hyper-parameters to estimate bulk viscosity using the Green–Kubo method. Although similar studies exist for shear viscosity, none has been reported so far specifically for bulk viscosity. The expected uncertainty in bulk viscosity for a given length and number of molecular dynamics trajectories used in statistical averaging is determined. The effect of system size, temperature, and pressure on bulk viscosity has also been studied. The study reveals that the decay of autocorrelation function for bulk viscosity is slower than that for shear viscosity and hence requires a longer correlation length. A novel observation has been made that the autocorrelation length required for convergence in the Green–Kubo method for both shear and bulk viscosity of dilute nitrogen gas is of the same mean collision time length units irrespective of simulation pressure. However, when the temperature is varied, the required autocorrelation length remains unaffected for shear viscosity but increases slightly with temperature for bulk viscosity. The results obtained from the Green–Kubo method are compared with experimental and numerical results from the literature with special emphasis on their comparison with the results from the nonequilibrium molecular dynamics-based continuous expansion/compression method. Although the primary focus and novelty of this work are the discussion on bulk viscosity, a similar discussion on shear viscosity has also been added.

Translational and rotational dynamics of a transporting particle in the free molecular gas flow regime are investigated using an in-house, multi-species, three dimensional Direct Simulation Monte Carlo (DSMC) solver. The DSMC algorithm is modified to study free molecular gas flows and validated against the analytical results for the dynamics of a spherical particle. A particular focus of this work is on estimating the effects of particle size, shape, and orientation on the dynamics of an ellipsoidal particle in free molecular gas and comparing them with that of a spherical particle. Properties such as particle speed, temperature, drag, and heat transfer coefficients are considered for comparison. The effect of particle shape on the aforesaid properties is qualitatively discussed and quantified through a comprehensive analysis taking care of lift, pitching moment, particle rotation, and the associated resistive torque. The relaxation of an ellipsoidal particle to surrounding gas conditions is simulated at zero and non-zero angles of attack to demonstrate the effect of particle orientation. Furthermore, the particle size effect on its translational and rotational dynamics is discussed. Finally, the trajectory of a spherical particle is compared with that of an ellipsoidal particle at different eccentricities.