Jack Chessa

An Extended Finite Element Method for Two-Phase Fluids

An extended finite element method with arbitrary interior discontinuous gradients is applied to two-phase immiscible flow problems. The discontinuity in the derivative of the velocity field is introduced by an enrichment with an extended basis whose gradient is discontinuous across the interface. Therefore, the finite element approximation can capture the discontinuities at the interface without requiring the mesh to conform to the interface, eliminating the need for remeshing. The equations for incompressible flow are solved by a fractional step method where the advection terms are stabilized by a characteristic Galerkin method. The phase interfaces are tracked by level set functions which are discretized by the same finite element mesh and are updated via a stabilized conservation law. The method is demonstrated in several examples

An ab initio study of the elastic behavior of single crystal group (IV) diborides at elevated temperatures

We report on an ab initio molecular dynamics study of the lattice parameters, thermal expansion coefficients, and elastic constants of ZrB2, TiB2, and HfB2 ceramics at ultrahigh temperatures (up to 2200 K). Equilibrium lattice parameters of the ceramics are determined at finite temperatures. A finite strain method is used to extract the stiffness tensor of the ceramics. The results obtained for ZrB2 and TiB2 agree well with experimental results reported in the literature. Our work demonstrate that accurate properties may be obtained from a statistical averaging of the lattice parameters alone neglecting phonon interactions.

Flame synthesis of coiled carbon nanotubes

Abstract A flame synthesis technique of coiled carbon nanotubes using acetylene jet flames and iron catalyst is presented. It was found that acetylene flames generate single wall coiled nanotubes within in a narrow pyrolysis zone above the burner for a limited range of fuel flow rates. The corresponding local convective time scale corresponding to the coiled nanotube formation zone is around 0.04 ms. The yield of the coiled carbon nanotubes was further increased by changing the local convective time scale with the addition of controlled turbulence.

Analytical and Finite Element Analysis of Thermal Stresses in TiN Coatings

The thermal residual stress and associated effects in TiN coatings with planar and nonplanar surface roughness on silicon (100) substrates were analyzed using both analytical and finite element (FE) modeling. The effect of growth temperature (Ts), thickness, and the modulus of elasticity on the stress evolution in TiN coatings is reported. The results indicate that the variable thermal stress in the TiN coatings is due to the existence of both positive and negative temperature gradients and thus the resultant existence of disparity of the coefficient of thermal expansion between the substrate and the coating.