Narrow Results

Tracer dispersion is governed by the velocity fluctuations that the particles are subjected to during their movement. The fluctuation of particle velocity is due to deviations from the mean velocity in the flow field and also to the change of the streamline caused by diffusion. The lattice-BGK method is a good tool to investigate the interaction of both of them, because it models the flow field in detail with even small flow structures. A serious drawback of direct simulations are the requirements in computer time and memory. For spatially periodic media, this can be overcome by using the generalized Taylor-dispersion method to calculate the asymptotic effective dispersion from a solution in an elementary cell. This solution is obtained by simulations with an FHP-BGK-lattice gas. Joining the two methods yields a tool to study the effective dispersion constant of a given periodic geometry.

In this paper, we consider the macroscopic quantity, namely the dispersion tensor associated with a periodic structure in one dimension (see Refs. 5 and 7). We describe the set in which this quantity lies, as the microstructure varies preserving the volume fraction.

We consider periodically heterogeneous fluid-saturated poroelastic media described by the Biot model with inertia effects. The weak and semistrong formulations for displacement, seepage and pressure fields involve three equations expressing the momentum and mass balance and the Darcy law. Using the two-scale homogenization method, we obtain the limit two-scale problem and prove the existence and uniqueness of its weak solutions. The Laplace transformation in time is used to decouple the macroscopic and microscopic scales. It is shown that the seepage velocity is eliminated from the macroscopic equations involving strain and pressure fields only. The plane harmonic wave propagation is studied using an example of layered medium. Illustrations show some influence of the orthotropy on the dispersion phenomena.

A stable matrix method is presented for studying acoustic wave propagation in thick periodically layered anisotropic media at high frequencies. The method enables Floquet waves to be determined reliably based on the solutions to a generalized eigenproblem involving scattering matrix. The method thus overcomes the numerical difficulty in the standard eigenproblem involving cell transfer matrix, which occurs when the unit cell is thick or the frequency is high. With its numerical stability and reliability, the method is useful for analysis of periodic media with wide range of thickness at high frequencies.

Numerical analysis of wave propagation in composite structures needs large-scale computation which is not feasible in practice. This paper investigates the possibility of applying a wave finite element method (WFEM) to composite structure. The key feature of the WFEM is its capability of accurately analyzing the discontinuities in stress wavefront along with the discontinuities in velocities and strains. In addition, the numerical analysis of the WFEM is unconditionally stable and reduces numerical computation. This paper studies a periodically layered composite rod to analyze the dispersion and propagation of stress waves using the WFEM. The numerical results are compared with other numerical/analytical solutions, paying attention to the accuracy of computing the strong discontinuities in the stress wavefront as well as the dispersion of pulses in a heterogeneous elastic rod. It is shown that the WFEM can be used as an affordable tool for numerically solving wave propagation in composite structures.