Influence of Boundary Roughness on Magnetohydrodynamic Kelvin-Helmholtz Instability in Couple-stress Fluid

The Kelvin-Helmholtz instability (KHI) occurs at the interface amongst two fluids, which are in relative motion with a common boundary. The growth rate of waves occurs whenever the relative velocity is greater as compared with the critical relative velocity. In the present paper, the influence of boundary roughness on KHI under the impact magnetic field in a couple-stress fluid layer bounded by a rigid surface at the lower side and upper side by a fluid saturated porous layer. Using suitable surface and boundary conditions, we have derived the dispersion relation and results are depicted graphically. As observed in presence of sharp interface, magnetic field exhibits stabilizing effect however, destabilizing effect is shown by the buoyancy force on KHI. Also, noted that the growth rate of interface reduces, as there is a rise in roughness parameter value.

An Analytical Simulation of Boundary Roughness for Incompressible Viscous Flows

The intention of this paper is to investigate the boundary roughness of a mounted obstacle which is inserted into an incompressible, external and viscous flow field of a Newtonian fluid. In particular, the present study focuses on the cross – sectional area of the obstacle, which is assumed to be a non deformable body (rigid object) with a predefined shape of random roughness. For facility reasons and without violating the generality, one may select the cross – section of the body which contains its center of gravity and is perpendicular to the main flow direction. The boundary of this cross – sectional area is mathematically simulated as the polygonal path of the length of a single – valued continuous function. Evidently, this function should be of bounded variation. The novelty of this work is that the formulation of the random roughness of the boundary has been carried out in a deterministic manner.

The effect of boundary roughness on dense granular flows

In the field of granular rheology, an important open question is to understand the influence of boundary conditions on granular flows. We perform experiments in a quasi-2D annular shear cell subject to 6 different boundaries with controlled roughness/compliance. We characterize the granular slip at the boundaries to investigate which aspects of a dense granular flow can be controlled by the choice of boundary condition. Photoelastic techniques are implemented to measure the stress fields P(r) and τ(r) throughout the material. A full inverse-analysis of the fringes within each disk provides the vector force at each contact. This allows us to measure the continuum stress field by coarse-graining internal forces. We have observed that boundary roughness and compliance strongly controls the flow profile v(r) and shear rate profile γ˙(r). We also observed that boundary roughness and compliance play a significant role in the pressure profile P(r) and shear stress profile τ(r).

Optics of Inhomogeneous Thin Films with Defects: Application to Optical Characterization

This review paper is devoted to optics of inhomogeneous thin films exhibiting defects consisting in transition layers, overlayers, thickness nonuniformity, boundary roughness and uniaxial anisotropy. The theoretical approaches enabling the inclusion of these defects into formulae expressing the optical quantities of these inhomogeneous thin films are summarized. These approaches are based on the recursive and matrix formalisms for the transition layers and overlayers, averaging of the elements of the Mueller matrix using local thickness distribution or polynomial formulation for the thickness nonuniformity, scalar diffraction theory and Rayleigh-Rice theory or their combination for boundary roughness and Yeh matrix formalism for uniaxial anisotropy. The theoretical results are illustrated using selected examples of the optical characterization of the inhomogeneous polymer-like thin films exhibiting the combination of the transition layers and thickness nonuniformity and inhomogeneous thin films of nonstoichiometric silicon nitride with the combination of boundary roughness and uniaxial anisotropy. This characterization is realized by variable angle spectroscopic ellipsometry and spectroscopic reflectometry. It is shown that using these optical techniques, the complete optical characterization of the mentioned thin films can be performed. Thus, it is presented that the values of all the parameters characterizing these films can be determined.

Effect of boundary roughness on nonlinear saturation of Rayleigh-Taylor instability in couple-stress fluid

The boundary roughness effects on nonlinear saturation of Rayleigh-Taylor instability (RTI) in couple-stress fluid have been studied using numerical technique on the basis of stability of interface between two fluids of the system. The resulting fourth order ordinary nonlinear differential equation is solved using Adams-Bashforth predictor and Adams-Moulton corrector techniques numerically. The various surface roughness effects and surface tension effects on nonlinear saturation of RTI of two superposed couple-stress fluid and fluid saturated porous media are well investigated. At the interface, the surface tension acts and finally stability of the problem is discussed in detail.

EXPERIMENTAL STUDY OF THE REFLECTION OF ELASTIC WAVES FROM ROUGH BOUNDARIES

The results of physical modeling show that the influence of boundary roughness on its reflectivity depends on the ratio of roughness characteristic dimension and wavelength. The boundary roughness leads to a decrease in the effective reflection coefficients at subcritical and to their increase at supercritical angles of incidence. A change in the azimuth of the plane of incidence with respect to the roughness direction appreciably affects the reflection coefficients only at supercritical angles.