A. Sameen

Bio: A. Sameen is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Reynolds number & Vortex. The author has an hindex of 12, co-authored 44 publications receiving 454 citations. Previous affiliations of A. Sameen include Indian Institute of Science & Jawaharlal Nehru Centre for Advanced Scientific Research.

Dynamic effects in transition from regular to Mach reflection in steady supersonic flows

Abstract: The effect of rapid wedge rotation on the transition from regular (RR) to Mach reflection (MR) is investigated. This unsteady shock reflection transition is compared with the steady-state transition. The dependence of various flow features such as the unsteady Mach stem height, position of the reflection point, and shock angle at the reflection or triple point on the wedge angle for a fixed Mach number is compared at various rotation rates. The study is further extended to compare the dynamic effects for various Mach numbers in the strong shock reflection domain at higher wedge speeds. Transition lines corresponding to different rotation speeds are obtained similar to the detachment transition line in steady cases. It is found that the pivot point has only marginal effect on the transition point, but it substantially affects the Mach stem growth and the movement of the reflection point, specifically at higher Mach numbers. The location of the transition from the inlet also depends on the pivot point and the rate of rotation.

Scale-space energy density function transport equation for compressible inhomogeneous turbulent flows

Abstract: The scale-space energy density function and can be considered as the generalization of the spectral energy density function concept to inhomogeneous flows. In this work, we derive the scale-space energy density function transport equation for compressible flows to develop a better understanding of scale-to-scale energy transfer and the degree of non-locality of the energy interactions. Specifically, the effects of variable-density and dilatation on an energy cascade are identified. It is expected that these findings will yield deeper insight into compressibility effects on canonical energy cascades, which will lead to improved models (at all levels of closure) for mass flux, density variance, pressure-dilatation, pressure–strain correlation and dilatational dissipation processes. Direct numerical simulation (DNS) data of mixing layers at different Mach numbers are used to characterize the scale-space behaviour of different turbulence processes. The scaling of the energy density function that leads to self-similar evolution at the two Mach numbers is identified. The scale-space (non-local) behaviour of the production and pressure dilatation at the centre-plane is investigated. It is established that production is influenced by long-distance (order of vorticity thickness) interactions, whereas the pressure dilatation effects are more localized (fraction of momentum thickness) in scale space. The analysis of DNS data demonstrates the utility of the energy density function and its transport equation to account for the relevance of various physical mechanisms at different scales.

Effect of vortex line distribution in superfluid plane Poiseuille flow instability

Abstract: The hydrodynamic stability of plane Poiseuille flow of superfluid is studied using modal and non-modal analysis. Two modes of instability are predicted, in normal mode stability analysis of the normal fluid, one caused by viscosity similar to the classical mode and another due to mutual friction between superfluid and normal fluid. The mutual friction mode occurs at high wavenumbers, which are stable wavenumbers in classical plane Poiseuille flow. A high superfluid vortex line density alone is not enough to induce instability in normal fluid; a localization of vortex lines is shown to play a major role. The extent of vortex line concentration required to cause instability depends on the density itself. Non-modal instability analysis shows that oblique waves are stronger than streamwise waves, unlike the scenario in classical plane Poiseuille flow.

Boundary Layer Theory

Abstract: The boundary layer equations for plane, incompressible, and steady flow are $$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

New perspectives in turbulent Rayleigh-Bénard convection.

Abstract: Recent experimental, numerical and theoretical advances in turbulent Rayleigh-Benard convection are presented. Particular emphasis is given to the physics and structure of the thermal and velocity boundary layers which play a key role for the better understanding of the turbulent transport of heat and momentum in convection at high and very high Rayleigh numbers. We also discuss important extensions of Rayleigh-Benard convection such as non-Oberbeck-Boussinesq effects and convection with phase changes.

Global Linear Instability

Abstract: This article reviews linear instability analysis of flows over or through complex two-dimensional (2D) and 3D geometries. In the three decades since it first appeared in the literature, global instability analysis, based on the solution of the multidimensional eigenvalue and/or initial value problem, is continuously broadening both in scope and in depth. To date it has dealt successfully with a wide range of applications arising in aerospace engineering, physiological flows, food processing, and nuclear-reactor safety. In recent years, nonmodal analysis has complemented the more traditional modal approach and increased knowledge of flow instability physics. Recent highlights delivered by the application of either modal or nonmodal global analysis are briefly discussed. A conscious effort is made to demystify both the tools currently utilized and the jargon employed to describe them, demonstrating the simplicity of the analysis. Hopefully this will provide new impulses for the creation of next-generation algorithms capable of coping with the main open research areas in which step-change progress can be expected by the application of the theory: instability analysis of fully inhomogeneous, 3D flows and control thereof.

Instabilities in Viscosity-Stratified Flow

Abstract: This review highlights the profound and unexpected ways in which viscosity varying in space and time can affect flow. The most striking manifestations are through alterations of flow stability, as established in model shear flows and industrial applications. Future studies are needed to address the important effect of viscosity stratification in such diverse environments as Earth's core, the Sun, blood vessels, and the re-entry of spacecraft.