Articles written in Sadhana
Volume 27 Issue 6 December 2002 pp 657-673
Mixing of coaxial jets within a tube in the presence of blockage has been numerically studied. This configuration is encountered during the modelling of flow and heat transfer in CVD (
Volume 38 Issue 6 December 2013 pp 1135-1171
Vapor-to-liquid phase change in the form of discrete drops on or underneath a substrate is called dropwise condensation. The process is hierarchical in the sense that it occurs over a wide range of length and timescales. As the associated heat transfer coefficient is much higher than the film and mixed mode of condensation, it is of considerable interest in applications. The present study is focused on mathematical modelling of dropwise condensation process at multiple scales. The model includes formation of drops at the atomistic scale, droplet growth, coalescence, instability, slide off and fall-off, followed by fresh nucleation of liquid droplets. The model shows that the largest stable cluster size in the atomic model matches the minimum drop radius estimated from thermodynamic considerations. The minimum drop radius is insensitive to surface texturing and does not provide controllability at larger length and timescales. A closer examination of droplet distribution over the substrate reveals that small drops are locations of high heat transfer rates, which diminishes with increasing drop radius. The largest drop diameter depends on its stability and hence, the interfacial forces at phase boundaries. Therefore, drop instability controls the heat transfer coefficient in dropwise condensation. Enhancement of heat transfer necessitates that these drops grow with time, become unstable and be swept away as quickly as possible. Enhancement may be achieved either by
inclining the substrate or
by creating an interfacial force at the three-phase contact line by a wettability gradient over the horizontal substrate, inducing drop motion.
Wall heat transfer and shear stress under moving drops have been determined using a CFD model. A simple model of coalescence has been adopted in this work. Simulation studies on the effect of fluid properties, surface inclination and its wettability condition on drop size distribution, cycle time, heat transfer coefficient, and wall shear stress are comprehensively discussed in the present article.
Volume 40 Issue 3 May 2015 pp 625-625 Section I – Fluid Mechanics and Fluid Power (FMFP)
Volume 40 Issue 3 May 2015 pp 875-887 Section I – Fluid Mechanics and Fluid Power (FMFP)
Coil embolization is a mildly invasive endovascular method for treatment of a cerebral aneurysm. The presence of a coil reduces fluid loading of the blood vessel and delays further deformation of the walls. Its effectiveness depends on the coil porosity and permeability apart from the nature of flow pulsations and its geometry. In the present work, a three dimensional numerical study of pulsatile flow of blood through an artery with saccular cerebral aneurysm is reported. The flow is unsteady but is taken to be laminar and incompressible. The coil is treated as homogeneous and isotropic porous medium. A comparative study has been carried out on aneurysms with and without a coil insert considering blood as a non-Newtonian fluid. The simulation is carried out for Reynolds numbers $Re$ = 500 and 1500. Results show that the velocity magnitude within the coil embolized aneurysm becomes negligible after coil insertion. The wall shear stress within the aneurysm decreases to a great extent for both Reynolds numbers. Pressure levels remain relatively unchanged. Overall, reduced wall loading with a coil stabilizes the growth of the aneurysm and thus provides an advantage.
Volume 41 Issue 1 January 2016 pp 97-110
Although combining flows are common in natural streams, no comprehensive experimental data has been compiled to characterize the three-dimensional flow field within the compound channel confluence. The present study examines the time-averaged flow structure at confluence over a rigid bed. Current knowledge of channel confluence, based on laboratory observation indicates that cross flow interactions exert a significant influence on confluence events. Secondary current and turbulent stresses are reproduced well by the hydraulic model and found greater in the interface region as relative flow ratio decreases. Velocity fields in combining flow region arising from varying discharge ratios are presented. A zone of depression in surface elevation in compound channel junction is observed as well. The flow field in compound channel is seen to be moderately different from that of simple channel junction. This study contributes to a better knowledge of hydraulic key processes into fundamental aspect of combining flow dynamics.