Arakeri, Jaywanth H

In bioinspired pulse jet studies, the apparatus is typically stationary. But does this truly reflect natural flow dynamics where an animal is in motion We examine this experimentally using a clapping body and report differences in flow dynamics and body kinematics between the body that is allowed to propel forward freely (dynamic) and one that is constrained from moving forward (stationary). The experiments were done in quiescent water. The body consists of two interconnected plates hinged at one end with a "torsion"spring. Initially, a thread loop holds the plates apart at an interplate angle of 60∘. Cutting of the thread initiates the clapping motion, and if allowed, the body propels forward a certain distance. Experiments have been performed for three values of d∗(=depth/length): 1.5, 1.0, and 0.5. In both cases, vortex loops initially develop along the three edges of each plate, which reconnect by the end of the clapping motion resulting in the formation of an elliptical vortex loop in the wake for d∗=1.5, 1.0 bodies, and multiple connected rings for d∗=0.5 bodies. Three main and unexpected differences are observed between the "dynamic"and "stationary"bodies. In the dynamic case, the clapping action is faster compared to the stationary case, with the maximum angular plate velocity being twice as high. The mean thrust coefficient, CT¯, based on plate tip velocity, is higher for the stationary case. The value of CT¯ and the circulation in the starting vortices is almost independent of d∗ for the dynamic case, whereas it increases with d∗ for the stationary case. Control volume analysis showed that significant lateral flow in stationary cases leads to depth-dependent variations in circulation and thrust coefficient, while negligible sideways flow in the dynamic cases eliminates these variations.

Heat transfer in flows created by buoyancy, or natural convection, is a widely studied topic across various disciplines spanning natural flows as well as those with engineering applications. The convective heat transfer rate on a surface is commonly represented by the Nusselt number (Nu), a ratio of convective to diffusive transport, expressed often as R a n P r m , where R a is the Rayleigh number, the buoyancy forcing parameter, and P r the Prandtl number. Motivated by the observation that n ∼ 1 / 3 for turbulent convection, which implies the heat flux is independent of the length scale ( L , characteristic length related to the geometry), we propose an alternate and physically more meaningful non-dimensional heat transfer parameter, denoted by C q . C q is derived using only the near wall variables and does not contain L . For n = 1 / 3 , C q is constant. Even for laminar convection, where n ∼ 1 / 4 , C q ∼ R a − 1 / 12 , a weak function of R a . We show that for natural convection over several geometries and a wide range of Ra, the C q values within a narrow range while the corresponding Nu values span several orders of magnitude. We also show that C q is akin to the non-dimensional representation of wall shear stress, skin friction coefficient C f . We believe that just like C f , C q will be an equally useful non-dimensional measure of heat transfer in natural convection flows.

A model for obtaining scaling laws for Rayleigh-Bénard convection (RBC) at high Rayleigh numbers in tall, slender cells (cells with low aspect ratio, (Formula presented)) is presented. Traditional RBC ((Formula presented)) is characterised by large-eddy circulation scaling with the height of the cell, a near-isothermal core and almost all the thermal resistance provided at the horizontal walls. In slender RBC cells, on the other hand, away from the horizontal walls, tube-like convection with eddies scaling with the tube diameter and a linear temperature gradient driving the convective flow is present. The crux of our approach is to split the cell into two components: (i) ‘wall convection’ near the top and bottom horizontal walls and (ii) ‘tube convection (TC)’ in the central part away from the walls. By applying the scaling relations for both wall convection and TC, and treating the total thermal resistance as a sum of their contributions, unified scaling relations for Nusselt number, Reynolds number and mean vertical temperature gradient in slender RBC cells are developed. Our model is applicable for high enough Rayleigh numbers, such that convection both at the wall and in the tube are turbulent. Our model predictions compare well with the data from various studies in slender RBC cells where these conditions are satisfied. In particular, the effects of (Formula presented) and Prandtl number are well captured. We propose a scaled aspect ratio using which we obtain ‘universal’ correlations for the heat flux and for the fractional temperature drop in the tube that include the effects of Rayleigh and Prandtl numbers. The profiles of suitably scaled horizontal and vertical velocity fluctuations, along with estimates for boundary layer thickness near the horizontal walls, and the radial distribution of the velocity fluctuations in the tube part are also presented. © 2025 Elsevier B.V., All rights reserved.

The present work investigates the dynamical characteristics of single vapor bubble during saturated nucleate boiling of dichloromethane (DCM), one of the high volatile fluids. Boiling experiments have been performed for four heat flux conditions (25, 35, 50, and 65 kW/m²) under atmospheric pressure. Thin-film interferometry and high-speed rainbow schlieren deflectometry were employed in tandem to simultaneously capture the dynamics of microlayer and/or dry-patch and vapor bubbles. A detailed analysis of the temporal evolution of the equivalent diameter of DCM bubbles reveals distinct phases: inertia-controlled growth, a transitional regime, and diffusion-controlled expansion. Furthermore, the temporal evolution of various forces, such as buoyancy, contact pressure, surface tension, and growth forces are examined during the ebullition cycle. Unlike traditional force balance analyses, the spatio-temporally resolved whole-field experimental data is used for delineating the forces involved from bubble inception to departure time (td). Notably, in contrast to the conventional working fluids, for instance water, DCM displays a distinct bubble formation mechanism that is devoid of any microlayer, despite its high wettability with indium-tin-oxide coated glass. As the heat flux increases, there is a corresponding linear increase in bubble departure frequency, with no significant rise in growth time. The force balance analysis during the ebullition cycle of the vapor bubble of the considered high volatile fluid reveals that until 0.8td, the prevailing downward force promotes bubble growth while inhibiting departure. However, beyond 0.8td, the upward force takes precedence, counteracting the downward force and enables the bubble's departure from the nucleating surface. © 2025 Elsevier B.V., All rights reserved.

Moisture loss is a familiar and widely observed phenomenon perceived on a daily basis. Much of the water loss, due to the phase change from liquid to vapours, occurs from bare water surfaces, lands, and green cover. Understanding the magnitude of moisture loss and its physical mechanism in these three completely different types of surfaces is crucial as it forms an important component in hydrological balance and the water cycle. Evaporation is driven by the difference in the vapour concentration at the evaporating surface and in the ambient. A key aspect of evaporation (and transpiration) is obtained from an energy budget which helps in understanding the strong and non-linear coupling between the evaporation rate and the surface temperature. Apart from the optical properties of the surface, the rate of evaporation also depends on the wetness of a surface i.e., completely (water and fully saturated soils) or partially (unsaturated soils and leaves). Over a water body, evaporation occurs uniformly throughout its surface but in partially wet surfaces, it occurs from discontinuous local sites (like stomata in leaves or pores in soils). The open area percentage of stomata is quite low (1–10% only) but it can transpire at rates equivalent to that of a bare water surface. Soils have a slightly higher open area ratio (35–50%) compared to the leaves and the rate of evaporation depends on the saturation level. In the case of a discontinuous wet surface, three important length scales exist – opening size, opening-to-opening spacing, and concentration boundary layer thickness. Soil-like surfaces exhibit two key features – (a) formation of a wet patch consisting of a considerably large number of particles and (b) evaporation from within the pore; the latter is a pore-scale feature while the former is at a larger scale. Broadly speaking, soil surface water content and the moisture distribution inside in the vertical direction govern the rates of evaporation, apart from the environmental conditions. Assessing the conditions of a soil surface is hence of paramount interest for many purposes – agricultural sector, evaporation maps, and remote sensing. We discuss evaporation from various types of fully wet and discontinuously wet surfaces under natural convection conditions. Results are mainly from laboratory-scale experiments, under controlled IR heating. The surfaces include those of conventional porous media consisting of spherical particles, with different sizes, and non-conventional porous media, leaf-like surfaces, closely packed vertically or horizontally oriented circular rods, and closely packed vertical plates. We discuss the impact of particle size and inter-pore spacing on the evaporation dynamics. Finally, we comment on the issues of scaling up these results to larger scales, for example, those encountered in the atmosphere. © Indian Academy of Sciences 2025.