Articles written in Sadhana

    • Effect of external pulsation on kinematics of fluid particles in the field of Lamb–Oseen vortex pair


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      The effect of external pulsation on a pair of stationary Lamb–Oseen vortices of equal strength has been analyzed to investigate kinematic behavior of a fluid particle. The assumption of vortices being treated stationary or fixed vortex filaments is valid in a reference frame attached to the vortex system with axes along and perpendicular to the line of their centers. Also, it is assumed that change in core shape and size is much small, with least possibility of core merger. In such situations, periodic particle paths are observed and superposition of pulsation becomes beneficial. In the present work, motion of a representative fluid particle is modeled as a non-linear dynamical system by varying both amplitude and frequency of external pulsation. Effect of external pulsation has been brought out with the help of quantification of deviation from periodic paths by using the concept of total average deviation. Results are presented in terms of particle paths, velocity phase plots, velocity signals and their spectra for varying amplitude and frequency of external pulsation.

    • Onset of natural convection in a differentially heated layer of gray and non-gray gas mixtures


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      The onset of convection instability in a differentially heated layer consisting of gray and non-gray gaseous mixtures is studied numerically. The conditions investigated cover a wide range of Planck number values (Pl = ¼ KkT/ 4sT3 0 ), from the conduction-dominated regime of Pl > > 1 to the radiation-dominated regime of Pl < < 1. The linear stability theory is applied to mass, momentum and energy balance equations and the resulting linear stability equations are solved by Chebyshev spectral collocation method. The divergence of radiative flux is solved by the finite-volume-based discrete ordinates method. The Spectral Line Weighted sum of gray gas (SLW) model is used to represent the fine spectral variation of absorption coefficient for a non-gray gas medium. The results indicate that the critical Rayleigh number (Rac) for the onset of convection increases with mean temperature (T0) in the conduction-dominated regime at low values of T0. In the radiation-dominated regime (Pl << 1), Rac decreases with T0 for gray media. If the medium is non-gray, the critical Rac reduces to even lower values (as compared with those of gray gases) due to the dependence of gas absorptivity on temperature T0. A reduction in the wall emissivity value increases the stability of the fluid layer due to reflection of radiation from the wall, in the radiation-dominated regime. The reverse trend is seen for Pl >> 1. The critical parameters also significantly depend on the concentrations of radiatively participating gases in the mixture. The temperature profile in the fluid layer transforms from a linear profile in conduction regime to a stratified profile with steep gradients near the walls, in the presence of non-gray participating gases

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