Ganesh Subramanian

Field and laboratory experiments on aerosol-induced cooling in the nocturnal boundary layer

2017-02-21T10:25:22Z

Field and laboratory experiments on aerosol-induced cooling in the nocturnal boundary layer Mukund, V.; Singh, D. K.; Ponnulakshmi, V. K.; Subramanian, Ganesh; Sreenivas, K. R. Heat transfer processes in the nocturnal boundary layer (NBL) influence the surface energy budget and play an important role in many micrometeorological processes, including the formation of inversion layers, radiation-fog and in the control of air-quality near the ground. Under calm and clear-sky conditions, radiation plays an important role in determining the characteristics of the NBL. In this article, we report observations, close to ground, of hypercooling that has a radiative origin, and which leads to anomalous vertical temperature profiles with elevated minima. In addition, a laboratory experimental set-up is developed that is capable of capturing the thermal structure of the NBL, close to ground, under various conditions. Results from the laboratory experiments indicate that the high cooling rates near the ground, observed in the field experiments, arise from a near-surface heterogeneity in the (aerosol-laden) NBL; a feature ignored in radiation models used for atmospheric simulations. Many of these models nevertheless predict preferential near-ground cooling in apparent agreement with our field observations. However, the cooling is spurious, and arises from the use of an incorrect frequency-averaged transmittance in the radiation model. Based on our observations, a non-dimensional number is proposed that characterizes the evolution in the lowest metres of the NBL; in particular, the effect of radiation on the NBL thermal structure. Our results should help in parametrizing NBL transport process, and highlight the need to account for both the effects of aerosols close to the ground and a varying ground emissivity, via appropriate boundary conditions in general circulation and climate models. Restricted Access

Stochastic dynamics of active swimmers in linear flows

2017-02-21T10:25:15Z

Stochastic dynamics of active swimmers in linear flows Sandoval, Mario; Marath, Navaneeth K.; Subramanian, Ganesh; Lauga, Eric Most classical work on the hydrodynamics of low-Reynolds-number swimming addresses deterministic locomotion in quiescent environments. Thermal fluctuations in fluids are known to lead to a Brownian loss of the swimming direction, resulting in a transition from short-time ballistic dynamics to effective long-time diffusion. As most cells or synthetic swimmers are immersed in external flows, we consider theoretically in this paper the stochastic dynamics of a model active particle (a self-propelled sphere) in a steady general linear flow. The stochasticity arises both from translational diffusion in physical space, and from a combination of rotary diffusion and so-called run-and-tumble dynamics in orientation space. The latter process characterizes the manner in which the orientation of many bacteria decorrelates during their swimming motion. In contrast to rotary diffusion, the decorrelation occurs by means of large and impulsive jumps in orientation (tumbles) governed by a Poisson process. We begin by deriving a general formulation for all components of the long-time mean square displacement tensor for a swimmer with a time-dependent swimming velocity and whose orientation decorrelates due to rotary diffusion alone. This general framework is applied to obtain the convectively enhanced mean-squared displacements of a steadily swimming particle in three canonical linear flows (extension, simple shear and solid-body rotation). We then show how to extend our results to the case where the swimmer orientation also decorrelates on account of run-and-tumble dynamics Self-propulsion in general leads to the same long-time temporal scalings as for passive particles in linear flows but with increased coefficients. In the particular case of solid-body rotation, the effective long-time diffusion is the same as that in a quiescent fluid, and we clarify the lack of flow dependence by briefly examining the dynamics in elliptic linear flows. By comparing the new active terms with those obtained for passive particles we see that swimming can lead to an enhancement of the mean-square displacements by orders of magnitude, and could be relevant for biological organisms or synthetic swimming devices in fluctuating environmental or biological flows. Restricted Access

Rotational motion of a thin axisymmetric disk in a low Reynolds number linear flow

2017-02-21T10:25:12Z

Rotational motion of a thin axisymmetric disk in a low Reynolds number linear flow Singh, Vikram; Koch, Donald L.; Subramanian, Ganesh; Stroock, Abraham D. The motion of thin, torque-free axisymmetric rigid particles with fore-aft symmetry in low Reynolds number linear flows is investigated. The rotational motion of such a particle is fully determined by the effective aspect ratio (kappa(e)), defined as the aspect ratio of a spheroid having the same period of rotation as that of the particle. We determine the effective aspect ratio for a family of shapes given by, y(rho) =kappa(1 -rho(2))alpha where alpha is a positive parameter, rho is the radial distance from the particle center in polar coordinates, y is half the thickness of the particle, kappa is the aspect ratio of the particle defined as the ratio of the thickness (L) of the particle parallel to the axis of symmetry to its diameter (d) perpendicular to the axis of symmetry. This family includes an oblate spheroid (alpha = 1/2) and the shape approaches a blunt circular cylinder shape as alpha -> 0. For a thin particle, the effective aspect ratio scales like G(A)(1/2), where G(A) is the torque non-dimensionalized by mu gamma d(3) acting on a particle held in a fixed alignment in a simple shear flow with its longer dimensions in the flow-vorticity plane. Here, mu is the fluid viscosity and gamma is the shear rate. Starting with the integral representation of the Stokes flow, an analysis based on a matched asymptotic expansions approach is performed to determine the scaling of the torque acting on a stationary particle in simple shear flow with kappa as the small parameter. Using boundary element method simulations, the exact torques are calculated and the scaling obtained from the analysis is verified. We find that there are two regions of interest that contribute to the torque, a flat outer region covering most of the disk area and a boundary layer region of large slope at the edge. For alpha > 1/4, the torque is dominated by the stresses acting on the flat surface of the particle and is O(kappa(2)) to the leading order. For these shapes, the effective aspect ratio scales like the aspect ratio of the particle. On the other hand, for alpha <= 1/4, the leading order torque contribution comes from the boundary layer region. To the leading order, the torque scales as O(kappa(2)log kappa) for alpha = 1/4 and as O(kappa(3/2(1-alpha))) for alpha < 1/4. In general, a particle with a blunt edge is found to rotate faster than a particle of the same aspect ratio for which the slope of the edge varies slowly. The fastest rotating particle is identified to be a cylindrical disk for which kappa(e) = 1.12 kappa(3/4). (C) 2014 AIP Publishing LLC. Restricted Access

Linearized oscillations of a vortex column: the singular eigenfunctions

2017-02-21T10:25:10Z

Linearized oscillations of a vortex column: the singular eigenfunctions Roy, Anubhab; Subramanian, Ganesh In 1880 Lord Kelvin analysed the linearized inviscid oscillations of a Rankine vortex as part of a theory of vortex atoms. These eponymously named neutrally stable modes are, however, exceptional regular oscillations that make up the discrete spectrum of the Rankine vortex. In this paper, we examine the singular oscillations that make up the continuous spectrum (CS) and span the entire base state range of frequencies. In two dimensions, the CS eigenfunctions have a twin-vortex-sheet structure similar to that known from earlier investigations of parallel flows with piecewise linear velocity profiles. The vortex sheets are cylindrical, being threaded by axial lines, with one sheet at the edge of the core and the other at the critical radius in the irrotational exterior; the latter refers to the radial location at which the fluid co-rotates with the eigenmode. In three dimensions, the CS eigenfunctions have core vorticity and may be classified into two families based on the singularity at the critical radius. For the first family, the singularity is a cylindrical vortex sheet threaded by helical vortex lines, while for the second family it has a localized dipole structure with radial vorticity. The presence of perturbation vorticity in the otherwise irrotational exterior implies that the CS modes, unlike the Kelvin modes, offer a modal interpretation for the (linearized) interaction of the Rankine vortex with an external vortical disturbance. It is shown that an arbitrary initial distribution of perturbation vorticity, both in two and three dimensions, may be evolved as a superposition over the discrete and CS modes; this modal representation being equivalent to a solution of the corresponding initial value problem. For the restricted case of an initial axial vorticity distribution in two dimensions, the modal representation may be generalized to a smooth vortex. Finally, for the three-dimensional case, the analogy between rotational flows and stratified shear flows, and the known analytical solution for stratified Couette flow, are used to clarify the singular manner in which the modal superposition for a smooth vortex approaches the Rankine limit. Restricted Access