arXiv subjects

Sridhar Balasubramanian

Publications and source records attributed to Sridhar Balasubramanian.

7 recordsLinked to original sources

arXiv2026

Baroclinic wave dynamics in the Ekman-free rotating rectangular annulus with localized forced plume

We report numerical simulations of a rotating rectangular annulus that isolates the Ekman-free bulk of the cylindrical baroclinic annulus, subjected to bi-directional temperature gradients imposed by a uniformly cooled inner wall and a localized forced heated plume at the outer bottom. The finite-volume OpenFOAM solver is employed across combinations of source Richardson number $Ri_0 = 99, 4, 1$ and Rossby number $Ro = 0.3, 0.1, 0.07$. A non-dimensional scaling of the governing equations identifies geostrophic-hydrostatic balance as the leading-order bulk state, a result confirmed a posteriori by the $x$ and $z-$momentum budgets. Baroclinic waves of mode $m=2$ at $Ro=0.3$ transition to $m=3$ as $Ro$ decreases, consistent with the contraction of the Eady deformation radius $L_ρ= NH/f$; Complex Empirical Orthogonal Function (CEOF) analysis characterizes the wave regime and detects a Hopf-bifurcated vacillating state at $Ri_0 = 99,~Ro = 0.1$. The plume morphology, classified through the Morton length scale and source flux-balance parameter, transitions from weak, laterally-swept structures at $Ri_0 = 99$ to sustained columnar plumes traversing the full baroclinic depth at $Ri_0 \leq 4$. The plume entrainment coefficient $Γ(z)$ shows opposite rotational sensitivities at low and high $Ri_0$, which we organize through a local plume Rossby number $Ro_p = w/(2Ωb)$. A mixing-length argument predicts a bulk turbulent heat flux $\overline{u'T'} \propto Ri_0^{-1/2}$, anticipating an order-of-magnitude enhancement from $Ri_0 = 99$ to $Ri_0 = 1$, in agreement with the simulations. A regime map in the $(Ri_0, Ro)$ plane reveals that, within the explored range, the plume-regime and wave-selection problems are approximately separable: $Ri_0$ sets the plume regime while $Ro$ selects the dominant baroclinic wave mode.

physics.flu-dyn↗

arXiv2021

Regional modeling of surface and sub-surface dynamics in the Bay of Bengal using Modular Ocean Model

Regional dynamics of Bay of Bengal is studied using Open Boundary Condition (OBC) in Modular Ocean Model (MOM) to understand the effect of primary and secondary mesoscale features on various bulk ocean products and turbulent fluxes. A horizontal resolution of 0.1o is adopted to resolve a range of mesoscale features. In order to parametrize the vertical mixing, K-Profile Parametrization (KPP) scheme is implemented. The results show successful implementation of OBC in a regional domain with exchange of mass and energy and conservation of mass at the boundaries. The Sea Surface Temperature (SST) and Sea Surface Salinity (SSS) are validated with SODA reanalysis, which are found to be in good agreement. The Mixed Layer Depth (MLD) pattern is very well represented, albeit the magnitude is slightly under-predicted in comparison with SODA reanalysis. Additionally, analysis of flow energetics reveal regional differences in turbulent kinetic energy (K), production flux (P), buoyancy flux (B), and dissipation (ε). Our results clearly reveal the presence of inverse energy-cascade in the southern Bay of Bengal, wherein energy flows back into mean flow structures from the turbulent eddies. The re-energization of mean flow structures is likely to allow the large-scale circulation to persist for longer periods, thereby modifying the local dynamics. Further analysis shows that the B term is the major source of turbulence production in the north and central Bay of Bengal regions. These results convey the various mechanisms by which energy is produced and transferred in different regions of Bay of Bengal.

physics.ao-ph↗

arXiv2020

Super Cyclone Amphan: A Dynamical Case Study

Cyclone Amphan, a super cyclone in the Bay of Bengal after 21 years, intensified from a cyclonic storm (CAT 1) to a super cyclone (CAT 5) in less than 36 hours. It went on to make landfall over West Bengal as a Very Severe Cyclonic Storm (VSCS) with winds close to 155 kmph. Here, we analyze the dynamics that led to its rapid intensification, given that the system struggled to develop initially despite the presence of ripe conditions like high Sea Surface Temperature (SST) in the Bay. Our analysis clearly reveals that a Convectively Coupled Kelvin Wave (CCKW) from upper troposphere might have initiated strong instabilities in the tropopause, which then propagated vertically downward and interacted with surface disturbances to promote convective coupling with the Madden Julian Oscillations (MJO). Such convective coupling resulted in a burst of westerly winds along with enhanced vertical mixing and moisture convergence, which eventually led to the formation and intensification of super cyclone, Amphan.

physics.ao-ph↗

arXiv2020

Energetics and mixing efficiency of lock-exchange gravity currents using simultaneous velocity and density fields

A series of laboratory experiments on energy conserving gravity currents in a lock-exchange facility are conducted for a range of Reynolds numbers, $Re= \frac{U_Fh}ν =$ 485-12270. The velocity and density fields are captured simultaneously using a PIV-PLIF system. A moving average method is employed to compute the mean field and a host of turbulence statistics, namely, turbulent kinetic energy ($K$), shear production ($P$), buoyancy flux ($B$), and energy dissipation ($ε$) during the slumping phase of the current. The subsequent findings are used to ascertain the quantitative values of mixing efficiency, $Ri_{f}$, Ozmidov length-scale ($L_O$), Kolmogorov length-scale ($L_κ$), and eddy diffusivities of momentum ($κ_m$) and scalar ($κ_ρ$). Two different forms of $Ri_{f}$ are characterized in this study, denoted by $Ri_{f}^I=\frac{B}{P}$ and $Ri_{f}^{II}=\frac{B}{B+ε}$. The results cover the entire diffusive regime (3 $<Re_b<$ 10) and a portion of the intermediate regime (10 $<Re_b<$ 50), where $Re_b=\fracε{νN^2}$ is the buoyancy Reynolds number that measures the level of turbulence in a shear-stratified flow. The values of $P$, $B$, and $ε$ show a marked increase at the interface of the ambient fluid and the current, owing to the development of a shear-driven mixed layer. Based on the changes in the turbulence statistics and the length scales, it is inferred that the turbulence decays along the length of the current. The mixing efficiency monotonically increases in the diffusive regime ($Re_{b}<$10), and is found to have an upper bound of $Ri_{f}^{I}\approx$ 0.15 and $Ri_{f}^{II}\approx$ 0.2 in the intermediate regime. Using the values of $Ri_{f}$, the normalized eddy diffusivity of momentum is parameterized as $\frac{κ_m}{ν.Ri_{g}}$=1.2$Re_{b}$ and normalized eddy diffusivity of scalar as $\frac{κ_ρ}ν$=0.2$Re_{b}$

physics.flu-dyn↗

arXiv2018

Entrainment and mixing in gravity currents using simultaneous velocity-density measurements

Gravity currents modify their flow characteristics by entraining ambient fluid, which depends on a variety of governing parameters such as the initial density, $Δρ$, the total initial height of the fluid, $H$, and the slope of the terrain, $α$, from where it is released. Depending on these parameters, the gravity current may be designated as sub-critical, critical, or super-critical. It is imperative to study the entrainment dynamics of a gravity current in order to have a clear understanding of mixing transitions that govern the flow physics, the shear layer thickness, $δ_{u}$, and the mixing layer thickness, $δ_ρ$. Experiments were conducted in a lock-exchange facility in which the dense fluid was separated from the ambient lighter fluid using a gate. As the gate is released instantaneously, an energy conserving gravity current is formed, for which the only governing parameter is the Reynolds number defined as $Re=\frac{Uh}ν$, where $U$ is the front velocity of the gravity current, and $h$ is the height of the current. In our study, the bulk Richardson number, $Ri_{b}$=$\frac{g^{'}H}{U_{b}^{2}}$=1, takes a constant value for all the experiments, with $U_{b}$ being the bulk velocity of the layer defined as $U_{b}$=$\sqrt{g^{'}H}$. Simultaneous Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) measurement techniques are employed to get the velocity and density statistics. A flux-based method is used to calculate the entrainment coefficient, E$_{F}$, for a Reynolds number range of $Re\approx$400-13000 used in our experiments. The result shows a mixing transition at $Re\approx$2700 that is attributed to the flow transitioning from weak Holmboe waves to Kelvin-Helmholtz type instabilities.

physics.flu-dyn↗

arXiv2016

Dynamics of a buoyant plume in a linearly stratified environment using simultaneous PIV-PLIF measurements

The presence of stratified layer in atmosphere and ocean leads to buoyant vertical motions, commonly referred to as plumes. It is important to study the mixing dynamics of a plume at a local scale in order to model their evolution and growth. Such a characterization requires measuring the velocity and density of the mixing fluids simultaneously. Here, we present the results of a buoyant plume propagating in a linearly stratified medium with a density difference of 0.5%, thus yielding a buoyancy frequency of N=0.15 s^{-1}. To understand the plume behaviour, statistics such as centerline and axial velocities along varying downstream locations, turbulent kinetic energy, Reynolds stress, and buoyancy flux were measured. The centerline velocity was found to decrease with increase in height. The Reynolds stress and buoyancy flux profiles showed the presence of a unstable layer and the mixing associated within that layer.

physics.flu-dyn↗

arXiv2016

Convection in rotating flows with simultaneous imposition of radial and vertical temperature gradients

Laboratory experiments were conducted to study heat transport characteristics in a nonhomogeneously heated fluid annulus subjected to rotation along the vertical axis (z). The nonhomogeneous heating was obtained by imposing radial and vertical temperature gradient (ΔT). The parameter range for this study was Rayleigh number, Ra=2.43x10^8-3.66x10^8, and Taylor number, Ta=6.45x10^8-27x10^8. The working fluid was water with a Prandtl number, Pr=7. Heat transport was measured for varying rotation rates (Ω) for fixed values of ΔT. The Nusselt number, Nu, plotted as a function of Ta distinctly showed the effect of rotation on heat transport. In general, Nu was found to have a larger value for non-rotating convection. This could mean an interplay of columnar plumes and baroclinic wave in our system as also evident from temperature measurements. Laser based imaging at a single vertical plane also showed evidence of such flow structure.

physics.flu-dyn↗