Shamik Sarkar - Academia.edu (original) (raw)

Papers by Shamik Sarkar

Physics of the Earth and Planetary Interiors, 2018

Using an enthalpy based thermo-mechanical model we provide a theoretical evaluation of melt produ... more Using an enthalpy based thermo-mechanical model we provide a theoretical evaluation of melt production beneath mid-ocean ridges (MORs), and demonstrate how the melts subsequently develop their pathways to sustain the major ridge processes. Our model employs a Darcy idealization of the two-phase (solid-melt) system, accounting enthalpy (∆H) as a function of temperature dependent liquid fraction (φ). Random thermal perturbations imposed in this model set in local convection that drive melts to flow through porosity controlled pathways with a typical mushroom-like 3D structure. We present across-and along-MOR axis model profiles to show the mode of occurrence of melt-rich zones within mushy regions, connected to deeper sources by single or multiple feeders. The upwelling of melts experiences two synchronous processes: 1) solidification-accretion, and 2) eruption, retaining a large melt fraction in the framework of mantle dynamics. Using a bifurcation analysis we determine the threshold condition for melt eruption, and estimate the potential volumes of eruptible melts (~3.7x 10 6 m 3 /yr) and sub-crustal solidified masses (~1-8.8x 10 6 m 3 /yr) on an axis length of 500 km. The solidification process far dominates over the eruption process in the initial phase, but declines rapidly on a time scale (t) of 1 Myr. Consequently, the eruption rate takes over the solidification rate, but attains nearly a steady value as t > 1.5 Myr. We finally present a melt budget, where a maximum of ~5 % of the total upwelling melt volume is available for eruption, whereas ~19% for deeper level solidification; the rest continue to participate in the sub-crustal processes.

Critical Reviews in Biomedical Engineering, 2018

Diffuse axonal injury (DAI) is the most common pathological feature of brain injury which account... more Diffuse axonal injury (DAI) is the most common pathological feature of brain injury which accounts for half of the traumatic lesions in the United States. Although direct shear strain measures indicate DAI, it is localized and varies greatly in the brain. It has limitations when correlated with the possibility and severity of DAI in different brain regions along different planes for variable factors. Rather, a statistical strain measure such as peak shear strain possibility (PSSP) is proposed as a head injury criteria. In this study, computer tomography (CT) was used to derive a finite element model of the skull-brain complex including viscoelastic behavior for brain material. It was simulated under blunt impact for variable factors such as five impact directions, four impact velocities, and four head sizes. Nodal shear strain measures were obtained for seven brain regions along three planes. Proposed PSSPs for different shear strain level (10%-90%) were calculated. Considering 30% shear strain as the critical level, PSSPs were 0.49 for corpus callosum and 0.71 for the brain stem along the sagittal plane, and 0.63 for frontal impact. Among eighty simulation cases (240 strain measures), the corpus callosum and brain stem have the highest possibility (30%) of DAI. Frontal impact is the most dangerous direction, followed by side-back and back impact. For all impact directions, the highest PSSP along the sagittal plane indicates the predominance of rotational motion of the brain for causing DAI. Head size variation has the least effect on DAI possibility. At higher impact velocities, DAI possibility increases nonlinearly. Hence, these proposed criteria are expected to predict DAI under variable factors.

This paper presents a kinematic analysis for explaining the spatial and temporal variations of de... more This paper presents a kinematic analysis for explaining the spatial and temporal variations of deformational structures in the Cuddapah Proterozoic orogenic belt, South India. This belt shows virtually undeformed, flat-lying sedimentary rocks in its western margin, whereas strongly deformed and folded rocks in the eastern boundary. The structural variations indicate increasing deformation in the east direction, implying a relative tectonic convergence from the east side. Based on the corner flow theory, we use a viscous model to determine the heterogeneous strain field in the asymmetric convergent zone, and explain varying deformations from the foreland to hinterland. According to this model, the strain field in the hinterland part varies in a pulsating manner with progressive crustal thickening, giving rise to multiple deformational structures in the course of a single continuous tectonic movement. We also studied the strain field in finite element models considering the gravity fo...

Current science

Mantle plumes constitute a large-scale thermal advec- tion process of million-year timescale insi... more Mantle plumes constitute a large-scale thermal advec- tion process of million-year timescale inside the Earth. It has been inferred that they mostly initiate as ther- mal perturbations at the core–mantle boundary, and subsequently ascend through the mantle, giving rise to hotspots and large igneous provinces. Using volume- of-fluid (VOF) models, the present study provides a new insight into the issue – ballooning versus curling mode of plume ascent. Earlier models have predicted curling to ballooning transitions with increasing mantle– plume viscosity contrast. Our thermo-mechanical model simulations demonstrate this transition as a function of two independent physical variables: den- sity contrast (Δ ρ = ρ a / ρ p , ρ a and ρ p are mantle and plume density respectively) and material influx rate (normalized in terms of Reynolds number Re). The ballooning mode occurs in a condition of high Δ ρ (~ 1.2) and low Re (~ 6), which transforms into the curling mode as the condition is revers...

Tectonophysics, 2014

ABSTRACT Using a random thermal perturbation (RTP) model this study investigates the process of m... more ABSTRACT Using a random thermal perturbation (RTP) model this study investigates the process of magmatic segmentation along mid-oceanic ridge (MOR) axes as a function of the upwelling dynamics, controlled by coupled solidification–melting processes. The RTP model suggests that the variation in along-axis velocity (VL) fields constitutes the underlying mechanism of segmentation in natural MORs, showing temperature variations within a steady-state range, irrespective of large initial thermal perturbations imposed at the model base. The VL patterns are initially transient, characterized by multi-order segments, but attain a stable configuration with dominantly large segments (average size ~ 100 km) within a time scale of 2.3 Ma. Buoyant-melt driven thermal convection explains this transient segmentation. Small scale convection cells are found to be progressively consumed by larger cells, resulting in a stable convection structure over a similar time scale. Slow- and fast-spreading ridges (SSR and FSR) undergo upwelling with contrasting melt flow patterns. SSRs involve melt feeding into the ridge axis by horizontal flows from segment centers, trailing into large-scale conduits at an early stage. With time, vertical upwelling occurs throughout the segment. In the case of FSRs, both melt supply avenues prevail throughout their development. We also evaluate the variation of the across-axis flow velocity (VT) to investigate the mode of geometric evolution of MORs. Time series VT maps suggest that a ridge structure develops through localization of discrete axes (VT = 0) with offsets varying up to 15 km, which coalesce with one another to form a single axis. The matured ridge, however, retains higher-order offsets (up to 9 km).

International Journal of Multiphase Flow, 2014

Using multi-phase numerical experiments based on the Volume of fluid (VOF) methods we show the as... more Using multi-phase numerical experiments based on the Volume of fluid (VOF) methods we show the ascent behaviour of a fluid injected into another immiscible fluid with higher viscosity. Our VOF models reveal that the injected fluid forms jets characterized by a large head trailing to a slender tail, which do not break up, but ascend in two principal modes, Mode 1: continuous and Mode 2: pulsating. In Mode 1, their heads grow continuously either by volume expansion, forming balloon-shaped geometry (Mode 1a) or by curling, forming mushroomshaped geometry (Mode 1b). In contrast, Mode 2 jets ascend in pulses, forming multiple heads, leading to pinched-off geometry. In this study we show the viscosity ratio (R) between the injecting and the ambient fluids as a crucial parameter in controlling the ascent modes, and provide an estimate of critical R for the Mode 1 to 2 transition under varying fluid injection rates (non-dimensionalized as Reynolds number, Re). From the entrainment velocity field we explain that the transition of continuous to pulsating process results from the flow convergence at several locations along the upwelling direction. We have also discussed the effects of buoyancy on the ascent modes in case of plumes.

Journal of Structural Geology, 2014

ABSTRACT We performed a series of sandbox experiments to investigate the initiation of thrust ram... more ABSTRACT We performed a series of sandbox experiments to investigate the initiation of thrust ramping in tectonic wedges on a mechanically continuous basal decollement. The experiments show that the decollement slope (β) is the key factor in controlling the location of thrust initiation with respect to the backstop (i.e. tectonic suture line). For β = 0, the ramping begins right at the backstop, followed by sequential thrusting in the frontal direction, leading to a typical mono-vergent wedge. In contrast, the ramp initiates away from the backstop as β> 0. Under this boundary condition an event of sequential back thrusting takes place prior to the onset of frontal thrust progression. These two-coupled processes eventually give rise to a bi-vergent geometry of the thrust wedge. Using the Drucker-Prager failure criterion in finite element (FE) models, we show the location of stress intensification to render a mechanical basis for the thrust initiation away from the backstop if β> 0. Our physical and FE model results explain why the Main Central Thrust (MCT) is located far away from the Indo-Tibetan plate contact (ITSZ) in the Himalayan fold-and-thrust belts.

Geological Society, London, Special Publications, 2015

Abstract This study investigates the first-order Himalayan mountain topography from the perspecti... more Abstract This study investigates the first-order Himalayan mountain topography from the perspective of deep-crustal flow patterns in the Indo-Asia collision zone. Using a thin-viscous-sheet model we theoretically predict that flat hinterland topography with a stable elevation (Type I) can develop only when the lithospheric slab underthrusts with a threshold velocity (Vs*). For Vs>Vs*, the hinterland continuously gains in elevation, leading to Type II topography. This type is characterized by varying first-order surface slopes, but always facing the mountain front. Conversely, the elevated hinterland masses undergo gravity-driven subsidence, forming a topography (Type III) with characteristic backward surface slopes when Vs<Vs*. We evaluate Vs* as a function of: (i) the regional slope of the initial first-order surface topography (α); (ii) the angle of underthrusting (β); and (iii) the relative width of foreland plain (λ), assuming little effects of surface erosion. Our model shows two characteristic deep-crustal flow patterns: corner flow and vortex flow. The corner flow pattern, described by upwardly pointed hyperbolic streamlines, is responsible for Type II topography. Conversely, the vortex flow leads to Type III, whereas the transition between the two gives rise to Type I. This corner-to-vortex type flow transition commences on decreasing Vs.

Journal of the Geological Society of India, 2010

This paper presents results obtained from numerical model experiments to show different patterns ... more This paper presents results obtained from numerical model experiments to show different patterns of mantle flow produced by lithospheric movement in subduction zones. Using finite element models, based on Maxwell rheology (relaxation time ~ 10 11 S), we performed three types of experiments: Type 1, Type 2 and Type 3. In Type 1 experiments, the lithospheric slab subducts into the mantle by translational movement, maintaining a constant subduction angle. The experimental results show that the flow perturbations occur in the form of vortices in the mantle wedge, irrespective of subduction rate and angle. The mantle wedge vortex is coupled with another vortex below the subducting plate, which tends to be more conspicuous with decreasing subduction rate. Type 2 experiments take into account a flexural deformation of the plate, and reveal its effect on the flow patterns. The flexural motion induces a flow in the form of spiral pattern at the slab edge. Density-controlled lithospheric flexural motion produces a secondary flow convergence zone beneath the overriding plate. In many convergent zones the subducting lithospheric plate undergoes detachment, and moves down into the mantle freely. Type 3 experiments demonstrate flow perturbations resulting from such slab detachments. Using three-dimensional models we analyze lithospheric stresses in convergent zone, and map the belts of horizontal compression and tension as a function of subduction angle.

International Journal of Vehicle Safety, 2008

Subdural haematoma (SDH) is a very common injury associated with in-house as well as traffic acci... more Subdural haematoma (SDH) is a very common injury associated with in-house as well as traffic accidents. A high level of relative motion between the skull and the brain causes laceration in the cerebral bridging veins followed by acute SDH. This article describes finite element-based simulations and analysis of mechanical responses in the skull-brain complex in accident-like situations to study and assess SDH in the human head. A 3D finite element model of the human head was prepared by reconstructing CT images, and subsequently simulated at real life impact situations using explicit finite element codes. The Green-Lagrangian strain in bridging veins has been computed by post-processing finite element results for every possible orientation and position of bridging veins. It has been observed that occipital (back) impact is most likely to cause SDH, and that vein length plays the predominant role over vein angle in inducing strain. The possibility of vein rupture increases with head size.

Heat and Mass Transfer, 2013

ABSTRACT Using physical experiments we investigated the evolution of thermally driven melt patter... more ABSTRACT Using physical experiments we investigated the evolution of thermally driven melt patterns in a semi-infinite solid crystalline phase subjected to uniform heating from one side, maintaining melting temperature. We treat the melt initiation phenomenon theoretically in the perspective of two-phase interactions on the microscopic level, and propose a new reaction-diffusion model based on the prey-predator dynamics. This model predicts the fractal behavior of melt fronts observed in the experiments.

Applied Physics Letters, 2009

Page 1. Evolution of 180°, 90°, and vortex domains in ferroelectric films Manas Kumar Roy,a Shami... more Page 1. Evolution of 180°, 90°, and vortex domains in ferroelectric films Manas Kumar Roy,a Shamik Sarkar, and Sushanta Dattagupta Indian Institute of Science Education and ResearchKolkata, Mohanpur, PIN: 741252, India ...

Journal of Structural Geology, 2009

Non-cylindrical sheath folds often develop by the accentuation of inherent geometrical irregulari... more Non-cylindrical sheath folds often develop by the accentuation of inherent geometrical irregularities along the hinge line during progressive deformation. This paper presents an analysis of the growth of such non-cylindrical folds in pure shear as a function of a non-dimensional initial ...