Showing papers by "Somnath C. Roy published in 2018"

Effect of pulse rate variation on blood flow through axisymmetric and asymmetric stenotic artery models.

Abstract: The present work reports numerical simulations of blood flow patterns and wall shear stress (WSS) distributions in stenotic arteries, modelled as straight tubes. Inflow waveforms have been generated for different pulse rates considering constant volumetric flow during each pulsation cycle and a two-element windkessel model has been used to specify the outlet pressure. It is noticed that the non-Newtonian shear thinning rheology of blood produces more accurate and realistic predictions of the flow field as compared to the Newtonian assumption. Further, the effects of variation of pulse rates on the spatial and temporal distribution of WSS and oscillatory shear index (OSI) have also been studied for both axisymmetric and asymmetric stenosis. The changes in the mean flow features due to changes in pulsation frequencies have also been reported.

Quantum Mechanical Process of Carbonate Complex Formation and Large Scale Anisotropy in the Adsorption Energy of CO$_2$ on Anatase TiO$_2$ (001) Surface

Abstract: Adsorption of CO$_2$ on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity and suitable band edge positions, TiO$_2$ is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the CO$_2$ conversion efficiency on TiO$_2$ surfaces has not been maximized. While anatase TiO$_2$ (101) is the most stable facet, the (001) surface is more reactive and it has been experimentally shown that the stability can be reversed and a larger percentage (up to ~ 89%) of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the CO$_2$ adsorption on TiO$_2$ (001) surface. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the CO$_2$-TiO$_2$ interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of CO$_2$ primarily determines the BE. A conceptual experiment is devised where the CO$_2$ concentration and flow direction can be controlled to tune the BE within a large window of ~1.5 eV. The experiment also reveals that a maximum of 50% coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of ~0.9 eV.

Quantum-mechanical process of carbonate complex formation and large-scale anisotropy in the adsorption energy of C O 2 on anatase Ti O 2 (001) surface

Abstract: Adsorption of $\mathrm{C}{\mathrm{O}}_{2}$ on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity, and suitable band-edge positions, $\mathrm{Ti}{\mathrm{O}}_{2}$ is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the $\mathrm{C}{\mathrm{O}}_{2}$ conversion efficiency on $\mathrm{Ti}{\mathrm{O}}_{2}$ surfaces has not been maximized. While anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ (101) is the most stable facet, the (001) surface is more reactive, and it has been experimentally shown that the stability can be reversed and a larger percentage (up to $\ensuremath{\sim}89%)$ of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the $\mathrm{C}{\mathrm{O}}_{2}$ adsorption on $\mathrm{Ti}{\mathrm{O}}_{2}$ (001) surfaces. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the $\mathrm{C}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{O}}_{2}$ interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of $\mathrm{C}{\mathrm{O}}_{2}$ primarily determines the BE. A conceptual experiment is devised where the $\mathrm{C}{\mathrm{O}}_{2}$ concentration and flow direction can be controlled to tune the BE within a large window of $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The experiment also reveals that a maximum of $50%$ coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of $\ensuremath{\sim}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$.

Acceleration of a 3D Immersed Boundary Solver Using OpenACC

Abstract: Immersed-boundary methods (IBM) have been constantly gaining popularity and are increasingly expanding to new areas of applications in computational mechanics since last three decades due to the potentials of their application in modeling complex multiphysics phenomena which involves flow over complex and moving boundaries. The specific advantages of an immersed boundary method are due to its accuracy and simplicity. As this method uses a fixed structured Cartesian mesh, the complex grid generation processes can be fully avoided whereas the complex/moving boundary is described using another surface mesh. The computational overheads in an immersed boundary implementation can be very high due to expensive search and interpolation steps through which the effects of the boundary conditions on the surface mesh are translated to the fixed Cartesian volume mesh. Therefore, computationally efficient numerical implementation of an IBM solver is of extreme importance to researchers. This paper presents an accelerated discrete finite difference based immersed boundary (IB) solver that is used to study the external flow behavior around complex geometries. The flow is assumed to be incompressible. The immersed boundary solver is parallelized using OpenACC for quick acceleration with minimal code changes and to ensure performance portability across both GPUs and multicore CPUs. Our experimental results indicate that the OpenACC-based IB solver run on a NVIDIA Tesla P100 GPU is 21x faster than the sequential legacy solver and is 3.3x faster than the OpenACC-based IB solver run on a dual socket Intel Xeon Gold 6148, 20 core CPU. The recirculation lengths obtained for Reynolds numbers of 20 and 40 and the Strouhal number for Reynolds number 100, for a standard flow visualization problem over a fixed cylinder, are in accordance with the reported data in available literature, thereby validating the accuracy of the parallel solver. We also analyze the performance of the accelerated solver on different GPU architectures: Kepler, Pascal and Volta.