Kameswararao Anupindi

In the present work, a characteristic-based off-lattice Boltzmann method is used to study mixed convection heat transfer in a two-dimensional concentric annular cavity. First, the numerical method is verified using two benchmark cases: the Taylor–Couette flow and mixed convection in a concentric annular cavity. Thereafter, the effects of strength and the direction of rotation of the cylinders on the flow and heat transfer characteristics are analyzed. To this end, four different configurations of the cylinders, namely, the counter-rotating-cylinders, the co-rotating-cylinders, outer-rotating-cylinder with a stationary inner-cylinder, and inner-rotating-cylinder with a stationary outer-cylinder are considered. Further, a range of Rayleigh numbers, Ra=104,105 and 106 and a range of Reynolds numbers, Re=0 to 104 are considered for a Prandtl number of, Pr=0.71. It is observed that irrespective of the Reynolds number of the flow, the forced convection always results in a lower heat transfer rate. At a lower Rayleigh number of Ra=104, heat transfer in the natural convection is always higher than the mixed convection heat transfer for all the four rotation configurations considered. Both the co-rotating-cylinders and inner-cylinder-rotating configurations always result in a lower heat transfer rate than the natural convection heat transfer rate. However, up to certain Reynolds numbers, the counter-rotating-cylinders and outer-rotating-cylinder configurations result in a higher heat transfer rate for Ra=105 and 106. Beyond this Reynolds number, the heat transfer rate begins to decrease with an increase in the Reynolds number. Compared to other configurations, the co-rotating-cylinders configuration behaves similar to forced convection at lower Reynolds numbers. The profiles of the overall Nusselt number as a function of the Richardson number are studied for all four configurations, and the map thus obtained could be divided into three different zones. The distinguishing features of each of these zones are explained.

The present study numerically investigates the influence of introducing a spin-type mixer and different angular orientations of the mixer blades on the spray-wall interaction and mixing, following cross-stream injection of a pulsed spray into airflow in a circular duct. This is relevant to the Selective Catalytic Reduction system in diesel engines for exhaust gas after-treatment. The spin-type static mixer is located downstream of the injector and generates a swirling airflow in the duct. All simulations were carried out using ANSYS Fluent V18.0. The standard k-ω model is used to simulate the turbulent continuous phase flow, while the discrete phase model is employed to track the spray droplets. The Taylor Analogy Breakup and Kuhnke wall film models are adopted to model droplet breakup and wall-film formation, respectively. First, the swirling airflow characteristics without spray injection are validated against in-house particle image velocimetry measurements. Second, the spray computations are compared with the experiment. Overall, good agreement between simulation and experiment is achieved. Furthermore, the choice of water and urea water solution injection liquid on the in-channel spray characteristics is also studied. The main focus of the present work is on the study of the influence of spin mixer clocking on the post-impingement spray evolution, droplet redistribution and mixing, and wall-film characteristics. The results show that the choice of the angular orientation of the mixer governs the extent of droplet deposition and splashing on the mixer blades and, as a result, strongly influences the spatial uniformity of droplets and ammonia species at the channel exit.

Flow between differentially rotating cylinders, also known as counter rotating Taylor-Couette (CRTC) system exhibit a wide variety of flow states comprising of separate laminar and turbulent regions as well as flow states with co-existence of both of them. In the present work we focus on simulating incompressible turbulent flow in a CRTC system using large eddy simulation(LES) turbulence model available in OpenFOAM. The statistical features of the flow field such as time-averaged mean field and the root means square velocity fluctuations are computed for different Reynolds numbers and are validated from literature. The dynamical features of the flow such as instantaneous iso-surfaces of λ2 are computed from the simulations. Overall, the results obtained demonstrate the capability of LES to simulate strongly rotating wall bounded flows.

In the present work, the flow and thermal characteristics for flow through a wall-confined array of pin-fins are studied using large-eddy simulation (LES). The geometry considered here resembles the trailing-edge portion of a early-stage gas-turbine blade. The wall-confined array of low-aspect ratio cylindrical fins are arranged in a staggered manner, and the direction of the coolant is perpendicular to the axis of the fins. The analysis is performed for a fixed spacing between the fins and for a Reynolds number of 5900. In view of the actual running conditions, large temperature differences, ranging from 25∘C to 300∘C, are considered between the walls and the coolant. Together with this, the effect of higher pressure, ranging from 5 bar to 10 bar, on the cooling performance is analyzed. The numerical solver used is thoroughly validated with the reference experimental and LES data available in the literature. A detailed investigation of the mechanism for heating of the fluid, transport, and diffusion of the heat flux is presented by analyzing Nusselt number, turbulent heat fluxes and vorticity contours. It is observed that, an increase in Nusselt number is due to encapsulation of small-scale vortices around the pin-fin surfaces and a decrease in Nusselt number downstream is due to the lower frequency of high-energy vortex. The localized Nusselt number is higher over the fin surfaces compared to end-walls, and the base vortex is responsible for higher energy extraction at the junction of the pin-fin and the end-walls and its effect increases downstream. The spanwise component of velocity is responsible for localized mixing than the remaining two components, and the coolant gets heated inside the recirculation zone and diffusion of which occurs mostly at the free shear layer. The analysis of higher pressure and temperature difference shows higher Nusselt number at high pressure, whereas higher heat flux at lower pressure.

In the present work, turbulent flow in the annulus of a counter-rotating Taylor-Couette (CRTC) system is studied using large-eddy simulation. The numerical methodology employed is validated, for both the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature, for a range of Reynolds numbers from 500 to 4000. Thereafter, turbulent flow occurring in this system at Reynolds numbers of 8000 and 16000 are studied, and the results obtained are analyzed using mean and second-order statistics, vortical structures, velocity vector plots and power energy spectra. Further, the spatio-temporal variation of azimuthal velocity, extracted near the inner cylinder, shows the existence of herringbone like patterns similar to that observed in the previous studies. The effect of eccentricity of the inner cylinder with respect to the outer cylinder is studied, on the turbulent flow in the CRTC system, for two different eccentricity ratios of 0.2 and 0.5 and for two different Reynolds numbers of 1500 and 4000. The results of the eccentric CRTC are analyzed using contours of pressure, mean and second-order statistics, velocity vectors, vortical structures, and turbulence anisotropy maps. It is observed from the eccentric CRTC simulations that the smaller-gap region seems to contain higher amplitude fluctuations and more vortical structures when compared with the larger-gap region. The mean turbulent kinetic energy contours do not change qualitatively with the Reynolds number, however, quantitatively a higher turbulent kinetic energy is observed in the higher Reynolds number case of 4000.

In the present work, compact finite-difference lattice Boltzmann method (CFDLBM) is extended to simulate natural convection in square and concentric-circular-annulus cavities, in two-dimensions, for Rayleigh numbers in the range of 10 3 to 10 9 . Owing to the finite-difference discretization of the method, non-uniform and curvilinear grids with a finer near-wall grid resolution could be used in the present simulations. The use of high-order methods further enabled the simulations to be run on relatively coarse meshes when compared with simulations that used classical lattice Boltzmann method. The simulation results are analyzed with the help of streamlines, isothermal contours and temperature, velocity, Nusselt number variations along the walls. The average Nusselt number on the hot wall shows less than 1% error for natural convection in the square cavity study. The circular-annular-cavity is found to have laminar flow for Rayleigh numbers in the range of 10 3 –10 6 and it becomes unsteady and chaotic from 10 7 onwards. The instantaneous and time-average contours of isotherms and streamlines were used to study the unsteady behavior of the flow. A universal near-wall velocity profile is found to exist in all the unsteady natural convection studies in the annular cavity case. The location of the maximum velocities occurring in the annulus is analyzed for all the Rayleigh numbers. The spectra of velocity magnitude fluctuations and temperature fluctuations indicate that the unsteadiness in the cavity is dominant around 0.001 Hz. Natural convection in sine-walled and concentric star shaped cavities are simulated to demonstrate the ability of the present solver to handle complex geometries. The present results indicate that CFDLBM can be successfully used to simulate steady and unsteady chaotic natural convection flows in both rectangular and curvilinear geometries accurately.

In the present work, five different subgrid-scale (SGS) models and implicit large-eddy simulation (LES) are evaluated and compared against the DNS for the simulation of the planar turbulent wall-bounded jet with heat transfer. The SGS models tested are the classical constant coefficient Smagorinsky model and its dynamic version, the wall-adaptive local eddy-viscosity (WALE) model, the turbulent kinetic energy one-equation model, and its dynamic version. The effects of using variable turbulent Prandtl number and the near-wall damping function are also studied in these models. The mean, second-order flow and heat-transfer statistics with the evolution of Nusselt number along the jet downstream are used to assess the different SGS models. The quality of resolution of the present LES are evaluated using the activity parameter and the index of resolution quality. Among the models tested, the constant coefficient Smagorinsky model together with Van-Driest damping predicts the solution accurately in the near-wall region as well as in estimating the thermal parameters. However, the dynamic models performed better in evaluating the Reynolds stress profiles away from the wall in the outer region. Capabilities of the models to predict the turbulent kinetic energy budgets, pressure-velocity gradient correlations and triple velocity correlations are also studied. The implemented variable Prandtl number algorithm is noted to have minimal influence on the evolution of the solution.

In this work, a finite-difference-based axisymmetric off-lattice Boltzmann solver is developed to simulate blood flow through pathological arteries. The proposed solver handles arterial geometries using a body-fitted curvilinear mesh. The axisymmetric nature of the flow and the non-Newtonian behavior of blood are incorporated using external source terms. The solver is verified for spatially developing pulsatile inflow through an abdominal aortic aneurysm using reference data from literature. Thereafter, the effects of amplitude and frequency of an irregular-shaped stenosed artery are systematically studied. The results are analyzed using the instantaneous vorticity contours, streamlines, cycle-averaged and phase-averaged profiles of wall shear stress (WSS), and oscillatory shear index. Further, the correlation between the luminal surface concentration (LSC) of low-density lipoproteins and the WSS is studied to predict potential disease initiation and progression locations. It is noted that an increase in the amplitude of irregularity of the stenosis increases the magnitudes of maxima and minima of WSS profiles without altering their locations. On the other hand, an increase in the frequency of irregularity increases the magnitudes of WSS extrema while bringing the peaks closer together. Further, a positive correlation is found between the degree of irregularity as well as the number of locations of elevated LSC. The presence of irregularity creates additional vortices in the upstream section of the stenosis. Both the upstream and downstream sections of the stenosis are subjected to the opposing shear-layers with higher magnitudes, which may lead to endothelial damage. Finally, the shear-thinning effect of blood is studied using the power-law model. The magnitudes of the maxima and minima in WSS have a lower value for the shear-thinning model than the Newtonian case. Also, the vortices that were produced in the upstream section because of the irregularity get suppressed by the shear-thinning effect of the blood.

In the present work, flow around a heated circular cylinder, at a Reynolds number of Re=3900, is investigated using large-eddy simulation (LES). Large differences in temperature, 25 ∘C, 100 ∘C, 200 ∘C, and 300 ∘C between the cylinder and the oncoming flow are considered, and its effect on the flow and thermal characteristics in the near wake region are studied. The numerical methodology employed is validated for both, the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature. The results are analyzed using the mean temperature, velocity, Reynolds stresses, temperature variances, turbulent heat fluxes and energy spectra. The flow and thermal characteristics are studied along the center line in the wake, and in the transverse direction at two locations. The non-isothermal flow characteristics are compared with isothermal flow, to study the effect of temperature on the flow dynamics. Phase-averaging is performed to analyze the regions of turbulence production and convection of heat. It is observed that, the flow characteristics vary non-linearly with the temperature, and the effect is insignificant till a temperature difference of 100 ∘C, however, beyond this significant effect could be noticed. The effect of temperature difference is prominent in the thermal characteristics for all temperature differences, 25 ∘C to 300 ∘C, considered. The transverse component of shear stress fluctuations are observed to be dominant over the stream-wise components at both the locations downstream, thereby enhancing the local mixing of the fluid and hence, the heat transfer.

An axisymmetric compact finite-difference lattice Boltzmann method is proposed to simulate both Newtonian and non-Newtonian flow of blood through a lumen. The curvature of the arteries could be accurately resolved using body-fitted mesh owing to the proposed finite-difference formulation. The axisymmetric nature of the flow, as well as the non-Newtonian nature of blood, are incorporated into the lattice Boltzmann equation using separate source terms. Using Chapman-Enskog expansion it is shown that the resulting lattice Boltzmann equation with these additional source terms recovers the macroscopic axisymmetric hydrodynamic equations. The solver is verified for (1) steady inflow of a Newtonian fluid through a stenosed lumen, (2) temporally developing pulsatile flow (Womersley flow) through a straight lumen with Newtonian fluid, and (3) steady inflow of a non-Newtonian fluid through a straight lumen. The solver is then applied to simulate the steady flow of a non-Newtonian fluid through a stenosed lumen, and it was found that a smaller recirculation zone and lower WSS values are obtained when compared with the flow of a Newtonian fluid. The capability of the solver to simulate spatially developing (velocity-driven) pulsatile flow is then demonstrated by simulating physiological pulsatile flow through an axisymmetric abdominal aortic aneurysm. From this simulation, the cycle-averaged wall shear stress is observed to have a steep gradient going from a minimum (negative) to a maximum (positive) value towards the distal end of the aneurysm, which is prone to the risk of rupture. An iterative procedure to select the geometric and flow parameters for unsteady inflow condition in the lattice Boltzmann method framework is demonstrated that accurately resolves all the timescales to achieve incompressibility. Overall, the present solver seems to be promising to simulate axisymmetric flow of blood with steady and pulsatile inflows while considering the blood rheology.