Sankaranarayanan Vengadesan

Two-dimensional flow over a circular cylinder placed in-line behind a control rod of the same diameter is studied with and without the influence of a plane wall. Thermal patterns observed during various vortex shedding modes are discussed in detail. POD analysis of temperature data reveals that, with the decrease in wall height, modal structures are moved closer to the cylinder surface, which increases the heat transfer fluctuation. The plane wall increases the heat transfer from the bottom surface of the cylinder and decreases the heat transfer from the top surface, while the rotation of control rod contributes in an opposite manner. As the cylinder is moved closer to the wall, the time-averaged Nusselt number increases for a stationary control rod. In the case of a rotating control rod, the rotation of the control rod acts in the opposite sense to nullify this wall effect. © 2014 Taylor & Francis Group, LLC.

A method to resolve particle-particle and particle-wall unsteady conduction heat transfer is developed within the framework of the soft sphere collision model. The unsteady heat transfer model is incorporated in an existing CFD-DEM framework. The model is derived from the analytical solution of one-dimensional unsteady heat conduction between two semi-infinite objects. The model considers the area and time of contact with appropriate scaling for compatibility with the soft sphere model. The model is validated against experimental data available from the literature for three cases. The cases considered are pairwise particle-particle collision, single particle-surface collision, and that of the decay of temperature of a heated particle in a fluidized bed, including convection and particle-particle-wall conduction heat transfer. The results obtained are in good agreement with the experimental data. Next, the model is applied to a fluidized bed with an immersed tube to characterize the heat transfer mechanisms and the effective heat transfer coefficient. The influences of fluidization parameters such as gas velocity and particle diameter on heat transfer mechanism is investigated.

The complexities in the flow pattern in a cavo-pulmonary vascular system-after application of the Fontan procedure in the vicinity of the superior vena cava, inferior vena cava, and the confluence at the T-junction-are analysed. A characteristic-based split (CBS) finite element scheme involving the artificial compressibility approach is employed to compute the resulting flow. Benchmarking of the CBS scheme is carried out using standard problems and with the flow features observed in an experimental model with the help of a dye visualization technique in model scale. The transient flow variations in a total cavo-pulmonary connection (TCPC) under pulsatile conditions are investigated and compared with flow visualization studies. In addition to such qualitative flow investigations, quantitative analysis of energy loss and haemodynamic stresses have also been performed. The comparisons show good agreement between the numerical and experimental flow patterns. The numerically predicted shear stress values indicate that the pulsatile flow condition is likely to be more severe than steady flow, with regard to the long-term health of the surgically corrected TCPC. Copyright © 2008 John Wiley & Sons, Ltd.

This article presents the numerical study of laminar forced convective heat transfer from elliptic cylinders of various axis ratios (AR=0.1, 0.4, 0.6, 0.8, and 1.0), angles of attack (AOA=30°, 45° , 60° , and 90°), and Reynolds numbers (Re=50, 100, 150, and 200). Simulations are carried out for both isothermal and isoflux wall boundary conditions. A detailed study of flow field reveals distinct instantaneous and time-averaged flow patterns behind the elliptic cylinder. The effect of flow patterns on isotherms and, thus, on heat transfer, is analyzed in detail. Local and surface averaged Nusselt number (Nu and Nuavg) is computed and their variation due to change in AR, AOA, and Re is studied. It is observed that increasing AR and Re increases Nu avg monotonically, while increasing AOA decreases Nuavg. Finally, correlations are proposed for Nuavg with respect to AR, AOA, and Re with minimum rms error. Copyright © Taylor & Francis Group, LLC.

Fluid flow and heat transfer characteristics past four elliptic cylinders arranged in square configuration are numerically investigated in this study. An in-house finite element method code based on streamline upwind Petrov - Galerkin algorithm has been used for simulations of two-dimensional, incompressible and laminar flow of air. Analyses are carried out by changing geometric parameters such as spacing ratio, axis ratio and incident angle. Results in terms of vorticity and temperature contours, Strouhal number, drag and lift coefficients and Nusselt number are studied for all cases. These results show that fluid flow patterns are strongly influenced by geometric parameters, based on which flow can be categorized into different types. A critical spacing ratio is found for some of the configurations, where a steep increase in drag coefficient is encountered. From this study, it is observed that incident angle 22.5 degree gives a higher heat transfer rate compared to the other two configurations (0 and 45 degree). Also, elliptic cylinders (axis ratio of 0.8 and 0.5) have similar Nusselt number value with less drag force compared to circular cylinders (axis ratio of 1.0) which justifies the utilization of elliptic cylinders as an alternative for circular ones.

Ornithopters and entomopters should be insensitive to the gusty environment during outdoor operations. Hence, it becomes imperative to understand their behaviour under the influence of gust for ensuring stable flight. In light of this, the present numerical study focused on understanding the aerodynamics of flapping wings with five different planform shapes under the influence of a spatiotemporally varying frontal gust. 3D, unsteady, laminar, and incompressible Navier-Stokes equations were solved using finite volume formulation. A canonical case of asymmetric 1 degree of freedom (DoF) flapping kinematics was considered. Horizontal and vertical force patterns in constant and gusty inflow conditions were numerically computed and compared. Findings were analyzed quantitatively by comparing the differences in the instantaneous force patterns, ordinal scoring approach, and phase space plots. Qualitative comparisons were made based on plots of vortex structures and surface pressure contours for constant and gusty inflow conditions for wings with different planform shapes. Spanwise Lagrangian Coherent Structures (LCS) of all the five wings were also compared. Studies revealed that the elliptical wing exhibited low sensitivity and inverse semi-elliptical wing exhibited high sensitivity to the gusty inflow. Rectangular, triangular and semi-elliptical shaped wings were moderately sensitive to the gusty inflow. This finding, within the limitations of the flapping kinematics and simulation conditions considered for the present study, supported the fact that many natural flyers like forest raptors, non-migratory passerines, pheasants, and partridges have adopted elliptical wing planform for efficient flight.

Numerical solutions are presented for fluid flow past a rotating elliptic cylinder placed in a uniform flow for different non-dimensional rotation rates (α). The two dimensional, incompressible Navier-Stokes equations are solved using immersed boundary method (IBM) on a non-body conforming Cartesian grid. Reynolds number based on the perimeter of elliptic cylinder is 100 and axis ratio of the cylinder is 0.1. The periodic flow, after the cylinder is impulsively started, is studied. Vortex shedding is observed in all the cases with significant differences in shedding pattern. For α ≥ 1, we observe an interesting phenomenon of formation of 'Hovering vortex’. The factors contributing to hovering vortex and associated changes in the aerodynamic forces are studied. Significant increase in lift coefficient is observed with increase in rotation rate. For some cases we observe thrust generation for major portion of the cycle.

Numerical studies on rocket pumps are computationally expensive and hence the secondary passages such as sidewall clearance gaps, wear ring gaps, axial balancing mechanism, coolant/lubricant paths, etc. in the pump are usually not considered. In this study, a liquid propellant pump with and without secondary passages are modelled and flow simulation results are compared to analyse the differences in cavitation, vortices and radial force predictions arising due to the presence/absence of the secondary flow passages. Single-phase and multiphase simulations are conducted for the design and two off-design points. It is predicted that the presence of secondary passages has a significant effect on the type of cavitation instability predicted and on the volume of cavity generated in the inducer due to which the radial forces generated also differ significantly. The presence of secondary passages predicted large fluctuations from the average radial forces generated by the inducer. These are not obtained if the leakages are not considered. This type of underprediction during the design phase might cause severe wear and tear in the bearings during actual operation. Thus, this study stresses the need for incorporating the secondary passages even at the design stages. This study also highlights the possibility of cavitation instability type manipulation using some means in the inducer which would be significant for cavitation control research in rocket pumps.

Laminar flow past a rotating cylinder near a plane wall is studied numerically using a second-order accurate immersed boundary method. The wake-wall interactions are investigated in detail, for different wall heights and varying rotational rates. Flow patterns are classified based on the wake structure and the effect of rotation on critical wall heights is discussed for both clockwise and counterclockwise rotation. For the configuration studied i.e. rotating cylinder above a bottom plane wall, counterclockwise rotation of cylinder favors the wake-wall interactions, while clockwise rotation influences in an adverse manner. In addition to conventional analysis, the evolution of the vortical structures in the wake region are examined using Lagrangian analysis of individual vortical structures. The diffusion of the positive vortex shed from the wall facing side of the cylinder is accelerated due to the influence of boundary layer with net negative vorticity. The wall augmented diffusion of positive cylinder vortices is compensated by the creation of secondary wall vortices from the bottom wall boundary layer. Counterclockwise rotation of cylinder increases the shear inside boundary layer resulting in more pronounced diffusion.

Internal cooling of the gas turbine blade is critical for the durability of the blade material. One of the ways to accomplish this is by passing coolant through serpentine passages roughened with surface elements to enhance the heat transfer. In the present study, the traditional square rib (SQ-rib) placed normal to the flow direction is modified to a backward-facing step rib (BS-rib) and a forward-facing step rib (FS-rib). Large-eddy simulation (LES) is carried out for a square duct at Reb = 20000. Results show that the modified rib shapes result in substantial increase in heat transfer over the square rib with only a marginal increase in flow losses. The BS-rib shape produces the highest heat transfer augmentation followed by the FS-rib. The overall heat transfer augmentation for the BS-rib and FS-rib is 18% and 10% larger than the SQ-rib, respectively. Thermal-hydraulic performance is enhanced by 15%.