K. Arul Prakash

Forced Convective Heat Transfer from Unconfined Isothermal and Isoflux Elliptic Cylinders

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 Nu avg ) 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 Nu avg . Finally, correlations are proposed for Nu avg with respect to AR, AOA, and Re with minimum rms error.

Natural Convection from a Heated Elliptic Cylinder with a Different Axis Ratio in a Square Enclosure

A detailed study about the free convection over a heated elliptic cylinder, placed at the center of a square cavity having cooled walls, is performed. Simulations are carried out for three Rayleigh numbers (104, 105, and 106) and two cavity aspect ratios (CR = 2.5 and 5.0) for different axis ratio (AR). The effect of AR on fluid flow and heat transfer characteristics for varying Rayleigh number and cavity aspect ratio are analyzed. The influence of AR is phenomenal at higher Ra and lower CR. At higher Ra, thermal plumes are observed above the cylinder for different ARs. Bicellular vortices are formed at low Ra by changing CR. The surface-averaged Nusselt number (Nu avg ) increases with increasing AR and Ra. The value of Nu avg increases with decreasing CR, and a correlation for Nu avg in terms of AR is obtained for each CR.

Onset of laminar separation and vortex shedding in flow past unconfined elliptic cylinders

This article presents the numerical studies on predicting onset of flow separation and vortex shedding in flow past unconfined two-dimensional elliptical cylinders for various Axis Ratios (AR) and a wide range of Angles of Attack (AOA). An efficient Cartesian grid technique based immersed boundary method is used for numerical simulations. The laminar separation Reynolds number (Res) that marks separation of flow from surface and the critical Reynolds number (Recr) which represents transition from steady to unsteady flow are determined using diverse methods. A stability analysis which uses Stuart-Landau equation is also performed for calculating Recr. The shedding frequency (Stcr) that corresponds to Recr is calculated using Landau constants. The simulated results for circular cylinder are found to be in good agreement with the literature. The effects of AR and AOA on Res, Recr, and Stcr are studied. It is observed that the Res, Recr, and Stcr exhibit a direct/inverse relationship with AR depending upon th...

Effect of Axis Ratio on Fluid Flow Around an Elliptic Cylinder—A Numerical Study

The bluff body simulations over canonical forms like circular and square cylinders are very well studied and the correlations for bulk parameters like mean drag coefficient and Strouhal numbers for the same are reported widely. In the case of elliptic cylinder, the literature is very sparse, especially for moderate Reynolds number (Re). Hence, in this work, a detailed study about fluid flow characteristics over an elliptic cylinder placed in a free stream is performed. Simulations are carried out for different Re ranging from 50 to 500 with axis ratio (AR) varied between 0.1 to 1.0 in steps of 0.1. Immersed boundary method is used for the solid boundary condition implementation which avoids the grid generation for each AR and a single Cartesian grid is used for all the simulations. The effect of AR for various Reynolds numbers is also focused on using the in-house code. The influence of AR is phenomenal for all the Re and the values of wake length, drag coefficient, and Strouhal number decrease with decreasing AR for a particular Re. The critical ARs, for vortex shedding and wake formation, are identified for various Re. Detailed correlations for wake length, critical ARs for vortex shedding and wake formation, mean drag coefficient and Strouhal number, in terms of AR, are reported in this work. [DOI: 10.1115/1.4024862]

Numerical Prediction of Fluid Flow and Heat Transfer in the Target System of an Axisymmetric Accelerator-Driven Subcritical System

Thermal hydraulics related to the design of the spallation target module of an accelerator-driven subcritical system (ADSS) was investigated numerically using a streamline upwind Petrov-Galerkin (SUPG) finite element (FE) method. A large amount of heat is deposited on the window and in the target during the course of nuclear reaction between the proton beam and the molten lead-bismuth eutectic (LBE) target. Simulations were carried out to predict the characteristics of the flow and temperature fields in the target module with a funnel-shaped flow guide and spherical bottom of the container. The beam window was kept under various thermal conditions. The analysis was extended to the case of heat generation in the LBE. The principal purpose of the analysis was to trace the temperature distribution on the beam window and in the LBE. In the case of turbulent flows, the number of recirculation regions is decreased and the maximum heat transfer was found to take place downstream of the stagnation zone on the window.

Numerical analysis of laminar fluid flow characteristics past an elliptic cylinder

Purpose – The purpose of this paper is to study the effects of Angle of Attack (AOA), Axis Ratio (AR) and Reynolds number (Re) on unsteady laminar flow over a stationary elliptic cylinder. Design/methodology/approach – The governing equations of fluid flow over the elliptic cylinder are solved numerically on a Cartesian grid using Projection method based Immersed Boundary technique. This numerical method is validated with the results available in open literature. This scheme eliminates the requirement of generating a new computational mesh upon varying any geometrical parameter such as AR or AOA, and thus reduces the computational time and cost. Findings – Different vortex shedding patterns behind the elliptic cylinder are identified and classified using time averaged centerline streamwise velocity profile, instantaneous vorticity contours and instantaneous streamline patterns. A parameter space graph is constructed in order to reveal the dependence of AR, AOA and Re on vortex shedding. Integral parameter...

Numerical studies on fluid flow characteristics through different configurations of spiral casing

ABSTRACT The design of optimum spiral casing configuration is a difficult task and a big challenge in the field of turbomachinery. Computational fluid dynamics (CFD) analysis of fluid flow characteristics in a turn around spiral casing plays an important role in its design. The objective in this study is to propose an optimum spiral casing configuration by reducing the total pressure loss and increasing the spiral velocity coefficient and average radial velocity at the exit of spiral casing. For this, three different configurations of spiral casing, viz. accelerated, decelerated and free vortex type, with different aspect ratios (ARs) are numerically simulated. A Eulerian velocity-correction method based on the streamline upwind Petrov–Galerkin (SUPG) finite-element method is employed to solve complete Reynolds-averaged Navier–Stokes (RANS) equations governing fluid flow characteristics. The results show that the average radial velocity along the exit of spiral casings is more for elliptical cross-sectional spiral casings of AR>1 when compared to circular cross-sectional spiral casings. The total pressure loss is found to be at minimum for decelerated spiral casings. In the case of decelerated spiral casings, a further reduction in total pressure loss is obtained with elliptical cross-sections of AR>1. The spiral velocity coefficient is found to be at maximum for decelerated spiral casings with AR>1.

Robust Design of Savonius Wind Turbine

The Savonius wind turbine, a class of vertical axis wind turbine (VAWT), is simple and provides a better cost-benefit ratio. It works on the principle of differential drag and is effective in rooftop and ground mounting. Despite the advantages of Savonius wind rotors, they are not preferred due to their low aerodynamic performance levels. In order to address this, several experimental and numerical studies have been carried out in recent years. The primary aim of this work is to provide a simple methodology for the robust optimal design of the Savonius wind turbine. In the parameter design stage, the performance of the turbine is maximized using the traditional Taguchi method. An L27 orthogonal array is used considering five factors of three levels each, which affect C p . Wind speed is considered to be the noise factor. Signal-noise ratio (SNR) metric is used to find the optimal settings for robust design. The aerodynamic performance of the turbine is investigated through dynamic computational fluid dynamics (CFD) models of the design sets. The numerical models used for the simulations are also discussed.

Enhanced Cooling of Electronic Components Using Fluid Flow Under High Adverse Pressure Gradient

Heat transfer in electronic systems is studied by simulating flow in a two pass channel with the divider representing a circuit board. Bypass holes are introduced on the circuit board to obtain detailed physical insights of the reversed flows in the second pass and thereby improve the cooling effect. The time-dependent governing equations are solved using an in-house code based on Streamline upwind/Petrov-Galerkin finite element method for Reynolds number ranging from 100 to 900. It is observed that stagnant zones are formed in the return path along the upper heated wall due to the formation of primary recirculation region on the divider plate. These stagnant zones are convected downstream by introducing bypass slots thereby enhancing the convective cooling. A parametric study on the location and number of bypass slots reveals that for a particular combination, the flow becomes unsteady thereby the heat transfer is increased. The presence of multiple slot jets also reduces the overall pressure drop required to drive the flow and heat transfer is very high at the point of impingement between the slots.

Effect of splitter plate on fluid flow characteristics past a triangular cylinder

In the present numerical investigation, the effect of splitter plate length on fluid flow characteristics past an equilateral triangular cylinder apex facing the flow configuration is studied. Simulations are carried out by using in-house solver based on Streamline Upwind/Petrov-Galerkin (SUPG) finite element method in laminar regime. The range of Reynolds numbers (Re) based on side (b) of the triangular cylinder is varied from 50 to 200 and the splitter plate length (L) is varied from L/b = 0 to = 6. The effect of splitter plate length on parameters like lift and drag are studied. It is observed that drag decreases with the attachment of splitter plate. In addition, vortex shedding suppression is observed for Re = 50 − 150 whereas, for Re = 200 vortex shedding suppression is not observed even for plate length of L/b = 6. The obtained results indicate that for a specific value of Re drag is minimized at a particular splitter plate length. The drag is minimized approximately from 9% to 57% based on Re by attaching a splitter plate to the triangular cylinder.