IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

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 (10 4, 10 5, and 10 6) 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. © 2012 Copyright Taylor and Francis Group, LLC.

Department of Applied Mechanics

Numerical Heat Transfer; Part A: Applications

Natural convection from a heated elliptic cylinder with a different axis ratio in a square enclosure

The transition of square cylinder wake flow from two-dimensional (2-D) to three-dimensional (3-D) when inflow is subjected to linear shear is examined numerically. The value of the non-dimensional shear parameter (K) considered in this study are 0.0, 0.1, and 0.2. The range of Reynolds number (Re) defined based on the centerline velocity and cylinder width is from Re = 150 to 700. The transition of the wake flow from 2-D laminar to 3-D is marked by streamwise vortical structures. Unlike in uniform flow, in shear flow the transition is characterized by single mode of spanwise wavelength. The critical Reynolds number (Recrit), at which the transition from 2-D to 3-D occurs, is less in case of shear flow. The magnitude of the mean lift coefficient increases with increasing shear parameter on the positive side. The strength of the Karman vortices on the top side is higher and on the bottom side is lower when compared with the same in the uniform flow. © 2010 John Wiley &amp; Sons, Ltd.

International Journal for Numerical Methods in Fluids

Shear effect on square cylinder wake transition characteristics

This paper presents a computational fluid dynamics (CFD) approach to the calculation of translational added mass coefficient of axisymmetric underwater bodies in both longitudinal and transverse directions. The proposed method involves CFD simulations of accelerated as well as uniform flows past axisymmetric bodies based on Reynolds averaged Navier-Stokes (RANS) equations and makes use of the results of these simulations to obtain the inertia force acting on a body as a function of time. The translational added mass can be obtained from this inertia force history. Validation of the methodology is presented for the benchmark case of accelerated flow past sphere and infinitely long circular cylinder in cross-flow for which analytical solutions of the added mass problem are well known.

Department of Ocean Engineering

Journal of Ship Research

Translational added mass of axisymmetric underwater vehicles with forward speed using computational fluid dynamics

A transient haemodynamic study in a model cavopulmonary vascular system has been carried out for a typical range of parameters using a finite element-based Navier-Stokes solver. The focus of this study is to investigate the influence of non-Newtonian behaviour of the blood on the haemodynamic quantities, such as wall shear stress (WSS) and flow pattern. The computational fluid dynamics (CFD) model is based on an artificial compressibility characteristic-based split (AC-CBS) scheme, which has been adopted to solve the Navier-Stokes equations in space-time domain. A power law model has been implemented to characterize the shear thinning nature of the blood depending on the local strain rate. Using the computational model, numerical investigations have been performed for Newtonian and non-Newtonian flows for different frequencies and input pulse forms. The haemodynamic quantities observed in total cavopulmonary connection (TCPC) for the above conditions suggest that there are considerable differences in average (about 25-40%) and peak (about 50%) WSS distributions, when the non-Newtonian behaviour of the blood is taken into account. The lower WSS levels observed for non-Newtonian cases point to the higher risk of lesion formation, especially at higher pulsation frequencies. A realistic pulse form is relatively safer than a sinusoidal pulse as it has more energy distributed in the higher harmonics, which results in higher average WSS values. The present study highlights the importance of including non-Newtonian shear thinning behaviour for modelling blood flow in the vicinity of repaired arterial connections. © 2010 John Wiley & Sons, Ltd..

GENERAL

Department of Mechanical Engineering

Non-Newtonian blood flow study in a model cavopulmonary vascular system

This paper presents a computational fluid dynamics approach to study the forces and moments generated on the parent body due to attached fins in marine vehicles and also the velocity profile behind the fins. The conservation equations of mass and momentum are solved with Finite Volume Method by use of the software FLUENT. The turbulence closure model used is k-ω SST (shear stress transport). The study is carried out for single fin at various angles of attack and four fins attached to a cylinder.

World Academy of Science, Engineering and Technology

Numerical simulation of a fin at varying angle of attack

The Trapped Vortex Combustor (TVC) is a new design concept in which cavities are designed to trap a vortex flow structure established through the use of driver air jets located along the cavity walls. TVC offers many advantages when compared to conventional swirl-stabilised combustors. In the present work, numerical investigation of cold flow (non-reacting) through the two-cavity trapped vortex combustor is performed. The numerical simulation involves passive flow through the two-cavity TVC to obtain an optimum cavity size to trap stable vortices inside the second cavity and to observe the characteristics of the two cavity TVC. From the flow attributes, it is inferred that vortex stability is achieved by circulation and the vortex is trapped inside when a second afterbody is added.The Trapped Vortex Combustor (TVC) is a new design concept in which cavities are designed to trap a vortex flow structure established through the use of driver air jets located along the cavity walls. TVC offers many advantages when compared to conventional swirl-stabilised combustors. In the present work, numerical investigation of cold flow (non-reacting) through the two-cavity trapped vortex combustor is performed. The numerical simulation involves passive flow through the two-cavity TVC to obtain an optimum cavity size to trap stable vortices inside the second cavity and to observe the characteristics of the two cavity TVC. From the flow attributes, it is inferred that vortex stability is achieved by circulation and the vortex is trapped inside when a second afterbody is added.The Trapped Vortex Combustor (TVC) is a new design concept in which cavities are designed to trap a vortex flow structure established through the use of driver air jets located along the cavity walls. TVC offers many advantages when compared to conventional swirl-stabilised combustors. In the present work, numerical investigation of cold flow (non-reacting) through the two-cavity trapped vortex combustor is performed. The numerical simulation involves passive flow through the two-cavity TVC to obtain an optimum cavity size to trap stable vortices inside the second cavity and to observe the characteristics of the two cavity TVC. From the flow attributes, it is inferred that vortex stability is achieved by circulation and the vortex is trapped inside when a second afterbody is added.

Aeronautical Journal

Enhanced vortex stability in trapped vortex combustor

The unsteady flow structure and the time-varying aerodynamic forces acting on a 2D dragonfly model wing are studied by numerically solving the Navier-Stokes equations. The incompressible Navier-Stokes equations are discretized and solved on a non-body confirming Cartesian grid; the concept of immersed boundary method is made use of to impose the no-slip boundary condition on the surface of the wing. The objective of the present study is to investigate the influence of the following kinematic parameters on the flight performance of inclined stroke plane hovering: Reynolds number (Re), stroke amplitude, wing rotational timing and rotational duration. While the effects of the above mentioned parameters on the stroke averaged force coefficients are the same in both horizontal and inclined stroke plane motions, the spatiotemporal dynamics of vorticity which produce the effects are entirely different. Our results also indicate that the drag mechanism proposed for tiny insects does not seem to augment the vertical force generation in inclined stroke plane motion. © 2009 Elsevier Ltd. All rights reserved.

Computers and Fluids

Flight force production by flapping insect wings in inclined stroke plane kinematics

Navier-Stokes simulations over an idealized flapping insect wing reveal a new strategy for vertical force generation in inclined stroke plane hovering: insects use their wing as bluff and streamlined bodies during downstroke and upstroke, respectively. This strategy helps insects to stay aloft. Furthermore, qualitative and quantitative results from our 2-D simulations determine to what extent the delayed stall mechanism is significant for enhanced flight performance in inclined plane hovering. Copyright © Taylor & Francis Group, LLC.

The functional significance of delayed stall in insect flight

Large eddy simulation of planar shear flow past a square cylinder has been investigated. Dynamic Smagorinsky model has been used to model subgrid scale stress. The shear parameter, K, namely the nondimensional streamwise velocity gradient in the lateral direction, is 0.0, 0.1 and 0.2. Reynolds number based on the centerline velocity is fixed at Re=21400. The time and span-averaged velocity components, pressure coefficient, Reynolds stresses for uniform are in good agreement with the literature. In shear flow the calculated flow structure and mean velocity components are shown to be markedly different from those of the uniform flow. With increasing shear parameter, the cylinder wake is dominated by clockwise vortices. Both the velocity components in shear flow are compared with respective components in uniform flow. Comparison of normal and shear stresses between shear and no shear case have also been presented. © 2009 John Wiley & Sons, Ltd.

Large eddy simulation of bluff body wake in planar shear flow

Through two-dimensional numerical simulations, the fluid dynamic consequences of varying the Reynolds number and the corresponding changes in the aerodynamic forces acting on the flapping wings are studied. The incompressible Navier-Stokes equations are discretized and solved on a non-body conforming Cartesian grid; the concept of immersed boundary method is made use of to impose the no-slip boundary condition on the surface of the wing. The objective of the present work is to examine whether the drag mechanism acting on the inclined stroke plane motions can explain the flight of tiny insects. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

World University root node

Aerodynamic Force Generation in Tiny Hovering Insects

Unsteady three-dimensional (3-D) numerical simulations of linear shear flow past a square cylinder at moderate Reynolds number (Re=200) are performed. The shear parameter (K) considered in this study is varied as 0.0, 0.1, and 0.2. For the uniform flow (K=0.0) case, the chosen Re falls in the transition Reynolds number range. The low frequency force pulsations of square cylinder transition phenomena are observed to decrease with increasing shear parameter. The evolution of streamwise vortical structures indicates a mode A spanwise instability in the uniform flow. Unlike in uniform flow, mixed mode A and mode B spanwise instability is observed in the case of a shear flow. The autocorrelation function of the lift and the drag coefficients is improved for any particular separation distance with increasing K. © 2009 Elsevier Ltd. All rights reserved.

Journal of Fluids and Structures

Influence of inlet shear on the 3-D flow past a square cylinder at moderate Reynolds number

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.

Communications in Numerical Methods in Engineering

An investigation of pulsatile flow in a model cavo-pulmonary vascular system

A uniform flow past two unequal sized square cylinders arranged in a side-by-side pattern and at a Reynolds number of 50,000 has been investigated using large eddy simulation (LES) technique. The modelling of sub-grid scales of turbulence is done using the Smagorinsky model. The effect of the transverse gap ratio (T/D) on the flow characteristics has been studied. Numerical simulations are carried out for five different transverse gap ratios (T/D), namely 1.120, 1.250, 1.375, 1.750 and 2.500. Results in terms of the aerodynamic forces, Strouhal number, mean base pressure coefficient, streamlines, vorticity, surface pressure distribution, normal and shear stresses are presented. A shift in the stagnation point for the small square cylinder from the centre of its front face towards its gap side is seen at smaller T/D ratios. The presence of a jet-like flow seen in the gap side is more pronounced at T/D = 1.12. A biased gap side flow towards the near wake of the small square cylinder is seen at smaller T/D ratios. No interference effect is seen at T/D = 2.5. The flow behaviour is similar to that of the isolated square cylinder at this gap ratio. © 2009 Taylor & Francis.

International Journal of Computational Fluid Dynamics

Large eddy simulations of flow interference between two unequal sized square cylinders

This article describes a simple and elegant compact higher order finite-difference based numerical solution technique to the primitive variable formulation of unsteady incompressible Navier Stokes equations (UINSE) on staggered grids. The method exploits the advantages of the D'yakanov ADI-like scheme and a non-iterative pressure correction based fractional step method. Spatial derivatives are discretized to fourth order accuracy and the time integration is realized through the Euler explicit method. The fast and efficient iterative solution to the discretized momentum and pressure Poisson equations is achieved using a variant of conjugate gradient method. Spatial accuracy and robustness of the solver are tested through its application to relevant benchmark problems.

Department of Mathematics

A novel semi-explicit spatially fourth order accurate projection method for unsteady incompressible viscous flows

This paper deals with the CFD predictions of the three dimensional incompressible flow over a wall mounted cubic obstacle placed in fully developed turbulent flow along with the heat transfer calculations. Reynolds number considered in this study is 1870 based on cube height, h and bulk velocity Ub. Our main objective is to find out the appropriate two equation turbulence model for the complex flow structure which involves recirculation, separation and reattachment. We have used standard k-ε, low-Reynolds number k-ε, non-linear k-ε model, standard k-ω and improved k-ω models to solve the closure problem. The non-linear k-ε model and improved k-ω models along with standard models are validated with bench mark problem - flow through a backward facing step (BFS). Results showed that the improved k-ω model is giving overall better predictions of the flow field especially recirculation zone, mean streamwise velocity, and turbulent characteristics when compared to those by standard eddy viscosity models. The non-linear k-ε model is giving better prediction when compared to standard k-ε and low Reynolds number k-ε models. The complex vortex structure around the cube causes large variation in the local convective heat transfer coefficient. The maximum of the heat transfer coefficient occurred in the proximity of the reattachment points and the minimum is found at the recirculation zone. © 2007 Elsevier Ltd. All rights reserved.

International Journal of Heat and Mass Transfer

Performance of two equation turbulence models for prediction of flow and heat transfer over a wall mounted cube

Numerical simulations have been performed for flow past two equal-sized square cylinders in tandem arrangement subjected to incoming planar shear flow. Effect of L/d ratio and the shear parameter has been studied. The range of L/d ratio (ratio of center-to-center distance (L) to cylinder width (d)) is varied from 2 to 7 and the non-dimensional shear parameter (K) is varied from 0.0 to 0.4 in steps of 0.1. For all the cases the Reynolds number (Re) based on centerline velocity and cylinder width is fixed at 100. The results are compared with that of isolated square cylinder with uniform flow. Strouhal number decreases with increasing shear parameter. There are more than one shedding frequency at high shear parameters and L/d ratios. The mean drag coefficient is decreased with shear parameter and lesser than that of the single cylinder. The root mean square (RMS) value of both lift and drag coefficients is higher for the downstream cylinder for all values of shear parameter. With increasing L/d ratio, for both lift and drag, the RMS value increases and then decreases for upstream cylinder, whereas it continuously increases for the downstream cylinder. The stagnation point is moved towards the top leading edge with increasing shear. The critical L/d ratio, which is defined as the distance between two cylinders, beyond which the vortex shedding from the upstream cylinder occurs, decreases with increasing shear parameter. Copyright © 2007 John Wiley & Sons, Ltd.

Interference effect of two equal-sized square cylinders in tandem arrangement: With planar shear flow

The influence of the staggering position of a rotating rod on flow past a main circular cylinder is investigated numerically. The rod is rotated at a constant speed ratio of 3. The effect of the diameter ratio of the rotating rod is studied by considering two different diameter ratios. The investigation is carried out at a fixed pitch length of 1. The study is carried out for two Reynolds number, viz., 100 and 500. The momentum injection from the rod is found to alter the flow characteristics behind the main cylinder. For a certain arrangement of stagger angle and diameter ratio, the vortex shedding behind the main cylinder gets suppressed. The corresponding configuration for which minimum drag coefficient is achieved is suggested from this study. Copyright © 2008 by ASME.

Journal of Fluids Engineering, Transactions of the ASME

Influence of staggering angle of a rotating rod on flow past a circular cylinder

Incompressible linear shear flow across a square cylinder is numerically analyzed by solving unsteady 2-D Navier-Stokes equations. Simulations are carried out for three sets of shear parameters, 0.0, 0.1 and 0.2 and two sets of solid blockage ratios, 6.25% and 10%. The aim of the present simulations is to find out the influence of shear on the onset of periodic time dependent flow, which is observed using time varying lift coefficient. With increasing shear, the critical Reynolds number, at which flow becomes time dependent, is reduced. The mean drag coefficient decreases either with increasing shear for a particular Reynolds number or with increasing Reynolds number for a particular shear parameter. © 2008 Elsevier Inc. All rights reserved.

International Journal of Heat and Fluid Flow

Onset of vortex shedding in planar shear flow past a square cylinder

Numerical simulation of flow past a circular cylinder at sub-critical Reynolds number Re = 3,900 is performed using three-dimensional, unsteady, Reynolds-Averaged Navier-Stokes (URANS) equations. A nonlinear turbulence model based on the k-ε formulation is used to achieve the turbulent closure. The results obtained by the simulations are compared with experimental and previously reported numerical results. The grid used for the present simulation is reasonable, and the accuracy obtained is good considering the computational cost involved in carrying out large-eddy simulations (LES) for the same test case. The test flow is also simulated using standard k-ε model, and the results obtained by the nonlinear k-ε model are found to be better.

Three-dimensional simulation of flow past a circular cylinder by nonlinear turbulence model

A new combustor concept referred as the trapped vortex combustor (TVC) employs a vortex that is trapped inside a cavity to stabilise the flame. The cavity is formed between two axisymmetric disks mounted in tandem. TVC offers many advantages when compared to conventional swirl stabilisers. In the present work, numerical investigation of cold flow (non-reacting) through trapped vortex combustor is performed. The numerical simulation involves passive flow through TVC to obtain an optimum cavity size to trap stable vortices inside the cavity and to observe the important characteristics of TVC. One of the main objectives is to evaluate various two equation turbulence models for the aerodynamic predictions of TVC. Commercial CFD software Fluent is used for the present study. In addition to many models available, Non-linear k-ω and modified k-ω models are incorporated through user defined functions. Results obtained include streamlines, residence time and entrainments for all models. The reattachment length obtained by non-linear k-ω model closely matches with that obtained by DNS in the case of forebody-spindle alone. Non-linear k-ω model alone captures the comer vortices while all the other models failed to capture. From the entrainment characteristics study, it is inferred that the primary air needs to be injected for accommodating the decrease in oxidizer inside the cavity to obtain better performance from the TVC.