S. Vengadesan

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.

Flow Characteristics Behind Rectangular Cylinder Placed Near a Wall

Three-dimensional unsteady flow over a bluff body located parallel to a wall, kept at different gap height from the wall, has been studied numerically. The bluff body considered is a rectangular cylinder with two different aspect ratios, B/D = 1 and B/D = 2, where B and D are the width and height of the cylinder, respectively. The flow is considered as a laminar flow, and the Reynolds number based on the height of the cylinder cross section and oncoming reference velocity is 450. Numerical study is carried out by varying the distance of the cylinder from the wall, and the development of the vortex shedding phenomenon under the influence of the wall is investigated. From previous experiments, it is observed that as the distance between the wall and the cylinder decreases, the wake behind the cylinder becomes stationary and the vortex shedding is suppressed. The present numerical study confirms a similar trend. Periodic activity in the downstream of the flow is disturbed completely with decreasing gap between the wall and the cylinder.

3D unsteady RANS simulation of turbulent flow over bluff body by non‐linear model

Purpose – To perform 3D unsteady Reynolds Averaged Navier‐Stokes (URANS) simulations to predict turbulent flow over bluff body.Design/methodology/approach – Turbulence closure is achieved through a non‐linear k−e model. This model is incorporated in commercial FLUENT software, through user defined functions (UDF).Findings – The study shows that the present URANS with standard wall functions predicts all the major unsteady phenomena, with a good improvement over other URANS reported so far, which incorporate linear eddy viscosity models. The results are also comparable with those obtained by LES for the same test case.Originality/value – When comparing the computational time required by the present model and by LES, the accuracy achieved is significant and can be used for simulating 3D unsteady complex engineering flows with reasonable success.

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...

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

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.

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]

Influence of Staggering Angle of a Rotating Rod on Flow Past a Circular Cylinder

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.

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...

The Functional Significance of Delayed Stall in Insect Flight

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.