Numerical investigation of the wake flow of an ellipse cylinder for different angles of attack Eyub Canli (1) , Sercan Dogan (2) , Muammer Ozgoren (2) , Ozgur Solmaz (1) and Gokhan Ozseker (3) (1) Department of Mechanical Engineering, Technology Faculty, Selcuk University, Konya, Turkey +905072312828 ecanli@selcuk.edu.tr (2) Department of Mechanical Engineering, Engineering Faculty, Selcuk University, Konya, Turkey (3) Arcelik A.S. Refrigerator Plant, Eskisehir, Turkey 1. Introduction – Heat exchanger tubes are essential devices in various applications of the thermal-fluid energy conversion systems. One of the effecting parameters on the performance of heat exchanger is flow separation around the tubes which can cause vibration and hence noise, specifically when air is the case for external flow. Cross-section of the tube geometry such as square, triangle, circular or ellipse change considerably the flow characteristics and heat transfer capability. In this study, flow separation phenomena depending on the angle of attack is investigated for an ellipse cylinder using AnsysFluent software. The results with flow characteristics of only time-averaged vorticity and streamline topology were comparatively presented and discussed due to the page limitation. There are various studies in a very wide scientific area about flow around elliptical geometries and some are mentioned here. An elliptical cylinder is defined as a generic shape which represents a flat plate at its minor to major axis ratio (AR) limits of zero and infinity, and a circular cylinder at AR of unity by Faruquee et al. [1]. They conducted a numerical study in order to investigate the flow field around an elliptical cylinder over a range of ARs from 0.3 to 1 with the major axis parallel to the free-stream, at a Reynolds number of 40 based on the hydraulic diameter and proposed some quadratic correlations. Bourasa et al. [2] presented a report about the numerical study of the natural convection in a space annulus. The annulus was situated between two horizontal confocal elliptic cylinders. This space is filled by a Newtonian fluid. A CFD approach was used to investigate the effect of ten different tube cross- sections containing an ellipse geometry on heat transfer resistance, gas flow resistance and fouling [3]. An array of 18 elliptical tubes with minor to major outside axis ratio of 0.30 was oriented at zero angle of attack in a test section with the major axis parallel to the air flow an tested for heat transfer characteristics [4]. Reynolds Number was 1x10 3 < Re < 3.3x10 4 . They reported that the pressure coefficient across a single tube in the array was found to remain at approximately 0.16 for Re > 2x10 4 and they proposed a Nu-Re correlation. The effect of yield stress and fluid inertia on the momentum transfer characteristics at finite Reynolds numbers for a 100-fold variation in the aspect ratio of an elliptical cylinder was investigated numerically [5]. The authors also investigated the forced convection heat transfer characteristics in Bingham plastic fluids from a heated elliptical cylinder of various aspect ratios numerically [6]. Some important literature related to the topic can be found in [7, 8]. 2. Numerical Analysis - In this study, an ellipse cross section having a long diameter and a short diameter with 65 mm and 10 mm lengths respectively, was placed in a 2D computational domain. The aspect ratio of the ellipse cross section is 3.25 (long diameter over short diameter). 10 different orientation angles are selected for the CFD examination; namely 0 o , 5 o , 10 o , 15 o , 22.5 o , 30 o , 45 o , 60 o , 67.5 o and 90 o . ANSYS-FLUENT software was utilized for the two dimensional computational domain. Spatial data were used for post processing and results were prepared accordingly. Reynolds number was calculated according to the characteristic length of ellipse cylinder and numerical studies were performed between Re=500 and 6500 at constant free stream flow. Two dimensional computational domain was preferred prior to the analyses because cylinders can be defined two dimensional and flow characteristics along the cylinder do not change. Remaining flow structures data in the plane perpendicular to the cylinder axis is corresponding to the interests of this study. For modeling that plane, a 2D plane was prepared containing a solid contour in it. This solid contour i.e. cross section has the shape according to the investigation. For instance it was an ellipse with predefined dimensions and the angle of attack. The schematic model presentation and boundary conditions for the numerical problem are provided in Figure 1. The flow conditions before and after the cross sections are conditioned in order to examine the wake region. Especially rear part of the flow domain should be satisfactorily long in order to avoid back flow.