A COMPARISON BETWEEN THE WAKE BEHIND FINNED AND FOAMED CIRCULAR CYLINDERS IN CROSS-FLOW M.KHASHEHCHI 1,c , I.ASHTIANI ABDI, K.HOOMAN 1 , T.ROESGEN 2 1 School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia 2 Institute of Fluid Dynamics, ETH Zurich, 8092 Zurich, SWITZERLAND c Corresponding author: Tel.: +61733654187; Email: m.khashehchi@uq.edu.au KEYWORDS: Fluid dynamics, low speed air flow, finned and foamed tubes, Planar-PIV, turbulence ABSTRACT: The flow pattern behind a circular cylinder is associated with various instabilities. These instabilities are characterized by the Reynolds number and include the wake, separated shear layer and boundary layer. Depending on the physical application of the cylinder, increasing the level of turbulence on the surface of the cylinder would be a target for drag reduction or heat transfer enhancement. Particle Image Velocimetry (PIV) has been carried out to investigate the wake region behind a foamed and a finned cylinder. The purpose of this analysis is to investigate the flow characteristics for these two cases. The experiments are conducted for a wide range of Reynolds numbers (based on the mean air velocity and the cylinder diameter) from 1000 to 10000. Two dimensional results of planar PIV reveal the important aspects of the local flow features of the circular finned and foamed cylinders. These include turbulent boundary layer development over the surface and a delayed separation of the flow resulting in a smaller wake size at each speed. The application of Proper Orthogonal Decomposition (POD) to the PIV velocity fields of the two cylinder types is also discussed. The POD computed for the measured velocity fields for all cases shows that the first two spatial modes contain most of the kinetic energy of the flow, irrespective to the cylinder type. These two modes are also responsible for the large-scale coherence of the fluctuations. For three different cylinder types, the first four eigenmodes of the flow field were calculated and their structures were analysed. 1. Introduction During the last few decades, the mechanism of vortex shedding and the structure of the wake created behind circular cylinders have been extensively investigated since the large scale coherent structures in the mixing layer could be a source of drag, noise and heat transfer. Moreover, to have a better understanding of the mixing layer, it is necessary to analyze these structures in detail. Concern here is motivated not only by the desire to understand the fundamental characteristics of cylinder aerodynamics, but also by its direct impact on engineering applications such as heat exchangers. The ever-growing experimental capabilities such as PIV or other laser diagnostic methods enable a better understanding of details of the flow structures behind the cylinder and, consequently, the induced turbulence in the wake. Such techniques are also capable of resolving the restrictions associated with the presence of back flow in traditional point-wise measurement techniques such as hot-wire or pitot- tube.