THE ANALYSIS OF THE WAKE BEHIND A FOAMED AND A FINNED CYLINDER Morteza Khashehchi 1 , Kamel Hooman 1 1. Queensland Geothermal Energy Centre of Excellence, University of Queensland, QLD 4072, Australia m.khashehchi@uq.edu.au ABSTRACT 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 develop one- and two- point correlations and to investigate the flow characteristics for these two cases. The experiments are conducted for two Reynolds numbers (based on the mean air velocity and the cylinder diameter) 2000 and 8000. 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 in each case. 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 both 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 finned and foamed cylinder types, the first four eigenmodes of the velocity field were measured and their organizations were investigated. These eigenmodes disclose the overall mean flow structure, and the large- scale structure being essentially connected to the most robust flow motion. KEY WORDS PIV, Foamed and Finned cylinders, POD. 1. Introduction During the last few decades, the mechanism of vortex shedding structures and the structure of the wake created behind circular cylinders have been investigated in a wide variety of studies. 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 us to increase our understanding about the flow structures behind the cylinder and, consequently, the induced turbulence in the wake. Formation of coherent structures is normally observed when the flow passes the cylinder. These structures then will shed and be washed downstream with the flow. The characteristics of the wake, such as the shedding frequencies and mixing properties, are controlled by these flow structures. There have been numerous experiments conducted to examine the flow around circular cylinders in cross-flow (see e.g. Roshko [1] and Bearman and Harvey [2]) as well as the flow over the elliptical cylinder (see e.g. Daichin et. al. [3]). Roshko [1] defined the range of critical Reynolds numbers, which defines the fundamental problem for the scale model testing of curved structures in low speed wind tunnels. However, the concept of controlling the flow over circular cylinders is not yet fully understood, in spite of many studies on the effect of surface roughness on cylinders in the past. Bearman and Harvey [2] examined dimpled surfaces, while roughness on a cylinder was tested by Szechenyi [4]. Both of these studies showed that the pressure distribution around the cylinder could be altered through the addition of a roughness pattern. On the other hand, very limited research has been done looking at the application of attached finned and foamed cylinders in flow control strategies and heat transfer efficiency. Whilst fins are considered as vortex- spoilers as they disturb the shed vortices, making them less coherent and three dimensional (Zdravkovich [10]), several studies of vortex shedding of finned-cylinders show that the vortex shedding frequency is well correlated with the cylinder effective diameter, which is based on the projected frontal area of the cylinder (Mair et al. [11], Hamakawa et al. [12]). Several unresolved issues still need to be investigated in order to improve our fundamental understanding of the effect of fins on the turbulence behind the cylinder. Moreover, the role of the foam on the structures behind the cylinder seems to be different and has not been studied before. As mentioned, in contrast to the extensive consideration that has been dedicated to the flow around bare cylinders, the flow structures around the finned and foamed cylinders and the characteristics of the wake behind such surfaces has received relatively little Proceedings of the IASTED International Conference Power and Energy Systems (AsiaPES 2013) April 10 - 12, 2013 Phuket, Thailand DOI: 10.2316/P.2013.800-094 339