The flow structures behind a circular cylinder are associated with various instabilities. These instabilities are characterized by the Reynolds number and they include the wake, separated shear layer and boundary layer. Depending on the physical application of the cylinder, increasing the level of turbulence by wrapping the cylinder with metal foam would be a target for drag reduction or heat transfer enhancement.
In contrast to the extensive consideration that has been devoted to the flow around bare cylinders, the flow structures around the foam-covered cylinders and the characteristics of the wake behind such surfaces has received relatively little attention. Since the present study explores the possibility of using metal foams as replacements for ﬁns on heat exchanger tubes, investigation on the flow-field structures downstream of a foam-covered cylinder and compare it to the bare cylinder specifies the feasibility of using the foam-covered cylinders in heat exchangers. Moreover, the outcome of this study can be used to develop an appropriate boundary condition for the porous-air interface modelling & also increases the knowledge of turbulence in porous media.
As it will be discussed more in detail in the following chapters, pressure drop and drag coefficient are directly linked to the wake and recirculation region, and to study these two phenomena, it is necessary to have in depth study on the flow-field down-stream of the cylinder. Hence, the purpose of this study is to investigate the wake region behind a foam-covered cylinder by means of Particle Image Velocimetry (PIV) and Hot-Wire Anemometer. PIV is providing instantaneous whole-flow-field velocity vector measurements in a cross-section of a flow, which make it possible to study the instabilities of the flow-field and also the flow structures downstream of the model. By applying Proper Orthogonal Decomposition (POD) and Linear Stochastic Estimation (LSE) to the PIV results and comparing the results with what exits in literature for bare cylinder, we can conclude if the foam-covered cylinder increases the turbulence level.
Moreover, using hot-wire anemometry, to investigate the energy of the flow inside and outside of the foam-covered cylinder’s wake, let us know if a foam-covered cylinder can be treated as an obstacle to the incident flow with a rough surface or the whole foam can be considered as the combination of local jets that are coming out of the pores and disperse in random directions.