A COMPARISON BETWEEN THE SEPARATED FLOW STRUCTURES NEAR THE WAKE OF A BARE AND A FOAM-COVERED CIRCULAR CYLINDER

The aim of this study is to investigate and compare the separated flow structures behind bare and metal foam-covered single circular cylinders by means of Particle Image Velocimetry (PIV). The experiments are conducted for a range of Reynolds numbers from 2000 to 8000 based on the outer cylinders diameter and the air velocity upstream of the cylinder. The two dimensional planar PIV results as well as proper orthogonal decomposition (POD) analysis show the important effects of the inlet velocity and the foam cover on the separated structures. The results show a considerable increase of the wake size behind a foam-wrapped cylinder compared to that of a bare cylinder. Interestingly, the wake size increases with Re for a foamed cylinder while for the identical bare cylinder the converse is true. Furthermore, the shedding frequency is found to be around 10% higher in the case of the foam-wrapped cylinder where the wake size is approximately doubled for the former case compared to the latter. The swirl strength, however, is found to be strongly Reynolds-dependent at higher Reynolds numbers. With low Re values, the swirl strength remains the same for the two cases considered here while, on average, it is 15% higher with the foamed case. Our results showed that, with the same fluid velocity, the foarried-cylinders considered in this study cannot be modeled as a bare cylinder of the same size but with a rough surface. In addition, energy of the large scale separated structures spreads between modes and also the small scale structures contribute to the formation of the flow structures in foam cylinder making them a more efficient turbulent generator for the next rows when used in a heat exchanger tube bundle. On the other hand, higher energy level in such separated structures will translate into increased pressure drop compared to bare cylinders. Finally, the results of this study can be used as an accurate set of boundary conditions to model the flow field past such cylinders.