Morteza Khashehchi

A Comparison Between the Separated Flow Structures Near the Wake of a Bare and a Foam-Covered Circular Cylinder

The flow structures behind bare and aluminum foam-covered single circular cylinders were investigated using 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 analysis of the PIV data shows the important effects of the foam cover and the inlet velocity on the separated structures. The results show a considerable increase in the wake size behind a foam-covered cylinder compared to that of a bare cylinder. Furthermore, the turbulence intensity is found to be around 10% higher in the case of the foam-covered cylinder where the wake size is approximately doubled for the former case compared to the latter. The turbulence kinetic energy, however, is found to be less Reynolds dependent in the case of the foam-covered cylinder. In addition, small scale structures contribute to the formation of the flow structures in the foam-covered cylinder making them a more efficient turbulent generator for the next rows when used in a heat exchanger tube bundle. On the other hand, a 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 for modeling the flow field past such cylinders.

Investigation of Large-Scale Structures Behind a Single Tube (Finned and Foamed Tube) Using Two-Point Correlations

Flow structures downstream of a finned-tube are compared to those of an identical pipe; with the same diameter and length, covered with a foam layer. The standard case of cross-flow over a bare tube, i.e. no surface extension, is also tested as a benchmark. Experiments are conducted in a wind tunnel at Reynolds numbers of 4000 and 16000. Particle image velocimetry (PIV) was used for flow visualization on two different perpendicular planes. To characterize the size of the flow structures downstream of the tube, for each of the aforementioned case, two-point correlation, as a statistical analysis tool, has been used. It has been observed that by decreasing the Reynolds number, the flow structures are further stretched in streamwise direction for both bare and finned-tube cases. This is, however, more pronounced with the former. Interestingly, with a foam-wrapped tube the sizes of the flow structures are found to be independent of the Reynolds number. Finally, the structure sizes are smaller in the case of the foam-wrapped tube compared to those of finned-tube.

The analysis of the wake behind a foamed and a finned cylinder

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.