The Effects of Porous Surfaces on the Control of Flows over Bluff Bodies of Circular, Square and Rectangular Cross-Sections

Bluff bodies are an inextricable part of life. They are found abundantly in engineered structures. When exposed to high velocity fluid flows, bluff bodies cause the flow to separate from their surfaces, which result in vortices shed from the bodies. Vortex shedding gives rise to large unsteady forces in the wake of the bluff bodies. The control of such forces is of great importance in engineering design because they reduce drag forces, vibrations and the generation of noise. There is evidence that the application of porous media to the surfaces of bluff bodies immersed in turbulent fluid flows has a profound effect on the associated aerodynamic phenomena. This idea is explored in this thesis by performing a series of experiments and numerical simulations on cylinders treated with porous media. The findings contribute to existing knowledge by providing researchers in the field with a trove of new, and carefully obtained experimental data on the flow fields generated by bluff bodies that have been modified by incorporating regions of porous media. These data will provide theoreticians with benchmark cases against which they can evaluate their mathematical models. The study is significant for two principal reasons. Firstly, it provides new insights into the fundamental mechanisms of the fluid dynamics of flows around bluff bodies, and secondly it has useful practical implications for the design of a multitude of systems immersed in turbulent flow fields. The novelty of the research is that it develops the idea of porous treatment by investigating the effects of the replacement of a part of a bluff body with a porous material and determining the efficacy of such treatment in the control of flow. The benefits include reduction of drag, vibration and noise, which in turn leads to the reduction in fuel consumption of vehicles and therefore a decline in the emissions of greenhouse gases. In the experiments, cylinders that have circular, square and rectangular cross sections have been studied. Well-established benchmarks were set by measuring the flow fields around bare solid aluminium cylinders. The flow fields were also established around circular cylinders that had been encased in porous media, and which retained the same dimensions of the bare cylinder. The square and rectangular bluff bodies were modified so that their upstream and downstream halves consisted of solid and porous materials respectively. As a result, their leading and trailing edges were respectively solid and porous. The porous media consisted of open cell polyurethane. Three formulations were used that had permeabilities of 4.64×10−7 m2, 5.70×10−8 m2 and 6.87×10−8 m2. The corresponding porosities were 91.8%, 86.2% and 82.1%. Experiments were performed in a wind tunnel operating under conditions that resulted in a Reynolds number of about 53000 based on the diameter of the circular cylinder and the dimensions of the leading edges of the square and rectangular bodies. The flow streamlines, wake velocity profiles and Reynolds stresses were determined by means of Particle Image Velocimetry (PIV). The experimental investigations revealed that the porous media applied to the circular cylinder had the most effect on the wake, and the largest effect was observed in the wake generated by the most permeable material. The principal effect of the porous media was to cause the eyes of the recirculation region to be located further downstream. This effect is apparent in all three geometries considered, but the effect of the permeability is increasingly diminished in the square and rectangular geometries. The presence of the porous media also had a significant effect on the Reynolds stresses. For example, immediately downstream of the cylinders higher normal stresses in the direction of flow are generated by bare cylinders compared with those generated by the cylinders modified by porous media. The effect of the porous media applied to all three geometries is to damp the normal stresses further downstream of the bluff bodies. Experiments provided results, which were used as benchmarks for numerical simulations. A CFD code, CFX using an SST-RANS turbulence model was validated against the experimental results and further investigations were performed numerically. Square cylinders with and without porous materials were further examined and compared with each other to expand our understanding of the flow structures around the cylinders. A porous material with a measured permeability equal to 4.64×10−7 m2 and a resistance loss coefficient equal to 1658 m−1 was used. The same configurations in the experiments, namely the bare square cylinder and the square cylinder with its trailing half composed of porous material have been examined. Furthermore, the effect of the disposition of porous material on the flow behaviour and drag force was studied by examining a square cylinder consisted of porous material at its leading half. The flow behaviour in the wake and in the boundary region, pressure distributions and drag forces were obtained in each case and compared with each other. Investigations of the flow around the bare and porous-treated square cylinders disclosed that the trailing-half porous cylinder is the most effective in the control of flow. This was demonstrated through the location of the recirculation bubbles, and the drag force. Maximum shift in the recirculation bubbles occurred in the trailing-half porous cylinder. The biggest development in the boundary layer was also observed in the trailing half-porous cylinder. The presence of porous material both at the leading and trailing half of the cylinders resulted in a decrease in the drag force. The maximum reduction in the drag coefficient was achieved with the trailing-half porous cylinder by 13% compared to the bare cylinder. In addition, the use of porous materials significantly altered the pressure distributions around the cylinders, when the highest pressure gradient was found for the leading-half porous cylinder.