Experimental investigation of the flow around hexagonally arrayed circular cylinder group under steady flow conditions

Bridges have been practical structures through the history of civilization due to their useful role in the design of cost-effective transportation routes. As the bridge piers are in interaction with water, to investigate the potential impacts of the flow on the bridge pier has become an important subject for hydraulic engineers. When an obstacle, specifically a bridge pier, is exposed to flow, the presence of the structure significantly alters the flow domain in its vicinity. These alterations may cause increased sediment transport and result in scour around the structure. Since the instability caused by scour around bridge piers is one of the top reasons of bridge failures, the estimation of the scour depth and taking effective precautions have always been a key issue for engineers. Previous experimental researches about fluvial flow around vegetation elements were undertaken by using porous obstructions. Circular cylinder groups were substituted for vegetation elements. The experimental results showed that with the increasing porosity of the obstacle, the scour depth might be decreased to a considerable extent. In the light of this outcome, it was thought that designing a porous bridge pier might be a potential solution to diminish the scour depth. Moreover, a porous pier would block less flow and help reducing the flood risk. Considering the fact that the load bearing capacity of the bridge pier basically depends on its cross sectional area, the idea was feasible. From this point of view, different groups of circular cylinders were considered as potential porous bridge piers and the flow characteristics around them were investigated. Those experimental studies, investigating the flow structure and scour patterns around the circular cylinder groups, have formed a strong basis for the subject. Nevertheless, there is still progress to be made. The primary aim of the present study is to give a better understanding of the flow field around a group of pile, which may eventually be used for reducing scour around the structure. Within the scope of the thesis, fifteen sets of experiments were conducted in order to see how solid volume fraction and orientation (angle of attack of the flow) of different cylinder groups affect velocity and turbulence profile of the flow in the rear of the obstacles. Flume experiments have been carried out under steady flow conditions in a recirculating flume in the Hydraulics Laboratory of Istanbul Technical University, which has a horizontal bed of smooth concrete and sidewalls made of Plexiglas. Each experiment lasted for approximately 8 hours. Hexagonally arrayed circular cylinders (HACC) with four different circumambient diameters (12, 16, 20 and 24 cm) under three different orientations (regular, angled and staggered) and three solid cylinders with diameters of 12 cm, 16 cm and 20 cm were used as model piles. HACC is a porous element comprised of seven cylinders of 3.4 cm diameter. Six of them are hexagonally arrayed and one is placed at the center. In order to plot the velocity profile in the rear of each obstacle, 3D velocity values of constituent points of the lateral line closest to the obstacle have been recorded and the results were analyzed. In general, the behavior of the 12 cm HACC, which has the highest solid volume fraction, closely resembles the solid cylinder. With the decreasing solid volume fraction, flow patterns behind each HACC case show tendency to divergence from each other and their counterpart solid cylinder. Taken as a whole, the mean flow velocity and turbulence intensity values of surface measurements were higher compared to those of the bottom measurements. The normalized width-averaged bleed flow velocity values for the regular cases were always higher than other two cases owing to its smallest projected area. With the decreasing solid volume fraction, the role of the arrangement diminishs and the solid volume fraction becomes highly influential on the observed width-averaged bleed flow velocity. To sum up, the experimental results of the present study points towards the idea that both the orientation and the solid volume fraction of HACC play a crucial role in the flow structure behind the body.