LEE- Hidrolik ve Su Kaynakları Mühendisliği-Doktora
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Konu "varied flow" ile LEE- Hidrolik ve Su Kaynakları Mühendisliği-Doktora'a göz atma
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ÖgeExperimental study on interaction of unsteady flow with bridge piers with different cross sections(Graduate School, 2021-09-16) Gargari, Mehrnoush Kohandel ; Kırca Özgür, Veysel Şadan ; 501152502 ; Hydraulics and Water Resources EngineeringThe problem of interaction between a vertical cylindrical structure (such as a bridge pier or pile) and a gradually varying unsteady flow is addressed in this study. In practice not only circular cylinders, but also various hydraulically streamlined cross-sections are used in bridge piers. The flow structure around these obstacles are significantly altered which leads changes in bed shear stress and amount/geometry of scour that takes place around the bridge pier. In this thesis, the flow-pile interactions under the unsteady flow are investigated, and as such, the similarities and differences in comparison to the case of steady flow are determined. The spatial variations of Reynolds averaged velocity and turbulence characteristics around the cylindrical structures are determined as a useful tool to help us understand how the flow patterns in the wake of the cylinders reacts with change in the cross-sections both in unsteady and steady flows. Although many studies in the literature have dealt with the flow around similar structures, most of these studies are limited to circular cross-sections. Furthermore, due to the complex nature of unsteady flows, there is a gap in the literature regarding studying the effects of local and convective acceleration in the case of gradually-varied unsteady flows. The current experimental study will concentrate on the flow alterations in the wake of cylindrical structures with different cross-sections in the presence of unsteady flow (i.e. during the passage of a hydrograph) in help to fulfil the aforementioned knowledge gap in the literature. Therefore, in this thesis, an experimental study was conducted comprising rigid bed experiments in a 30m long and 1m wide recirculating flume equipped with a variable discharge pump. Circular cylinders with 9 cm diameter (D=9 cm) are used, and elongated cylinders with aspect ratios of L/D=2, 3 and 4 are also investigated. To understand the influence of accelerating and decelerating flow conditions, three unsteady cases with different unsteadiness degrees were tested as well as a reference steady flow case. The spatial and temporal variations of Reynolds-averaged velocity and turbulence characteristics around the pile, as well as undisturbed flow, were analysed. Findings show that there are distinct differences between the tested gradually-varied unsteady flow cases and the reference steady flow case. Three-dimensional velocity measurements were conducted via an Acoustic Doppler velocimeter (ADV) at more than 200 locations for each of the test conditions. Moreover, water level and hydraulics slope values were recorded by use of resistant-type water level sensors. The data were analyzed to obtain spatial and temporal variation of Reynolds-averaged velocities and turbulence characteristics (fluctuating components, Reynolds stresses, turbulent kinetic energy) under steady and unsteady flow case in a comparative manner. Findings show that there are distinct differences between steady and unsteady flow conditions around bridge piers. Considering circular bridge piers, the wake turbulence was observed to get significantly higher during the rising stage of the hydrograph compared to the falling stage, whereas the turbulence due to lateral flow contraction exhibits an inverse behavior. The near bed flow around the bridge pier was seen to react the changing pressure gradient much quicker compared to the main flow region, where the reaction was much delayed, causing a longer recirculation region during the falling stage of the hydrograph. It is concluded that the flow structures foreseen for steady flow becomes noticeably altered in the case of unsteady flow, and these alterations are suggested to be considered in the engineering practices. The Reynolds-averaged velocity vs. Turbulence kinetic Energy plots of undisturbed flow indicated a hysteresis effect, such that larger turbulence is generated during the falling stage of the flow compared to the rising stage. This hysteresis was considerably reduced in the pile wake, and even reversed hysteresis was seen at certain cases. The spatial variation of Reynolds-averaged velocity and turbulence in the peak instant of unsteady flow was qualitatively similar to that of steady flow, but quantitatively, turbulence, flow contraction, and velocity deficit in the near-wake region were smaller in the case of unsteady flow. Contrarily, the unsteady flow generated remarkably higher turbulence levels at further downstream in the pile wake. It is concluded that in the case of unsteady flow the pile behaves as if it has a more streamlined shape. The results were also interpreted from structure-bed interactions perspective, explaining the differences between the pile scour induced by steady and unsteady flow conditions.