Abdollahpour Kohnehshahri, Milad (2023) Numerical Study of Flow Downstream a Step with a Cylinder. [Tesi di dottorato]

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Abstract

The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics The nature of this flow, consisting of separation and reattachment, makes it a problem worthwhile to study. In this study, the effect of a cylinder placed downstream of the step on the 2D flow structure was investigated. The classical 2D BFSF was validated using OpenFOAM (Open-Source Field Operation and Manipulation). The BFSF characteristics such as, reattachment, recirculation zone, velocity profile, skin friction coefficient, and pressure coefficient were validated for a step-height Reynolds number in the range from 75 to 9,000, covering both laminar and turbulent flow. The numerical results at different Reynolds numbers of laminar flow and four RANS turbulence models (standard k-ɛ, RNG k-ɛ, standard k-ω, and SST k-ω) were found to be in good agreement with the literature data. Later, the effect on the 2D flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was investigated. In the laminar flow, different Reynolds numbers were considered. In the turbulent flow, the effect on the flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was comparatively analyzed. When the cylinder was positioned below the step edge mid-plane, flow over the step was not altered by a cylinder. However, in other locations of a cylinder, the added cylinder modified the structure of flow increasing the skin friction coefficient in the recirculation zone. Also, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point remained stable in the flow recovery process. Moreover, the presence of the step significantly influenced the dynamics of the vortex generation and shedding leading to an asymmetric wake distribution. As the mixing of concentration in streams is a significant problem in environmental fluid mechanics, concentration transport studied to understand how release a pulse concentration is affected by cylinder in backward-facing step. Also, it is important to analyze the performance of hydraulic indices of concentration transport that are used in the literature to determine which index yields the most reliable results for the assessment of the pulse injection efficiency in sudden lateral expansions channel with the presence of cylindric obstacles. The presence of step geometry at the bottom of rivers is of interest because of localized velocity gradients, affecting aquatic habitat. Finally, the effect of cylinder placement at different horizontal locations on the local variations of velocity distributions and habitat complexity metric downstream step were analyzed.

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