Date of Award

4-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Aerospace, Physics, and Space Sciences

First Advisor

Chelakara Subramanian

Second Advisor

Jian Du

Third Advisor

Hamid Hefazi

Abstract

The present research work focuses on investigating the effect of wake structure on the drag coefficient of a thin flexible wire. Previous researchers have investigated the flow characteristics around the cylinder and estimated the drag coefficient for different Reynold’s number range. The flow transition characteristics in the boundary layer and its effect on the drag coefficient were described. These studies were performed on a non-deformable cylinder. The present work focuses on the drag characteristics of thin flexible cylinders whose diameter is on the order of microns. The results showed up to 30% lower value than reported literature drag coefficient values for a non-deformable cylinder. Experiments such as wake survey using pitot static measurement and hot wire experiments were conducted for thin flexible wires. The range of Reynolds number consider is from 150 to 1250 (laminar boundary layer). The Reynolds number is varied based on both diameter of the flexible wire (i.e. diameter 500 microns to diameter 1000 microns) and velocity (i.e. v = 5 m/s to 18 m/s). The pitot static and hot wire measurements are used to estimate the drag coefficient and turbulence parameters in the wake. To further investigate the cause of the drag reduction with the flexible wire and the flow field around the wire, computational fluid dynamics simulation is used. The Direct numerical simulation for structured grid around the cylinder was performed for the similar range of Reynolds number as the experiments. The computation simulations for a rigid wire were performed using ANSYS 17.0 commercial code and ICEM – CFD was used for grid

generation. The grid independence study was performed in order to choose an appropriate number of grid elements for the batch simulations. All the results showed around ten percent lower drag coefficient values compared to the experiments. A case setup including the structural deformation was simulated to match with the actual experimental setup and Fluid Structural Interaction coupled simulations (FSI) were performed using ANSYS 17.0 and the results matched the experimental results with an uncertainty of ±3%. The investigation observed a subcritical laminar boundary layer drag dip which is hypothesized as due to wake flow transition phenomenon. This is confirmed by the near wake flow transition structures. The effect of the flexible wire deformation on the wake structure, which also effects the drag coefficients of the thin flexible wire are also explained.

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