WAKE CHARACTERISTICS DOWNSTREAM OF A RIGID UNSUBMERGED CYLINDER IN A RECTANGULAR CHANNEL

Author

Sanjeet -. Shrestha, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2014

Degree Name

Master of Science

Department

Civil Engineering

First Advisor

Wilkerson, Gregory

Abstract

Wakes form when a fluid flows past stationary objects. Within the wake, the velocity of the fluid is reduced, that is, there is a velocity defect. Limited information is available for predicting the magnitude of the velocity defects and the width of wakes that form behind rigid cylinders in open-channel flows. Therefore, the objective of this study is to develop relationships for predicting wake geometry downstream of a rigid unsubmerged cylinder. This study was conducted in a 0.46-m-wide rectangular channel with either medium sand (median size, d₅₀ = 0.33 mm) or fine gravel (d₅₀ = 6.35 mm) bed material. A wooden dowel (6.35 mm to 38.1 mm in diameter) was used as a rigid cylinder. Point velocity data was collected for varied hydraulic conditions; the data was used to compute the maximum velocity defect (u_max) and half-width of the wake (2b_1/2) at sections located different distances downstream from the cylinder (&Deltax). The data were also used to compute parameters for models that predict u_max and 2b_1/2 as a function of x. It was expected that the study results would yield two equations: one for predicting u_max and 2b_1/2; instead, the parameters varied with cylinder size and bed material. Since the results were not as expected, evaluation tests were performed to determine why the parameters varied. The evaluation tests (pump configuration tests, point-velocity repetition-tests, point-velocity sampling-time tests, and boundary-layer-development tests) revealed that the turbulent boundary layer (the layer of fluid where the effects of viscosity are significant) was not fully developed in most of the sections where point velocity data was collected. In addition, it was determined that the point-velocity sampling-time of 100 s was not sufficient for keeping the velocity measurement within 2% of the long-term mean velocity (the mean velocity based on a 500-s sample); to achieve this goal it is recommended that a 200-s sampling time should be used.