UBC Theses and Dissertations
The measurement of wake and surface effects in the subcritical flow past a circular cylinder at rest and in vortex-excited oscillation Ferguson, Nelson
A pressure transducer, sensitive to acoustic level pressures, was designed and used to measure amplitude, frequency and phase of fluctuating pressure on the surface of a three inch diameter circular cylinder at rest and exhibiting large-amplitude vortex-excited oscillation in a uniform incident wind flow. The phase of the fluctuating pressure relative to the cylinder motion and the cylinder amplitude and frequency were recorded. A disc probe connected to the pressure transducer was used in wake surveys for the stationary and oscillating cylinder. Measurements, made in the Reynolds number range 1.5(10⁴) <N(R)< 4.1(10⁴), indicated the following: Fluctuating pressures on the surface of both a stationary and a vortex-excited circular cylinder experience amplitude modulation, being random for the stationary cylinder and critically dependent on wind speed for the vortex-excited cylinder. Fluctuating pressures for both a stationary and an oscillating cylinder are in phase over one side of the cylinder and 180° out of phase with the opposite side. For a vortex-excited cylinder, the vortex frequency is 'captured' by the cylinder frequency over a discrete range of wind speed. The amplitude of fluctuating pressure on the surface of a vortex-excited cylinder increases as the resonant wind speed (wind speed corresponding to maximum cylinder amplitude) is approached, but before that actual wind speed is reached, an abrupt decrease occurs, and the amplitude modulation nearly disappears. A sudden change of phase between cylinder motion and fluctuating pressure occurs near the resonant wind speed and the pressure wave form becomes asymmetrical. The longitudinal spacing of vortices in the wake of a stationary and a vortex-excited cylinder initially increases as the vortices are swept downstream from the formation zone. Vortex-excited oscillation of a cylinder, as the resonant wind speed is approached, produces an increase in longitudinal vortex spacing and a corresponding abrupt decrease in lateral spacing. At the resonant wind speed, the wake loses its coherent periodicity.
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