In the present thesis the problem of an oscillating flow past a circular cylinder bounded by two parallel endplates has been studied experimentally. The problem is complicated because it involves with many length scales. This leads to the dependence of the characteristics of the oscillating flows on three independent dimensionless parameters, namely, the cylinder length (2H) to diameter (D) aspect ratio Γ, the ratio of oscillation amplitude to cylinder diameter KC and the square of the ratio of cylinder diameter to the thickness of oscillatory Stokes layer β_D. The objective of present research is to understand more clearly the physical phenomena of this problem, such as the effect of endplate on Honji instability and oscillating flow pattems over a wide range of these parameters....[ Read more ]

In the present thesis the problem of an oscillating flow past a circular cylinder bounded by two parallel endplates has been studied experimentally. The problem is complicated because it involves with many length scales. This leads to the dependence of the characteristics of the oscillating flows on three independent dimensionless parameters, namely, the cylinder length (2H) to diameter (D) aspect ratio Γ, the ratio of oscillation amplitude to cylinder diameter KC and the square of the ratio of cylinder diameter to the thickness of oscillatory Stokes layer β_D. The objective of present research is to understand more clearly the physical phenomena of this problem, such as the effect of endplate on Honji instability and oscillating flow pattems over a wide range of these parameters.

To determine the flow conditions at far away from the cylinder, Laser Doppler velocimetry system (LDV) was employed to measure the amplitude and phase of velocity oscillations inside the Stokes boundary layer both in flat plate and narrow channels. The LDV data were reduced by a phase-average scheme to determine amplitudes and phase angles. The agreements between experimental data and laminar theoretical solutions lead to the conclusion that the flow pattern far away from the cylinder in present experimental domain is laminar.

Laser-Induced fluorescence (LIF) system was setup in the present experiments to visualize the patterns of oscillating flows. The test models were fabricated and installed with a dye injection system onto the oscillation mechanism on a water tank. The data reduction scheme was established based on Matlab. The experiments were performed over parametric ranges focusing on the studies of Honji instability and overall flow structure.

The effects of two parallel endplates on Honji instability were investigated by visualizing the mushroom-shaped vortices along the cylinder crown on the plane perpendicular to the flow oscillation direction. Two main effects of the endplate on the Honji instability are found. Firstly, the occurrence of the mushroom-shaped vortices caused by Honji instability is periodic along the cylinder crown and the spacing period of two nearby mushroom-shaped vortices is an integer divider of the length of the cylinder. Secondly, the mushroom-shaped vortex structures near the endplates are quite different from those in the core region, because the formation of mushroom-shaped vortices near the endplates is highly suppressed by the endplates.

To obtain a complete picture of the oscillating past the cylinder with endplates, different flow pattems are visualized in the mid-plane perpendicular to the cylinder axis over a wide range of (Γ, KC, β_D). It is found that the flows are stabilized by decreasing aspect ratio Γ number when KC and β_D numbers are kept constant. Three flow regimes around the circular cylinder are observed in the ranges of 0.3D< 16600 for small Γ numbers (Γ=0.05 and 0.03). The secondary steady streaming regime happens at small KC number; flow separation regime with vortex bubbles occurs at the intermediate KC number; and at large KC and/or large β_D the flows around the cylinder become transition to turbulent regime.

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