Visualization of irregular oscillatory flow around a stationary cylinder has been carried out, using the aluminum powder technique. The process of vortex shedding is analysed. It has been shown that the regular oscillatory flow vortex-flow regimes quickly disappear with an increase of the irregularity of the flow. Vortex pair, which is characteristic for a flow started from rest, is the dominating flow regime under irregular oscillatory flow conditions. Consequences of this on values of hydrodynamic coefficients are discussed in the paper.
Hydrodynamic forces on cylindrical structural members in regular waves are usually described by the Monson equation: (Equation is shown in the paper) The main purpose of this study is to investigate the motion of vortices around a stationary cylinder under two-dimensional irregular oscillatory flow conditions by visual analysis of each half period of the How. The effect of a wind-driven 3-dimensional seas is not taken into consideration in the present study. The results are used to explain the behaviour of the force coefficients for irregular oscillatory flows observed by Longoria et al. (1991).
The tests were carried out in a water tank (the length: 1.50 m, the width: 0.50 m, and the depth: 0.60 m). The carriage technique was used in the experiments to simulate the two-dimensional oscillatory flow around a cylinder. The cylinder was mounted to a carriage which oscillated by means of a hydraulic system. Three plastic cylinders with diameters 9, 3 and 1.5 cm were used (Table 1). The surfaces of the cylinders were smooth. The test setup is illustrated in Fig. 1. The aluminum powder technique was used to visualize the flow around the cylinder. The motion of vortices was traced in a stationary frame of reference by a video camera placed above the equilibrium position of the cylinder.
