Free-surface effects on turbulent boundary layer and near-wake around a surface-piercing body

Abstract

In this study, free-surface effects on boundary layer and near-wake around a slender free-surface piercing body were investigated by flow field and wave elevation measurements. To provide three different wave conditions, three Froude number (Fr) conditions were applied for model tests: 0.126, 0.282, and 0.400. In addition, models of three different sizes were chosen to range Reynolds number (Re) from 34,200 to 1,080,000. Wave elevation was measured by capacitance type wave height gauge and observation. Wave breaking and bubbly free-surface were observed for Fr = 0.400, as reported in previous studies on the surface piercing body. At the bubbly free-surface region, the fluctuation of the free-surface elevation in certain frequency range appeared. Flow field were measured by towed underwater stereoscopic particle image velocimetry (SPIV) system. Towed SPIV measured three-components of velocity on a two-dimensional planes, which were perpendicular to the longitudinal direction of the model. By the SPIV measurement, free-surface effects on the boundary layer development for Fr = 0.126, where wave was hardly generated, were identified first. At the juncture of the free-surface and the model surface, flow fluctuated in normal direction to the free-surface and it reduced the flow velocity. The turbulence strength and isotropy increased near the juncture. In the near-wake, the free-surface delayed the wake recovery and turbulence dissipation. The free-surface wave was developed in the intermediate Fr condition. It was steady and smooth, thus orbital motion of water particles in waves was well observed at outside of the boundary layer. The boundary layer restrained orbital motion due to no-slip wall effects. Behind the trailing edge of the model, the free-surface fluctuated and knobs were observed. At the free-surface and model surface juncture, reverse flow appeared and turbulence kinetic energy increased along the boundary of the stationary flow region, because of strong shear strain. As Fr increased, the knobby free-surface region expanded upstream and bubbly free-surface affected the flow underneath it. The turbulence induced by the violent free-surface behavior was omnidirectional and decreased anisotropy near the free-surface. In the near-wake region, the knobby free-surface stimulated momentum transportation and turbulence dissipation. As the violent free-surface behavior dissipated turbulence in near-wake, dominant turbulence dynamics in low frequency decreased rapidly in downstream. In addition, testing models of different sizes followed. By wave elevation measurement, it was confirmed that the wave elevation decreased in low Re conditions as viscous force became significant. Free-surface fluctuation also reduced and capillary wave appeared as Re decreased. Viscous force effects were also visible in flow fields. The boundary layer thickness ratio increased, but flow separation along the trailing edge in high Fr condition decreased and localized near the free-surface only.