Experimental investigation of the wave-induced flow around a surface-touching cylinder

The wave-induced flow around a circular cylinder near both a rigid wall and an erodible bed is experimentally investigated using Particle Tracking Velocimetry (PTV). The aim of this study is to gain quantitative information on the local mean flow, the vorticity dynamics and the evolution of the erodible bed. The flow is characterized in terms of the Keulegan–Carpenter (KC), Reynolds (Re) and Ursell (Ur) numbers. The effects of changing these parameters over the ranges 1<KC<31, 3×103<Re<2.6×104 and 1.5<Ur<152 are investigated. For KC<1.1 the flow does not separate. When KC increases, the flow becomes unstable and large-scale vortical structures develop. The dimensionless intensity (|Γ|) depends non-monotonically on KC, with a local maximum at KC=17, and the dimensionless area of the same macrovortex (A) follows a somewhat similar law. Although the dimensionless boundary layer thickness (δ) exhibits some discontinuities between KC regimes, it decreases with KC at x/D=0.5, as x/D=1 weakly depends on KC and can be regarded as constant (δ=0.7) and then, increases with KC when moving away from the cylinder. These findings are used to interpret the physics governing the flow around a cylinder touching a wall and are compared with available results from the literature (Sumer et al., 1991). The evolution of the scour mechanism occurring over an erodible sandy bed is also investigated. The validity of some empirical formulas in the literature is also tested on the basis of the available dataset. The empirical relationships of Cevik and Yuksel (1999) and Sumer and Fredsøe (1990) for the dimensionless scour depth (S/D) agree well with our results. The dimensionless scour width (Ws/D) is predicted well by Sumer and Fredsøe's (2002) empirical equation for KC<23, whereas Catano-Lopera and Garcia's (2007) formula is more accurate for higher values of KC.