||Characteristic Time, Length and Velocity Scales of Transverse Flows in the Surf Zone
Otsuka, Junichi ,
Coastal engineering journal
, p.1550006 , 2015-06-08 , World Scientific Publishing
An ultrasonic velocity profiler (UVP) acquires instantaneous distributions of axial velocity along the ultrasonic beam emitted from the transducer. In this paper, organizations of the transverse flows over a span of a laboratory wave flume in the surf zone were identified experimentally on the basis of UVP measurements. Ensemble mean divergent and convergent flows were observed over the span of the flume in the surf zone regardless of breaker conditions. By analogy with the flow evolution in a previous computation, by Watanabe and Saeki , we identified that the organization of the wave-breaking-induced counterrotating vortices determined the spatial and temporal variations in the transverse flows at the early stages of the breaking process regardless of the widths of wave flumes and friction on the sidewalls and bottom. The observed transverse flows were driven by pairs of counter-rotating vortices produced under the breaking waves and the change in the orientation during the passage of breaking waves. The fundamental features of the transverse flows evolving in the breaking process, depending on the breaker type, are discussed statistically to parameterize velocity, time, and length scales of the variations of the transverse velocity. The dimensionless length and time scales had negative linear correlations with the Froude number (defined by relative transverse convection with respect to gravity) as well as a surf similarity parameter, which indicates that the gravity effect associated with the projections of the breaking wave crests defines the primary roller vorticity and shear intensity at the plunging location, resulting in destabilization of the flows in the transverse direction to form organized transverse flow structures.