Abstract

For surface gravity wave groups, the well-known Stokes drift and transport are complemented by a set-down (or set-up) of the free surface and an Eulerian return flow first described by Longuet-Higgins and Stewart (1962). We present experimental validation of these classical second-order theoretical predictions for uni-directional wave groups in a laboratory flume at the University of Plymouth COAST -lab and for directionally spread wave groups in the University of Edinburgh FloWave facility. Firstly, we present detailed PTV (particle tracking velocimetry) measurements of the Lagrangian transport and trajectories of near-neutrally buoyant particles underneath two-dimensional surface gravity wave groups in a laboratory flume. By focussing our attention on wave groups of moderate steepness, we confirm the predictions of standard second-order multi-chromatic wave theory, in which the body of fluid satisfies the potential flow equations. Particles near the surface are transported forwards and their motion is dominated by Stokes drift. Particles at sufficient depth are transported backwards by the Eulerian return current. Secondly, we present detailed measurements of the surface elevation of the second-order long bound-waves in directionally spread groups. We demonstrate that the set-down becomes a set-up for sufficiently large degrees of directional spreading using measurements in the newly built FloWave facility.