The Defrees hydraulics lab is equipped with three main experimental facilities, a Wave Tank, a Wide Meandering Wave-Current Flume, and a Tilting Wind Water Tunnel. Companion data acquisition and analytical measurement equipment includes a selection of quantitative imaging systems such as digital cameras, light source (lasers) and associated optical components for illumination, computers for data acquisition, and image processing software. Currently, two kinds of experimental researches are being carried out: (i) water surface wave interactions with seafloors of a variety of rheology and (ii) breaking waves.
Wave-induced flows in seabed of a variety of rheologies
In the nearshore region, where the water depth is relatively shallow in comparison with the wavelength, interactions between propagating water waves and the seabed can be significant. On the one hand, the seabed can cause wave energy attenuatio
n and can also change the direction of wave propagation. On the other hand, water waves can generate seabed deformation and sediment transport by directly exerting stress field or by indirectly inducing flows inside the seabed. Since coastal sediments have a wide spectrum of rheological properties, it is difficult to describe them with a single constitutive model. Instead, different models can be applied to different sediment material. Four fundamental rheological models have been investigated as follows:
- a rigid, impermeable rock bottom;
- an unsaturated, permeable seabed;
- a viscous muddy seabed;
- a viscoplastic muddy seabed.
Researches on breaking waves in the near-shore region have focused on mean flow fields of periodic incident waves or a single runup-rundown process of a solitary wave. In these approaches, however, interactions between successively breaking waves are obscure or absent, and it is our objective to investigate the interactions using a train of solitary waves. With the new long-stroke wavemaker in the DeFrees Hydraulics Laboratory at Cornell University, we can control the number of the solitary waves as well as separation between waves. Particle Image Velocimetry (PIV) technique with fluorescent seeding particles and an optical filter to exclude scattered laser light from broken surface and air bubbles is employed and the flow field in the surf zone is obtained. So far two-wave cases have been conducted and it is found that as long as the two waves are close enough, they merge into one lump of water body in the surf zone and only one reflected wave is observed in the offshore region. Detailed measurements on water surface profiles, instantaneous velocity fields, depth-averaged velocities and bed shear stresses under breaking waves and the resulting turbulent bores have been carried out. Hydraulic jumps during rundown are carefully observed and their relevance to the offshore vortices is being studied. Further experimental results with three or more waves in a train will be conducted soon.