The enhanced wave-induced drift of large floating objects

Thesis

At the ocean surface, mass, momentum and energy are transferred between the atmosphere and the ocean. Ocean surface waves, in particular, drive the transport of floating objects such as sediments, pollutants, and man-made structures. In irrotational and progressive surface gravity waves, water particles exhibit a non-closed path over one wave cycle, leading to Stokes drift. The drift can be modified, and indeed enhanced, when an object deviates from being an ideal Lagrangian tracer. This thesis examines the wave-induced drift of two-dimensional (2D) floating objects by surface gravity waves, specifically, deep-water regular waves. A combination of analytical, numerical, and experimental approaches are employed to analyze the drift of objects with varying sizes and shapes under different wave steepness.

A hybrid numerical model encompassing both viscosity and diffraction effects is used to investigate the influence of changing size, shape and wave steepness on the object drift. The drift deviates from the standard Stokes drift and depends on objects’ size and shape. Larger objects with less streamlined shapes, resembling box-like structures, exhibit enhanced drift. The enhancement is attributed to the standing wave pattern generated as a result of diffraction of the wave field, and viscosity.

A diffraction-modified Stokes drift model is proposed to predict object drift. The model introduces additional terms to the standard Stokes drift, accommodating incident, diffracted, and radiated wave fields. The results are compared to both experimental measurements and numerical results, demonstrating a close agreement when the object is not too large. The model offers an in-depth understanding of one of the mechanisms contributing to drift enhancement.

A 2D experiment has been conducted in a laboratory flume. The floating objects are specially designed to ensure the experiment is two dimensional. The enhanced drift, along with the associated standing wave pattern, predicted by the model is also observed in experimental data, which validates the theoretically predicted enhanced drift. Distinct drift behaviours are identified for small and large objects at low and high wave steepness. The data reveals that the scaling relationship between the object drift and steepness is characterized by a mixture of linear and second-order terms (in steepness) and is dependent on object size.

The influence of object corner shape on drift is further explored using the hybrid numerical model. In particular, the drift of a bluff body with sharp and round corners has been investigated. It is found that an object’s sharp corners can induce vorticity and thus change the pressure distribution around the moving objects, resulting in a significantly altered drift trajectory and unsteadiness in the object’s drift.

Authors: Xiao, Q

• DOI: 10.5287/ora-xrype4yxe
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: wave diffraction; Qale-FOAM; surface gravity waves; RANS turbulent simulation; 2D experiment for drift; wave-induced drift; vorticity; mass transport; wave-body interaction
• Subjects: Wave-induced drift