Sound, Sand and Turbulence: Stress in the Boundary Layer Above Evolving Sand Ripples

Alex Hay, Ph.D.

Department of Oceanography
Dalhousie University
Halifax, Nova Scotia

Friday, Apr. 30, 2010, 3:00pm
Chase 130

This talk will focused on observations of the vertical structure of the flow and Reynolds stress within the turbulent oscillatory boundary layer above evolving sand ripples. The velocity measurements were made with a newly-developed multi-frequency acoustic Doppler profiler, which produces turbulence-resolving velocity profiles through the boundary layer with better than 1 mm range resolution at an ensemble-averaged rate of O(100 Hz). Bed elevation profiles with mm accuracy were obtained with a digital camera and laser light sheet system. Results are presented from laboratory experiments using an oscillatory flow apparatus for both fixed-roughness and mobile beds, and compared to canonical theories for oscillatory boundary layers and semi-empirical relations for the turbulence-related parameters such as the bottom drag coefficient. Questions addressed in the talk will be: (1) how close to the bed can velocities be measured with this system?; and  (2), how realistic are the estimates the ensemble-mean structure of the boundary layer, including stress? The talk concluded with a brief discussion of plans for the near future.


Alex Hay’s research interests are in oceanographic processes on the continental shelf from the nearshore to the base of the continental margin, with a particular interest in sediment dynamics. He uses and develops high-frequency acoustic systems for investigating momentum and material fluxes in the bottom boundary layer and for imaging the temporal evolution of the sediment-water interface. After his doctoral studies (of turbidity currents) at the University of British Columbia, he joined the Department of Physics at Memorial University of Newfoundland, and in 1996 took up a chair in ocean acoustic technology in the Department of Oceanography at Dalhousie University. His current research is focused mainly on sediment dynamics in wave-dominated environments.