FLOW AND TURBULENCE OVER A RIPPLED SEABED AND BIOPHYSICAL INTERACTIONS IN THE INNER PART OF THE COASTAL BOTTOM BOUNDARY LAYER
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Date
2015-02-18
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Johns Hopkins University
Abstract
Characterization of the near-bed flow structure is critical in understanding the coastal bottom boundary layer (BBL) dynamics, which has important implications to sediment transport, nutrient mixing and shelf circulation. This thesis reports on field experiments undertaken to study the interaction of waves and currents with a rippled seabed using simultaneous particle image velocimetry, acoustic Doppler velocimeter and bottom roughness measurements. A large database spanning varying mean current, wave and seabed states has been recorded and analyzed.
Mean velocity profiles collapse with appropriate scaling in the log layer, but vary substantially in the roughness sublayer. When wave induced motions are similar or greater than the mean current, the hydrodynamic roughness determined from velocity profiles is substantially larger than directly measured values. The roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range, but decays in the log layer for larger roughness elements. Reynolds shear stress profiles peak in the lower parts of the log layer, diminishing below it, and gradually decaying at higher elevations. In contrast, wave shear stresses are negligible within the log layer, but become significant within the roughness sublayer. This phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave-induced motion. No single boundary layer length scale collapses the Reynolds stresses, but both the Prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation, especially near the seabed. Within the log region, profiles of shear production and dissipation rates of turbulence converge. Below it, dissipation rapidly increases, peaking near the seabed. Conversely, the shear production decays near the seabed, in agreement with the eddy viscosity model, but in contrast to both laboratory and computational rough wall studies.
The presence of a benthic aggregation of mysids in several datasets enables us to study their interaction with the near-bed flow. The mysid layer varies in thickness from 3-9 cm depending on the flow conditions. The majority of the individuals in the swarm are seen to exhibit positive rheotaxis, i.e. they face into the incoming flow. The mysid concentration with elevation is inversely proportional to the instantaneous flow velocity, i.e. they tend to exhibit depth seeking behavior and are aggregated near the seabed during high velocities. This response to the oscillatory flow is clearly observed by the matching frequencies of the peaks in the wave and mysid concentration spectra. No discernible changes that could be attributed to the presence of mysids are seen in the mean velocity structure, or the Reynolds stress, shear production and dissipation rate profiles in the overlying flow.
Advisor: Dr. Joseph Katz
Readers: Dr. Thomas Haine and Dr. Tamer Zaki
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Keywords
Coastal Bottom Boundary Layers, Particle Image Velocimetry, Turbulence