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Thomas F. Gross

Abstract

The law of the wall for turbulent boundary layer flow provides an analytic solution for the velocity profile and predictions of the bed shear stress. Most formulations and usages of the law of the wall require knowledge of the physical configuration of the seabed, which is input to the theory in the form of the z 0 length scale, the putative height above the bed where the logarithmic velocity profile goes to zero. Models of current–wave interaction and sediment transport attempt to predict the value of z 0 by modeling a physical length scale active near the bed, that is, the wave boundary layer height and height of particle saltation. Recent work with flow through forest canopies has put forth a different approach where the effect of drag on individual trees is to reduce the mean velocity momentum and increase the rate of dissipation of turbulent kinetic energy. Simulations with momentum-energy closure models show that a z 0 is developed that depends on the amplitude of the drag coefficient and the areal extent of the drag elements. Applications of this technique to oceanographic models of sediment transport and wave–current interaction are possible by accounting for a momentum sink and enhanced energy dissipation due to these processes. Oceanic measurements of velocity profiles will not be able to demonstrate the validity of this approach. Direct measurements of energy dissipation and turbulence structure functions will be required to validate the approach.

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Thomas F. Gross
and
Arthur R. M. Nowell

Abstract

The simple scaling of a tidal bottom boundary layer by the shear velocity, u *, and the wall to the wall describes well the mean Bow field. To test the full extent of this scaling measurements were made of the turbulence spectra in a natural tidal flow and a steady canal flow. The scaling of the turbulence spectra by the distance to the wall works only when the ratio of spectral rate to the mean shear is large. Under these conditions estimates of shear velocity u *, based on the dissipation derived from the magnitude of the inertial range of the spectra were found to agree to within 10 percent with estimates of the shear velocity from the mean velocity profiles and Reynolds stress. Within the 10 percent error bounds no effects ascribable to time dependence in the tidal spectra discerned, and hence the simple scaling of the momentum field by u * and z may be used for higher-order moments.

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Thomas F. Gross
and
Francisco E. Werner

Abstract

Tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth. Using the depth-averaged vorticity equations, the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined. The authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity. Specific cases are considered on the northern California shelf, Georges Bank, and the U.S. South Atlantic Bight.

The generation of residual vorticity is a strong function of the length scales at which roughness or depth vary. Length scales of bottom roughness variation are commonly within the range of greatest effect (e.g., sand patchiness, cobbly outcrops, etc.). The site-specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability. The implication for numerical modeling studies is that resolution of roughness variability is as important as resolution of topography at length scales comparable to the tidal excursion. Therefore higher-resolution models that seek to resolve flow patterns due to tidal scale topographic variability will also require similarly resolved bottom roughness variability.

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Albert J. Williams 3rd
,
John S. Tochko
,
Richard L. Koehler
,
William D. Grant
,
Thomas F. Gross
, and
Christopher V. R. Dunn

Abstract

A vertical array of acoustic current meters measures the vector flow field in the lowest 5 m of the oceanic boundary layer. By resolving the velocity to 0.03 cm s−1 over 15 cm paths, it samples the dominant turbulent eddies responsible for Reynolds stress to within 50 cm of the bottom. Profiles through the inner boundary layer, from six sensor pods, of velocity, turbulent kinetic energy, and Reynolds stress can be recorded for up 10 four months with a 2 Hz sample rate and 20 min averaging interval. We can study flow structure and spectra from as many as four event-triggered recordings of unaveraged samples, each lasting one hour, during periods of intense sediment transport. Acoustic transducer multiplexing permits 24 axes to be interfaced to a single receiving circuit. Electrical reversal of transducers in each axis eliminates zero drift. A deep-sea tripod supports the sensor array rigidly with minimum flow disturbance, yet releases on command for free vehicle recovery.

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