Mixing Layer Dynamics in Separated Flow Over an Estuarine Sill with Variable Stratification

Authors

Stefan A. Talke, Portland State UniversityFollow
Alexander R. Horner-Devine, University of Washington - Seattle CampusFollow
C. Chris Chickadel, University of Washington - Seattle CampusFollow

Sponsor

This research was funded by a MURI grant from the Office of Naval Research (ONR), award N00014‐05‐1‐0485.

Published In

Journal of Geophysical Research

Document Type

Article

Publication Date

9-2010

Subjects

Estuaries, Snohomish River Watershed (Wash.) -- Stratified flow -- Mathematical models, Estuarine oceanography

Abstract

We investigate the generation of a mixing layer in the separated flow behind an estuarine sill (height H ∼ 4 m) in the Snohomish River, Washington as part of a larger investigation of coherent structures using remote and in situ sensing. During increasing ebb flows the depth d and stratification decrease and a region of sheared flow characterized by elevated production of turbulent kinetic energy develops. Profiles of velocity and acoustic backscatter exhibit coherent fluctuations of order 0.1 Hz and are used to define the boundaries of the mixing layer. Variations in the mixing layer width and its embedded coherent structures are caused by changes to both the normalized sill height H/d and to a bulk Richardson number Ri_h defined using the depth of flow over the sill. Entrainment E_T and the mixing layer expansion angle increase as stratification and the bulk Richardson number decrease; this relationship is parameterized as E_T = 0.07Ri_h^−0.5 and is valid for approximately 0.1 < Ri_h < 2.8. Available comparisons with literature for inertially dominated conditions (Ri_h < 0.1) are consistent with our data and validate our approach, though lateral gradients may introduce an upwards bias of approximately 20%. As the ratio H/d increases over the ebb, the free surface boundary pushes the mixing layer trajectory downward, reduces its expansion angle, and produces asymmetry in the acoustic backscatter (coherent structures). Three-dimensional divergence, as imaged by infrared video and transecting data, becomes more prominent for H/d > 0.8 due to blocking of flow by the sill.

Description

This is the publisher's final PDF. Reproduced here with author and publisher permission.

Originally published in Journal of Geophysical Research.

Copyright 2010 by the American Geophysical Union.

DOI: 10.1029/2009JC005467

DOI

10.1029/2009JC005467

Persistent Identifier

http://archives.pdx.edu/ds/psu/11206

Citation Details

Talke, S. A., Horner-Devine, A. R., & Chickadel, C. C. (2010). Mixing layer dynamics in separated flow over an estuarine sill with variable stratification. Journal Of Geophysical Research, 115(C9)