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RESEARCH> Turbulence ModelingTurbulence Modeling |
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Quantifying and understanding turbulent fluxes of salinity, temperature and momentum is a fundamental problem in physical oceanography. Regional ocean models such as POM (Princeton Ocean Model) and ROMS (Regional Ocean Modeling Systems) cannot resolve small-scale turbulent processes and rely on turbulence closure models to parameterize the unresolved processes. However, these parameterization schemes sometimes fail to properly represent important turbulent mixing processes. For example, Mellor-Yamada model is known to produce too little mixing in stratified fluids. We are using Large Eddy Simulation (LES) model to investigate stratified shear flows in shallow-water estuaries. Our goal is to develop improved mixing parameterization schemes. |
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| Figure 1. Comparison of turbulence fields between ebb (top panel) and flood tide (bottom panel) in a LES simulation of a partially mixed estuary. Distributions of (a, c) vertical velocity and (b, d) salinity in a vertical section aligned in the along-stream direction. Turbulence in the bottom boundary layer appears to be driven by the bottom friction during the flood tide whereas localized shear-induced turbulence dominates in the outer part of the boundary layer during the ebb tide. |
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| Observations have shown that stratification and turbulent mixing exhibit a flood-ebb tidal asymmetry in estuaries and continental shelf regions affected by horizontal density gradients. We use a Large Eddy Simulation (LES) model to investigate the penetration of a tidally driven bottom boundary layer into stratified water in the presence of a horizontal density gradient. Turbulence in the bottom boundary layer is driven by bottom stress during flood tides, with low gradient (Ri) and flux (Rf) Richardson numbers, but by localized shear during ebb tides, with Ri= ¼ and Rf=0.2 in the upper half of the boundary layer. If the water column is unstratified initially, the LES model reproduces periodic stratification associated with tidal straining. The model results show that the energetics criterion based on the competition between tidal straining and tidal stirring provides a good prediction for the onset of periodic stratification, but the tidally averaged horizontal Richardson number Rix has a threshold value of about 0.2, which is lower than 3 suggested in a recent study. Although the tidal straining leads to negative buoyancy flux on flood tides, we find that for typical values of horizontal density gradient and tidal currents in estuaries and shelf regions, buoyancy production is much smaller than shear production in generating turbulent kinetic energy. |
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 Figure 2. Along-channel distributions of (a, d) along-channel barotropic tidal current velocity, (b, e) salinity and (c, f) logarithm of vertical diffusivity (m2s-1) at time when the tide is in the flood phase in the lower and middle Bay (day 109.2) or when the tide is in the ebb phase in the lower Bay (day 109.4). |
 Figure 3. Along-channel distributions of detided residual current (ms-1) at (a) day 109.2 (a), (b) day 109.4 and (c) tidally averaged current. |
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