Numerical Simulations of a Stratified Oceanic Bottom Boundary Layer. John R. Taylor - MIT Advisor: Sutanu Sarkar - UCSD

Transcription

1 Numerical Simulations of a Stratified Oceanic Bottom Boundary Layer John R. Taylor - MIT Advisor: Sutanu Sarkar - UCSD

2 Motivation Objective I: Assess and improve parameterizations of the bottom boundary layer in ocean models. Velocity magnitude, MITgcm (Chris Hill personal communication) Numerical ocean models are very important to accurate weather and climate prediction. Ocean models cannot resolve three!dimensional turbulence in the bottom boundary layer.

3 Oceanic Bottom Boundary Layer! 5! 1 Mean / Tidal currents U Outer layer, Stable Stratification ρ(z)! 15 Depth (m)! 2! 25 Logarithmic Layer? l z Bottom mixed layer O(1m)! 3! 35 Q = Viscous (Roughness) Sublayer! 4

4 Field Observations Objective II: Provide a database to help interpret field data Data from: Johnson et al. JPO 24, 1994 Meters above bottom Mixed layer height The seafloor stress is often estimated by fitting a logarithmic profile to the observed velocity profile. 1! Velocity (m/s)

5 Field Observations Data from: Johnson et al. JPO 24, 1994 Meters above bottom Mixed layer height Best fit 5-2m u*=8.56 cm/s τ w = ρ u 2 =7.5 N/m 2 Best fit -5m u*=4.83 cm/s τ w = ρ u 2 =2.4 N/m 2 1! Velocity (m/s) Shear is larger at the top of the mixed layer. Can lead to a dramatic overestimate in the wall stress.

6 Computational Domain Open Boundary: Radiation Condition + Sponge layer Free-stream Velocity U Temperature Gradient dθ Impose: U,, f dz Geostrophy: fu = dp dy dθ dz z Ri = N 2 /f 2 Periodic in x, y x y Seafloor: Non-sloping, hydrodynamically rough wall Ekman Spiral dθ dz = ( N 2 = αg dθ dz )

7 Turbulent Ekman Layer

8 Density, Velocity Mixed layer, Ekman height are limited by stratification.! Associated with a small increase in the drag coefficient z/" N/f = 32 N/f = 75 z/! N " /f = N " /f =75 Steady state balance: f < v > dz = τ w Drag Coefficient N /f u /U <!>/(" d<!>/dz # ) 2 4 <v>/u * <u>/u *

9 Density, Velocity Gradients Stratification causes an increase in the mean shear..3 Density gradient.3 Φ = ( ) κz d u u dz N/f = N/f = 32 N/f = 75 z/".15 z/".15.1 N/f = 32 N/f = d<#>/dz d<#>/dz $ !

10 Friction Velocity Estimates Profile Method, fit to: u = u κ log ( z z ) Modified-Profile Method 1 l = 1 κz + N u, d u dz = u l u = u κ log ( z z ) + z N(z )dz.12.1 u * /U! u * /U! Profile Method Modified-Profile Method Profile Method (upper 1/2 ML) Modified-Profile Method (upper 1/2 ML) N! /f

11 Outer-layer Internal Waves N/f = 32 dw/dz ω 2 = N 2 cos 2 (Θ)+f 2 sin 2 (Θ) z/δ Studies of turbulence-generated IWs Grid-generated Turbulence (Linden 1975, E&Hopfinger 1986, Dohan+Sutherland 23,25, etc.) Shear Layers (Sutherland & Linden 1988, Sutherland et al. 1994, Basak & Sarkar 26, etc.) Gravity Currents (Flynn & Sutherland 24, etc.) Rough Topography BL (Aguilar & Sutherland 26, etc.) Wakes (Bonneton et al. 1993, Gourlay et al. 21, Spedding 22, Diamessis et al. 25, etc.) Θ 42-55º 45-6º 41-64º 4-46º x/δ

12 Viscous Decay Model 1. Start with the equations for the turbulent kinetic energy and the perturbation potential energy. 2. Obtain an expression for the rate of change in wave energy owing to viscous dissipation (neglecting wavewave interactions) 3. Given an initial wave amplitude and propagation speed, what is the expected amplitude at a height z following viscous dissipation.

13 Viscous Decay Model Define wave energy, W=KE+PE W t + (W c g)=ν d2 W dz 2 2ν k 2 W Neglect viscous diffusion, assume c g (linearized) DW Dt = 2ν k 2 W D/Dt is time derivative following c g In a stationary frame in terms of vertical velocity amplitude: A(z) =A k k exp[ νω k h (ω 2 f 2 ) 1/2 z k 4 (N 2 ω 2 ) 1/2 dz ] Given the initial amplitude, A (k h, ω), we can predict A(k h, ω,z)

14 Viscous Decay Model.8 N/f=1 ( k ( w z ) 2 ) 1/2.8 N/f= z /"=.45.6 z /"= z/"= !/f 45 LES.2 z/"= !/f 45 Viscous Decay Model

15 Looking Ahead... Consider a time-dependent outer layer flow (tides or waves). How is the bottom boundary layer affected by lateral density gradients? Study the stability of a directional shear in a stratified fluid. Apply the internal wave model to other flows, e.g. stratified wakes, topographic generation.

16 Development of a CFD Code Developed in collaboration with Prof. Tom Bewley User selected combination of pseudo-spectral, finite differences. Large Eddy Simulation model (LES) using dynamic eddy-viscosity and/or scale-similar terms. Capable of considering an arbitrary number of passive and/or active scalars Parallelized using MPI Source code available from: numerical-renaissance.com/diablo.html

17 Acknowledgments Advisor: Sutanu Sarkar Thesis committee: Tom Bewley, Paul Linden, Rob Pinkel, Bill Young Collaborator: Vincenzo Armenio Family: Erin, George + Cindy, Annie + Tim Funding from NDSEG Fellowship ONR, Physical Oceanography

18 References Taylor J.R., S. Sarkar, and V. Armenio (25). Large eddy simulation of stably stratified open channel flow. Phys. Fluids 17, Taylor J.R., and S. Sarkar (27). Internal gravity waves generated by a turbulent bottom Ekman layer. Journal of Fluid Mechanics 59, 1, Taylor J.R., and S. Sarkar (27). Direct and large eddy simulations of a bottom Ekman layer under an external stratification. Int. J. Heat and Fluid Flow, 29, 3, Taylor J.R., and S. Sarkar (27). Stratification effects in a bottom Ekman layer. Journal of Physical Oceanography, in press. Taylor J.R., and S. Sarkar (27). Internal wave generation by a turbulent bottom boundary layer. Proceedings of the Fifth International Symposium on Environmental Hydraulics. Taylor J.R., and S. Sarkar (27). Near-wall modeling for LES of an oceanic bottom boundary layer. Proceedings of the Fifth International Symposium on Environmental Hydraulics. Taylor J.R., and S. Sarkar (27). Large eddy simulation of a stratified benthic boundary layer. Turbulence and Shear Flow Phenomena-5 Proceedings. Taylor J.R., S. Sarkar, and V. Armenio (25) Open channel flow stratified by a surface heat flux. Turbulence and Shear Flow Phenomena-4 Proceedings.