Numerical Study of Island Wakein Deep Water

Changming (Charles) Dong

James McWilliams

Alexander Shchepetkin

IGPP/UCLA, Los Angels,USA

Acknowledgements: J. Molemaker, C. Zhang, M. Blass

• Introduction

• Model Configuration

• Basic Experiment

• Sensitivity Tests

• Summary

Introduction

2. Wake Classic Fluid Dynamics

3. Wake in geophysical fluid dynamics

1. Observational and numerical evidence

a) no separation, laminar boundary layer

b) vortex pair with central return flow

c) wake formation with wave disturbances

along the current/wake interface

d) von Karman vortex street

(From M. Tomczak, 2000)

1. Shallow water

Wolanski (1984) Signell and Geyer (1991) Davies (1995)

2. Deep water

Heywood et al (1996) Coutis and Middleton (2002)

Two Categories (Tomczak,1988)

Basic Experiment Rectangular Domain: 180km x 80km , Water Depth : 500m

Island Diameter D= 10km

Spatial Resolution : DX = 500 m (160 x 360 x20)

Boundary Condition

a). Upstream BC (incident flow)

b). Downstream BC

c). BC neighboring the island

Downstream BC:

1. Modified Orlanski radiation (Marchesiello et al , 2001)

2. Specified BC with sponge layer

Island BC: Non-slippery with mask

Strouhal Number: St=nD/U=0.207

Time Series of Lateral Boundary Layer

Sensitivity Tests

1. Reynolds number

2. Rotation

3. Island Scale

4. Vertical Shear

5. Stratification

Background Horizontal Viscosity μ

Grid Reynolds Number

Re=dx*U/μ

• Implicit diffusion associated with upstream-biased advection scheme

• If Re> 10, scheme diffusion dominates

• If Re<=10, physical diffusion dominates

Re=200

Re=100

Re=25

Re=10

Unknown Re

Sensitivity Tests

1. Reynolds Number

2. Rotation

3. Island Scale

4. Vertical Shear

5. Stratification

6. Grid size

Sensitivity Tests

1. Reynolds

2. Rotation

3. Island Scale

4. Vertical Shear

5. Stratification

St=0.18 St = 0.20 St=0.23

Sensitivity Tests

1. Reynolds

2. Rotation

3. Island Scale

4. Vertical Shear

5. Stratification

Basic Case Weaker Shear

Weaker ShearBasic Case

Sensitivity Tests

1. Reynolds

2. Rotation

3. Island Scale

4. Vertical Shear

5. Stratification

Weaker StratificationBasic Case

Weaker StratificationBasic Case

Summary

1. ROMS is applied to study the ideal island wake in the dynamically deep water with rotation and stratification.

2. Background eddy viscosity should be chosen appropriately higher spatial resolution show finer structure of eddy activities in the wake. 3. Rotation, island scale, vertical shear and stratification affect the wake structure.

The work is still in progress!