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ISLAND WAKES GENERATED ISLAND WAKES GENERATED BYBY
AN ELLIPTICAL TIDAL FLOWAN ELLIPTICAL TIDAL FLOW
Philippe EstradePhilippe Estrade
Jason MiddletonJason Middleton
University of New South WalesUniversity of New South Wales
Outline
➢ introduction➢ model set-up➢ some 2D results➢ some 3D results➢ concluding remarks
Introduction
• Wake flows often govern near shore environmental processes (sediments, nutrients, pollutants, ...)
• Wakes generated by constant upstream flows are well documented however oceanic flows are unsteady and non-uniform (topography, tidal currents, forcing variability, ...)
• Transient wake generated by unidirectional tidal flow have been addressed for headland (Signell & Geyer, 1991) & inner shelf island (Rattray island) but not for mid & outer shelf island for which the tidal flow is not polarized
• Impact of an elliptical tidal current on island wake structure ?
• ROMS has been used in idealized configurations to address this question ...
Dynamical context :
“isolated outer shelf island in shallow water” (e.g. Great Barrier Reef) :homogeneous fluid flowing around a topographic obstacle both in 2D & 3D
3D :
2D :also barotropic mode in 3D
Dynamicalpreconditioning :
Topography :
(circular island with or without surrounding bathymetry)
+ additional eddy viscosity within sponge layers(600 m wide - up to 10 m²/s at open boundaries)
3D :
cyl gauss
Forcing and boundaryconditions : all boundaries are open with :
2D & 3D : FS_CHAPMAN & M2_FLATHER(η,u,v specified analytically)
3D : M3_RADIATION & T_RADIATION(u,v,w,T not specified)
η v u given by basic [2D – flat bottom – linear – inviscid] theory :
solution can be written as a linear combinationof inertia gravity waves (igw) :
where are input parametersfor each tidal component of interest
wave number(s) given by the dispersion relation :
η u
η v
η u
η v
Elliptical tidal forcing :
Study limited to 1 component : meridional (kx=0, k=ky) & semi-diurnal wave (period T=12h) ==> 2 cases :
Progressive wave : Standing wavesingle wave (k) incident (k) + reflected wave (-k)
similar elliptical forcing can be found with adequate choice of ysw & tsw
η0 (m) latitude (°N)
e1 0.5 15
e2 0.455 30
e3 0.265 60
flood/ebbin phase vs out of phase
high/low
a “control” run :2D / flat bottom (no island) / e1 grid : 200*300 ; res=50 m ; dt=1s
“error” < 5% except : when t/T ~ n+1/4 or n+3/4 ( η & v ~ 0 )
when t/T ~ n or n +1/2 ( u ~ 0 ) the numerical solution propagates almost like the linear igw (used to force the model at the 4 open boundaries) despite non-linearity (advection, bottom friction)
12 experiments :
Spin-up :(insignificant difference
between 2D/3D& cyl/gauss)
grid :2D cyl e1 400*450
/ / e2 450*5503D gauss e3 600*600
2D & 3D : horizontal resolution 50 m barotropic mode 1 s
3D : baroclinic mode 48 s 20 σ levels
initial conditions : from rest
some 2D results (vorticity & circulation) :
thin ellipse (e1 & e2) : transient 2 eddies structuresduring flood & ebb phases & dissipation in between(stronger activity with varying bottom topography)
large ellipse : vorticity filamentscontinuously progressing
(weak sensitivity to topography)
about the free surface elevation (2d/gauss/e1) :
<η> :
η-ηpw :
residual :
Larger negative “anomalies” in the flow separation regions leads to residual depression along minor axis despite residual flow convergence
residual circulationand vorticity (2D & 3D) :
the tidal current rotation favors the development of the eddy rotating in the
same direction and weakens the development of the second eddy
no qualitative difference in η, u, v between 2D & 3D modelling
(for this particular range of parameters !)
some 3D results (vertical velocity) :
z = -H/2
Upwellings are : stronger with a varying topography weaker with larger ellipse
Which mechanisms drive these vertical motions ?
Secondary circulation & vertical motions :
Alaee et al (2004) : flow curvature can generate significant vertical motions by convergence/divergence of the secondary circulation
(u’,v’)t/T=5.1
3D/gauss/e1
vertically integrating the continuity equation from the bottom (or from the surface) to depth z and then replacing (u,v) by (u’,v’) + (u,v) gives w = wp + ws :
Example vertical velocity decomposition :
t/T = 5.1z = -3/4.H
Vertical motions mainly stem from :
flow curvature (i.e convergence/divergence of the secondary circulation) for a cylindrical island
Combination of wp & ws
for a varying topography
Residual vertical velocities :
z = -3/4.H
3D/gauss/e1 & e2 :
wide & strongresidual upwelling
along the major axis
consistent feature ?
Concluding remarks :
➢ further sensitivity studies needed to identify wake regimes vs relevant dimensionless numbers (h0/[Cd.R], V0/[ω.R], ω/f, others ?)
➢ similar tidal forcing method can be applied for headland or innershelf island wake studies (Kelvin waves propagating along a closed boundary) but limited to idealized configurations as well (flat bottom near open boundaries)
➢ study motivated by transient wake observations around LEI (Lady Elliot Island – Great Barrier Reef)
➢ current studies : adding missing LEI ingredients (stratification, buoyancy flux, wetting & drying, wind, neap/spring M2 & S2, ...) and applying GST tools
➢ heading for realistic modelling of LEI ...