A brief history…
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Nature’s drag queens: how vegetation impacts aquatic flows † Marco Ghisalberti Centre for Water Research, University of Western Australia DIALOG VII SYMPOSIUM † formerly known as “Momentum and scalar transport in vegetated shear flows”
description
Nature’s drag queens: how vegetation impacts aquatic flows † Marco Ghisalberti Centre for Water Research, University of Western Australia DIALOG VII SYMPOSIUM † formerly known as “Momentum and scalar transport in vegetated shear flows”. A brief history…. Velocity profile. Slope 1:10000 - PowerPoint PPT Presentation
Transcript of A brief history…
Nature’s drag queens: how vegetation
impacts aquatic flows†
Marco Ghisalberti
Centre for Water Research,University of Western Australia
DIALOG VII SYMPOSIUM
†
formerly known as “Momentum and scalar transport in vegetated shear flows”
A brief history…
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Velocity (cm/s)
Hei
ght
(cm
)
Slope 1:10000
4% plant volume
Vegetated
Bare
(taken from Defina and Bixio,
Water Resour. Res., 2005)
• Current models fail to predict in-canopy:
1.Velocity profile2.Vertical mixing
Diffusivity (m2/s)
Velocity profile
Mixing
A brief history…
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 30
Velocity (cm/s)
Hei
ght
(cm
)
Slope 1:10000
4% plant volume
Vegetated
Bare
(taken from Defina and Bixio,
Water Resour. Res., 2005)
• Current models fail to predict in-canopy:
1.Velocity profile2.Vertical mixing
Diffusivity (m2/s)
Velocity profile
Mixing
“Model”
Questions that needed an answer
WHAT’S GOING ON IN THESE FLOWS?
• Why does the traditional treatment yield such poor results?
HOW CAN WE CHARACTERIZE FLUXES
OF NUTRIENTS/SEDIMENT/GASES?• Why the sharp mixing gradient?
HOW CAN WE USE THIS PHYSICALINSIGHT?• Can we develop a general, rather
than canopy specific, framework?
Understanding
Prediction(taken from piscoweb.org)
Experimental design
Model vegetation (7 m)
H = 47 cm
Velocity meters (acoustic Doppler)
Cylinder array
Flow straightener
Flow
• Canopy defined by its:
height: h
drag coefficient: CD
density: a
• Vertical transport dominated by coherent vortex structures
• Vortices generate strongly oscillatory flow and transport• Mixing is more rapid than above a flat bed
Salient hydrodynamic features:
1. The vortexVertical transport
High
Low
Canopy top
FlowFlow
• Vortices separate the canopy into two distinct zones.
• Upper zone: “Exchange zone” D ≈ 1/50 × vortex size × rotation ~ O(10 cm2/s)
• Lower zone: “Wake zone” D ≈ 1/400 × flow speed × stem diameter × % wakes. ~ O(0.1―1 cm2/s)
Salient hydrodynamic features:2. Hydrodynamic stratification
Exchange
Wake
Velocity profile
e/(CDa)-1
CDah
Closed symbols – Cylinders in water (♦ Ghisalberti and Nepf [2004], ● Vivoni [2000], ■ Dunn et al. [1996], ▲Tsujimoto et al. [1992])
Open symbols – Cylinders/strips in air (○ Seginer et al. [1976], Raupach et al. [1996], ◊ Brunet et al. [1994])
Extrapolation to other vegetated flows
e ≈ 0.2 / CDa
(i.e. less penetration into dense, drag-exerting canopies)
What do we understand ?
• Experiments have given us a much better idea of:- Residence times and vertical gradients of passive tracers in canopies.- Fluxes in/out of canopies- Brief but intense nature of mixing events.
• To what extent does the hydrodynamics control the chemistry & biology?
[NH4+] (M)
z (m
)
Flushing?
What don’t we understand ?
• How does plant waving impact nutrient uptake & particle capture ?
