Title: What a drag! A numerical model of time-dependent f low over topography
1What a drag!A numerical model of time-dependent
f low over topography
- Sally Warner
- Advised by Parker MacCready
- December 6, 2006
2What is drag?
- Drag is a force from a body acting parallel to
the direction of relative f luid motion.
Friction drag
Form (pressure) drag
large wake
smooth surface
medium wake
rough surface
small wake
3Why do oceanographers care about form drag?
- gtgt Form drag works to
- dissipate tidal energy
- generate eddies and internal waves
- produce turbulence and mixing
- gtgt Unlike frictional drag, form drag is not well
- resolved in coastal and larger scale models
4Form drag in the Puget Sound
- In their Puget Sound tidal model, Lavelle et al.
(1988) originally used a frictional drag
coefficient of CD 3x10-3. - But they found that some areas needed a much
larger coefficient of CD 20x10-3 to match
tidal observations.
5Form drag at Three Tree Point
- Three Tree Point is a 1 km headland in the
Main Basin of the Puget Sound - MacCready and others found the form drag to be 20
times larger than frictional drag
MacCready
6Motivating questions
gtgt Why is there such a large form drag in
places like Three Tree Point? gtgt How can I
parameterize the form drag into a new drag
coefficient (CD_FORM)?
Solution approach
gtgt Idealized numerical model
7The model basic setup
8The model Gaussian bump
First experiment 15 different bump sizes
9The model stratif ication
N 0.0125 s-1
10The model grid size
11The model tidal forcing
Forced with a propagating tidal wave Fluid
entering boundaries has original stratification
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13Vertically averaged velocity
14Surface height
A 3.6 phase lag between left and right makes
sense for a 20 km channel with a surface gravity
wave speed of c 44 m/s.
15How do I measure form drag?
- pB bottom pressure
- bottom slope
- AB bottom area
H
L
MacCready
2 tidal cycles 24 lunar hours
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17Fr gt1
18Power drag velocity
Bump height is 20 of total depth
Bump height is 10 of total depth
?drag 102
?drag 130
19 Average Power
Results in the same range as the 0.7 MW of power
dissipated at Three Tree Point.
20Questions to answer
- What are the factors that create the very large
form drag measured in places like Three Tree
Point? - How does form drag contribute to the generation
of internal tides and eddies? - How does the magnitude of the form drag compare
with the magnitude of the frictional drag? - What role does form drag play in the energy
budget of coastal regions?
21Future goal parameterization
- CD_FORM function( bump height,
- bump width,
- stratification,
- tidal velocity,
- tidal period,
- channel width,
- channel length,
- channel height )
22Thank you
- Parker MacCready
- David Darr
- Tom Connolly
- Natalia Stefanova
- Neil Banas
- Betty Bottler (ARCS Fellowship donor)