fluid flow in rivers
DESCRIPTION
Fluid Flow in Rivers. Outline Flow uniformity and steadiness Newtonian fluids Laminar and turbulent flow Mixing-length concept Turbulent boundary layer Mean boundary shear stress Velocity distribution and the “Law of the Wall” Depth-averaged velocity. Flow in Rivers. - PowerPoint PPT PresentationTRANSCRIPT
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Fluid Flow in RiversOutline1. Flow uniformity and steadiness2. Newtonian fluids3. Laminar and turbulent flow4. Mixing-length concept5. Turbulent boundary layer6. Mean boundary shear stress7. Velocity distribution and the “Law of the Wall”8. Depth-averaged velocity
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Flow in Rivers
• Rivers are nonuniform, unsteady, Newtonian, hydraulically rough turbulent flows
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(Bridge, 2003)
Uniform Flow: 0xAU
Nonuniform Flow: 0xAU
Open Channel Flow
UAUdwQ
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Steady Flow: 0
tAU
Unsteady Flow: 0tAU
Open Channel Flow
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(Bridge, 2003)
Fluid Viscosity
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(Bridge, 2003)
Types of fluids
Molecular viscosity is independent of the magnitude of shear (rivers)
Molecular viscosity is independent of the magnitude of shear once yield strength is exceeded (mud and lava flows)
Molecular viscosity is dependent on shear and it has a yield strength (paint)
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(Hornberger et al., 1998)
Laminar flow, Re < 2000 (viscous forces dominate)
Turbulent flow, Re > 4000 (turbulent forces dominate)
Ud
forces viscousforcesturbulent Re
Large masses of fluid “eddies” are being transported
Types Flows
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Laminar and Turbulent Flow
• For 1 m deep flow:Re = 300, U ~ 0.0003 m/sRe = 3000, U ~ 0.003 m/s
• Susquehanna River near Waverly, PAd ~ 2 m, U ~ 1 m/s, Re ~ 2,000,000
Ud
forces viscousforcesturbulent Re
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(Bridge, 2003)
Turbulent velocity time series
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Turbulent Velocities
01
01
1d1
1
2
1
10
n
iirms
n
ii
i
n
ii
T
uun
uu
uun
u
uuu
un
tuT
u
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(Bridge, 2003)
22
yulvu
Reynolds Stress
Mixing length concept
l: the vertical distance over which the momentum of a fluid parcel is changed (mixing length)
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(Bridge, 2003)
Turbulent boundary layer
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(Bridge, 2003)
Viscous sublayer streaks
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(Allen, 1985; Bridge, 2003)
Turbulent “bursting” process
~70% of all turbulence in open channel flows is due to the bursting process
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(Julien, 1998)DU
g*Re
Grain roughness effects on turbulent boundary layer
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(Falco, 1977)
Large-scale Turbulent Motions
(Nakagawa & Nezu, 1981)
(Ferguson et al., 1996)
(Belanger et al. 2000)
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(Bridge, 2003)
gdS 0
Boundary Shear Stress
dy 10
Conservation of downstream momentum:Impelling force (downstream component of weight of water) = resistive force
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sin''
xpwu
yy vis
“Law of the Wall”
0*
* ln1 ;yy
uu
yu
yu
0* u
• Derived from Reynolds-averaged Navier-Stokes equations (Gomez, 2006)– Nearly universal use
• Important assumptions:– Prandtl’s mixing length
theory– Zero slope (?!)– Constant shear stress (no
vertical gradient; ?!)
0neg.(Taylor, 1921; Prandtl, 1925, 1926)
yuLwu
dd''
0
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ln y (m)
u (m/s)
slope
interceptln
slope:data regression From
ln1
wall" theof Law" theofn Applicatio
0
*
0*
y
u
yy
uu
regression
ln y (m)
u (m/s)
slope
interceptln
slope:data regression From
ln1
wall" theof Law" theofn Applicatio
0
*
0*
y
u
yy
uu
regression
Utility of the “Law of the Wall”
0
0
2*0
2.30height roughnessvelocity -zero
yky
u
s
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Distance (m)
4 5 6 7 8
WS
E (m
)
-0.03
-0.02
-0.01
0.00
S=0.0026u*=0.051 m/s
Depth-slope productFor a mobile upper stage plane bed (mm-scale bedwaves), the boundary is essentially flat. Thus, the mean shear stress determined using a Reynolds stress projection, the law of the wall, and the depth-slope product (relative water surface elevation; WSE) are nearly identical (from Bennett et al., 1998).
u'v'
0.0 0.5 1.0 1.5 2.0 2.5
y/d
0.0
0.2
0.4
0.6
0.8
1.0
u*=0.045 m/s
Reynolds stress
U (m/s)
0.5 0.6 0.7 0.8 0.9 1.0 1.1
ln y
-6.0
-5.0
-4.0
-3.0
-2.0u*=0.045 m/s=0.33ks=2.0 mm
Law of the wall
RS LW DS
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(P
a)
0.0
0.5
1.0
1.5
2.0
2.5
Distance from Reattachment (m)
0.0 0.1 0.2 0.3 0.4 0.5
Hei
ght (
m)
0.00
0.02
0.04
0.06
Dune Profile
Law of the wallNear-bed Reynolds stress
Crest
Trough
Reynolds Stress vs. Velocity Gradient Shear Stress over a Fixed Dune
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Depth Averaged Velocity
i=4
tsmeasuremen ofnumber theis ;;0, :where
1
2
:form Specific
d1 :form General
: velocityflow integrated-Depth
1100
1
1
1
1
0
nuudyuy
yud
U
uuuyyy
yud
U
nnn
n
iii
iii
iii
d
y
y3,u3
y4,u4
344 yyy
2344 uuu
u (m/s)
y (m)
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Fluid Flow and Stream Restoration
• Conforms to physics• Based on steady, uniform flow conditions, there
are three (3) methods to determine near-bed shear stress
• Distributions of velocity should be predictable
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Fluid Flow in Rivers
Conclusions1. River flow is unsteady, non-uniform, turbulent,
and hydraulically rough2. River flow can be treated as a boundary layer,
and its distribution of velocity can be determined
3. Turbulence is derived from bursting process4. Bed shear stress can be determined from bulk
hydraulic parameters and velocity profiles