abldd lecture 1-2
TRANSCRIPT
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TODAY
A very brief introduction to measuring turbulent flows..........To back up some techniques used in
papers today....
see last weeks handout for fuller list
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Laboratory
1. Flow Visualisation - dye, particles
2. Hydrogen bubbles
3. Constant temperature anemometry
4. Laser Doppler anemometry
5. Acoustic Doppler velocity profiling
6. Particle Imaging velocimetry
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Field
1. Rotary current meters
2. Electromagnetic current meters
3.Acoustic Doppler instruments
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2. Hydrogen bubbles
•Principle: Uses electrolysis in water•pass a current through water to liberate hydrogen at cathode and oxygen at anode•Produces hydrogen that can be used as a ‘flow tracer’ in a small area before buoyancy effects become large
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flow
H2 sheet
Platinum wire (cathode)
Hydrogen bubbles - modes of operation
Sheet Pulsed Pulsed & speck insulated
timelines square bubbles!
…..can give quantitative visualisation
Horseshoe hairpin
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9mm sediment bed
‘inrush’
‘ejection’
TBL work of Tony Grass
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H2 bubble visualisation in front of bridge pier
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2. Hydrogen bubbles
Advantages:•excellent quantitative visualisation
•can image large parts of whole flow•wire can be used in complex
topographiesa note: H2 bubble technique yielded some of the great early progress in TBL studies: Kline
and Grass
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2. Hydrogen bubbles
Disadvantages:•difficult/impossible to use in high
velocity/Re # flows• bubbles have limited travel distance
before rising•need electrolyte in water
•analysis can be slow/complex
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3. Constant temperature anemometry (CTA)
Principle: Uses heat loss from a heated wire/film to measure velocity
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3. Constant temperature anemometry (CTA)
1,6
1,8
2
2,2
2,4
5 10 15 20 25 30 35 40
U m/s
E v
olt
s
Velocity U
Current I
Sensor (thin wire)
Sensor dimensions:length ~1 mmdiameter ~5 micrometer
Wire supports (St.St. needles)
•heat wire up•flow cools wire•monitor drop in voltage and reheat to a constant temperature•change in voltage therefore gives velocity (need calibration)
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3. Constant temperature anemometry (CTA)
1D
2D
3D
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3. Constant temperature anemometry (CTA)
Sampling of CTA, LDA & PIV
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3. Constant temperature anemometry
Advantages:•excellent spatial and temporal
resolution•can use multi-probes
•can be 1, 2 or 3D probes•relatively cheap!
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3. Constant temperature anemometry
Disadvantages:•intrusive
•single at-a-point•often need to control temperature of
flow•calibration can be very difficult
•probes are fragile (don’t like sediment grains)
•contamination of probe (dirt, bubbles)
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4. Laser Doppler anemometry (LDA)
Principle: Uses Doppler shift from scattered light to calculate velocity
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The Doppler EffectThe apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light.
How much the frequency changes depends on how fast the object is moving toward or away from the receiver.
Johaan Christian Doppler1803-1853
Sound wav
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Laser
Signalprocessing
Transmittingoptics
Receiving opticswith detector
Signalconditioner
Flow
HeNe
Ar-Ion
Nd:Yag
Diode
Gas
Liquid
Particle
PC
4. Laser Doppler anemometry (LDA)
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Measurement of wake flow around a ship model in a towing tank
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Measurement of U-component of flow over a dune
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4. Laser Doppler anemometry
Advantages:•non-intrusive
•superb spatial and temporal resolution
•no calibration (Doppler shift)•can be 1, 2 or 3D
•can be used in complex geometries
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4. Laser Doppler anemometry
Disadvantages:•need clear flows (non-opaque)•need good laser light intensity
•considerations of tracer particle (signal) drop-out (i.e. may not be a
continuous signal)•safety
•expensive to establish
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5. Acoustic Doppler velocity profiling (ADV, UDVP)
Principle: Uses Doppler shift from scattered sound to calculate velocity
Uses one or several transducers to emit a sound pulse. Detects
frequency of sound from scatterers in the flow and use change in frequency (Doppler shift) to
calculate velocity
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Ultrasonic Doppler Velocity Profiling (UDVP) transducer
Transducer: 4 MHz 5mm diameterProbe: 8mm diameterMeasuring range: 5-194 mmAccuracy: ± 4 mm s-1
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Principles of Ultrasonic Doppler Velocity Profiling (UDVP)
• Velocity: detection of Doppler shift
V = cfD/2foc = velocity of ultrasound; fD = Doppler frequency
shift;fo = ultrasound frequency
• Profile (128 points): detection of Doppler shift at gated time intervals
x = ct/2x = distance; t = time lapse between emission and
reception of ultrasound pulses
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3 4 0 3 4 5 3 5 0 3 5 5 3 6 0 3 6 5 3 7 0 3 7 5 3 8 0 3 8 5 3 9 0
9 0
8 0
7 0
6 0
5 0
4 0
3 0
2 0
1 0
t i m e , s e c o n d s
di
st
an
ce
d
ow
ns
tr
ea
m,
m
m
- 3 0- 2 0- 1 00 1 02 03 04 05 0
v e l o c i t y , c m / s e c
U-component of flow in lee of dune at 128 points
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flow
5 cm
P1
P2
P3
0 105cm
0
5
flow
25 20 15 10 5 0
0
20
40
60
80
100
distance (cm)
tim
e (
s)
0 10 20 30 40 50
U (cm/s)
25 20 15 10 5 0
0
20
40
60
80
100
distance (cm)
tim
e (
s)
-15 -10 -5 0 5 10 15 20
U (cm/s)
25 20 15 10 5 0
0
20
40
60
80
100
distance (cm)
tim
e (
s)
0 10 20 30 40 50
U (cm/s)
P1 P2 P3
P1
P2
P3
0 105cm
0
5
flow
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5. Acoustic Doppler velocitimeters
Uses three transducers focused onto one point to give 3D measurements
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5. Acoustic Doppler velocity profiling
Advantages:• non-intrusive & good S/T resolution
• robust
• quantification of sediment-laden flows
• multipoint flow-field mapping (with profiler)
• instantaneous profiles
• can track evolution of coherent flow structures
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5. Acoustic Doppler velocity profiling
Disadvantages:•beam spread gives changing sampling volume• different frequencies needed for different depths (lower frequency=greater sound penetration)•profiler is 1D•ADV is at-a-point
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6. Particle Imaging Velocimetry (PIV)
Principle: Uses change in position of tracer particles between two
video/photo images to calculate velocity:
velocity = distance/time
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PIV optical configuration
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principles of PIVt1
neutrally-buoyant particles &double-pulsed laser light sheet(particles track the flow)
t2
U = x/t
x
xx
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dx
dxdy
dy
CCD detectorarea
Interrogationregion
principles of PIV
Peak detection on correlation
plane
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some results of PIV..flow around a cube …Mark Lawless
v velocity.aviseeding.avi
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6. PIV
Advantages:• non-intrusive
• whole flow field mapping (WOW!)
• 1,2 and 3D (use 2 cameras and parallax)
• fair spatial resolution (~mm2)
• temporal resolution ok - 15 Hz (new systems up to 4000 Hz)
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6. PIV
Disadvantages:•need clear flows (non-opaque)•temporal resolution lower than CTA & LDA•considerations of lighting geometry•safety (v. powerful lasers)•expensive to establish
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Reading:
Clifford, N.J. & French, J.R. 1993Monitoring & Modelling Turbulent Flow: Historical & Contemporary Perspectives, In: Turbulence: Perspectives on Flow & Sediment Transport (Eds: Clifford, N.J., French, J.R. & Hardisty, J.), 1-34.
Apologies as its not in library – I have copies available
• Papers in rest of course• Search the web!!
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From www.cortana.com
Turbulent boundary layer structure
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Shear velocity, u*
u* = √τo/ρ
0 hR S
0 hR S = boundary shear stress0 hR S = fluid density
0 hR S = slope (gradient)0 hR S = hydraulic radius
0 hR S = hydraulic radius = cross-sectional area/wetted perimeter
Shear velocity, u*
u* = √o/ρ
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Turbulent boundary layer structure over a FLAT bed
Classic research by the groups of Kline (Stanford) and Grass (UCL)
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Tony Grass (UCL)JFM 1971
Used H2 bubbles over
different bed roughness
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Bursts and sweeps
Grass, 1971
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+U’-U’
+v’
-v’
If U’ and v’ are deviation of downstream and vertical velocity from their mean (+ve v = upwards)
12
3 4
2 = bursts4 = sweeps1 = outward interactions3 = inward interactions
Define a ‘hole’ size
to exclude small
events Quadrant
Analysis
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Tb=fU/Y~5
Jackson, 1975, 1976
Burst period
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Smith and Metzler, 1983
Planform characteristics…Smith and Metzler, 1983
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Smith and Metzler, 1983
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H2 bubblewire
low-speedstreaks
flow
time
Looking down onto the channel bed
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Low speed streak spacing, l+:
l+ = l.u*/n
where l = streak spacingu* = shear velocity
n = kinematic viscosity
l+ = l.u*/n 100
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Smith and Metzler, 1983
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Smith and Metzler, 1983
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Smith and Metzler, 1983
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Smith and Metzler, 1983
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The burst-sweep cycle (from Allen, 1984)
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The earlier work of
Kline and
colleagues
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Smith et al., 1991
Generation of secondary hairpin vortices (Smith et al., 1991)
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Turbulent Boundary Layer Structure (Robinson, 1991)
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Grass, 1971
The influence of roughness (Grass 1971)
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Grass, 1971
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Links to Large-Scale-Motions (Falco, 1977)
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Links to Sediment Entrainment (Grass, 1971)
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References
Grass, A.J. (1971) Structural features of turbulent flow over
smooth & rough boundaries, J. Fluid Mechanics, 50, 233-255.
Kline, S. J., Reynolds, W. C., Schraub, F. A. & Runstadler, P. W.
(1967) The structure of turbulent boundary layers. Journal of
Fluid Mechanics, 30, 741-773.
Robinson, S. K. (1991) Coherent motion in the turbulent
boundary layer. Ann. Rev. Fluid Mech. 3, 601-639.
Smith, C.R. and Metzler, S.P. (1983) The characteristics of low-
speed streaks in the near-wall region of a turbulent boundary
layer, Journal of Fluid Mechanics, 129, 27-54.
Smith, C.R. (1996), Coherent flow structures in smooth-wall
turbulent boundary layers: Facts, mechanisms and speculation.
in Coherent Flow Structures in open channels edited by P.J.
Ashworth, S.J. Bennett, J.L. Best, and S.J. McLelland, pp. 1-39,
John Wiley and Sons.
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Next weeks seminars
Frank: Adrian, R. J., C. D. Meinhart and C. D. Tomkins
(2000), Vortex organization in the outer region of the
turbulent boundary layer, Journal of Fluid Mechanics, 422, 1-
54.
Nathaniel: Head, M.R., and P. Bandyopadhyay (1981), New
aspects of turbulent boundary layer structure, Journal of
Fluid Mechanics, 107, 297-338.
Kevin: Acarlar, M. S. & Smith, C. R. (1987) A study of hairpin
vortices in a laminar boundary layer. Part 1. Hairpin vortices
generated by a hemisphere protuberance. Journal of Fluid
Mechanics 175, 1-41.
NOTE: These are large papers!: start with Intro,
Conclusions and Discussion: reviewers can then focus in on
papers