ABLDD Lecture 1-2

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

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

Field

1. Rotary current meters

2. Electromagnetic current meters

3.Acoustic Doppler instruments

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

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

9mm sediment bed

‘inrush’

‘ejection’

TBL work of Tony Grass

H2 bubble visualisation in front of bridge pier

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

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

3. Constant temperature anemometry (CTA)

Principle: Uses heat loss from a heated wire/film to measure velocity

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)

3. Constant temperature anemometry (CTA)

1D

2D

3D

3. Constant temperature anemometry (CTA)

Sampling of CTA, LDA & PIV

3. Constant temperature anemometry

Advantages:•excellent spatial and temporal

resolution•can use multi-probes

•can be 1, 2 or 3D probes•relatively cheap!

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)

4. Laser Doppler anemometry (LDA)

Principle: Uses Doppler shift from scattered light to calculate velocity

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

Laser

Signalprocessing

Transmittingoptics

Receiving opticswith detector

Signalconditioner

Flow

HeNe

Ar-Ion

Nd:Yag

Diode

Gas

Liquid

Particle

PC

4. Laser Doppler anemometry (LDA)

Measurement of wake flow around a ship model in a towing tank

Measurement of U-component of flow over a dune

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

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

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

Ultrasonic Doppler Velocity Profiling (UDVP) transducer

Transducer: 4 MHz 5mm diameterProbe: 8mm diameterMeasuring range: 5-194 mmAccuracy: ± 4 mm s-1

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

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

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

5. Acoustic Doppler velocitimeters

Uses three transducers focused onto one point to give 3D measurements

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

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

6. Particle Imaging Velocimetry (PIV)

Principle: Uses change in position of tracer particles between two

video/photo images to calculate velocity:

velocity = distance/time

PIV optical configuration

principles of PIVt1

neutrally-buoyant particles &double-pulsed laser light sheet(particles track the flow)

t2

U = x/t

x

xx

dx

dxdy

dy

CCD detectorarea

Interrogationregion

principles of PIV

Peak detection on correlation

plane

some results of PIV..flow around a cube …Mark Lawless

v velocity.aviseeding.avi

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)

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

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!!

From www.cortana.com

Turbulent boundary layer structure

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

Turbulent boundary layer structure over a FLAT bed

Classic research by the groups of Kline (Stanford) and Grass (UCL)

Tony Grass (UCL)JFM 1971

Used H2 bubbles over

different bed roughness

Bursts and sweeps

Grass, 1971

+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

Tb=fU/Y~5

Jackson, 1975, 1976

Burst period

Smith and Metzler, 1983

Planform characteristics…Smith and Metzler, 1983

Smith and Metzler, 1983

H2 bubblewire

low-speedstreaks

flow

time

Looking down onto the channel bed

Low speed streak spacing, l+:

l+ = l.u*/n

where l = streak spacingu* = shear velocity

n = kinematic viscosity

l+ = l.u*/n 100

Smith and Metzler, 1983

Smith and Metzler, 1983

Smith and Metzler, 1983

Smith and Metzler, 1983

The burst-sweep cycle (from Allen, 1984)

The earlier work of

Kline and

colleagues

Smith et al., 1991

Generation of secondary hairpin vortices (Smith et al., 1991)

Turbulent Boundary Layer Structure (Robinson, 1991)

Grass, 1971

The influence of roughness (Grass 1971)

Grass, 1971

Links to Large-Scale-Motions (Falco, 1977)

Links to Sediment Entrainment (Grass, 1971)

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.

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