TODAY A very brief introduction to measuring turbulent flows... To back up some techniques used in papers today...

Transcription

1 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

2 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

3 Field 1. Rotary current meters 2. Electromagnetic current meters 3.Acoustic Doppler instruments

4 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

5 Hydrogen bubbles - modes of operation Sheet Pulsed Pulsed & speck insulated flow H 2 sheet timelines square bubbles! Platinum wire (cathode)..can give quantitative visualisation Horseshoe hairpin

6 TBL work of Tony Grass ejection 9mm sediment bed inrush

7 H 2 bubble visualisation in front of bridge pier

8 2. Hydrogen bubbles Advantages: excellent quantitative visualisation can image large parts of whole flow wire can be used in complex topographies a note: H 2 bubble technique yielded some of the great early progress in TBL studies: Kline and Grass

9 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

10 3. Constant temperature anemometry (CTA) Principle: Uses heat loss from a heated wire/film to measure velocity

11 E volts 3. Constant temperature anemometry (CTA) Current I Sensor dimensions: length ~1 mm diameter ~5 micrometer Velocity U Sensor (thin wire) Wire supports (St.St. needles) 2,4 heat wire up flow cools wire monitor drop in voltage and reheat to a constant temperature change in voltage therefore gives velocity (need calibration) 2,2 2 1,8 1, U m/s

12 3. Constant temperature anemometry (CTA) 1D 3D 2D

13 3. Constant temperature anemometry (CTA) Sampling of CTA, LDA & PIV

14 3. Constant temperature anemometry Advantages: excellent spatial and temporal resolution can use multi-probes can be 1, 2 or 3D probes relatively cheap!

15 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)

16 4. Laser Doppler anemometry (LDA) Principle: Uses Doppler shift from scattered light to calculate velocity

17 The Doppler Effect The 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 Doppler Sound wav

18 4. Laser Doppler anemometry (LDA) Flow Laser Transmitting optics Receiving optics with detector HeNe Ar-Ion Nd:Yag Diode Gas Liquid Particle PC Signal processing Signal conditioner

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

20 Measurement of U-component of flow over a dune

21 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

22 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

23 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

24 Ultrasonic Doppler Velocity Profiling (UDVP) transducer Transducer: 4 MHz 5mm diameter Probe: 8mm diameter Measuring range: mm Accuracy: ± 4 mm s -1

25 Principles of Ultrasonic Doppler Velocity Profiling (UDVP) Velocity: detection of Doppler shift V = cf D /2f o c = velocity of ultrasound; f D = Doppler frequency shift; f o = ultrasound frequency Profile (128 points): detection of Doppler shift at gated time intervals x = ct/2 x = distance; t = time lapse between emission and reception of ultrasound pulses

26 d i s t a n c e d o w n s t r e a m, U-component of flow in lee of dune at 128 points t i m e, s e c o n d s v e l o c i t y, c m / s e c

27 time (s) time (s) time (s) flow 5 0 cm flow P3 P2 P1 0 P1 P2 P cm cm flow P3 P2 P distance (cm) distance (cm) distance (cm) U (cm/s) U (cm/s) U (cm/s)

28 5. Acoustic Doppler velocitimeters Uses three transducers focused onto one point to give 3D measurements

29 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

30 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

31 6. Particle Imaging Velocimetry (PIV) Principle: Uses change in position of tracer particles between two video/photo images to calculate velocity: velocity = distance/time

32 PIV optical configuration

33 principles of PIV t1 t2 neutrally-buoyant particles & double-pulsed laser light sheet (particles track the flow) x x x U = x/ t

34 principles of PIV CCD detector area Interrogation region d y d x Peak detection on correlation plane d y d x

35 some results of PIV..flow around a cube Mark Lawless seeding.avi v velocity.avi

36 6. PIV Advantages: non-intrusive whole flow field mapping (WOW!) 1,2 and 3D (use 2 cameras and parallax) fair spatial resolution (~mm 2 ) temporal resolution ok - 15 Hz (new systems up to 4000 Hz)

37 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

38 Reading: Clifford, N.J. & French, J.R Monitoring & Modelling Turbulent Flow: Historical & Contemporary Perspectives, In: Turbulence: Perspectives on Flow & Sediment Transport (Eds: Clifford, N.J., French, J.R. & Hardisty, J.), Apologies as its not in library I have copies available Papers in rest of course Search the web!!

39 Turbulent boundary layer structure From

40 RS 0 h RS h h RS 0 h RS = boundary shear stress 0 h = hydraulic radius RS 0 h = fluid density = slope (gradient) Shear velocity, u * u * = τ o /ρ RS= hydraulic radius = cross-sectional area/wetted perimeter Shear velocity, u * u * = o /ρ

41 Turbulent boundary layer structure over a FLAT bed Classic research by the groups of Kline (Stanford) and Grass (UCL)

42 Tony Grass (UCL) JFM 1971 Used H 2 bubbles over different bed roughness

43 Bursts and sweeps Grass, 1971

44 If U and v are deviation of downstream and vertical velocity from their mean (+ve v = upwards) 2 = bursts 4 = sweeps 1 = outward interactions 3 = inward interactions 2 +v 1 -U +U Define a hole size to exclude small events 3 4 -v Quadrant Analysis

45 Burst period T b =fu/y~5 Jackson, 1975, 1976

46 Planform characteristics Smith and Metzler, 1983 Smith and Metzler, 1983

47 Smith and Metzler, 1983

48 H 2 bubble wire time low-speed streaks flow Looking down onto the channel bed

49 Low speed streak spacing, l + : l + = l.u * /n where l = streak spacing u * = shear velocity n = kinematic viscosity l + = l.u * /n 100

50 Smith and Metzler, 1983

51 Smith and Metzler, 1983

52 Smith and Metzler, 1983

53 Smith and Metzler, 1983

54 The burst-sweep cycle (from Allen, 1984)

55 The earlier work of Kline and colleagues

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

57 Turbulent Boundary Layer Structure (Robinson, 1991)

58 The influence of roughness (Grass 1971) Grass, 1971

59 Grass, 1971

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

61 Links to Sediment Entrainment (Grass, 1971)

62 References Grass, A.J. (1971) Structural features of turbulent flow over smooth & rough boundaries, J. Fluid Mechanics, 50, Kline, S. J., Reynolds, W. C., Schraub, F. A. & Runstadler, P. W. (1967) The structure of turbulent boundary layers. Journal of Fluid Mechanics, 30, Robinson, S. K. (1991) Coherent motion in the turbulent boundary layer. Ann. Rev. Fluid Mech. 3, Smith, C.R. and Metzler, S.P. (1983) The characteristics of lowspeed streaks in the near-wall region of a turbulent boundary layer, Journal of Fluid Mechanics, 129, 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.

63 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, Nathaniel: Head, M.R., and P. Bandyopadhyay (1981), New aspects of turbulent boundary layer structure, Journal of Fluid Mechanics, 107, 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, NOTE: These are large papers!: start with Intro, Conclusions and Discussion: reviewers can then focus in on papers