Hydrogen Bubbles: Uses Electrolysis in Water - Lecture Slides | GEOG 595, Lab Reports of Geography

Download Hydrogen Bubbles: Uses Electrolysis in Water - Lecture Slides | GEOG 595 and more Lab Reports Geography in PDF only on Docsity! 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 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 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 o lt s Velocity U Current I Sensor (thin wire) Sensor dimensions: length ~1 mm diameter ~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 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 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 1803-1853 Sound wav 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 Principles of Ultrasonic Doppler Velocity Profiling (UDVP) • Velocity: detection of Doppler shift V = cfD/2fo c = velocity of ultrasound; fD = Doppler frequency shift; fo = 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 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 d i s t a n c e d o w n s t r e a m , m m - 3 0- 2 0- 1 0 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) ti m 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) ti m 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) ti m 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 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 cylindrical dichroic lens polarization spherical Nd:YAG lasers splitter lens prism 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 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 Burst period © %e @_8@ . C8 © 0% é gs laboratory Reynolds number = U,dp/y Jackson, 1975, 1976 Traversing platform To storage tank Exit tank Variable-speed traverse motor Main pump Video camera ~_ (plan v Traversing platform | Linear ~, motion bearing Plexiglas flow., channel Steel supports SSeS SY Test plate Traversing strobe Inlet contraction fr Flow straightener / and screens Distribution manifold Inlet tank Traversing mechanism Variable-speed pump motor ~Video camera (side view) \ Hydrogen-bubble wire probe Smith and Metzler, 1983 a 8 " *y is t Smith and Meizler, 1983 _. manne OL 6 ia fe * 6 A ———— ee ee ee a ee ee ee 1000 2000 3000 4000 5000 6000 7000 Re 8 Smith and Meizler, 1983 Smith and Metzler, 1983 200 = 300 nt X= 146 | ¥, = 0-46 200 = 300 nt Smith and Meizler, 1983 The earlier work of Kline and colleagues Fievre 10. The mechanics of streak breakup. Smith et al., 1991 Generation of secondary hairpin vortices (Smith et al., 1991) Turbulent Boundary Layer Structure (Robinson, 1991) Links to Large-Scale-Motions (Falco, 1977) tema ar yRpical eddie ———} —+_ Ty superburst large scale motions Links to Sediment Entrainment (Grass, 1971) 3 a Acs] + 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.