Alluvial Boundary Layer Processes and Deposits. Bruce Rhoads and Jim Best Tuesdays :30 p.m. 329 Davenport Hall

Transcription

1 Alluvial Boundary Layer Processes and Deposits Bruce Rhoads and Jim Best Tuesdays :30 p.m. 329 Davenport Hall

2 Syllabus January 18 th : Introductions and backgrounds (from all of us!). Structure of course; aims, methods of assessment. Research Project topics. Overview of linkages among flow, sediment transport and channel morphology in rivers; scales, processes and feedbacks. Techniques for measurement. JB January 25 th : Techniques for measurement (contd) and Turbulent Boundary Layer (TBL) structure. JB February 1 st : Smooth and rough boundaries; methods of shear stress determination. Research topic discussions and choices. Writing a scientific paper. BR/JB SEMINARS February 8 th : Flow Separation; types of secondary flows. JB/BR SEMINARS February 15 th : Analysis of turbulent time series data: turbulence statistics, event recognition, spectral analysis and wavelets. BR SEMINARS February 22 nd : TBL structure and sediment entrainment; defect initiation and the initiation of bedforms in unidirectional flows. JB SEMINARS March 1 st : Types of bedforms and bar forms in rivers: generative mechanisms, self-organization, dynamics and stability. JB SEMINARS March 8 th : Turbulence modulation and sediment-fluid feedbacks. The influence of sediment on flow turbulence enhancement, attenuation; transitional and laminar flows. How does changing sediment concentration influence bedform dynamics? JB SEMINARS March 15 th : Flow under a river bed: what s going on in the hyporheic zone and how does it influence free-stream flow? JB SEMINARS Research Project Meetings SPRING BREAK March 19 th 27 th March 29 th : Research Project Meetings April 5 th : Flow and turbulence through vegetated boundaries BR SEMINARS April 12 th : Seminars Flow and turbulence in meander bends BR SEMINARS April 19 th : Shallow shear flows and mixing layers BR SEMINARS April 26 th : Flow and turbulence in confluences BR/JB SEMINARS May 3 rd : Final Research Project Meetings Seminars will commence from Week 3 Other individual projects meetings will be scheduled as necessary

3 Class format and schedule EACH week, the class will consist of: A c. 50 (ish) minute lecture from Bruce or Jim, which will introduce a topic and set reading for the next week. Two or three papers will be given as reading each week and ALL will be expected to read and critique these. One person will be assigned to present a short summary seminar on each paper. This should consist of a 10-minute (maximum) PowerPoint presentation (8 slides maximum) and a two-side (pdf) printed summary. These must be ed/given to Jim on the Monday before the Tuesday class, for inclusion on the course website and you should also print a copy of your pdf document for us to xerox for distribution. This will then allow approximately 10 minutes for discussion/questions on each topic/paper you will all be expected to contribute. You will all also have the opportunity to chair these sessions..keeping the speakers on time and helping guide discussion! EACH session will thus last approximately 2 hours.

4 Assessment Seminars: 10% each: each person will present two seminars (TOTAL 20%) Class contribution: 20% - based on attendance and input to group discussions Term paper: 60% - in the style of a scientific paper for JGR-ES. Maximum of 12 sides JGR-ES sides. Date due: Thursday May 5 th 12.00

5 Research Topics Projects can be lab-based, use field data, perform a detailed literature review, theoretical The flow dynamics of interacting particles and bedforms (x2) Mean flow and turbulence over a dune field in the Missouri River (x1) Interacting shear layers (x1) Flow in the hyporheic zone in a gravel bed (1-2) Flow over the ripple:dune transition turbulence and vorticity (x1) Flow and bed morphology in a meander bend (x1) Flow at a large river junction (x1) Flow separation at confluences (x2) The flow dynamics of bendway weirs (x2) Flow structure and turbulence within meander bends (x2) Your possible ideas for projects?...we are very keen to encourage projects with your own data or develop your own ideas for a topic, which can be worked into this format...

6 Sediment Transport Bed Morphology WHY?? the holy trinity Fluid Flow (Turbulence) turbulent flow structures turbulence modulation Fluid Turbulence large-scale structures in flow? flow generated over bed morphology local sediment transport rate flow separation

7 .at a range of scales.. from grains to channels

8 Applications landscape evolution environmental management pollution engineering modeling interpreting ancient environments ALLUVIUAL, YES, BUT we all live in a TBL that interacts with form...so lets think wider

9 TODAY A very brief introduction to measuring turbulent flows...

10 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

11 Field 1. Rotary current meters 2. Electromagnetic current meters 3.Acoustic Doppler instruments 4. MBES

12 1. Flow Visualisation - dye, particles Advantages: can tell your eyes a lot excellent for whole flow field/complex flows guides later measurement can be used to construct streamlines etc

13 1. Flow Visualisation - dye, particles Disadvantages: often qualitative often intrusive need to ensure dye/particles track flow (i.e must not inject at different velocity) diffusion of dye/3d movement of particles

14 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

15 Hydrogen bubbles - modes of operation Sheet Pulsed Pulsed & speck insulated flow H 2 sheet timelines square bubbles! Platinum wire (cathode)..can give quantitative visualisation timeline avi timeline2 avi

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

17 H 2 bubble visualisation in front of bridge pier

18 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

19 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

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

21 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) E volts 2,2 2 1,8 1, U m /s

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

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

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

25 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)

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

27 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

28 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

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

30 Measurement of U-component of flow over a dune

31 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

32 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

33 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

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

35 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

36 U-component of flow in lee of dune at 128 points distance downstream, mm time, seconds velocity, cm/sec

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

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

39 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

40 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

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

42 PIV optical configuration

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

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

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

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

47 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

48 Field 1. Flow Visualisation - dye, floats 2. Rotary current meters 3. Electromagnetic current meters 4. Acoustic Doppler velocimeters 5. Acoustic Doppler current profilers 6. MBES and field PIV

49 2. Rotary current meters rotation of blade gives velocity Ads: rugged, mean velocity Disads: no turbulence data, slow response, intrusive, at-apoint

50 3. Electromagnetic Current Meters (ECM) Principle: Faraday s principle of electromagnetic induction: when an electrically conductive fluid moves through a magnetic field, an electromotive force is induced both normal to the magnetic field and the fluid. Different ECM heads

51 Sensing volume ~2.5D ECM probe with diameter D & electrodes Flow induces electromotive force normal to electrodes Measure the change; calibrate with U & V velocity

52 3. ECM Advantages: robust 1 & 2D good temporal resolution (~30 Hz)

53 3. ECM Disadvantages: intrusive (sensor alters flow field?) large sensing volume (~2.5D) electrical interference sensor contamination (i.e. organics etc)

54 5. Acoustic Doppler Current profiling (ADCP) Principle: Uses Doppler shift from scattered sound to calculate velocity uses three/four transducers to obtain 3D measurements

55 ADCP three transducers 25º deviation bin height = 0.25m sampling time = 5s diameter of near-bed sampling area ~ 12m Cell 1 Cell 2 Cell 3 Cell 4 Cell N

56 Echo Sounder 1.5 MHz ADCP Use DGPS to get boat velocity and subtract from Doppler velocity use sensor to correct for pitch/roll of boat

57 ADCP field quantification: June 1999 downstream lateral velocity, cm s -1 vertical

58 5. Acoustic Doppler Current Profiler (ADCP) Advantages: non-intrusive multipoint 3D works in sediment-laden flows works in deep flows (and now also shallow flows)

59 5. Acoustic Doppler Current Profiler (ADCP) Disadvantages: low sampling frequency (1-0.1 Hz) beam spread gives changing sampling volume ideally need excellent DGPS correction different frequencies needed for different depths (lower frequency=greater sound penetration)

60 6. MBES and field PIV New uses of Multibeam Echo Sounding (MBES)

61 RESON SeaBat 7125 MBES 200/400 khz 256 or 512 beams 10 Hz Tilt, roll, pitch, gyro Bottom detect and water column RTK dgps with PPS input Swath parallel to flow Range bins ~0.02m in length Calibrated swath Amplitude ran ge (m ) Phase ran ge (m ) -180

62

63 Methods Calculation of MBES Reverberation Level Inputs: 1. Backscatter Magnitude at approximately 2cm sampling interval along 2 dimensional fan of beams 2. Radius for averaging For each individual sample determine location of all samples within the given radius (0.13m) Allocate the mean/median value of all the values within the radius to the given sample point Repeat for all pings in data set Repeat for all samples in the fan Temporal moving average between successive pings option Apply sonar equation to obtain reverberation levels using spreading, range cell volume size, TVG correction Calibrate linear reverberation level to concentration samples, assuming uniform grainsize distribution

64 MBES Results -5 0m 5 flow -7

65 Methods Calculation of Flow Velocities Interpolate the polar averaged reverberation values along the beams to a rectangular grid for two successive pings In the first ping determine the samples forming a rectangle about a central sample point Correlate the samples in the rectangle with the same sized rectangles centred within a given distance of the central sample point in the next sequential ping The velocity vector is determined by drawing a line from the centre of the original rectangle to that of the rectangle with the highest correlation in the second ping and dividing by the time delay between the two pings Repeat for a two dimensional grid of vector origins within the swath Repeat for next successive ping PIV for Apply temporal moving average for each vector position MBES!

66 MBES Results One ping Motivation Gravels Dunes Field Applications

67 MBES Results Motivation Gravels Dunes Field Applications

68 MBES Results comparison of velcoities 0-2 ADCP MBES 0-2 ADCP MBES Depth (m) -4 Depth (m) U (m/s) V (m/s) Possible sources of error: Slightly different spatial locations Non-concurrent adp-mbes sampling MBES mount angle?

69 6. MBES and field PIV Advantages: non-intrusive multipoint whole flow field works in sediment-laden flows works in shallow and deep flows

70 6. MBES and field PIV Disadvantages: Vessel must be fixed at-a-point 2D Spatial and temporal resolution fair Assumes sediment packets follow the flow

71 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.), Papers in rest of course Search the web!!