ADVANCED MEASUREMENT TECHNIQUES IN HYDRODYNAMICS. Chittiappa Muthanna

Transcription

1 ADVANCED MEASUREMENT TECHNIQUES IN HYDRODYNAMICS Chittiappa Muthanna

2 Outline Why use these techniques? Constant temperature anemometry Laser Doppler Velocimetry PIV Measuring Shapes and Deformations 2

3 3 Description of fluid flow Reynolds terms

4 State of the art CTA system 11/8/2017 Copyright Dantec Dynamics. All rights reserved 4

5 Constant Temperature Anemometry principle Consider a thin wire mounted to supports and exposed to a velocity U. When a current is passed through wire, heat is generated (I2Rw). In equilibrium, this must be balanced by heat loss (primarily convective) to the surroundings. Velocity U Current I Sensor (thin wire) Sensor dimensions: length ~1 mm diameter ~5 micrometer Wire supports (St.St. needles) If velocity changes, convective heat transfer coefficient will change, wire temperature will change and eventually reach a new equilibrium. 11/8/2017 Copyright Dantec Dynamics. All rights reserved 5

6 E volts du/de/u volts^-1 Velocity response - King s Law Current I Sensor dimensions: length ~1 mm diameter ~5 micrometer Velocity U Sensor (thin wire) Wire supports (St.St. needles) 2,4 5 2, ,8 3 1, U m/s U m/s Output voltage as fct. of velocity Voltage derivative as fct. of velocity 11/8/2017 Copyright Dantec Dynamics. All rights reserved 6

7 Velocity Calibration Calibration in gases or liquids Must have a known flow (pre calibrated) Low turbulent free jet Circulating water tunnel Or any other know unidirectional low turbulence flow 11/8/2017 Copyright Dantec Dynamics. All rights reserved 7

8 Film Probes Film Probes Thin metal film (nickel) deposited on quartz body. Thin quartz layer protects metal film against corrosion, wear, physical damage, electrical action Fiber-Film Probes Hybrid - film deposited on a thin wire-like quartz rod (fiber) split fiber-film probes. 11/8/2017 Copyright Dantec Dynamics. All rights reserved 8

9 Multiple Sensor Probes Film Probes X-probes for 2D flows 2 sensors perpendicular to each other. Measures within ±45 o. Split-fiber probes for 2D flows 2 film sensors opposite each other on a quartz cylinder. Measures within ±90 o. Tri-axial probes for 3D flows 3 sensors in an orthogonal system. Measures within 70 o cone. 11/8/2017 Copyright Dantec Dynamics. All rights reserved 9

10 10 Example flow through a compressor cascade

11 11 Example flow through a compressor cascade

12 12 Example flow through a compressor cascade

13 13 Example flow through a compressor cascade

14 14 Example flow through a compressor cascade

15 Laser Doppler Velocimetry Measurement of flow field around a 1:5 scale car model in a wind tunnel 15 Photo courtesy of Mercedes-Benz, Germany

16 Characteristics of LDV Invented by Yeh and Cummins in 1964 Velocity measurements in Fluid Dynamics (gas, liquid) Up to 3 velocity components Non-intrusive measurements (optical technique) Absolute measurement technique (no calibration required) Very high accuracy Very high spatial resolution due to small measurement volume Tracer particles are required

17 LDV - Fringe model Focused laser beams intersect and form the measurement volume Plane wave fronts: beam waist in the plane of intersection Interference in the plane of intersection Pattern of bright and dark stripes/planes

18 Velocity = distance/time Flow with particles Signal Processor d (known) t (measured) Detector Time Bragg Cell Laser backscattered light measuring volume

19 60 mm and 85 mm FiberFlow probes

20 Measurement of water flow inside a pump model Photo courtesy of Grundfos A/S, DK

21 Measurement of wake flow around a ship model in a towing tank Photo courtesy of Marin, the Netherlands

22 Measurement of air flow field around a ship model in a wind tunnel Photo courtesy of University of Bristol, UK

23 Measurement of flow around a ship propeller in a cavitation tank

24 Particle Image Velocimetry 24 TMR 7 lab test 3: Particle Image Velocimetry

25 Basic Principle of the PIV Technique What is Particle Image Velocimetry? It is a non-intrusive, whole field optical measurement technique used to get velocity information of the flow field. The basic principle is to take two snapshots of the flowfield at a known time interval, and evaluating the velocity frame 1 frame 2 Δt V V s t

26 Why use PIV? Consider a pitot-probe or hot-wire probe. To get the pictures on the right, we would have to take data one point at a time and then average them to get the velocity plots. We thus lose information as to how the flow develops around that point, and for time evolving flows, we have no idea how the flow behaves Note : to be able to get velocity vectors, special hot-wire probes, or 2 component LDA systems have to be used thus increasing complexity

27 Why use PIV? With PIV, we can actually take snapshots of the entire flow field at each instance, and thus are able to measure the evolving flowfield. Averaging these pictures, we thus can get the same average flow field we get with a single point measurement system. Note : Due to the low sampling frequency, PIV is very poor for spectral results.

28 Basic principle & components The main components are: Powerful light source (laser) Shaping optics lightsheet Seeding material Camera(s) recording particle images Synchronization unit Software to control acquisition and postprocessing 28 TMR 7 lab test 3: Particle Image Velocimetry

29 Components: light source + optics Requirements: bright!, monochromatic (single wavelength),ability to be bundled, redirected and formed into a sheet which implies we have to use a Laser light source 1. Laser source 1. Continous wave 1. To obtain short flashes of light ( freeze motion), CW-lasers need rotating mirrors/prisms 2. Pulsed 2. Light sheet optics 29 TMR 7 lab test 3: Particle Image Velocimetry

30 Components: light path & light sheet optics Laser periscope for submerged PIV

31 Components: light path & light sheet optics 31 TMR 7 lab test 3: Particle Image Velocimetry

32 Components: Cameras Requirements: fast image rate, good resolution (typically 1k by 1k, up to 4k by 4k = 16Mpixels Two sensor techniques commonly available, CCD cameras CMOS cameras 32

33 Components: Seeding The P in PIV Remember: We measure particle motion, not fluid motion! Requirements: - small: follow the flow neutrally - same density as fluid, neither buoyant nor sinking - big and reflective: give good scattered light image - cheaply available or easy to generate - homogenous in size and behaviour - non-toxic, chemically inactive Common particles in air: Smoke ( 1 μm) DEHS olive oil (0,5-1,5 μm)he filled soap bubbles (1-3 mm) Common particles in water: polystyrene/polyamide (10-90 μm), silver coated hol. glass spheres (10-90 μm), air bubbles (5-500 μm two-phase flow!) 33 TMR 7 lab test 3: Particle Image Velocimetry

34 Components: Seeding particles Common particles in water: - polystyrene/polyamide (10, 50,90 μm) - silver coated hollow glass spheres (15μm)

35 Components: Software 1.: Image acquisition adjust measurement parameters control hardware devices create image database with acqu. information 2.: Data analysis image processing / enhancement (if necessary) masking of unwanted areas analysis: image pair > vector map visualization, statistics, export & 2. might come in one packet ---

36 Correlation analysis 1600 by 1200px full image U frame #1 36 TMR 7 lab test 3: Particle Image Velocimetry

37 Correlation analysis 1600 by 1200px full image U frame #2 37 TMR 7 lab test 3: Particle Image Velocimetry

38 Correlation analysis Zoomed in: 64 by 64px section Green grid: 24 by 24px interrogation areas frame #1 frame #2 38 TMR 7 lab test 3: Particle Image Velocimetry

39 Correlation analysis frame #1 frame #2 39 TMR 7 lab test 3: Particle Image Velocimetry

40 Correlation analysis 40

41 Comparisons CTA LDV PIV Intrusive measurement technique must take care not to influence the flow. Point measurement technique Non-intrusive optical measurement technique Point measurement technique with high spatial resolution Non-intrusive optical measurement technique Full field flow measurement technique able to evaluate instantaneous spatial information Calibration is critical must have a known flow to calibrate the instrument against. No calibration needed Calibrations is critical Capable of high sampling frequencies (>10 khz) Can get turbulence spectral information fairly easily. Sampling frequency highly dependent on seeding density and flow properties high sampling frequencies are possible (>1 KHz) Possible to measure near wall flow able to determine shear stress if correctly set up Depending on seeding and equipment, capable of >1Khz sampling frequencies Depending on setup 3D volumetric flow measurements are possible 41 Very fragile not too suited for harsh environments Access can be challenging Needs optical access Needs optical access Very safe technique Uses lasers safety considerations Laser safety is critical as lasers are quite powerful. Fairly expensive Somewhat expensive Very expensive

42 Example: Flow around straked cylinders 42

43 #1 #2 #3 #2 #3 #5 #6 #4 #5 #6 #8 #9 #7 #8 #9 ure 1: Mean, normalized velocity magnitude from the 9 measurement planes indicated in Figure 43

44 HTA flat plate benchmark 44

45 HTA flat plate benchmark 45

46 46 Warrick, D. R. et al: Aerodynamics of the hovering hummingbird, 2005, Nature 435, TMR 7 lab test 3: Particle Image Velocimetry

47 Teknologi for et bedre samfunn