Measurements in Fluid Mechanics 058:180:001 (ME:5180:0001) Time & Location: 2:30P - 3:20P MWF 218 MLH Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui

2 Lecture 20. Particle image displacement methods and others

3 Particle image displacement methods - Optical, non- or minimally-intrusive, fluid flow measurement technique; - Instantaneous flow measurements in two-dimensional (2D) area or three-dimensional (3D) volume field of views; - Basic procedure of particle image displacement methods 1. Flow visualization 2. Image recording 3. Image evaluation - Flow field seeded with small tracer particles - Particles usually illuminated by a laser light sheet - Particle images captured by an imaging system - Saved in photographic film or digital image file - Young’s fringes method - Particle image identification - Correlation-based algorithm

4 Particle image displacement methods Example:

5 Particle tracking velocimetry (PTV) Particle image displacement methods Laser speckle velocimetry (LSV) Particle image velocimetry (PIV) - flow seeded with tracer particles of very low concentration - very low image number density in photo or video recordings - single particle can be identified in image recording - particle image tracking possible from frame to frame - low information density in measurement plane - flow seeded with tracer particles of very high concentration - very high image number density in photo or video recordings - single particle can not be identified in image recording - particle image tracking impossible from frame to frame - high information density in measurement plane - flow seeded with tracer particles of high concentration - high image number density in photo or video recordings - single particle can be identified in image recording - particle image tracking impossible from frame to frame - high information density in measurement plane Three groups of methods

6 Single frame image recordings Particle image displacement methods Single exposure - Long exposure time - Velocity determined by trajectory - Direction ambiguity - Low particle number density required Double exposure - Short exposure time - Velocity determined by displacement - Direction ambiguity - Methods to avoid direction ambiguity: a. color/intensity tagging b. Image shifting techniques Multi-exposure - Short exposure time - Velocity determined by displacement - Direction ambiguity - Used to increase particle image number - Limited in steady flow

7 Multi frame image recordings Particle image displacement methods - velocity determined with particle image displacement between frames - double/Multi exposure used to increase image number in steady flow

8 Frequently used evaluation methods Particle image displacement methods LID – low image density (PTV) HID – high image density (PIV)LS – laser speckle mode (LSV)

9 Data reduction Particle image displacement methods Image plane Objective Lens Laser light sheet Image plane Objective Lens S S’ L L’ Scale factor:  = L/L’ Time interval:  t Velocity:V=S/  t=  ·S’/  t

10 Evaluation methods Particle image displacement methods Particle trajectory identification Image recording - single frame - single long time exposure - low image density - film or digital recording Evaluation - read film recordings with a microscope system - identify particle trajectories in digital recording yy xx

11 Evaluation methods Particle image displacement methods Young’s fringes method Image recording - positive film - single frame - double/multiple exposed - HID & LS mode laser PC 2D traverse system CCD camera frosted glass Young’s fringes system - S M inversely proportional to S A - fringes perpendicular to particle image displacement

t1t1 t2t2 ox y ox y 12 Evaluation methods Particle image displacement methods Particle image tracking PIV recording - Minimum 2 frames - Single exposure - LID mode - Film or digital recording Evaluation - Identify particle images & determine position of each particle image center - Pairing particles in two frames (many algorithms) - Velocity determined by position difference of paired particles &  t (x 2, y 2 ) (x 1, y 1 )

13 Evaluation methods Particle image displacement methods Correlation-based interrogation m n  (m, n) -S S o Auto- correlation Cross-correlation (m’,n’)

14 Standard 2D PIV Particle image displacement methods Measurement volume Laser Lens t=t 0 Light sheet Lens system & Camera Fluid flow seeded with small tracer particles Double exposed recording Exposure #1 t=t 0 Image #1 Single exposed recording

15 Standard 2D PIV Particle image displacement methods Measurement volume Lens t=t 0 t=t 0 +  t Lens system & Camera Light sheet Single exposed recordingDouble exposed recording Fluid flow seeded with small tracer particles Image #1 t=t 0 +  t Image #2 Exposure #1 Exposure #2 Laser

16 Micro-scale PIV (MPIV) Particle image displacement methods Flood Illumination =532 nm = 610 nm Nd:YAG LASER MICROSCOPE BEAM EXPANDER CCD CAMERA MCROFLUIDIC DEVICE Nd:YAG Laser Micro Device Flow in Flow out Glass cover CCD Camera (1280x1024 pixels) Beam Expander Epi-fluorescent Prism / Filter Cube Microscope Focal Plane Micro-PIV image pair Micro-Fluidics Lab Purdue University

17 Stereo PIV (SPIV) Particle image displacement methods - 3 velocity components in a plane - Two cameras - Translation systems (lateral displacement) - Rotational systems (angular displacement) Scheimpflug condition

18 Holographic PIV (HPIV) Particle image displacement methods a. Hologram recordingb. Hologram reconstruction - 3 velocity components in a 3 dimensional volume - Complex and precise illumination

19 Other image-based methods Particle image displacement methods –Defocusing PIV (Pereira et al. 2000) Allow images to become defocused Single camera/ color CCD, particle image tracking –Multiple-sheet PIV (Raffel et al.,1995 ) Multiple laser light sheet, single camera –3D scanning PIV (Brücker, 1997) Scanning a 3D volume with a laser beam Single high speed camera –X-ray & Echo PIV –Molecular Tagging Velocimetry –Temperature measurement with particle Brownian motion –More

20 Measurement of wind velocity Sonic anemometers Cup anemometers Propeller anemometersVane anemometers

21 Homework - Questions and Problems: 9 on page Read textbook on page Due on 10/12