Measurements in Fluid Mechanics 058:180 (ME:5180) Time & Location: 2:30P - 3:20P MWF 3315 SC Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui Phone: (Lab), (Cell)

2 Lecture 36. Micro-scale velocimetry

3 Used to carry heat around a circuit - on-chip IC cooling, micro heat pipes Used to create forces - micro thrusters Used to transmit powers - micro pumps and turbines Used to transport materials - distribute cells, molecules to sensors Micro-scale Fluids

4 Need for Microfluidic Diagnostics Even though Re«1, flows still complicated Large surface roughness Imprecise boundary conditions Two-phase, non-Newtonian fluids Coupled hydrodynamics and electrodynamics Non-continuum effects

5 Full-field Microfluidic Velocimetry X-ray microimaging Lanzillotto, et al., Proc. ASME, 1996, AD52, Molecular-Tagging Velocimetry (MTV) Paul, et al., Anal. Chem., 1998, 70, Micro-Particle Image Velocimetry (  PIV) Santiago, et al., Exp. Fluids, 1998, 25(4),

6 X-ray Microimaging X-rays Positives Can image inside normally opaque devices Negatives low resolution ~20-40  m depth averaged (2-D) requires slurry to scatter x-rays Phosphor screen

7 Molecular-Tagging Velocimetry Positives minimally intrusive better with electrically- driven flows Negatives low resolution ~20-40  m depth averaged (2-D) greatly affected by diffusion UV laser Blue laser

8 Micro-Particle Image Velocimetry Positives high resolution ~1  m small depth average ~2-10  m minimally intrusive Negatives requires seeding flow particles can become charged Pulse laser CCD microscope

9 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 Typical MPIV System

10 –Micro-scale resolution Dimension of investigated flow structure in region of 1  m – 1 mm Nano-scale particles used –Volume (flood) illumination Micro-scale light sheet not available 2D measurement in focus plane of microscope objective –Fluorescent technique Fluorescent particles e.g. excited by =532nm and emitting =610nm Low-pass or band-pass optical filters used to reduce noises Typical MPIV System

11 –Typical problems Low signal to noise ratio because of –Low light intensity of nano-scale particles –Low light intensity of back scattering imaging –Illuminated particles out of focus plane Low particle image concentration Brownian motion of nano-scale particles Diffraction of nano-scale particles Large particle image displacement because of high magnification and time interval limit etc Typical MPIV System

12 2 mm longest vector~2.25 mm/s (Provided by Micro Fluidics Lab at Purdue University) Example: Microcantilever Driven Flow

13 Typical MPIV Image Microthruster: Magnification 40X Particle size 700 nm 500  m - Background image filtered - Particle image size d p =5  8 pixels - Image displacements S= 15  40 pixels - Image number density 3 in 32x32-pixel window

14 MPIV Image Filter Typical MPIV image features - High single-pixel random noise level because of low light intensity scattered/emitted by nano-scale particles - High low-frequency noise level because of particle images out of the focus plane - Big particle images (d p >4 pixels, d p <4 pixels for standard PIV) because of high imaging magnification MPIV filter: For SP noiseFor LF noise - Filter radius r big enough so that useful particle image information not be erased

15 MPIV Image Filter - Reduce influence of LF noises on the evaluation function Evaluation samplesEvaluation functions - Overall effect of MPIV in a micro-channel flow measurement Mean velocity profileStandard deviation

16 Correlation functions of replicated measurements at one point in the steady flow: - position of the main correlation peak not change - height and position of correlation peaks resulting from noises vary randomly Average evaluation function method (Meinhart, Wereley and Santiago, 2000) - average instantaneous evaluation functions to increase the signal-to-noise rato - only for steady laminar flows + ++ = Average Correlation Function

Long-distance Forward-Scattering MPIV Problem/solution for applying PIV in micro-scale air jet flow 1. Seeding - more difficult than in liquid flow 2. Working distance - long for micro-scale air jet flow 3. Illumination - insufficient for sub-micron particles 4. High velocity - limited by high imaging magnification 5. Low image number density & unsteady flow - average correlation impossible - smoke particles (Raffel et al.: d p <  m) - long-distance microscope (QUESTAR QM 100: WD>100 mm) - forward-scattering configuration (Raffel et al.:  10 3 ) - advanced imaging system (PCO200: ∆t=200 ns) - individual image pattern tracking 17

18 Long-distance Forward-Scattering MPIV Experimental setup

19 Long-distance Forward-Scattering MPIV Test & data acquisition Reduced image size 1024  256 pix for 60 fps (30 image pairs per second) 3 partitions in 4-GB memory for 3 axial positions in each test case Working distance 120 mm for measurement area 960  240  m 2 (0.94  m/pixel ) 1676 recording pairs in each group Time interval 200 ns PCO2000 camera 14-bit dynamic range 4-GB image memory  2048 pix Questar QM 100 Working distance up to 350 mm New Wave Solo II nm Beam diameter: 2.5 mm Repetition Rate: 30 Hz

Sample PIV recordings pairs (red: 1st image, green: 2nd image) Vector maps obtained by individual particle image pattern tracking 20 Long-distance Forward-Scattering MPIV

21 Long-distance Forward-Scattering MPIV Overlapped sample PIV recordings pairs (50 pairs) Overlapped vector maps (50 vector maps)

22 Long-distance Forward-Scattering MPIV Remove erroneous vectors by using a median filter Calculate local mean, fluctuation & correlation on a regular grid (Test at y/D = 1.5, Re  3200, 1676 vector maps, raw vectors, valid vectors)

23 Long-distance Forward-Scattering MPIV Mean velocity and velocity fluctuation at 3 positions along the jet axis (D=500 μm, Re  3200) High-speed air jet test results

24 Meinhart CD, Wereley ST, Gray MHB (2000) Volume illumination for two- dimensional particle image velocimetry. Meas. Sci. Technol. 11, pp Wereley ST, Gui L, Meinhart CD (2002) Advanced algorithms for microscale velocimetry, AIAA Journal, Vol. 40, #6 References