Imaging Techniques for Flow and Motion Measurement Lecture 4 Tracer Particle & Illumination Lichuan Gui University of Mississippi 2011 1 Tracer Particle Fluid mechanical properties Spherical particle in still fluid Gravitational induced velocity (Ug) STOKES drag law for low Re U g d p2 p 18 g dp – particle diameter Ug p – particle density – fluid density – dynamic viscosity g – gravity acceleration 2 Tracer Particle Fluid mechanical properties Velocity lag of spherical particle in continuously accelerating fluid Us Accelerating flow U Estimated Us for low Re: Us U p U d 2 p p 18 Up a Us dp – particle diameter p – particle density – fluid density – dynamic viscosity a – fluid acceleration Up – particle velocity U Up Decelerating flow U – fluid velocity 3 Tracer Particle Thermo fluid properties Brownian motion - Considerable for sub-micro particles in slow flows Quantified by the mean square distance of diffusion <s2> Diffusion coefficient D first derived by Einstein (1905) s 2 2 D t D T 3d p dp – particle diameter – dynamic viscosity – Boltzman’s constant T – absolute temperature 4 Tracer Particle Light scattering behavior Factors influencing the scattered light power - Light source power - Ratio of refractive index of particles to that of surrounding medium - Particle size - Particle shape and orientation - Polarization and observation angle - Others 5 Tracer Particle Light scattering behavior MIE’s scattering (dp>) for spherical particles Back scattering Side scattering Forward scattering Light scattering by a 1 m oil particle in air with 532 nm laser 6 Tracer Particle Light scattering behavior MIE’s scattering of different particle sizes 1 m glass particle in water 10 m glass particle in water 30 m glass particle in water 7 Tracer Particle Light scattering behavior Rayleigh scattering (dp</10) 8 Tracer Particle Particles used in liquid flows (Melling 1997, Meas. Imaging techniques for fluidSci. flowTechnol.) measurements 9 Tracer Particle Particles used in liquid flows - No special seeding system necessary in small and circulating systems - Seeding system required for open channel or big tank - Example: seeding system in a 3x3x100 m3 water tank Seeding system CONTAINER FOR PARTICLES RAKE 15m GLASS SPHERES 10 Tracer Particle Particles used in gas flows Gas flows more difficult to seed than liquid flows - Small liquid droplets will either evaporate (bad) or coat flow apparatus (bad) - Particles must be smaller because of the greater density difference (Melling 1997, Meas. Sci. Technol.) 11 Tracer Particle Particles used in gas flows Gas flows seeders - Oil droplets generator Seeding oil generator Sketch of a laskin nozzle 12 Tracer Particle Particles used in gas flows Gas flows seeders - Solid powder seeder Seeding system in a supersonic mixing layer facility 13 Illumination Light sources usually used in PIV systems Helium-Neon lasers – Continuous wave laser – Extremely monochromatic with wave length of =632.8 nm – High temporal coherence (typical coherence length of 1030 cm) – Spatially coherent – Unidirectional, parallel to the body of the laser – Beam of Gaussian intensity distribution – Cheap but not very powerful – Traditionally used for evaluation of PIV images 14 Illumination Light sources usually used in PIV systems Argon-ion lasers – Gas laser – Continuous wave – Multiple wavelengths with very narrow bandwidths – two dominant wavelengths, 514nm and 488nm, make up about 67% of the total beam output power – Single line operation possible by inserting prisms, diffraction gratings and other optical devices to "filter out" the unwanted wavelengths – Powerful enough to illuminate particles in PIV tests 15 Illumination Light sources usually used in PIV systems Copper-vapor lasers (Cu lasers) – High pulse speed, can be considered either CW or individual pulses for PIV particle illumination – Wavelength within the yellow and green spectrum – High average power (Typically 130 W) – Properties of a commercial Cu laser Wavelength: Average power: Pulse energy: Pulse duration: Peak power: Pulse frequency: Beam diameter: Beam divergence: 510.6 nm and 578.2 nm 50 W 10 mJ 15 ns – 60 ns <300 kW 5 kHz – 15 kHz 40 mm 0.6·10-3 rad 16 Illumination Light sources usually used in PIV systems Nd:YAG laser – Most popular solid-state laser for PIV – Available wavelengths: 1064, 532, 355, 266 nm etc. – Short laser pulses (~5 ns) – Slow repeat rate (10-15 Hz) – Operated in triggered mode with quality switch (Q-switch) – Dual-cavity configuration enables short time interval between laser pulses 17 Illumination Light sources usually used in PIV systems • White light • – For flood illumination in MPIV systems – Example: - Xe (xenon) flash lamps have high rep rates (kHz) and short pulse durations - Overhead projector lamps using CCD camera or spinning wheel to gate LED’s (Light Emitting Diodes) – Available in various colors – Short pulses possible 18 Illumination Light sheet optics With three cylindrical lenses (one with negative focal length) - Diverging lens used first to avoid focal lines Limited light sheet height 19 Illumination Light sheet optics With two spherical lenses and one cylindrical lens - More versatile illumination system Focal line exists but has a relatively large extension 20 Illumination Light sheet optics With three cylindrical lenses - Independently change of light sheet height and thickness Smaller light sheet than beam diameter possible Not suitable for high power lasers 21 Homework Practice with EDPIV software 1. Read EDPIV help manual to know details in “Imaging scale/time interval” window 2. Determine imaging scale of target image “target.bmp”. EDPIV software and help manual are available at http://www.edpiv.com/ 22