Measurement of Flow Velocity Prof. Václav Uruba IT ASCR, CTU Praha, UWB Plzeň

Resolution Time Space Velocity components Mean value Instantaneous values Independent Time Resolved Space 0D (point) 1D (line) 2D (plane) 3D (volumetric) Velocity components 1 2 3 TIME CORRELATION SPACE CORRELATION December 28, 2018

Methods Pressure measurement (M or TR, 0D, 1-3c) Thermal anemometry (TR, 0D, 1-3c) Optical methods LDA (TR, 0D, 1-3c) PIV (I or TR, 2D or 3D, 2-3c) December 28, 2018

Velocity Pressure measurement December 28, 2018

Pressure Probes Total pressure – Pitot Static pressure Dynamic pressure – Prandtl (Pitot-static) probe December 28, 2018

Incompressible Flow air upto 50 (100) m/s Bernoulli equation December 28, 2018

Subsonic Compressible Flow isentropic December 28, 2018

Supersonic Compressible Flow nonisentropic isentropic December 28, 2018

Multihole probes - direction Evaluated quantity Total pressure Static pressure 2-3 velocity comp. 3-6holes A.a. 30-45° 7-12 holes sphere A.a. upto 180° December 28, 2018

Multihole probes - direction Φ 3mm 5 holes Omnidirectional Φ 9.5mm 12 holes December 28, 2018

Fast response Φ 6.3mm 5 holes Fast response Φ 1.6mm 5 holes December 28, 2018

Thermal anemometry December 28, 2018

Thermal Anemometry Hot Wire or Film Measures any fluid quantity depending on heat transfer (velocity, temperature, concentration, …) Measuring “point”: The only method for more then 10kHz (upto 200kHz) December 28, 2018

Constant Temperature Anemometry December 28, 2018

Frequency response December 28, 2018

Directional sensitivity December 28, 2018

Directional ambiguity December 28, 2018

Sensor Wire Film Φ 1 - 10μm Nickel th. less 1μm December 28, 2018

Probes – wires December 28, 2018

Probes - films December 28, 2018

Calibration Cooling law Velocity set using pressures December 28, 2018

Thermal anemometry Small measuring point Good sensitivity High precision (depending on calibration) High frequency Range of velocities (air: 0.1m/s – 5M) Sensitivity to other quantities (T, p, concentration) Intrusive method Fragile probe Problems in harsh environment Velocity orientation ambiguity Sensitivity to other quantities (T, p, concentration) Calibration necessary December 28, 2018

Velocity Optical methods December 28, 2018

Optical Methods Laser Doppler Anemometry (LDA, PDA) Particle Image Velocimetry (PIV) December 28, 2018

Laser Doppler Anemometry December 28, 2018

LDA - 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 December 28, 2018

LDA Velocity = distance/time measuring volume backscattered light Flow with particles t (measured) Signal Time Processor d (known) Detector measuring volume Bragg Cell Laser backscattered light December 28, 2018

Measurement volume Length: Width: Height: Fringe separation: δz Fringe separation:  Z No. of fringes: δx δf X December 28, 2018

LDA system December 28, 2018

Application examples December 28, 2018

Particle Dynamics Analyzer December 28, 2018

LDA High precision No calibration Nonintrusive Up to 3 components Small measuring point Velocity orientation Particles necessary Unevent sampling Expensive December 28, 2018

Particle Image Velocimetry Velocity vector fields - space correlation December 28, 2018

PIV f = 1 – 100 Hz TR: f = 500 – 2000 Hz Δt = 0.2 – 1000 μs time December 28, 2018

Velocity evaluation December 28, 2018

PIV evaluation Correlation Particle tracking December 28, 2018

Image A December 28, 2018

Image B December 28, 2018

Image B December 28, 2018

Vector field December 28, 2018

Vectors + vorticity December 28, 2018

PIV variants Classical PIV Time Resolved PIV (high frequency) Plane 2 velocity components Low frequency Time Resolved PIV (high frequency) Stereo PIV (3 comp.) Tomographic PIV, 3D PIV (volume, 3 comp.) Micro PIV (<1mm) Mega PIV, Large Scale PIV (LSPIV) (1-10m) December 28, 2018

Stereo PIV True displacement Displacement seen from left seen from right Focal plane = Centre of light sheet Left camera Right December 28, 2018

Volumetric PIV December 28, 2018

PIV Spatial correlation No calibration Nonintrusive 2 to 3 components Velocity orientation Particles necessary Lower precission Expensive December 28, 2018

Seeding particles December 28, 2018

Seeding: ability to follow flow Particle Fluid Diameter (m) f = 1 kHz f = 10 kHz Silicone oil atmospheric air 2.6 0.8 TiO2 atmospheric air 1.3 0.4 TiO2 oxygen plasma 3.2 0.8 (2800 K) MgO methane-air flame 2.6 0.8 (1800 K) December 28, 2018

Particles Dynamics Important parameters in particle motion trajectory Fluid pathline Important parameters in particle motion Particle shape Particle size Relative density of particle and fluid Concentration of particles in the fluid Body forces Acc. Drag Pressure Added mass History December 28, 2018

Repetition rate December 28, 2018