MI 362Measurement and Instrument11/3/2014Lecture 26Flow MeasurementFluid VelocityVolume fow rateMass fow rateVisualization of fow characteristicsStream lines, path lines, streak lines etc.VorticesLaminar and turbulence behaiourFlow VisualizationGet an impression of fluid flow around a scale model of a real object, without any calculationsDevelopment of new and better theories and models of fluid flowTime lines, streak lines and path lines play an important role in flow visualizationAnalysis of a design: deposits of airborne materials dust, bacteria! in regions of slow, re"circulating #ow in building$s air"conditioning To understand detailed structure of flow field: boundary layers separation, circulation, turbulenceFlow VisualizationAddition of Foreign MaterialSmoke (v17)DyeSuspended solid tracer particlesTiny helium or hydrogen bubblesSmall bubbles (v18)Air bubbles (v19)Soap ilms (v!")Laser induced Fluorescence (video24) A spectroscopic method used or studying lo# visuali$ation and measurementsFluorescence emission of li!ht by a substance that has absorbed some form of electroma!netic radiation"his method allows for the isualization of phenomenon such as fow turbulence, waes, wakes and the dispersion of li#uids or particulates Laser induced Fluorescence fuorescent compound is introduced into the li#uidhi!hly isible illumination dependin! on density of the fuorescent compoundShadow!raphs$ased on the e%ect that a chan!e in density causes chan!e in refraction inde&' parallel li!ht beam throu!h is passed throu!h a moin! fuid.(ensity ariations cause some of li!ht rays to be refracted.)efraction of some of li!ht beams results in dark and li!ht areasSchlierenVery similar to shadow!raph' parallel li!ht beam throu!h is passed throu!h a moin! fuid.(ensity ariations cause some of li!ht rays to be refracted.Velocit!meter!La!ran!ian*followin! or trackin! a fuid particle that is +marked, or identi-ed in some way .dye in li#uids, smoke in air, tiny soap bubbles in air, tiny particles in water tiny particles moe with the fuid .neutrally buoyant/0 .seeding)"he elocity of the fuid particle is calculated by di%erentiation of position, 1&ample* a foatin! ball in a rier* timin! how lon! it takes to moe a certain distance2lacier Velocity measurement"article Image Velocimetry [PIV] (v23)% %luid is saturated with small and neutrally buoyant particles with different inde& of refraction% glass beads, aluminum flakes% '( to '(( micrometers% determines the velocity field by recording the light reflected from particles, illuminated by a sheet of laser need not be in visible spectrum!% Double pulse laser%The motion of the seeding particles is used to calculate speed and direction of the flow being studied% fundamentally )agrangian, the final result is a velocity field, which is an *ulerian result+ "article Image Velocimetr!,rinciple of ,-. operation,articles in flow scatter light Two images, per camera, are taken within a small time of one another /t+,atterns of particles are traced: -mage ,rocessing .elocity 0 direction 1 distance a particle travels2 /t!Available for both 3D and 4D33Velocit!meter!1ulerianinole a probe -&ed at a location in fow. measure the elocity of fuid fowin! past sensor1&ample* Laser (oppler Velocimetery .L(V/Laser #o$$ler Velocit!meter!Also called )aser Doppler anemometry )DA!+ )ike ,-., )D. measures velocity of seed particles ,-. is a )agrangian techni5ue, )D. is an *ulerian techni5ue: velocity is measured at a fi&ed point in the flow+4rocedure*laser beam is split by half5silered mirror .beam splitter) into two parallel laser beams $oth beams pass throu!h a coner!in! lens that focuses the beams at a +point, in the fow waes interfere* frin!e pattern V 6easurement volume %ringe lines y Laser (oppler VelocitymetryLaser li!ht is scattered by small seed particles* intensity is bri!ht5dark5bri!ht5dark 6 as particle moes throu!h frin!e pattern scattered laser li!ht is collected by a lens andphotodetector fuctuations in li!ht intensity 78 fuctuatin! olta!e 78 fre#uency f of voltage signal=> frequency of scattered li!ht.Bragg cell: shift frequency of one beam, causingfringe pattern to moe, makin! it possible to distin!uish positie and ne!atie elocities throu!h the focal olume.MI 362Measurement and Instrument14/3/2014Lecture 27 %ot &ire Anemometr!4rinciple* rate of conectie heat transfer from a hot ob9ect to the surroundin! fuid increases as the speed of the fuid fowin! around the ob9ect increases.1lectrically heated sensorFast time responsesensin! element is a wire .typically a few microns in diameter and about a mm lon!/"itot 'u(e"itot 'u(einfer elocity by measurin! pressure tube with a pressure tap at the sta!nation point that measures sta!nation pressure. 4itot5static probe has both a sta!nation pressure tap and seeral circumferential static pressure taps, and measures both sta!nation and static pressuresmust be ali!ned strai!ht into fow* else si!ni-cant errorsnot ery responsie to fuctuatin! elocity -elds* poor fre#uency responsedi:cult to measure sta!nation pressure in real, due to frictionmeasured sta!nation pressure is always lesser0 corrected by an empirical factor ;"itot 'u(e'ctual 4randtl tube used in 'ir$us* '