Prologue to Laser Doppler Velocimetry

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Prologue to Laser Doppler Velocimetry. Ken Kiger Burgers Program For Liquid Elements Turbulence School Park, Maryland, May 24-27. Laser Doppler Anemometry (LDA). Single-point optical velocimetry strategy. 3-D LDA Estimations on a 1:5 Mercedes-Benz

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Slide 1

Prologue to Laser Doppler Velocimetry Ken Kiger Burgers Program For Fluid Dynamics Turbulence School College Park, Maryland, May 24-27

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Laser Doppler Anemometry (LDA) Single-point optical velocimetry strategy 3-D LDA Measurements on a 1:5 Mercedes-Benz E-class show auto in wind burrow Study of the stream between turning impeller cutting edges of a pump

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Phase Doppler Anemometry (PDA) Single point molecule estimating/velocimetry strategy Droplet Size Distributions Measured in a Kerosene Spray Produced by a Fuel Injector Drop Size and Velocity estimations in an atomized Stream of Moleten Metal

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Laser Doppler Anemometry LDA A high determination - single point method for speed estimations in turbulent streams Basics Seed stream with little tracer particles Illuminate stream with at least one cognizant, captivated laser pillars to frame a MV Receive scattered light from particles going through MV and meddle with extra light sources Measurement of the resultant light force recurrence is identified with molecule speed A Back Scatter LDA System for One Velocity Component Measurement (Dantec Dynamics)

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LDA more or less Benefits Essentially non-nosy Hostile situations Very exact No adjustment High information rates Good spatial & transient determination Limitations Expensive gear Flow must be seeded with particles if none normally exist Single point estimation procedure Can be hard to gather information exceptionally close dividers

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Review of Wave Characteristics General wave spread A = Amplitude k = wavenumber x = spatial arrange t = time = precise recurrence e = stage

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Electromagnetic waves: soundness Light is radiated in "wavetrains" Short term, D t Corresponding stage move, e (t); where e may differ on scale t> D t Light is lucid when the stage stays consistent for an adequately long time Typical span ( D t c ) and proportional proliferation length ( D l c ) over which a few sources stay reasonable are: Interferometry is just handy with rational light sources Source l nom (nm) D l c White light 550 8 m Mercury Arc 546 0.3 mm Kr 86 release lamp 606 0.3 m Stabilized He-Ne laser 633 ≤ 400 m

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Electromagnetic waves: irradiance Instantaneous power thickness given by Poynting vector Units of Energy/(Area-Time) More helpful: normal over circumstances longer than light freq. Recurrence Range 6.10 x 10 14 5.20 x 10 14 3.80 x 10 14

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LDA: Doppler impact recurrence move Overall Doppler move due two separate changes The molecule "sees" a move in occurrence light recurrence because of molecule movement Scattered light from molecule to stationary identifier is moved because of molecule movement

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LDA: Doppler move, impact I Frequency Observed by Particle The primary move can itself be part into two impacts (a) the quantity of wavefronts the molecule sits back D t , as if the waves were stationary… Number of wavefronts molecule goes amid D t because of molecule speed:

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LDA: Doppler move, impact I Frequency Observed by Particle The principal move can itself be part into two impacts (b) the quantity of wavefronts passing a stationary molecule position over a similar length, D t … Number of wavefronts that pass a stationary molecule amid D t because of the wavefront speed:

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LDA: Doppler move, impact I The net impact because of a moving eyewitness w/a stationary source is then the distinction: Number of wavefronts that pass a moving molecule amid D t because of joined speed (same as utilizing relative speed in molecule outline): Net recurrence saw by moving molecule

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LDA: Doppler move, impact II An extra move happens when the light gets scattered by the molecule and is seen by the indicator This is the situation of a moving source and stationary finder (great prepare shriek issue) collector focal point Distance a scattered wave front would go amid D t toward locator, if u were 0: Due to source movement, the separation is changed by a sum: Therefore, the successful scattered wavelength is:

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LDA: Doppler move, I & II consolidated Combining the two impacts gives: For u << c, we can rough

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LDA: issue with single source/indicator Single pillar recurrence move relies on upon: speed size Velocity heading perception edge Additionally, base recurrence is very high… O[10 14 ] Hz, making direct recognition very troublesome Solution? Optical heterodyne Use obstruction of two bars or two locators to make a "beating" impact, similar to two somewhat off key guitar strings, e.g. Need to rehash for optical waves P

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Optical Heterodyne Repeat, yet consider diverse frequencies…

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How would you get distinctive diffuse frequencies? For a solitary bar Frequency relies on upon bearings of e s and e b Three normal techniques have been utilized Reference shaft mode (single diffuse and single bar) Single-pillar, double scramble (two perception edges) Dual bar (two occurrence bars, single perception area)

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Dual bar strategy Real MV framed by two bars Beam crossing edge g Scattering edge q "Forward" Scatter Configuration

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Dual bar strategy (cont) Note that so:

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Fringe Interference depiction Interference "borders" seen as standing waves Particles going through edges dissipate light in districts of helpful impedance Adequate clarification for particles littler than individual edges L

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Gaussian bar impacts A solitary laser bar profile Power dispersion in MV will be Gaussian formed In the MV, genuine plane waves happen just at the point of convergence Even for an immaculate molecule direction the quality of the Doppler "burst" will differ with position Figures from Albrecht et. al., 2003

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Non-uniform shaft impacts Particle Trajectory Centered Off Center DC AC DC+AC Off-focus direction brings about debilitated flag perceivability Pedestal (DC some portion of flag) is evacuated by a high pass channel after photomultiplier Figures from Albrecht et. al., 2003

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Multi-segment double pillar ^ x g ^ x b Three autonomous headings Two – Component Probe Looking Toward the Transmitter

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Sign vagueness… Change in indication of speed has no impact on recurrence X g u xg > 0 shaft 2 bar 1 u xg < 0

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Velocity Ambiguity Equal recurrence bars No distinction with speed bearing… can't distinguish turned around stream Solution: Introduce a recurrence move into 1 of the two bars X g Bragg Cell f b2 = f bragg + f b bar 1 f b = 5.8 e14 bar 2 f b1 = f b New Signal If D f D < f bragg then u < 0 Hypothetical move Without Bragg Cell

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Frequency move: Fringe depiction Different recurrence causes an evident speed in edges Effect aftereffect of impedance of two voyaging waves as somewhat extraordinary recurrence

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Directional equivocalness (cont) D f D s - 1 f bragg u xg (m/s) l = 514 nm, f bragg = 40 MHz and g = 20 ° Upper breaking point on positive speed restricted just by time reaction of locator

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Velocity inclination examining impacts LDA tests the stream in view of Rate at which particles go through the recognition volume Inherently a flux-weighted estimation Simple number weighted means are one-sided for insecure streams and should be rectified Consider: Uniform seeding thickness (# particles/volume) Flow moves at unfaltering rate of 5 units/sec for 4 seconds (giving 20 tests) would gauge: Flow that moves at 8 units/sec for 2 sec (giving 16 tests), then 2 units/sec for 2 second (giving 4 tests) would give

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Laser Doppler Anemometry Velocity Measurement Bias n th minute Mean Velocity Bias Compensation Formulas - The inspecting rate of a volume of liquid containing particles increments with the speed of that volume - Introduces a predisposition towards testing higher speed particles

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Phase Doppler Anemometry The general stage contrast is corresponding to molecule breadth Multiple Detector Implementation The geometric variable, b - Has shut frame answer for p = 0 and 1 just - Absolute esteem increments with y ( rise edge in respect to 0 °) - Is free of n p for reflection Figures from Dantec

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