Manish Shandilya 2010me20778

LASER DOPPLER VELOCIMETER

Deepak Vijay

2010me20914

By:-

The most recent advancement in the area of flow measurement

It is also known as laser Doppler anemometer

Measures instantaneous velocities These velocity measurements are of very

importance in boundary layer studies and turbulence studies.

It uses Doppler effect principle.

INTRODUCTION

The instrument measures the velocity at a point in the fluid, flowing in a glass walled channel, by detecting the Doppler shift in the frequency of the scattered light.

Doppler EffectIt says if there is a relative motion

between source and observer then actual frequency will change by Doppler shift.

PRINCIPLE

Numerator – Receiver (observer) Toward + Away –

Denominator – Source Toward – Away + Doppler shift = f’ - f

Source and receiver moving

r

r

s S

v1 c vcf f fv c v1c

∆f = (v/µ)[( cosθ -1 )cosβ - sinθ sinβ] If the incident and scattered beams are

equally inclined to a normal to the flow direction,

β=(π/2-θ) Then ∆f = (2v/µ)sin(θ/2) so velocity of particle is v=(∆f.µ)/(2sin(θ/2))

DESIGN AND METHODOLOGY

Principle of LDV, differential beam technique

Laser

Signalprocessing

Transmittingoptics

Receiving opticswith detector

Signalconditioner

Flow

He-NeAr-IonNd-YagDiode

Beamsplitter, Achrom.Lens

Gas,LiquidParticle

Achrom. LensSpatial FilterPhotomultiplierPhotodiode

Spectrum analyserCorrelatorCounter, Tracker

AmplifierFilter

PC

The instrument has four components (i) Laser source He-Ne (ii) Beam splitter

(iii) Light pick-up unit (iv) Signal processor. He-Ne laser is generally usedThe beam inclination θ could be raised Intensity of reference beam could be

attenuated .

Light--->photodiode--->optical mixing---> weak signal(f=∆f)--->amplifier---->signal processor unit ----->filter---->amplifier---->f–v converter

The instantaneous output voltage can be recorded on a strip chart recorder or can be digitized and stored on a floppy disk or Compact disc

The mean voltage, averaged over a few seconds, is indicated by a panel meter

Sensor- photodiodeTransducer – filter and f-v converterModifier – amplifierTerminator- meter reading

COMPONENTS

There is no transfer function involvementNon contact type of measurementVery high frequency response upto megahertzVery high accuracyVery small sensing volumeSuitable in both gas and liquid flow

Why is it preferred?

Investigation of boundary layer Measurement of flow between blades of

turbineCombustion and flame phenomenon in gas

turbine and jet propulsion systemsIn measurement of blood flowWind velocities

Areas in which it is preferred

It has very high accuracy and precisionsAccurate upto 0.2%There is no need of calibration so its

measurement is very precise.

Accuracy and precision

non-intrusiveit can be used in both Laminar and turbulent

flowsSurface velocity and vibration measurementIts measurements are not afected by

temperature and terbiditywide range of velocities (~m/s – km/s)can be used in hard environments (high T,

corrosive, etc)1D, 2D, and 3D measurements

ADVANTAGES:

needs optical access (not steal walls, only transparent fluids)

single-point measurementsneeds seeding$$$$

DISADVANTAGES: