Laser for Measurement of Velocity

Laser for Measurement of Velocity

Principle

Measurement of the velocity of fluid flow can be performed by scattering a laser beam from a liquid or gas. The laser beam interacts with smaii particles carried along by the flowing fluid. The particles scattered light is slightly shifted by the Doppler effect. The magnitude of the frequency shift is proportional to the velocity of the fluid.Measurement of the frequency shift directly gives the flow velocity.

Construction:

The measurement techniques basically consist of a focusing laser light at a point within the flowing fluid. Light scattered from the fluid or from particles entrained within the fluid flow is collected and focused on an optical detector. Signal processing of the detector output yields the magnitude of the Doppler frequency shift and hence the velocity of flow.

Working:

The approach towards measurement is called Dual beam approach. Light from a continous laser is split into two equal parts by a beam splitter. Ligtv from a continous laser is split into two equal parts by a beam splitter. The lens focuses the beam to the same position in the fluid. The place where the two beams crosses in the fluid, they interfere to form fringes consisting of alternating regions of high and low intensity.when the particle transverse the fringe patteren, it will scatter light when it passes through regions of high intensity.

The scattering will be reduced when a particle is passing through regions of low intensity. Liught scattered by a particles in the fluid and arriving at the detector will produce a varying output, the frequency of which is proportional to the rate at which particle transverses the interferences fringes. 

Advantages

No critical contacts with a fluid. Flow is not disturbed.

Hot or corrosive fluids can be measured without problems.

The nano spectral width of the laser light makes accurate measurement possible. It is possible to measure the flow velocity ranging from cm/s to 100s of m/s. Speed of response is high.

Makes it possible to perform measurements related to transient conditions and to investigate turbulent flow characteristics

Disadvantages

These require the necessity of having scattering entrained in the fluid. Impossible to measure flow rate of very cleaned fluid.

Additional cost is encountered because of the introduction of scattering centers in case very clean fluids.

It is possible to seed the flow with scattering particles. But the constraint is that particles seeded into the flow must be very small so as to follow the flow faithfully.