Introduction


Laser Doppler Anemometers, or LDAs, have been widely accepted as tools for analyzing fluid dynamics in gases and liquids and have been in use for more than three decades. This is a well-established method of determining the flow velocity.

Because of its non-intrusive principle and directional sensitivity, it is especially suitable for applications with reversing flow, chemically reacting or high-temperature media, and rotating machinery where physical sensors would be difficult or impossible to use. It requires tracer particles in the flow.

In particular, this method offers non-intrusive measurements, high spatial and temporal resolution, no calibration requirements, and the ability to measure reversing flows.

Measurement Principles of LDA


As light is reflected from a moving object, the frequency of the scattered light is shifted by a factor proportional to its speed.

Operation of Laser Doppler anemometer


1. A laser source emits a beam that is split by a beam splitter into two beams.

2. The two parallel beams are focused by lens L1 so they intersect at a point within the test section where flow ( with particles ) exists. Interference fringes are formed at the point of intersection.

3. The particles (carried by the flow) that pass through the intersection of the beams scatter the light from both beams.

4. Flow velocity is directly proportional to the Doppler shift in frequency of scattered light.

5. A photomultiplier tube (PMT) collects light. The device is constructed in such a way that the direct and scattered beams travel in the same optical path, causing interference at the PMT that is proportional to the frequency shift. The shift is then used to determine flow velocity.

6. A signal processor is used to retrieve velocity data from the PMT.


Advantages of Laser Doppler anemometer


1. The device does not disturb the flow during measurement

2. The device measure velocity directly

3. Volume of sensing part can be very small

4. Highly accurate

5. Can be used to measure the flow of both gases and liquids.

Disadvantages of Laser Doppler anemometer


1. Requires transparent channels

2. Cannot be used on clean flows


Applications of Laser Doppler anemometer



1. Studies of wind tunnels

2. Measurement of blood flow

3. Detection of wind velocity

4. Used in the combustion industry