The Doppler effect is a method to measure linear velocity. When a narrow laser beam (or radio beam or ultrasonic beam) is focused on an abject, the beam will be reflected back to the source. But if the object moves, the frequency of the received signal will differ from that of the transmitted signal. This frequency difference (Doppler shift) becomes a measure of the speed of the object.
LDA is used to measure the flow (especially, high frequency turbulence fluctuations). This device can measure the flow velocity accurately and not disturb the flow during the measurement process.
Description of Laser Doppler anemometer
The main parts of this device are as follows
1. A laser source (an argon laser or He-Ne laser).
2. A beam splitter that splits the laser beam into two equal intensity beams.
3. A lens to focus the two split lazer beams at an intersection point.
4. A photomultiplier tube (PMT)
For a flow whose velocity is to be determine, it is to be noted that the flow should contain small particles to scatter the light. The particle concentration should be very small.
Operation of Laser Doppler anemometer
1. The laser source sends a beam that is divided by a beam splitter into two beams.
2. The two parallel beams are focused by lens L1 so that they intersect at a point in the test section where there is flow (with particles). In the intersection region, interference fringes are formed.
3. The particles (carried along with the flow) that pass through the intersection of the beams scatter the light of both beams.
4. The scattered light experiences a Doppler shift in frequency that is directly proportional to the velocity of the flow.
5. The light is collected by a photomultiplier tube (PMT). The device is constructed in such a way that the direct and scattered beams travel in the same optical path, so that an interference in the PMT that is proportional to the frequency change will be observed. This change gives an indication of the flow velocity.
6. A signal processor is used to retrieve PMT velocity data
Advantages of Laser Doppler anemometer
1. The device does not interrupt the flow during measurement
2. The device measures the speed directly
3. The volume of the detection part can be very small
4. Highly accurate
5. It can be used to measure the flow of 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. Wind tunnel studies
2. Blood flow measurement
3. Sensing of wind velocity
4. Used in the field of combustion