Introduction
Laser Doppler Anemometry (LDA) is a technology used to measure velocities (of small particles) in flows. The technique is based on the fact that the intensity of the light scattered by a particle, crossing the intersection of two laser beams, is oscillating. The frequency of this oscillation is proportional to the velocity of the particle.
The technology has numerous advantages over other techniques. There is for instance no need for physical contact with the flow, so no disturbances occur and the technique can be applied to flows of highly reactive or extremely hot fluids and the like. Furthermore a relatively high spatial resolution can be obtained by focusing the two laser beams. These characteristics make LDA a valuable measuring technique with many applications. For example airflow measurements within combustion engines and airplane engines to improve fuel efficiency, reduce pollution and airplane noise.
Theoretical Background
LDA can be described in either classical or relativistic way. From a classical point of view the two laser beams will interfere at the intersection which is the measurement volume. This results in a series of light and dark surfaces or fringes, parallel to the optical axis of the system. A particle passing these fringes will scatter light. The intensity of this light has a specific frequency that is proportional to the velocity of the particles.
From a relativistic point of view one uses the frequency shift that light waves undergo when scattered by moving particles, i.e. the Doppler shift. The concepts as Doppler shift and interference pattern are visualized and interactively presented in this experiment by means of applets. Furthermore a simulation of the whole experiment can be performed.
Experimentation Method
A laser beam is split into two equal parallel beams. With a positive lens a measurement volume is created in the intersection of the two laser beams. By placing this measuring volume at a certain point in the flow the velocity of a small particle in the flow at that point can be measured by detecting the scattered light. The intensity of this light is recorded as a function of time. The frequency-spectrum is calculated from this time-spectrum by Fast Fourier Transformation from which the particle velocity and therefore the local flow velocity can be determined. By changing the position of the measuring volume it is possible to the determine velocity profiles.
Motivation to perform this experiment remotely
In optical experiments much time is needed for a good alignment of components. If the alignment is not a learning objective it is better to start with a working setup which can not be disturbed accidentally by the students. It gives the student more time to investigate the flow phenomena.
Because of a fast access to the setup the flow measurements can be used as a demonstration experiment in relevant courses.
Setup
Short description
The system is composed of three main parts. The first part consists of the components (laser, beam splitter, mirror and lens) that are used to create a measurement volume. The second part of the system consists of the optical detection apparatus (diaphragm, lenses and photodiode) for measuring the scattered light and the last part is the signal processing component (a data acquisition board and a computer) that is used for the processing of the output of the photodiode.
Furthermore the position of the measurement volume in the flow can be changed by moving the position of the flow tube with a spindle (see schematic view). The flow can be controlled by the speed of the pump and the temperature of the water can be set at a stabilized value.
Schematic view
Remote Interface
Variables that can be controlled remotely
- Position of the measuring volume in the flow tube.
- Pump speed.
- Temperature of the liquid.
Variables that can be measured remotely
- Light intensity of the collected scattered light.
- Flow speed.
- Position of the measuring volume in the flow tube.
- Temperature of the liquid.
What is visualized remotely?
- Intensity of the scattered light as a function of time.
- Frequency spectrum
- Data acquisition parameters
- A web cam image of the measuring volume in the flow tube.
Educational
Learning Objectives
- Gain knowledge of a modern technology to perform flow velocity measurements.
- Interpret measured data correctly to establish flow profiles.
- Gain knowledge of flow regimes (laminar, turbulent)
Target Group
- Bachelor student in physics.
- Bachelor student in natural medical science
Professional Relevance
- LDA is a valuable measuring technique with many applications. For example flow measurements within blood or around objects in a wind tunnel
- LDA is a good example to illustrate the usefulness and limitation of a modern measuring technique.