482 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
processing unit converts the variations in signal frequency into voltages
which are then converted to digital form and stored on a computer disk.
There are two basic configurations for the laser Doppler velocimeter.
In the forwarding scattering LDV, the scattered light is received by optics
located on the side of the wave flume opposite from the laser source. This
configuration requires glass side walls on both sides of the flume, but it has
the advantage that high-quality data can be acquired using a low-power
laser because more light is scattered in the forward direction.
The second configuration is the backscattering LDV in which the receiving optics are located in nearly the same position as the transmitting
optics. This method requires only one glass partition; but the backscattered light is much weaker than the forwarded scattered light, and a much
more powerful (and expensive) laser is required so the backscattered light
will have sufficient amplitude to be detected by the photodetectors.
The laser beams are very thin and the point of beam crossing occupies
a very small volume, which is the sampling volume of the velocity. The
result obtained at the crossing point is the velocity vector component lying
in the same plane as the two beams and perpendicular to a line that bisects
the angle between the two beams. A second velocity component at the
same point can be obtained by focusing another pair of laser beams having
different frequency. Normally, the two pairs of laser beams are oriented at
right angles to each other. It is possible to obtain a third velocity component
by introducing a third beam pair.
Wessels, et al. (1990) listed the key advantages of the laser Doppler
velocimeter:
1. The LDV system components can be physically located
external to the flow, thus the flow will not be disturbed.
2. The LDV sampling volume is very small (below 1 mm3)
which is very close to being a true “point measurement”
of velocity.
3. The LDV frequency response can be very high (above 1000
Hz) which enables measurements of very rapid flow variations.
4. The LDV is capable of differentiating whether the measured velocity component of reversing flows is in the positive or negative direction.
5. The measuring range can be varied between 1 mm/s to 30
m/s and higher (Burgi 1987).
6. Calibration of the LDV is a function of the optics geometry
and the speed of light emitted by the laser. No empirical
calibration is required.
processing unit converts the variations in signal frequency into voltages
which are then converted to digital form and stored on a computer disk.
There are two basic configurations for the laser Doppler velocimeter.
In the forwarding scattering LDV, the scattered light is received by optics
located on the side of the wave flume opposite from the laser source. This
configuration requires glass side walls on both sides of the flume, but it has
the advantage that high-quality data can be acquired using a low-power
laser because more light is scattered in the forward direction.
The second configuration is the backscattering LDV in which the receiving optics are located in nearly the same position as the transmitting
optics. This method requires only one glass partition; but the backscattered light is much weaker than the forwarded scattered light, and a much
more powerful (and expensive) laser is required so the backscattered light
will have sufficient amplitude to be detected by the photodetectors.
The laser beams are very thin and the point of beam crossing occupies
a very small volume, which is the sampling volume of the velocity. The
result obtained at the crossing point is the velocity vector component lying
in the same plane as the two beams and perpendicular to a line that bisects
the angle between the two beams. A second velocity component at the
same point can be obtained by focusing another pair of laser beams having
different frequency. Normally, the two pairs of laser beams are oriented at
right angles to each other. It is possible to obtain a third velocity component
by introducing a third beam pair.
Wessels, et al. (1990) listed the key advantages of the laser Doppler
velocimeter:
1. The LDV system components can be physically located
external to the flow, thus the flow will not be disturbed.
2. The LDV sampling volume is very small (below 1 mm3)
which is very close to being a true “point measurement”
of velocity.
3. The LDV frequency response can be very high (above 1000
Hz) which enables measurements of very rapid flow variations.
4. The LDV is capable of differentiating whether the measured velocity component of reversing flows is in the positive or negative direction.
5. The measuring range can be varied between 1 mm/s to 30
m/s and higher (Burgi 1987).
6. Calibration of the LDV is a function of the optics geometry
and the speed of light emitted by the laser. No empirical
calibration is required.