11.4 Laser Doppler Velocimetry or Anemometry
247
the axis of the probe volume and these are collected in the side scatter mode. The
intensity of light in the forward scatter mode is about 100 times higher than the other
modes.
– In the forward scatter mode, the receiving optics is positioned in the opposite
side of the transmitting optics (incident light), where each element is placed on
each sides of the working section. This set-up is more advantageous in terms of
optical performance and it gives a higher signal to noise ratio and is recommended
for measurement in high speed flows.
– In the back scatter mode the transmitting and receiving optics are on the same
side of the wind tunnel and are usually mounted within the same compact mount
which makes the alignment easier and hence ease of use. The main issue is the
signal to noise ratio which deteriorates with flow speed and usually limits the use
of this mode for speeds up to 300 m/s.
In order to determine the direction of the velocity component, the laser beams
are passed through a Bragg cell (acousto-optics modulator) which in modern system
acts as the beam splitter as well and applies a frequency shift to one of the beam.
For an observer fixed in space, the oscillation in the fringes is equivalent to a shifted
frequency corresponding to a velocity, U r = i × f r . The frequency, f r , is chosen in
such a way that the relative speed, U m , which is deduced from the frequency, fm,
measured in the interference fringe pattern is always positive. Therefore the absolute
velocity in the wind tunnel frame of reference is given by:
U a = U m + U r = i( f m + f r )
Bragg cells are used even in the absence of reverse flows as it ensures that velocity
components of small magnitude does not affect the accuracy of the measurement and
also the particle crosses through sufficient number of fringes.
11.4.4 Multi-components Measurements
Two velocity components are measured by intersecting the pair of fringe patterns
from two different lasers within the measurement volume and in this configuration a
total of 4 beams are generated as shown in Fig. 11.9. Each pair of beams is different
in colour and wavelength so that the collection optics can separate the two signals, a
process essential for resolving the two velocity components. In more recent compact
system both pairs could be transmitted from the same ‘head’ unlike in Fig. 11.9,
including the collections optics used in back scatter mode. For the third velocity
component, another pair of beam or fringe patterns is needed and this is usually
transmitted from a separate head mounted off-axis from the two-component head.
The set-up and alignment for 3-components measurement is more time consuming
as all six beams have to intersect within the probe volume very precisely. Due to the
increase in the measurement volume validation routine is employed to reject signals
247
the axis of the probe volume and these are collected in the side scatter mode. The
intensity of light in the forward scatter mode is about 100 times higher than the other
modes.
– In the forward scatter mode, the receiving optics is positioned in the opposite
side of the transmitting optics (incident light), where each element is placed on
each sides of the working section. This set-up is more advantageous in terms of
optical performance and it gives a higher signal to noise ratio and is recommended
for measurement in high speed flows.
– In the back scatter mode the transmitting and receiving optics are on the same
side of the wind tunnel and are usually mounted within the same compact mount
which makes the alignment easier and hence ease of use. The main issue is the
signal to noise ratio which deteriorates with flow speed and usually limits the use
of this mode for speeds up to 300 m/s.
In order to determine the direction of the velocity component, the laser beams
are passed through a Bragg cell (acousto-optics modulator) which in modern system
acts as the beam splitter as well and applies a frequency shift to one of the beam.
For an observer fixed in space, the oscillation in the fringes is equivalent to a shifted
frequency corresponding to a velocity, U r = i × f r . The frequency, f r , is chosen in
such a way that the relative speed, U m , which is deduced from the frequency, fm,
measured in the interference fringe pattern is always positive. Therefore the absolute
velocity in the wind tunnel frame of reference is given by:
U a = U m + U r = i( f m + f r )
Bragg cells are used even in the absence of reverse flows as it ensures that velocity
components of small magnitude does not affect the accuracy of the measurement and
also the particle crosses through sufficient number of fringes.
11.4.4 Multi-components Measurements
Two velocity components are measured by intersecting the pair of fringe patterns
from two different lasers within the measurement volume and in this configuration a
total of 4 beams are generated as shown in Fig. 11.9. Each pair of beams is different
in colour and wavelength so that the collection optics can separate the two signals, a
process essential for resolving the two velocity components. In more recent compact
system both pairs could be transmitted from the same ‘head’ unlike in Fig. 11.9,
including the collections optics used in back scatter mode. For the third velocity
component, another pair of beam or fringe patterns is needed and this is usually
transmitted from a separate head mounted off-axis from the two-component head.
The set-up and alignment for 3-components measurement is more time consuming
as all six beams have to intersect within the probe volume very precisely. Due to the
increase in the measurement volume validation routine is employed to reject signals