Method for the simultaneous measurement of three velocity components by means of laser doppler anemometry

The subject matter of the invention is a technique which allows three components of the velocity of fluid flows to be measured simultaneously by means of methods based on the use of laser Doppler anemometry. In the present case an Argon-Ion laser serves as a light source for a conventional two-component system which produces with the blue line at 488 nm and with the green line at 515 nm two interference fringe patterns in the probe volume. The interference fringes are in this case oriented at a high angle to each other. The same Argon-Ion laser is used as a light source for pumping the dye laser which emits at a wavelength of approximately 600 nm. With this beam of light a third interference fringe system is generated which is also at a high angle with respect to the other two ones. The separation according to the three colors of the scattered light emanating from the probe volume is achieved by means of usual interference filters or colored filters.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for the simultaneous measurement of three 
velocity components with the aid of laser Doppler anemometry using two 
continuous-wave lasers for the production of three interference fringe 
systems of different colours in a probe volume. This method is 
particularly used for measuring all three components of the velocity of a 
fluid flow. 
2. Description of the Prior Art 
In many flow studies the simultaneous measurement of all three components 
of the fluid flow velocity is of interest. For the majority of fluid 
flows, the shear stress is for instance one of the most important 
characteristics, which can only be determined by simultaneously measuring 
all three components of the velocity vector. In laser Doppler anemometry, 
in which small particles are used as flow velocity indicators, this means 
that either all velocity components of each individual particle have to be 
recorded or that three components of different particles which pass within 
a short time interval have to be acquired. The admissible duration of the 
time interval is in this case dependent on the time scale of the 
turbulence in the flow. For supersonic flows, for instance, the duration 
of this time interval must not exceed a few microseconds. 
Laser Doppler anemometry, which is the basis of the invention described 
herein, is occasionally also termed "Doppler-difference method" or "fringe 
anemometry" (B. Lehmann, H. J. Pfeifer, H. D. vom Stein, DE-OS No. 16 73 
403). 
The reason for this is that two partial beams emanating from a laser are 
simultaneously directed onto the moving object the velocity of which has 
to be measured. Because of the difference in the angles, the two beams 
undergo slightly different Doppler shifts in the scattering process and 
this difference is used as a measure of the velocity. An equivalent 
explanation starts from the fact that the two partial beams generate an 
interference fringe pattern in the intersection volume and as an object 
passes through this fringe system, it appears alternately bright and dark 
and the modulation frequency of the light scattered in this way from the 
object is proportional to the velocity component in the plane 
perpendicular to the interference fringe system. In the simplest case such 
a fringe anemometer can therefore detect only a single velocity component. 
In the past several optical arrangements have been desribed, which allow 
three velocity components to be determined simultaneously with the aid of 
laser Doppler anemometry. They are all extensions of two-component systems 
working with two colours of an Argon-Ion laser. With these two-component 
techniques the two strong lines emitted at the wavelengths 488 nm and 515 
nm are filtered out and used to generate two interference fringe patterns 
in the probe volume, the fringes of different colours thereby being normal 
to each other at a large angle, in general 90 degrees. For both the 
production of the interference fringe system and the recording of the 
light scattered, the filtering process is either performed by dichroic 
reflectors or by interference filters and dispersion prisms. 
Existing three-component systems may be divided into various categories. In 
the first one, simply only a third wavelength of the Argon-Ion laser is 
used to produce a third interference fringe system, the orientation of 
which is at a high angle with respect to the other two (W. J. YANTA, A 
Three Dimensional Laser Doppler Velocimeter for Use in Wind Tunnels, 
ICIASF"79 Record, IEEE Publication 79 CH 1500-8 AE, pp. 294-301, 1979). 
This method suffers from two drawbacks. On the one hand the output power 
of the third strongest line of the Argon laser is strongly below that of 
the two aforementioned lines. On the other hand this line at a wavelength 
of 477 nm is separated from the line at 488 nm by 11 nm only. Dichroic 
elements are no longer capable of separating the two lines. Interference 
filters are in the present case subject to strong losses and dispersion 
optics need long optical ways in the case of such a small line spacing, 
causing thereby instabilities in the whole arrangement. The same 
statements are true for the 497 nm line which is nearly as strong. 
The second device for the simultaneous measurement of three components uses 
only the two strongest lines of the Argon-Ion lasr. In addition to the 
four beams of the two-component system a fifth laser beam either at 488 nm 
or at 515 nm is directed into the probe volume ("LDV System 9100-11 for 
Three Component Measurement", paper published by the TSI firm, Inc. 500 
Cardington Road, St. Paul, Minn. 55164 USA). This partial beam produces a 
third fringe system along the propagation direction of the laser beams. 
The separation of the signals is in the present case provided by 
electronic means. A substantial disadvantage of this device is that for 
physical reasons in one interference fringe pattern the fringe spacing 
must be extremely small, i.e. of the order of 1 to 2 microns. Otherwise 
the fringe spacing becomes too large in the third system. Due to this 
small fringe spacing, this technique is only suited for very low 
velocities and it can never be used for velocities above 100 m/s. 
Also in a third existing system for three-component measurements two 
colours of an Argon-Ion laser are used. With one of these colours, two 
orthogonal fringe systems are generated and separated by polarization. The 
third component is measured by a fringe system made of the second colour 
("Laser Doppler Anemometry", pages 44 and 45, paper published by the DISA 
Elektronik A/S, Mileparken 22, 2740 Skovlunde, Denmark). The separation of 
two components by means of polarization, however, is only applicable if 
the particles do not change the polarization direction in the scattering 
process. However, this is very often the case so that the risk of an 
interaction between the two measurements cannot be excluded. 
A similar set-up also uses one colour to determine two components. In this 
case two Bragg cells, operating at two different frequencies, provide at 
the same time the splitting of the beam into four partial beams and the 
different displacement speeds of the interference fringes generated in the 
probe volume and displayed nearly orthogonally to each other (F. L. 
Eltsley, F. L. Crosswy and D. Brayton, Transonic Wing/Store Flow Field 
Measurement Using a Laser Velocimeter, Technical Report AEDC-TR-80-54 
1980, Arnold Engineering Development Center, Arnold Air Force Station, 
Tenn. 37389, USA). Separation of the two velocity components is carried 
out by electronic means again. The third component is recorded by the 
second colour again. In flows of high turbulence intensity, which are of 
major interest to the investigations conducted in fluid mechanics, 
separation of the two aforementioned signals is difficult and in many 
cases not possible. 
Another technique should be mentioned here which allows the third component 
to be measured by the direct Doppler effect. The light scattered back from 
the particles is in part superposed to the initial beam of light emanating 
from the laser. This leads to a different in the frequencies, which allows 
the velocity component to be measured directly along the propagation 
direction of the laser beams. This method works only in the back 
scattering mode and is therefore limited to low velocities. With this 
method only large-size inert particles can be detected. 
The German Pat. No. 31 06 025 (B. Lehmann) indicates that it is also 
possible to detect at the same time three velocity components with the aid 
of the direct Doppler technique according to Smeets. In this case 
essential characteristics of the fringe type anemometer are lost, for 
instance the possibility of observing single particles, working under the 
favorable conditions offered by the forward scattering mode, and using 
simple lasers presenting a short coherence length. 
Finally a method should be mentioned in which two Argon-Ion lasers are 
used. In this case, one of the lasers produces the usual two-colour system 
at wavelengths of 488 nm and 515 nm. The second laser generates an 
interference fringe system at 477 nm (A. Boutier, "Three Dimensional Laser 
Velocimetry: A Review", Proc. Second Intern. Symposium on Applications of 
Laser Anemometry to Fluid Mechanics, paper No. 10.5, 1984, Instituto 
Superior Tecnico, Mech. Engn. Dept., 1096 Lisboa codex, Portugal). This 
technique differs from the aforementioned methods in that a relatively 
weak laser is used for the two-colour system whereas a strong laser is put 
into operation for the third colour. However, the aforementioned 
disadvantages inherent is this three-colour system, and pertaining to 
wavelengths which are too close together cannot be eliminated. In addition 
arrangements of this type including an Argon laser are obviously quite 
expensive. The only advantage over the first method mentioned above is 
that the amplitude of the scattered light is nearly same for all the three 
colours. 
SUMMARY OF THE INVENTION 
Therefore the invention has as its object to avoid the above-mentioned 
drawbacks of the prior art. 
More particularly it is intended to improve the technique for the 
simultaneous measurement of three velocity components based on the use of 
laser Doppler anemometry as indicated above, in such a manner that the 
measuring accuracy can be strongly increased without much effort and that 
this measuring accuracy becomes to the largest possible extent independent 
of the magnitude of the velocity of the flow to be measured. 
Starting from the aforementioned technique for the simultaneous measurement 
of three velocity components according to the invention the beam of light 
emanating from a laser is split into two partial beams one of which is 
used in a two-component system while the other is employed for pumping a 
second laser. In this manner three interference fringe systems of 
different colours are generated in a probe volume, which present a 
separation of the colours of more than 25 nm and which are oriented at a 
high angle with respect to one another. 
According to the invention it is also possible to use two different 
continuous-wave lasers for the production of three interference fringe 
systems. To avoid the problems associated with Argon laser wavelengths 
which are too close together, a third colour is used according to the 
invention, which is the wavelength of another laser (dye laser). For 
instance this wavelength can be close to 600 nm. This additional laser is 
pumped with part of the Argon laser beam while the other part of the Argon 
laser beam serves to generate two interference fringe patterns of 
different colours in the probe volume. 
According to the invention this technique allows the intensity of all the 
three interference fringe patterns in the probe volume to be set to very 
high levels on the one hand, but also to attain approximately identical 
levels such that for all the three components scattered light signals are 
generated which have all the same high quality. An interference between 
different signals is therefore completely excluded. In spite of the use of 
an additional laser the optical set-up is a relatively simple one. It is 
at least much simpler than that used in other well-known techniques. 
Finally the number of optical items to be used in the optical set-up can 
be strongly reduced. 
Particularly advantageous configurations as well as improvements inherent 
in this invention are subject of the subclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawing, the beam of light 1 delivered by an Argon laser 2, 
with an output power above 5 W in all colours, is focused onto a beam 
splitter plate 3. The latter let pass 80% of the laser output power in the 
direction of the dye laser 4 and directs the remaining 20% toward the 
two-component system 5. The two-component system is a conventional one, 
i.e. it separates the lines with the wavelengths 488 nm and 515 nm by 
means of dichonic mirrors. Thus three beams 7 are directed into the probe 
volume 6. One beam has a wavelength of 488 nm, the other beam has a 
wavelength of 515 nm, and the third beam presents simultaneously the two 
wavelengths. Thus two interference fringe systems of different colours are 
generated in the probe volume 6, which are oriented at a high angle to 
each other. 
The yellow output beam of the dye laser 4 with a wavelength of 
approximately 600 nm is directed via the mirrors 8 and 9 into a 
conventional one-component system 10 which produces two partial beams 11 
of identical power. Thus a third interference fringe system is produced in 
the probe volume 6, which is also at a large angle with respect to the 
other two ones. 
The dye laser has to fulfill one condition only, i.e. it must produce an 
output power. Other requirements such as frequency stability or power 
stability have not to be fulfilled so that the simplest model of a laser 
of this type can be used. 
For the production of all three interference fringe systems Bragg cells are 
used such that it is possible to determine also the sign of the velocity 
vector for each of the three components. 
Due to the large spacing separating the three colours used in the present 
case, the three velocity components can easily be separated in the 
recording of the scattered light signals. For the blue colour at 488 nm 
and for the gree colour at 515 nm, an unblocked interference 12 filter is 
used for each colour while a simple edge filter 13 will do for the yellow 
colour at 600 nm. 
Since in the invention described here, all the three interference fringe 
patterns generated in the probe volume have a very high and nearly 
identical intensity, the scattered light signals generated for the three 
components all have the same high quality. Any interference between 
signals is therefore completely excluded. In spite of the use of an 
additional laser the optical set-up is much simpler than that used in all 
the techniques described so far. Also the number of optical items will be 
strongly reduced. 
Consequently the invention is concerned with a technique which allows three 
components of a fluid flow velocity vector to be measured simultaneously 
by means of laser Doppler anemometry. It is based on the use of an 
Argon-Ion laser 2 which serves as the light source in a conventional 
two-component system 5 which produces with the blue line at 488 nm and 
with the green line at 515 nm two interference fringe patterns in the 
probe volume 6. The interference fringes are in this case oriented at a 
high angle to each other. The same Argon-Ion laser is used as a light 
source for pumping the dye laser 10 which emits at a wavelength of 
approximately 600 nm. With this beam of light a third interference fringe 
pattern is generated which is also at a high angle with respect to the 
other two ones. The separation according to the three colours of the 
scattered light emanating from the probe volume 6 is achieved by means of 
usual interference filters or colored filters.