A method and apparatus are disclosed for measuring a velocity component of a moving fluid. Coherent light at two different wavelengths is directed into the moving fluid to create a space-and-time modulated optical field within a sample volume and to cause a chemical component of the moving fluid to fluoresce. Measurement of temporal and/or spatial characteristics of the resulting fluorescence is employed to determine the velocity component of the moving field.

The present invention relates to systems for the measurement of the 
velocity of a moving fluid and more particularly relates to a method and 
apparatus for determining the velocity of a moving fluid by measuring 
fluorescence in the moving fluid induced by a space-and-time modulated 
optical field. 
Several types of laser velocimeters have been developed for the measurement 
of fluid velocities. Known systems include particle scattering systems 
using crossed laser beams referred to as laser Doppler velocimeters, and a 
system that measures the Doppler shift in optical frequency of the 
fluorescence from a moving molecule. Each of these systems has been 
successfully applied in a wide variety of experimental flow conditions, 
but there are also important flow regimes for which they are not suitable. 
Successful application of the system which measures the Doppler shift of 
the fluorescence of a molecule requires relatively high (hypersonic) 
velocities and low temperatures to produce a wavelength shift of the 
molecular fluorescence which is significant compared to thermal Doppler 
broadening and pressure broadening. In addition, it requires very high 
resolution tunable dye laser sources to resolve the lineshape of the 
fluorescing molecule. 
The crossed beam laser Doppler velocimeter has been applied over a much 
broader range of flow regimes, but this system depends on the light 
scattered from particles passing through the crossed laser beams. The 
particles may occur naturally in the fluid or they may be introduced into 
the flow in sufficient quantity to produce the required number of 
scattering events. In either case, there is an implicit assumption that 
the particle velocity is the same as the fluid in which it is embedded. 
The degree to which the particle actually follows the fluid motion depends 
upon the acceleration of the flow and the particle size. As the 
acceleration of the flow increases, the particle size must decrease in 
order to reasonably follow the flow, and for strongly accelerated flows 
(e.g., shock waves, jet expansion, and wake flows), particles which are 
sufficiently small are difficult to introduce into the flowfield and the 
scattered light is often insufficient to provide reliable measurements. 
Particles of practical size cannot follow the flow without significant 
lag. The generation of particles of appropriate size, and the introduction 
of these particles into particular regions of interest in the flow, often 
result in significant experimental complications. 
Moreover, the signals produced by particle scattering systems are 
complicated to analyze since they consist of short bursts of modulation as 
the particle passes through the probe volume, and the signal processor 
must separate these bursts and determine their frequency. Sophisticated 
electronic circuits are required to detect situations when more than one 
particle contributes to the burst in order to eliminate the measurement of 
erroneous frequencies. In the laser Doppler velocimeter, the burst signal 
has the same wavelength as the probe beams, and scattered radiation from 
windows and surfaces placed in the flow often limit how close to a surface 
reliable measurements can be obtained. One spot and two spot systems which 
depend upon particle scattering suffer the same limitations. 
Accordingly, a need has arisen for a method and apparatus for determining 
the velocity of moving fluids without the limitations of known systems. 
More particularly, a need has arisen for a method and apparatus that is 
useful for a wide range of flow velocities and conditions and which 
obviates the need for the presence of particles in the flow. 
In accordance with the invention generally, there is provided a method and 
apparatus for measuring a velocity component of a moving fluid. Coherent 
light at two different wavelengths is directed into the moving fluid to 
create a space-and-time modulated optical field within a sample volume and 
to cause at least one chemical component of the moving fluid to fluoresce. 
The fluorescence induced by the modulated optical field is measured to 
determine flow velocity. In particular, the spatial and/or temporal 
characteristics of the fluorescence are measured to determine flow 
velocity. It is preferred to measure the phase, a spatial characteristic, 
to determine velocity, but visibility, a temporal characteristic, also 
provides velocity information. 
In accordance with a more particular form of the method and apparatus 
invention, the fluorescent light from said sample volume is detected and 
produces a modulated signal corresponding to the fluorescence. At least 
one characteristic of the modulated signal is measured to determine the 
velocity component of the moving fluid. 
In accordance with another more particular form of the invention, a phase 
reference signal corresponding to the difference in wavelength between the 
two wavelengths of coherent light is generated and a phase shift between 
the modulated signal relative to the phase reference signal is measured to 
determine the velocity component. 
In accordance with another more particular form of the invention, a value 
of the modulated signal corresponding to the visibility of the 
fluorescence is measured to determine the velocity component.

Referring now to the drawings in which like reference characters designate 
like or corresponding parts, a laser fluorescence velocimeter 10 embodying 
one form of the present invention is illustrated in FIG. 1. The apparatus 
10 depicted operates to measure the velocity of a moving fluid with the 
velocity component measured being indicated by arrow 12. As will become 
apparent hereinafter, any desired velocity component of the moving fluid 
can be observed and the application of the invention need not be limited 
to the measurement of the velocity in the overall direction of flow. As 
used herein, "velocity component" is used in its broadest name. Since all 
velocity has direction, measurement of an absolute magnitude of velocity 
is considered a particular velocity component. The same velocity would 
have other velocity components in other directions. 
As illustrated in FIG. 1, the laser velocimeter 10 includes a coherent 
light source 14 for producing coherent light at two different wavelengths. 
The coherent light source 14 depicted includes a single frequency tunable 
dye laser 16 for producing laser light at a precise wavelength which is 
converted into light of two different wavelengths. Light from the laser 16 
is directed into an acousto-optic modulator device referred to as a Bragg 
cell 18 to produce the two different wavelengths. 
The Bragg cell 18 is supplied with a driving signal from a Bragg cell 
driver 19 which is converted into acoustic waves in a deflection medium 
inside the cell. The light from the laser 16 is split and one beam is not 
affected by the Bragg cell 18 whereas the other beam is deflected within 
the Bragg cell by the deflection medium moving with the modulating signal. 
Due to the Doppler effect, a frequency displacement of this beam results 
from the movement of the deflecting medium. A first beam 20 thus emerges 
from the Bragg cell 18 essentially unaffected by the Bragg cell and a 
second frequency-shifted beam 22 emerges with a small angle of divergence 
between the beams. The two diverging beams 20 and 22 are caused to 
converge and intersect at a desired region of the moving fluid by lens 24 
and turning mirror 26. 
As depicted, the first beam 20 and the second frequency-shifted beam 22 
converge and cross in a region within which it is desired to measure fluid 
velocity. As will be explained in more detail hereinafter, the convergence 
of the two beams produces a space-and-time modulated optical field 
consisting of moving parallel sheets of optical intensity which vary 
periodically in both space and time. The region of the moving fluid in 
which this occurs will be referred to hereinafter as the sample volume and 
is designated generally by the reference character 28. As will also be 
explained, the first beam 20 and the second beam 22 are directed into the 
moving fluid such that a bisector 30 of the angle between the two beams is 
normal to the component of fluid velocity to be measured. Any velocity 
component of the fluid can be measured by adjusting the beams accordingly. 
The wavelength of the coherent light produced by the coherent light source 
14 is such that the space-and-time modulated optical field in the sample 
volume 28 causes fluorescence of at least one chemical component of the 
moving fluid. The fluorescing chemical component can occur naturally in 
the fluid. For example, nitrogen is suitable for use as the fluorescing 
chemical component in air. Alternately, the fluid can be seeded with a 
substance which will fluoresce with a chosen light source. It is 
preferable for the fluorescence to be at a different wavelength than the 
light source to facilitate detection of the fluorescence. It will be 
understood that the word light as referred to in this patent application 
is not intended to be restricted to light of the visible spectrum but also 
includes other wavelengths which are capable of inducing fluorescence. 
The temporal and spatial characteristics of the fluorescence induced by the 
space-and-time modulated optical field on the fluid in the sample volume 
28 are explained by the following, assuming that an infinite resevoir of 
fluorescing molecules are available. 
In a space and time modulated probe volume created by two laser beams whose 
frequency differs by .nu.f and that intersect at the angle .phi., the 
optical intensity in the probe volume is given by, 
##EQU1## 
where x is the distance into the probe volume in a direction perpendicular 
to the fringes, .lambda.f is the fringe spacing and I.sub.o (x) is the 
envelope of intensity in the probe volume. The fringe spacing is given by, 
##EQU2## 
where .lambda. is the average wavelength of the laser beams. The fringes 
move at a velocity giveny by, 
EQU .nu.f=.lambda.f.nu.f (3) 
Molecules which can be excited at the wavelength .lambda. are embedded in a 
fluid that moves with a velocity component of u in the positive x 
direction. For the infinite reservoir model, the equation governing the 
population of the excited molecules is, 
##EQU3## 
where a is the rate of excitation of the ground state molecules of 
(constant) density, n.sub.o and A is the total loss rate due to radiation 
and collision. 
It will be convenient to define variables for time, distance and velocity 
such that, 
EQU .tau.=.nu.ft (5a) 
EQU .epsilon.=x/.lambda.f (5b) 
EQU .mu.=u/.nu.f (5c) 
The position of the fluid element at the time .tau. is given by, 
EQU .epsilon.=.epsilon..sub.0 +.mu..tau. (6) 
where .epsilon..sub.0 is the initial location of the fluid element at .tau. 
equal to zero. Thus, the equation governing the density of excited 
particles becomes, 
##EQU4## 
The intensity experienced by this fluid element at time .tau. is given by 
##EQU5## 
where the intensity has been assumed to be constant for .epsilon. greater 
than zero and .tau..sub.0 is the time it takes the fluid element starting 
at .epsilon..sub.0 to reach the edge of the probe volume located at 
.epsilon. equal zero, 
##EQU6## 
The trajectory of the fluid element and the solution domain are represented 
on a time-distance (.tau.-.epsilon.) plot shown in FIG. 2. 
The initial condition for equation (7) is, 
EQU .eta.(.epsilon..sub.0,.tau..sub.0)=0 (10) 
and the solution for equations 7 through 10 is given by, 
##EQU7## 
Evaluation of the integral yields the result, 
##EQU8## 
where the visibility is defined by, 
##EQU9## 
and the phase is defined by, 
##EQU10## 
where .OMEGA. has been defined as equal to 2.pi..nu.f. 
To observe the population at a particular point within the fringe system 
(.epsilon.&gt;0) we note that the initial location of the fluid element is 
related to the position of observation and to the time of observation by, 
EQU .epsilon..sub.0 =.epsilon..sub.0 +.mu..tau. (15) 
and equation 12 can be written, 
##EQU11## 
The intensity within the probe volume can also be expressed in terms of the 
location of the observation point .epsilon..sub.0 by use of equation 15 in 
equation 8 to give, 
EQU I(.epsilon..sub.0,.tau.)=I.sub.0 {1-cos [2.pi.(.tau.-.epsilon..sub.0)]}(17) 
Equation 16 consists of a steady-state modulation represented by the first 
terms, and the second term is a transient which decays exponentially over 
distances of the order of u/A fringes. When .epsilon..sub.0 is greater 
than u/A, then the power radiated due to fluorescence of the excited 
molecules approximately is given by, 
##EQU12## 
where .lambda. is the wavelength of the fluorescence, h is Planck's 
constant, c is the speed of light, and A.sub..lambda. is the transition 
probability transition at wavelength .lambda.. 
Examination of these equations reveals that the fluorescence radiation, 
measured at a fixed location within the fringe system, has the frequency 
.sup..nu. f which is the same as the difference in frequency of the two 
beams creating the modulated optical field, but the visibility and the 
phase are dependent on the velocity of the fluid flow through equations 13 
and 14. The phase and visibility both depend on the value of a single 
parameter, 
##EQU13## 
consisting of two factors. The first factor is the ratio of the modulation 
frequency to the total depopulation rate of the excited state, and the 
second factor contains the velocity dependence. FIGS. 3 and 4 graphically 
illustrate these relationships for phase and visibility, respectively. 
In accordance with the relationships described above, the apparatus 10 
further includes a fluorescence detection and measurement system 32 for 
detecting and measuring the fluorescence to determine fluid velocity. 
Temporal and spatial characteristics of the fluorescence induced by the 
space-and-time modulated optical field in the sample volume 28 are 
utilized to determine flow velocity. In the preferred embodiment 
illustrated, the phase shift of the fluorescent light is measured since it 
is dependent on the fluid velocity as explained above. Visibility, which 
is essentially the depth of modulation of the signal, can also be measured 
to determine velocity but it will be affected by unmodulated fluorescence 
emitted from regions of laser beams outside of the sample volume and also 
by the modulation transfer function of the collecting optics. It will be 
understood, however, that for some applications visibility is a useful 
parameter. 
The detection and measurement system 32 in the apparatus 10 includes an 
optical arrangement 34 for receiving fluorescent light from the sample 
volume 28 and directing it into the system 32. The optical arrangment 34 
includes a collecting lens 38 which has an optical axis placed normal to 
the plane containing the beams 20 and 22 which intersect to form the 
sample volume 28. The optical arrangement 34 also includes an entrance 
slit 36 having a slit axis parallel to the bisector 30 of the two beams. 
Fluorescent light passing through the slit enters a spectrometer 40 which 
separates the light from the sample region according to wavelength and 
only transmits light corresponding to the wavelength of the fluorescence 
and eliminates any light scattered from the two laser beams in the sample 
region 28. In the preferred embodiment, a tunable grating monochrometer is 
employed as the spectrometer 40. Light transmitted by the spectrometer 40 
passes into an optical detector 41, preferably a photomultiplier, which 
produces a modulated electrical signal corresponding to the intensity of 
the fluorescent light received. 
The detection and measuring system 32 further includes a signal processing 
system 42 which receives the modulated electrical signal from the optical 
detector 41 and which receives a reference signal from the Bragg cell 
driver 19 which corresponds to the driving signal supplied to the Bragg 
cell 18. The signal processing system 42 compares the modulated electrical 
signal to the reference signal to determine a phase shift. The signal 
processing system converts the phase shift to a signal representing the 
magnitude of the velocity component of the moving fluid which is supplied 
to visual readout 44. It will be understood that for some applications, 
instead of a visual readout, a data acquisition device be employed to 
record the observed phase shift over a period of time so that such data 
can subsequently undergo more detailed analysis. Phase lock detection of 
the electrical signal corresponding to the fluorescence is referenced to 
the signal from the Bragg cell driver 19 and integration of the signal 
over many periods provides an accurate measurement of phase shift. The 
magnitude of this phase shift provides direction information as to a 
particular velocity component. That is, the apparatus measures a 
particular velocity component which includes two opposite directions. 
Molecules moving to the left along the axis of the velocity component will 
produce a phase shift magnitude different than molecules moving to the 
right, and, thus, the phase shift includes information as to the direction 
of the velocity component of the molecules. 
In operation of the preferred embodiment depicted, coherent light at two 
different wavelengths in beams 20 and 22 are directed by the coherent 
light source 14 into the sample volume 28 with the bisector 30 of the 
beams being normal to the velocity component of the fluid to be measured. 
The space-and-time modulated field produced causes the selected chemical 
component in the fluid to fluoresce. The optical arrangement 38 of the 
detection and measurement system 32 directs the resulting fluorescent 
light through entrance slit 36 and into the spectrometer 40 which 
transmits light corresponding to the wavelength of the fluorescence to the 
optical detector 41 to produce a modulated electrical signal. The signal 
processing system receives the modulated electrical signal and compares it 
to the reference signal from the Bragg cell driver 19 to produce a signal 
which is supplied to the visual readout 44 which displays a reading for 
the velocity component. 
In an alternate embodiment, the velocimeter 10 uses a Bragg cell 18 that is 
operable to shift the frequency of either of beams 20 and 22. In this 
embodiment, the cell 18 operates in a first mode by shifting the frequency 
of beam 20 and leaving beam 22 at the original frequency of laser 16. 
Then, the cell 18 operates in a second mode by shifting the frequency of 
beam 22 and leaving beam 20 at the original frequency. The modulated 
optical field at the sample volume 28 that is created by the first mode of 
operation moves in an opposite direction from the optical field that is 
created by the second mode of operation. Thus, the two modes of operation 
will produce different phase shifts in the fluorescent light detected by 
the spectrometer 40, and the velocity of the fluid may be determined by 
comparing the phase of the fluorescent light created by the two different 
modes of operation. The advantage of this embodiment is that the 
fluorescent light generated at the sample volume provides its own phase 
reference signal, but a disadvantage is that the left/right direction of 
the fluid velocity component is not determined by this embodiment. 
The method and apparatus of the present invention has a number of 
advantages when compared to the conventional, particle-based laser Doppler 
velocimeter. An important advantage is the elimination of the requirement 
that the flow contain particles and the invention is well suited for 
continuously flowing fluids and complex situations such as shock waves, 
jet expansion, and wake flows. Instead of particles, only the appropriate 
atomic or molecular species for producing the fluorescence need be present 
in the flow. In most flows of practical interest, a chemical species can 
be selected which follows the flow within which it is embedded. The 
invention is most advantageously employed when the chosen species produces 
strong fluorescing lines at a wavelength different from the absorbing 
line. Using the tunable dye lasers which are presently available, there 
are numerous candidates for the fluorescing species--both as naturally 
occurring flow constituents or as added seed material. 
The signal produced consists of a continuous signal of constant frequency. 
Compared with a laser Doppler velocimeter which produces burst signals, a 
continuous signal has a number of advantages. First, since the signal is 
of fixed frequency, a higher degree of electronic discrimination is 
possible using phase lock detection. This has the advantage of 
discriminating between fluorescence signals which arise from the probe 
volume and fluorescence which arises from other regions along the laser 
beams. Secondly, since the signal is continuous, it is possible to 
integrate over a number of periods of the signal in order to improve the 
precision of the electronic measurement. In addition, use in the preferred 
embodiment of a fluorescent signal at a wavelength different from that 
used to excite the fluorescing component of the moving fluid, makes it 
possible to use wavelength discrimination to reduce problems which arise 
due to scattering from optical surfaces or from naturally occurring 
particles within the probe volume. Furthermore, the single frequency 
tunable dye laser makes it possible to scan the exciting wavelength to 
obtain a measurement of the linewidth of the absorbing species within the 
probe volume. 
While a preferred embodiment of the invention has been shown and described 
in the foregoing detailed description, there is no intent to limit the 
invention to this embodiment and it will be understood that the invention 
is capable of numerous modifications without departing from the spirit of 
the invention as set forth in the appended claims.