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In-flight laser anemometry for aerodynamic investigations on an aircraft
Optics and Lasers in Engineering 21 (1997) 571-586 0 1997 Else&r Science Limited All rights reserved. Printed in Northern Ireland 0143~8166/97/$17~00 PII: s0143-8166(%)ooo64-4
ELSEVIER
In-flight Laser Anemometry for Aerodynamic Investigations on an Aircraft* M. Beversdorff”, “German “National
Aerospace Aerospace
W. Fiirster”,
R. Schodl” & H. W. Jentinkb
Research
Establishment DLR, Institute for Propulsion Technology, D-51170 Koln, Germany Laboratory NLR, Aircraft Instrumentation Department, P.O. Box 90502, 1006 BM Amsterdam, The Netherlands
(Received
1 December
1995; accepted
26 June 1996)
ABSTRACT A differential laser Doppler anemometer (LDA) of NLR and a laser-two-focus (L2F) anemometer of DLR were selected to be installed in a research aircrafr to demonstrate their capabilities for in-flight flow investigations. A ground test with the anemometers was performed before the flight tests. Experience with parallel operation of the anemometers was gained in a free jet and properties of both systems were investigated. During the flight tests the aircraft was flown in different atmospheric conditions to investigate whether the atmospheric seeding conditions are sufficient for measurements. Measurement volumes were traversed through the flow perpendicular to the fuselage. In this manner, the anemometers measured mean velocities, turbulence levels and the flow direction. The experiment showed that laser anemometry can be applied successfully for in-flight flow measurements. 0 1997 Elsevier Science Ltd.
1 INTRODUCTION Laser anemometry is a measurement technique applied in many flow investigations on the ground. The application of the technique in in-flight research is in the development stage. The technique has several characteristics that may solve current measurement problems. Measurement of flows at a considerable distance in front of the aircraft is one item. This may increase the safety of flight because clear air turbulence, windshear and wake vortices can be detected before entering the adverse atmospheric phenomena. Calibrations of airborne equipment for measuring * Part
of this text is based on a presentation OH, July 1%21,1995.
at the ICIASF’95 571
Conference,
held in Dayton.
572
M. Beversdorff et al.
airspeed can also be deduced from this information. Reference beam laser anemometers have been developed for this purpose. Early versions were flown by NASA’ and RAE/RSRE*. Many other anemometers have been developed and flown since. Measurement of air flow very close to the aircraft gives information about the aerodynamics around the aircraft structures. Using laser anemometry for aerodynamic investigations on the full scale aircraft has potential advantages over conventional measurement techniques in having no disturbance of the flow under investigation, a large bandwidth, a high spatial resolution and the potential to measure at spots with difficult access with mechanical instruments. Examples of difficult measurements potentially being investigated better with laser anemometry are flow investigations behind propellers and turbulence research. Measuring air flow close to the aircraft with laser anemometry can also be used to determine the airspeed of the aircraft. Smart3 has developed an instrument for this application. Enhanced in-flight aerodynamic research using laser anemometry was investigated in 1977 by ISL.4 They used a pulsed laser and a measuring distance of 2 m. A major problem with this setup was the limited data rate acquired with the system. Later work was directed to smaller systems using diode lasers and a much smaller working distance.5 The university of Erlangen continued the work on in-flight laser anemometry by developing a laser Doppler anemometer (LDA) specifically for in-flight6 measurements and by developing a miniaturized version suitable for, amongst others, in-flight applications. 7 The latter anemometer was tested in collaboration with NLR in flight.8 Laser-two-focus (L2F) anemometry has been developed since the early 70s and was applied on the ground, with much success, to turbomachinery flow research and to a lot of other high speed flow experiments.’ The most important characteristic of this technique is the high light concentration in the measurement volume which is advantageous if laser power is limited. A diode-L2F system was developed at DLR, Cologne (see Refs 10 and 11) for airborne applications. This system was also optimized regarding the characteristics of the laser-two-focus method. Practical experience with in-flight applications could not be collected before this program of collaboration between NLR and DLR was started. 2 LASER
ANEMOMETERS
Similarities of the two laser anemometers in this study were: the use of a diode laser as a light source; an avalanche photodiode as detector; and a
In -flight laser anemometry
573
personal computer (PC) for data acquisition, data storage processing. A description of the anemometers is given below. 2.1 The laser-two-focus
and data
anemometer
The laser-two-focus technique is a non-intrusive method for flow velocity measurements. It is based on a time-of-flight measurement of small particles contained in the flow crossing two highly focused parallel laser beams. With their known separation, s, the flow velocity in the plane perpendicular to the optical axis can be determined. By successive measurements with stepwise turning the beam plane of the two foci it is possible to obtain also the flow direction, see Fig. 1. In order to get the mean flow velocity, mean flow angle and turbulence intensities a statistical sample of time-of-flight measurements has to be performed and evaluated by statistical analysis. A detailed description is given in Ref. 9. Standard L2F systems are operated with an Ar-laser, which needs water cooling and electrical power of more than 10 kW to operate. In-flight measurements in small aircraft require small and low power consuming instruments. The use of laser diodes fulfils this demand and the electrical power consumption is reduced to a few Watts. To optimize the diode L2F-system for in-flight applications special optics were required to achieve maximum signal-to-noise ratio, because the typical laser diode beam characteristic is divergent and elliptical.“.” Moreover, the focusing qualities are reduced compared to an Ar-laser due to the longer wavelength of the diode. In Fig. 2 the arrangement of the L2F optical head is shown. The laser beam from a SDL-diode (100 mW at 830 nm wavelength) is collimated and formed circular by a pair of anamorphic prisms. A Wollaston prism splits the beam in two and the measurement volume is generated by the Beams’
Fig. 1.
Principle
Cross-section of the laser beams
of the laser-two-focus
velocimeter.
M. Beversdorff et al.
574 LD
APD
C
P
PH
PR
L4
Ml
WP
Ll
L2
L3
MV
M2
Fig. 2. Optical components in the diode laser L2F system. APD = avalanche photodiode; C = collimator; Ll-L4 = lenses; LD = laser diode; Ml,M2 = mirrors; MV = measurement volume; P = anamorphic prisms; PH = pinhole; RP = A/4 retarder plate; WP = Wollaston prism.
lens Ll and imaged by lenses L2 and L3. Only the centre part of the optical beam path was used for transmitting the light into the measurement volume. The outer part of the lenses, the Wollaston prism and the mirrors imaged the scattered light from the particles through a pinhole into an avalanche photo-diode (APD), RCA 39019 (40 MHz).To align the plane spread by the beam pair with the flow direction, the prism was rotated by a stepper motor. The measurement volume could be traversed by moving the front lens with a second stepper motor up to 120 mm in the direction of the optical axis. A portable pc was used for system control, data-acquisition and evaluation. It contained a board for driving the laser diode and the APD, a signal-processing board, which was developed by the University of Restock, and the two stepper motor controller boards. The signalprocessing board used a special leading and trailing edge trigger method to determine the individual particle flight times.12 The complete system could be operated with a total electrical power of about 130 W. 2.2 The differential laser Doppler anemometer A LDA-system measures a component of the velocities of particles passing the intersection volume of two laser beams. The measured velocity component is in the plane of the two light beams, perpendicular to the bisector between the light beam propagation directions. The use of diode lasers instead of gas lasers in differential LDAs has the same advantages as described for L2F systems. Extremely small LDAs have been constructed with all optical components in a 52mm length and 10 mm
In-jlight laser anemometry
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diameter housing. However, the working distance of such an instrument is limited (4.4 mm).13 The LDA applied in this study was developed by the University of Erlangen and Invent7, and marketed by Invent under the name DFLDA. The DFLDA has three units, a measurement probe which is placed close to the flow under investigation, an optical component unit connected to the probe with three optical fibres and an electronics unit in which power supplies and data processing electronics are housed. The probe is small (115 mm long and 30mm in diameter) and robust, which makes the application of the system relatively simple and suitable for in-flight applications. The optical component unit is sensitive to mechanical stress due to alignment requirements for the components. Especially the coupling of light in monomode glass fibres depends on careful alignment. The unit can be placed in a stable position due to the fibres used. A disadvantage of the use of fibres is the limited coupling efficiency of light into the fibres even if the coupling is optimized. The light power in the measurement volume was between 35 and 43 mW during the experiments, while the light emitted by the diode laser was 120mW. Signals are interfaced to a pc with an analog to digital card in these experiments. The card, Gage type CompuScope 250, acquires samples at rates up to 100 million samples per second. The signal from one particle is digitized in these experiments with &bits resolution and in 64 samples. The digitized signals are stored in the pc. This process is required to run fast, because it is necessary to acquire as many particle velocities as possible to increase the statistics and, herewith, the accuracy of measurements. Moreover, stable conditions are required in many in-flight experiments, which are more easily maintained if measurement durations are short. Using a Kontron 486, 33 MHz clock frequency pc a maximum of 800 bursts per second was collected. Additional software was developed to calculate the velocities from the stored data. The processing of data was executed after the experiment.
3 GROUND
TESTS
To get experience with parallel operation and to compare both anemometers before the flight tests, a careful examination of both systems was undertaken in the DLR laboratory. First the properties of the anemometers, e.g. laser power and measurement volume geometry, were investigated and summarized in Table 1. These values will be used for further considerations. Measurements in a small free jet (nozzle diameter D = 10 mm) were performed in the free jet
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TABLE 1 of L2F- and LDA-Anemometers
Specifications
L2F
LDA
830 nm 104mW No
854 nm 120 mW Yes
67 mW 11.5 pm 108 pm -
40 mW 56 pm -
Laser Wavelength Power P Fibre fink Measurement
A
volume
Light power Ppv l/e*-beam diameter d Beam separation s Beam crossing angle 2a l/e’-axial length 1
7.62 830 pm
300 pm
Receiver Outer diameter dA Percentage of usable receiving area sR Focal length f Working distance Fibre link/efficiency r, Dimensions optical head’
Electrical power consumption ‘For the L2F system including traversing mechanism.
traversing
48 mm 50% 120 mm 98 mm No/l.0 width 130 mm height 210 mm length 450mm 130 w mechanism
24 mm 80% 120 mm 115 mm Yes/O.8 diameter 30 mm length 115 mm 120 w
and for the LDA without
core about 10mm from the nozzle exit at different speeds. Since with increasing speed the signal amplitude on both systems decreases the changing system parameters were observed while varying the flow speed between 30 and 130 m/s. The turbulence level, defined as the root-meansquare of velocity fluctuations divided by the mean velocity, was varied by setting the measurement location to different axial distances z from the nozzle exit. Setting z = 10 mm led to a turbulence level of about O-5%, z = 50 mm to 5-6% and at z = 100 mm the turbulence level was about 20%. Measurements with natural particles contained in the laboratory air were possible, but to get higher data rates glycerine droplets produced by a DLR particle generator were added. Their size was below 1 pm. Both anemometers were operated with maximum laser power and APDsensitivity. To compare the anemometers’ effectiveness not only the mean speed and turbulence intensities, but also parameters like signal-to-noise ratio SIN and data rates were measured. In this study we defined these
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571
quantities as follows: . S is the electrical signal amplitude from the avalanche photodiode recorded once per second in a certain measurement situation; . N is the maximum electrical signal amplitude from the avalanche photodiode if no particles are in the measurement volume of the anemometer; . data rate is the rate of acquiring particle velocity measurements. As the ratio SIN is discussed in the following, we are not concerned about amplification factors of electronics, which are in the signal as well as in the noise. The following results present an example of the collected data. Example 1: measurement
location:
mean speed
turbulence
level
data rate signal amplitude noise amplitude
centre of free jet 10 mm from nozzle exit L2F
LDA
108.1 m/s 108.5 m/s 108.9 m/s O-61% 0.62% 0.62% 8000 Is 750 mV 4mV
108.6 m/s 108.9 m/s 109-4 m/s 1.65% 1.80% 1.40% 30 /s 70 mV 20 mV
Example 2: measurement
location:
mean speed turbulence
level
50 mm down stream nozzle exit 74.02 m/s 74.76 m/s 5.86% 5.84%
74.2 m/s 75-l m/s 5.1% 5.1%
The results show a good agreement under all test conditions, except at the highest speed of 130m/s, where the LDA has a lower measured velocity compared with the L2F of about 2.6%. This may be related to the
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fact that the LDA due to its less favourable optical characteristic possibly detected only larger particles with a significant lag to the flow velocity. Another reason could be a negative velocity bias caused by a reduced visibility of the faster particles.14 The indicated turbulence levels were also within the accuracy limits of the instruments. Only at turbulence levels below 2% the LDA measured higher levels than the L2F system. The limited number of samples for determining the particle velocity (64) induces a higher uncertainty in LDA measurements. We identified quantities determining the signal-to-noise ratio of both systems on the basis of the measured system parameters shown in Table 1. The calculation scheme is shown in Table 2. At first we calculated the maximum light intensity in the measurement volume. Since the lasers of both systems operated at only slightly different wavelengths, we neglect the wavelength influence on the particle response i(h). The effective collection aperture was determined by the collimator outer diameter, the receiver efficiency &Rand the focal length f. The value l-&R indicates how much area of the total receiver area is used for the transmitted laser beams. Only the LDA was instrumented with a receiving fibre, the transmission efficiency is given. The bandwidth SF of the photodetectors also influences the S/N-ratio. S/N’s proportionality to quantities is SIN - Im,,i(h)na,ff~f l*F
Estimation
of Laser Anemometer
TABLE 2 Performances
L2F
Maximum intensity in measurement volume I,,, (lo6 W/m2) Particle response to light at the used wavelength i(h) (difficult to estimate, depends on particle size distribution) Effective collection naeff = ~~ (dA)*/f2 numerical aperture Receiving fibre efficiency 2, Reciprocal noise of PM-bandwidth l/V’(aF) S/N(L2F)/S/N(LDA)
Ppv/2(z/4)d2
(1)
Concerning
the S/N-Ratio
LDA
= 323
2 Ppv/(n/4)d2 = 32.4
Ratio LZFILDA 9.97
i(830 nm)
i(854 nm)
about 1.0
0.08
0.032
2.5
no fibre 1.0 l/d/(40 MHz)
with fibrelink 0.8 l/q/(125 MHz) _
1.25 1.77 5.5
In -flight laser anemometry
579
To achieve the total ratio of the L2F and the LDA signal-to-noise ratio all factors in the right-hand column were multiplied. The result of 55 agrees very well with the mean value of all experimental data.
4 FLIGHT
TESTS
4.1 Installation
of the anemometers
in the aircraft
The two laser anemometers were installed in the Cessna Citation II research aircraft of NLR. The window of the emergency exit was replaced with a steel plate. Two glass inserts were integrated in the steel plate and a mounting rack was fixed to the plate and emergency exit. The L2F system and the probe of the LDA were mounted on the rack such that the anemometers had optical access through the windows to the flow outside the aircraft (see Fig. 3). The optical unit of the LDA and the pcs for data acquisition are installed in the cabin near seats. The position of the L2F measurement volume could be changed between 0 and 85 mm outwards of the glass insert. The LDA probe was mounted on a traversing mechanism
Fig. 3.
Photographs
of the exterior and interior of the instrumented NLR research aircraft.
escape hatch of the
580
M. Beversdorff
et al.
which enabled the manual traverse of the measurement volume between 0 and 100 mm from the glass insert. The traversing mechanism was available from other experiments, was somewhat large (length 227 mm and diameter 45 mm), but adequate for traversing the miniature probe. The traversing direction was perpendicular to the glass surface. The flow in the boundary layer was measured by traversing the measurement volumes from the glass surface through the boundary layer on the fuselage. The measurement of flow parameters in the boundary layer was chosen for the demonstration of capabilities of the anemometers, because the access to this flow field was relatively simple. The L2F system measured the magnitude of the flow and the flow direction in the measurement volume. The angles are given relative to the longitudinal axis of the aircraft within good approximation. No precise alignment procedure was applied to install the anemometers which results in an uncertain bias in angles of several degrees. Positive angles correspond to a flow with a component directed upward. The LDA measures the velocity component of the airspeed parallel to the longitudinal axis of the aircraft. 4.2 The flight test program The flight test program was composed to demonstrate the capabilities of anemometers under different atmospheric conditions and to measure local velocities at a range of positions in boundary layers and at several flight conditions. The number of particles in the air is an important quantity for these experiments. A large amount of time has been spent to find sufficient variation in this quantity. Flights were executed on 2 days over the North Sea during a longer meteorologic high pressure period. This induced a considerable temperature inversion in the atmosphere. Concentrations of pollution were visible in this layer when flying through it downwind of industrial areas. Clear air was found almost everywhere between zero and 10 km altitude. Clouds with very limited sizes were found after a long-continued search. In the pollution under the inversion layer and in the clouds velocity profiles of the boundary layer were measured. An extra opportunity to fly only the LDA emerged later. On this day there were many clouds around 2000m altitude. During a flight over the centre of the Netherlands additional boundary layer data were measured. The change of the boundary layer introduced by flying the aircraft at several speeds and with considerable slip was measured. 4.3 Results Clear air did not contain sufficient large particles for velocity measurements with adequate signal to noise ratio. Below the inversion tayer
In-flight laser anemometry
581
TABLE 3 Overview of the Data Rates (In Particle Velocity Measurements Per Second]) Acquired During the Flights with the Anemometers. Measurements Above 2000 m Altitude were Partially Disturbed by Condensation of Water on the Inside of the Glass Windows Due to the Large Temperature Difference Across the Windows. However, also with Clear Windows the Measurements were of Insufficient Quality at These Altitudes During These Flights Air layer description
During and after start Above chimney Close to inversion layer Above inversion layer In clouds In clouds Under clouds In cirrus clouds
Altitude (m)
O-100 300 300 400 1500 3 000 2900 10 000
L2F data rate (/s)
LDA data rate (/s)
100-200 200-300 0 2 000-10 000 100-900 <20 <20
10-100 l-10 l-10 0 100-800 0 0
increased data acquisition rates were obtained. After signal analysis it appeared that the L2F signals were of good quality, whereas the trigger level of the LDA had to be adjusted too close to the noise level. Noise was recorded with the LDA. No clouds were encountered during the flight on the first day. In the flight on the second day measurements in the inversion layer confirmed the earlier results. On the western part of the North Sea clouds were encountered with limited extent. In these clouds the data acquisition rates were high. Table 3 gives data rates of measurements in different air layers. L2F and LDA measurements in the boundary were acquired for 180 and 240 kts indicated airspeed (IAS) at 1500 m altitude. Figure 4 shows the results at 180 kts IAS for the L2F measurements in the inversion layer. Turbulence levels are calculated in this part of the study as the root-mean-square of velocity fluctuations in a run divided by the true airspeed TAS measured with the digital air data computer (DADC). Figure 5 gives the mean velocities in the boundary layer as measured with LDA in clouds at different IAS of the aircraft. Mean velocities were normalized on the TAS. The normalized velocity distribution in the boundary layer depends considerably on IAS. We do not give an explanation for the behaviour, as it is difficult to say anything conclusive about the complex flow. The velocity as far as possible from the aircraft is not equal to the TAS, which is acceptable, because the position is still in the influence of the aircraft.
M. Beversdorff
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et al.
(a) 120
20
00 0
_
b
n
q
0.8 0.9 0.7 L2F velocrty nom,al~udon TAS
0.6
1.0
(b) 120
a
00
2
4
6
8
sol 10
12
14
LZF flow angle [degree1 CC) 120
Velocities (a); flow angles (b); and turbulence intensities (c), measured in the boundary layer flying at 180 kts IAS. Measurements were gathered with the L2F system in the inversion layer. Velocities and turbulence intensities were normalized on the TAS measured with the DADC to compensate the difference in aircraft speeds during the experiment as well as possible. Fig. 4.
In-flight laser anemometry (a)
Fig. 5.
583
120
Velocities normalized
on TAS (a); and turbulence levels (b), measured with LDA in the boundary layer at 180 kts.
The differences between velocities measured with the L2F system and LDA are larger than expected from the ground tests. This cannot be explained by the difference in the measured quantities with a L2F system and LDA. The L2F measurement of flow angle shows that it changes in the boundary layer. This is an interesting feature, but it is not inducing a significant difference between the velocity component along the longitudinal axis of the aircraft and the magnitude of the flow velocity. The reduced accuracy can be contributed to the fact that the turbulent boundary layer was not fully developed on this part of the aircraft. Measurement locations of the L2F system and the LDA were 105 mm apart which may cause a boundary layer difference. Furthermore, the particles with a wide size distribution encountered in the clouds compared with the controlled size distribution in the laboratory may also have
584
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Fig. 6. Velocity differences between LDA measurements at 180 kts IAS while the aircraft slips with the nose about 15” to the left (A), with 15” slip to the right (0) and without slip (0). Lines demonstrate the approximate shape of the boundary layer.
contributed to reduced performances. Another effect distorting especially the turbulence measurements is fluctuation in airspeed during a measurement. The influence of slip on the velocity distribution in the boundary layer is displayed in Fig. 6. It appears that the boundary layer on the escape hatch, located on the right-hand side of the aircraft, is thinner if the aircraft’s nose is turned to the left, which is as expected. Lines demonstrate the approximate shape of the boundary layers. The restrictions of the application of these laser anemometers with respect to the particle distribution in air are not always sustainable in flight experiments. Artificially seeding air may in some investigations form a solution. However, in many investigations it should be avoided, because it is not easy to apply without disturbing the flow under investigation. The quantities influencing the performance of an anemometer given in Section 3 show that the use of a diode laser pumped Nd-Yag laser working at 532 nm with 400 mW light power improves the performance of the laser anemometers considerable. The larger light power and the different wavelength of the light may increase the S/N ratio discussed in Section 3 with a factor of about 30. In this factor it is assumed that the light scattered by a particle is dependent on A4, which is the small particle (Rayleigh) limit. The shorter wavelength causes a smaller beam diameter in the probe volume and, therefore, an increase of intensity with a factor of 2.5. On the other hand, the avalanche photodiode is less sensitive for the 532 nm light, which leads to a factor about O-5. Increase of performance of anemometers results in a large effect on the data
In-flight laser anemometry
585
acquisition rate as smaller particles are much more numerous in the atmosphere. This means that experienced limitations of the anemometers for in-flight applications can be relaxed using dedicated instruments.
5 CONCLUSIONS Laser-two-focus anemometry and laser Doppler anemometry can be applied for in-flight aerodynamic investigations. This was demonstrated by determining velocity distributions, turbulence levels and, for the L2F system, local flow directions in the boundary layer on the fuselage of a research aircraft. The performance of anemometers can be increased using recently developed lasers, which is expected to result in relaxed operational limitations with respect to the required particles in the air. Quantities determining the performance of laser anemometers were identified and prospects were discussed.
REFERENCES 1. Munoz, R. M., Mocker, H. W. & Koehler, L., Airborne laser Doppler velocimeter. Appl. Optics, 13 (1974) 2890. 2. Woodfield, A. A. & Vaughan, J. M., Using an airborne CO2 cw laser for free stream airspeed and windshear measurement. AGARD Flight Mechanics Panel Symposium on Flight Test Techniques, Lisbon, Portugal, 2-5 April, 1984, paper 22. 3. Smart, A. E., Optical velocity sensor for air data applications. Opt. Engng, 31 (1992) 166. 4. Mainone, D. & Bouis, X., Verwendung eines Farbstofflasers mit hoher Spitzenleistung und langer Pulsdauer zur Messung der Luftgeschwindigkeit vom Flugzeug aus. Z. Flugwiss. Weltraumforsch., 2 (1978) 3. 5. Damp, S., Battery-driven miniature LDA-system with semiconductor laser diode. 4th Int. Symp. on Appl. of Laser Anemometry to Fluid Mechanics, Lisbon, Portugal, 1988, paper 5. Laser Doppler 6. Durst, F., Miiller, R. & Naqwi, A., A semiconductor Anemometer for applications in aerodynamic research. ICZASF, 18-21 September 1989, Gottingen, Germany, p. 215. 7. Stieglmeier, M. & Tropea, C., Mobile fiber-optic laser Doppler anemometer. Appl. Optics, 31 (1992) 4096. 8. Jentink, H. W., Stieglmeier, M. & Tropea, C., In-flight velocity measurements using laser Doppler anemometry. J. Aircraft, 31 (1994) 444. 9. Schodl, R., Laser-two-focus velocimetry. In Advanced Instrumentation for Aero Engine Components, Philadelphia, USA, 1986, AGARD-CP-399,
paper 7. 10. Ziemann,
E., Konzeption Geschwindigkeitsmessgeriites
stitut ftir Antriebstechnik,
eines Signaloptimierten Laser-Zwei-Fokus auf Laserdiodenbasis. Diplomarbeit DLR, In-
Ruhr Universitat,
Bochum, 1992.
586
M. Beversdorff
11. Schulze,
R.,
Konstruktion
eines
et
al.
Laser-Zwei-Fokus
Velocimeters
auf
. Diplomarbeit DLR, Institut fiir Antriebstechnik/Technische Hochschule Darmstadt, 1992. 12. Htibner, K. & Kramer, R., Entwicklung, Aufbau und Erprobung eines Laser Laufzeit Anemometers fur den Einsatz an Abgas-Turboladern. Dissertation Uni Restock, 1990. 13. Jentink, H. W., Helsdingen, M. A., de Mul, F. F. M., Suichies, H. E., Aarnoudse, J. G. & Greve, J. On the construction of small differential laser Doppler velocimeters using diode lasers. Znt. J. Optoelectron., 4 (1989) 405. 14. Durao, D. F. G. & Whitelaw, J. H., Relationship between velocity and signal quality in laser Doppler anemometry. J. Phys. E, Sci. Intr., 12 (1979) 47. Laserdiodenbasis
ELSEVIER
In-flight Laser Anemometry for Aerodynamic Investigations on an Aircraft* M. Beversdorff”, “German “National
Aerospace Aerospace
W. Fiirster”,
R. Schodl” & H. W. Jentinkb
Research
Establishment DLR, Institute for Propulsion Technology, D-51170 Koln, Germany Laboratory NLR, Aircraft Instrumentation Department, P.O. Box 90502, 1006 BM Amsterdam, The Netherlands
(Received
1 December
1995; accepted
26 June 1996)
ABSTRACT A differential laser Doppler anemometer (LDA) of NLR and a laser-two-focus (L2F) anemometer of DLR were selected to be installed in a research aircrafr to demonstrate their capabilities for in-flight flow investigations. A ground test with the anemometers was performed before the flight tests. Experience with parallel operation of the anemometers was gained in a free jet and properties of both systems were investigated. During the flight tests the aircraft was flown in different atmospheric conditions to investigate whether the atmospheric seeding conditions are sufficient for measurements. Measurement volumes were traversed through the flow perpendicular to the fuselage. In this manner, the anemometers measured mean velocities, turbulence levels and the flow direction. The experiment showed that laser anemometry can be applied successfully for in-flight flow measurements. 0 1997 Elsevier Science Ltd.
1 INTRODUCTION Laser anemometry is a measurement technique applied in many flow investigations on the ground. The application of the technique in in-flight research is in the development stage. The technique has several characteristics that may solve current measurement problems. Measurement of flows at a considerable distance in front of the aircraft is one item. This may increase the safety of flight because clear air turbulence, windshear and wake vortices can be detected before entering the adverse atmospheric phenomena. Calibrations of airborne equipment for measuring * Part
of this text is based on a presentation OH, July 1%21,1995.
at the ICIASF’95 571
Conference,
held in Dayton.
572
M. Beversdorff et al.
airspeed can also be deduced from this information. Reference beam laser anemometers have been developed for this purpose. Early versions were flown by NASA’ and RAE/RSRE*. Many other anemometers have been developed and flown since. Measurement of air flow very close to the aircraft gives information about the aerodynamics around the aircraft structures. Using laser anemometry for aerodynamic investigations on the full scale aircraft has potential advantages over conventional measurement techniques in having no disturbance of the flow under investigation, a large bandwidth, a high spatial resolution and the potential to measure at spots with difficult access with mechanical instruments. Examples of difficult measurements potentially being investigated better with laser anemometry are flow investigations behind propellers and turbulence research. Measuring air flow close to the aircraft with laser anemometry can also be used to determine the airspeed of the aircraft. Smart3 has developed an instrument for this application. Enhanced in-flight aerodynamic research using laser anemometry was investigated in 1977 by ISL.4 They used a pulsed laser and a measuring distance of 2 m. A major problem with this setup was the limited data rate acquired with the system. Later work was directed to smaller systems using diode lasers and a much smaller working distance.5 The university of Erlangen continued the work on in-flight laser anemometry by developing a laser Doppler anemometer (LDA) specifically for in-flight6 measurements and by developing a miniaturized version suitable for, amongst others, in-flight applications. 7 The latter anemometer was tested in collaboration with NLR in flight.8 Laser-two-focus (L2F) anemometry has been developed since the early 70s and was applied on the ground, with much success, to turbomachinery flow research and to a lot of other high speed flow experiments.’ The most important characteristic of this technique is the high light concentration in the measurement volume which is advantageous if laser power is limited. A diode-L2F system was developed at DLR, Cologne (see Refs 10 and 11) for airborne applications. This system was also optimized regarding the characteristics of the laser-two-focus method. Practical experience with in-flight applications could not be collected before this program of collaboration between NLR and DLR was started. 2 LASER
ANEMOMETERS
Similarities of the two laser anemometers in this study were: the use of a diode laser as a light source; an avalanche photodiode as detector; and a
In -flight laser anemometry
573
personal computer (PC) for data acquisition, data storage processing. A description of the anemometers is given below. 2.1 The laser-two-focus
and data
anemometer
The laser-two-focus technique is a non-intrusive method for flow velocity measurements. It is based on a time-of-flight measurement of small particles contained in the flow crossing two highly focused parallel laser beams. With their known separation, s, the flow velocity in the plane perpendicular to the optical axis can be determined. By successive measurements with stepwise turning the beam plane of the two foci it is possible to obtain also the flow direction, see Fig. 1. In order to get the mean flow velocity, mean flow angle and turbulence intensities a statistical sample of time-of-flight measurements has to be performed and evaluated by statistical analysis. A detailed description is given in Ref. 9. Standard L2F systems are operated with an Ar-laser, which needs water cooling and electrical power of more than 10 kW to operate. In-flight measurements in small aircraft require small and low power consuming instruments. The use of laser diodes fulfils this demand and the electrical power consumption is reduced to a few Watts. To optimize the diode L2F-system for in-flight applications special optics were required to achieve maximum signal-to-noise ratio, because the typical laser diode beam characteristic is divergent and elliptical.“.” Moreover, the focusing qualities are reduced compared to an Ar-laser due to the longer wavelength of the diode. In Fig. 2 the arrangement of the L2F optical head is shown. The laser beam from a SDL-diode (100 mW at 830 nm wavelength) is collimated and formed circular by a pair of anamorphic prisms. A Wollaston prism splits the beam in two and the measurement volume is generated by the Beams’
Fig. 1.
Principle
Cross-section of the laser beams
of the laser-two-focus
velocimeter.
M. Beversdorff et al.
574 LD
APD
C
P
PH
PR
L4
Ml
WP
Ll
L2
L3
MV
M2
Fig. 2. Optical components in the diode laser L2F system. APD = avalanche photodiode; C = collimator; Ll-L4 = lenses; LD = laser diode; Ml,M2 = mirrors; MV = measurement volume; P = anamorphic prisms; PH = pinhole; RP = A/4 retarder plate; WP = Wollaston prism.
lens Ll and imaged by lenses L2 and L3. Only the centre part of the optical beam path was used for transmitting the light into the measurement volume. The outer part of the lenses, the Wollaston prism and the mirrors imaged the scattered light from the particles through a pinhole into an avalanche photo-diode (APD), RCA 39019 (40 MHz).To align the plane spread by the beam pair with the flow direction, the prism was rotated by a stepper motor. The measurement volume could be traversed by moving the front lens with a second stepper motor up to 120 mm in the direction of the optical axis. A portable pc was used for system control, data-acquisition and evaluation. It contained a board for driving the laser diode and the APD, a signal-processing board, which was developed by the University of Restock, and the two stepper motor controller boards. The signalprocessing board used a special leading and trailing edge trigger method to determine the individual particle flight times.12 The complete system could be operated with a total electrical power of about 130 W. 2.2 The differential laser Doppler anemometer A LDA-system measures a component of the velocities of particles passing the intersection volume of two laser beams. The measured velocity component is in the plane of the two light beams, perpendicular to the bisector between the light beam propagation directions. The use of diode lasers instead of gas lasers in differential LDAs has the same advantages as described for L2F systems. Extremely small LDAs have been constructed with all optical components in a 52mm length and 10 mm
In-jlight laser anemometry
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diameter housing. However, the working distance of such an instrument is limited (4.4 mm).13 The LDA applied in this study was developed by the University of Erlangen and Invent7, and marketed by Invent under the name DFLDA. The DFLDA has three units, a measurement probe which is placed close to the flow under investigation, an optical component unit connected to the probe with three optical fibres and an electronics unit in which power supplies and data processing electronics are housed. The probe is small (115 mm long and 30mm in diameter) and robust, which makes the application of the system relatively simple and suitable for in-flight applications. The optical component unit is sensitive to mechanical stress due to alignment requirements for the components. Especially the coupling of light in monomode glass fibres depends on careful alignment. The unit can be placed in a stable position due to the fibres used. A disadvantage of the use of fibres is the limited coupling efficiency of light into the fibres even if the coupling is optimized. The light power in the measurement volume was between 35 and 43 mW during the experiments, while the light emitted by the diode laser was 120mW. Signals are interfaced to a pc with an analog to digital card in these experiments. The card, Gage type CompuScope 250, acquires samples at rates up to 100 million samples per second. The signal from one particle is digitized in these experiments with &bits resolution and in 64 samples. The digitized signals are stored in the pc. This process is required to run fast, because it is necessary to acquire as many particle velocities as possible to increase the statistics and, herewith, the accuracy of measurements. Moreover, stable conditions are required in many in-flight experiments, which are more easily maintained if measurement durations are short. Using a Kontron 486, 33 MHz clock frequency pc a maximum of 800 bursts per second was collected. Additional software was developed to calculate the velocities from the stored data. The processing of data was executed after the experiment.
3 GROUND
TESTS
To get experience with parallel operation and to compare both anemometers before the flight tests, a careful examination of both systems was undertaken in the DLR laboratory. First the properties of the anemometers, e.g. laser power and measurement volume geometry, were investigated and summarized in Table 1. These values will be used for further considerations. Measurements in a small free jet (nozzle diameter D = 10 mm) were performed in the free jet
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TABLE 1 of L2F- and LDA-Anemometers
Specifications
L2F
LDA
830 nm 104mW No
854 nm 120 mW Yes
67 mW 11.5 pm 108 pm -
40 mW 56 pm -
Laser Wavelength Power P Fibre fink Measurement
A
volume
Light power Ppv l/e*-beam diameter d Beam separation s Beam crossing angle 2a l/e’-axial length 1
7.62 830 pm
300 pm
Receiver Outer diameter dA Percentage of usable receiving area sR Focal length f Working distance Fibre link/efficiency r, Dimensions optical head’
Electrical power consumption ‘For the L2F system including traversing mechanism.
traversing
48 mm 50% 120 mm 98 mm No/l.0 width 130 mm height 210 mm length 450mm 130 w mechanism
24 mm 80% 120 mm 115 mm Yes/O.8 diameter 30 mm length 115 mm 120 w
and for the LDA without
core about 10mm from the nozzle exit at different speeds. Since with increasing speed the signal amplitude on both systems decreases the changing system parameters were observed while varying the flow speed between 30 and 130 m/s. The turbulence level, defined as the root-meansquare of velocity fluctuations divided by the mean velocity, was varied by setting the measurement location to different axial distances z from the nozzle exit. Setting z = 10 mm led to a turbulence level of about O-5%, z = 50 mm to 5-6% and at z = 100 mm the turbulence level was about 20%. Measurements with natural particles contained in the laboratory air were possible, but to get higher data rates glycerine droplets produced by a DLR particle generator were added. Their size was below 1 pm. Both anemometers were operated with maximum laser power and APDsensitivity. To compare the anemometers’ effectiveness not only the mean speed and turbulence intensities, but also parameters like signal-to-noise ratio SIN and data rates were measured. In this study we defined these
In-flight laser anemometry
571
quantities as follows: . S is the electrical signal amplitude from the avalanche photodiode recorded once per second in a certain measurement situation; . N is the maximum electrical signal amplitude from the avalanche photodiode if no particles are in the measurement volume of the anemometer; . data rate is the rate of acquiring particle velocity measurements. As the ratio SIN is discussed in the following, we are not concerned about amplification factors of electronics, which are in the signal as well as in the noise. The following results present an example of the collected data. Example 1: measurement
location:
mean speed
turbulence
level
data rate signal amplitude noise amplitude
centre of free jet 10 mm from nozzle exit L2F
LDA
108.1 m/s 108.5 m/s 108.9 m/s O-61% 0.62% 0.62% 8000 Is 750 mV 4mV
108.6 m/s 108.9 m/s 109-4 m/s 1.65% 1.80% 1.40% 30 /s 70 mV 20 mV
Example 2: measurement
location:
mean speed turbulence
level
50 mm down stream nozzle exit 74.02 m/s 74.76 m/s 5.86% 5.84%
74.2 m/s 75-l m/s 5.1% 5.1%
The results show a good agreement under all test conditions, except at the highest speed of 130m/s, where the LDA has a lower measured velocity compared with the L2F of about 2.6%. This may be related to the
M. Beversdorff et al.
578
fact that the LDA due to its less favourable optical characteristic possibly detected only larger particles with a significant lag to the flow velocity. Another reason could be a negative velocity bias caused by a reduced visibility of the faster particles.14 The indicated turbulence levels were also within the accuracy limits of the instruments. Only at turbulence levels below 2% the LDA measured higher levels than the L2F system. The limited number of samples for determining the particle velocity (64) induces a higher uncertainty in LDA measurements. We identified quantities determining the signal-to-noise ratio of both systems on the basis of the measured system parameters shown in Table 1. The calculation scheme is shown in Table 2. At first we calculated the maximum light intensity in the measurement volume. Since the lasers of both systems operated at only slightly different wavelengths, we neglect the wavelength influence on the particle response i(h). The effective collection aperture was determined by the collimator outer diameter, the receiver efficiency &Rand the focal length f. The value l-&R indicates how much area of the total receiver area is used for the transmitted laser beams. Only the LDA was instrumented with a receiving fibre, the transmission efficiency is given. The bandwidth SF of the photodetectors also influences the S/N-ratio. S/N’s proportionality to quantities is SIN - Im,,i(h)na,ff~f l*F
Estimation
of Laser Anemometer
TABLE 2 Performances
L2F
Maximum intensity in measurement volume I,,, (lo6 W/m2) Particle response to light at the used wavelength i(h) (difficult to estimate, depends on particle size distribution) Effective collection naeff = ~~ (dA)*/f2 numerical aperture Receiving fibre efficiency 2, Reciprocal noise of PM-bandwidth l/V’(aF) S/N(L2F)/S/N(LDA)
Ppv/2(z/4)d2
(1)
Concerning
the S/N-Ratio
LDA
= 323
2 Ppv/(n/4)d2 = 32.4
Ratio LZFILDA 9.97
i(830 nm)
i(854 nm)
about 1.0
0.08
0.032
2.5
no fibre 1.0 l/d/(40 MHz)
with fibrelink 0.8 l/q/(125 MHz) _
1.25 1.77 5.5
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579
To achieve the total ratio of the L2F and the LDA signal-to-noise ratio all factors in the right-hand column were multiplied. The result of 55 agrees very well with the mean value of all experimental data.
4 FLIGHT
TESTS
4.1 Installation
of the anemometers
in the aircraft
The two laser anemometers were installed in the Cessna Citation II research aircraft of NLR. The window of the emergency exit was replaced with a steel plate. Two glass inserts were integrated in the steel plate and a mounting rack was fixed to the plate and emergency exit. The L2F system and the probe of the LDA were mounted on the rack such that the anemometers had optical access through the windows to the flow outside the aircraft (see Fig. 3). The optical unit of the LDA and the pcs for data acquisition are installed in the cabin near seats. The position of the L2F measurement volume could be changed between 0 and 85 mm outwards of the glass insert. The LDA probe was mounted on a traversing mechanism
Fig. 3.
Photographs
of the exterior and interior of the instrumented NLR research aircraft.
escape hatch of the
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et al.
which enabled the manual traverse of the measurement volume between 0 and 100 mm from the glass insert. The traversing mechanism was available from other experiments, was somewhat large (length 227 mm and diameter 45 mm), but adequate for traversing the miniature probe. The traversing direction was perpendicular to the glass surface. The flow in the boundary layer was measured by traversing the measurement volumes from the glass surface through the boundary layer on the fuselage. The measurement of flow parameters in the boundary layer was chosen for the demonstration of capabilities of the anemometers, because the access to this flow field was relatively simple. The L2F system measured the magnitude of the flow and the flow direction in the measurement volume. The angles are given relative to the longitudinal axis of the aircraft within good approximation. No precise alignment procedure was applied to install the anemometers which results in an uncertain bias in angles of several degrees. Positive angles correspond to a flow with a component directed upward. The LDA measures the velocity component of the airspeed parallel to the longitudinal axis of the aircraft. 4.2 The flight test program The flight test program was composed to demonstrate the capabilities of anemometers under different atmospheric conditions and to measure local velocities at a range of positions in boundary layers and at several flight conditions. The number of particles in the air is an important quantity for these experiments. A large amount of time has been spent to find sufficient variation in this quantity. Flights were executed on 2 days over the North Sea during a longer meteorologic high pressure period. This induced a considerable temperature inversion in the atmosphere. Concentrations of pollution were visible in this layer when flying through it downwind of industrial areas. Clear air was found almost everywhere between zero and 10 km altitude. Clouds with very limited sizes were found after a long-continued search. In the pollution under the inversion layer and in the clouds velocity profiles of the boundary layer were measured. An extra opportunity to fly only the LDA emerged later. On this day there were many clouds around 2000m altitude. During a flight over the centre of the Netherlands additional boundary layer data were measured. The change of the boundary layer introduced by flying the aircraft at several speeds and with considerable slip was measured. 4.3 Results Clear air did not contain sufficient large particles for velocity measurements with adequate signal to noise ratio. Below the inversion tayer
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581
TABLE 3 Overview of the Data Rates (In Particle Velocity Measurements Per Second]) Acquired During the Flights with the Anemometers. Measurements Above 2000 m Altitude were Partially Disturbed by Condensation of Water on the Inside of the Glass Windows Due to the Large Temperature Difference Across the Windows. However, also with Clear Windows the Measurements were of Insufficient Quality at These Altitudes During These Flights Air layer description
During and after start Above chimney Close to inversion layer Above inversion layer In clouds In clouds Under clouds In cirrus clouds
Altitude (m)
O-100 300 300 400 1500 3 000 2900 10 000
L2F data rate (/s)
LDA data rate (/s)
100-200 200-300 0 2 000-10 000 100-900 <20 <20
10-100 l-10 l-10 0 100-800 0 0
increased data acquisition rates were obtained. After signal analysis it appeared that the L2F signals were of good quality, whereas the trigger level of the LDA had to be adjusted too close to the noise level. Noise was recorded with the LDA. No clouds were encountered during the flight on the first day. In the flight on the second day measurements in the inversion layer confirmed the earlier results. On the western part of the North Sea clouds were encountered with limited extent. In these clouds the data acquisition rates were high. Table 3 gives data rates of measurements in different air layers. L2F and LDA measurements in the boundary were acquired for 180 and 240 kts indicated airspeed (IAS) at 1500 m altitude. Figure 4 shows the results at 180 kts IAS for the L2F measurements in the inversion layer. Turbulence levels are calculated in this part of the study as the root-mean-square of velocity fluctuations in a run divided by the true airspeed TAS measured with the digital air data computer (DADC). Figure 5 gives the mean velocities in the boundary layer as measured with LDA in clouds at different IAS of the aircraft. Mean velocities were normalized on the TAS. The normalized velocity distribution in the boundary layer depends considerably on IAS. We do not give an explanation for the behaviour, as it is difficult to say anything conclusive about the complex flow. The velocity as far as possible from the aircraft is not equal to the TAS, which is acceptable, because the position is still in the influence of the aircraft.
M. Beversdorff
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et al.
(a) 120
20
00 0
_
b
n
q
0.8 0.9 0.7 L2F velocrty nom,al~udon TAS
0.6
1.0
(b) 120
a
00
2
4
6
8
sol 10
12
14
LZF flow angle [degree1 CC) 120
Velocities (a); flow angles (b); and turbulence intensities (c), measured in the boundary layer flying at 180 kts IAS. Measurements were gathered with the L2F system in the inversion layer. Velocities and turbulence intensities were normalized on the TAS measured with the DADC to compensate the difference in aircraft speeds during the experiment as well as possible. Fig. 4.
In-flight laser anemometry (a)
Fig. 5.
583
120
Velocities normalized
on TAS (a); and turbulence levels (b), measured with LDA in the boundary layer at 180 kts.
The differences between velocities measured with the L2F system and LDA are larger than expected from the ground tests. This cannot be explained by the difference in the measured quantities with a L2F system and LDA. The L2F measurement of flow angle shows that it changes in the boundary layer. This is an interesting feature, but it is not inducing a significant difference between the velocity component along the longitudinal axis of the aircraft and the magnitude of the flow velocity. The reduced accuracy can be contributed to the fact that the turbulent boundary layer was not fully developed on this part of the aircraft. Measurement locations of the L2F system and the LDA were 105 mm apart which may cause a boundary layer difference. Furthermore, the particles with a wide size distribution encountered in the clouds compared with the controlled size distribution in the laboratory may also have
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Fig. 6. Velocity differences between LDA measurements at 180 kts IAS while the aircraft slips with the nose about 15” to the left (A), with 15” slip to the right (0) and without slip (0). Lines demonstrate the approximate shape of the boundary layer.
contributed to reduced performances. Another effect distorting especially the turbulence measurements is fluctuation in airspeed during a measurement. The influence of slip on the velocity distribution in the boundary layer is displayed in Fig. 6. It appears that the boundary layer on the escape hatch, located on the right-hand side of the aircraft, is thinner if the aircraft’s nose is turned to the left, which is as expected. Lines demonstrate the approximate shape of the boundary layers. The restrictions of the application of these laser anemometers with respect to the particle distribution in air are not always sustainable in flight experiments. Artificially seeding air may in some investigations form a solution. However, in many investigations it should be avoided, because it is not easy to apply without disturbing the flow under investigation. The quantities influencing the performance of an anemometer given in Section 3 show that the use of a diode laser pumped Nd-Yag laser working at 532 nm with 400 mW light power improves the performance of the laser anemometers considerable. The larger light power and the different wavelength of the light may increase the S/N ratio discussed in Section 3 with a factor of about 30. In this factor it is assumed that the light scattered by a particle is dependent on A4, which is the small particle (Rayleigh) limit. The shorter wavelength causes a smaller beam diameter in the probe volume and, therefore, an increase of intensity with a factor of 2.5. On the other hand, the avalanche photodiode is less sensitive for the 532 nm light, which leads to a factor about O-5. Increase of performance of anemometers results in a large effect on the data
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585
acquisition rate as smaller particles are much more numerous in the atmosphere. This means that experienced limitations of the anemometers for in-flight applications can be relaxed using dedicated instruments.
5 CONCLUSIONS Laser-two-focus anemometry and laser Doppler anemometry can be applied for in-flight aerodynamic investigations. This was demonstrated by determining velocity distributions, turbulence levels and, for the L2F system, local flow directions in the boundary layer on the fuselage of a research aircraft. The performance of anemometers can be increased using recently developed lasers, which is expected to result in relaxed operational limitations with respect to the required particles in the air. Quantities determining the performance of laser anemometers were identified and prospects were discussed.
REFERENCES 1. Munoz, R. M., Mocker, H. W. & Koehler, L., Airborne laser Doppler velocimeter. Appl. Optics, 13 (1974) 2890. 2. Woodfield, A. A. & Vaughan, J. M., Using an airborne CO2 cw laser for free stream airspeed and windshear measurement. AGARD Flight Mechanics Panel Symposium on Flight Test Techniques, Lisbon, Portugal, 2-5 April, 1984, paper 22. 3. Smart, A. E., Optical velocity sensor for air data applications. Opt. Engng, 31 (1992) 166. 4. Mainone, D. & Bouis, X., Verwendung eines Farbstofflasers mit hoher Spitzenleistung und langer Pulsdauer zur Messung der Luftgeschwindigkeit vom Flugzeug aus. Z. Flugwiss. Weltraumforsch., 2 (1978) 3. 5. Damp, S., Battery-driven miniature LDA-system with semiconductor laser diode. 4th Int. Symp. on Appl. of Laser Anemometry to Fluid Mechanics, Lisbon, Portugal, 1988, paper 5. Laser Doppler 6. Durst, F., Miiller, R. & Naqwi, A., A semiconductor Anemometer for applications in aerodynamic research. ICZASF, 18-21 September 1989, Gottingen, Germany, p. 215. 7. Stieglmeier, M. & Tropea, C., Mobile fiber-optic laser Doppler anemometer. Appl. Optics, 31 (1992) 4096. 8. Jentink, H. W., Stieglmeier, M. & Tropea, C., In-flight velocity measurements using laser Doppler anemometry. J. Aircraft, 31 (1994) 444. 9. Schodl, R., Laser-two-focus velocimetry. In Advanced Instrumentation for Aero Engine Components, Philadelphia, USA, 1986, AGARD-CP-399,
paper 7. 10. Ziemann,
E., Konzeption Geschwindigkeitsmessgeriites
stitut ftir Antriebstechnik,
eines Signaloptimierten Laser-Zwei-Fokus auf Laserdiodenbasis. Diplomarbeit DLR, In-
Ruhr Universitat,
Bochum, 1992.
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R.,
Konstruktion
eines
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auf
. Diplomarbeit DLR, Institut fiir Antriebstechnik/Technische Hochschule Darmstadt, 1992. 12. Htibner, K. & Kramer, R., Entwicklung, Aufbau und Erprobung eines Laser Laufzeit Anemometers fur den Einsatz an Abgas-Turboladern. Dissertation Uni Restock, 1990. 13. Jentink, H. W., Helsdingen, M. A., de Mul, F. F. M., Suichies, H. E., Aarnoudse, J. G. & Greve, J. On the construction of small differential laser Doppler velocimeters using diode lasers. Znt. J. Optoelectron., 4 (1989) 405. 14. Durao, D. F. G. & Whitelaw, J. H., Relationship between velocity and signal quality in laser Doppler anemometry. J. Phys. E, Sci. Intr., 12 (1979) 47. Laserdiodenbasis