Experimental investigation of aircraft trailing vortices ...

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Lille, in order to investigate chronologically the structure and motion of trailing vortices behind a free flying Airbus model in a ground-based frame of reference.
8th International EALA Conference on Laser Anemometry - Advances and Applications, September 6-9, 1999, Rome (Italy)

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Experimental investigation of aircraft trailing vortices in a catapult facility using PIV L. Dieterle, R. Stuff, G. Schneider, J. Kompenhansa - P. Coton, J.C. Monnierb a

DLR, Institut für Strömungsmechanik, Bunsenstr. 10, 37073 Göttingen, Germany, E-mail: [email protected] b ONERA, Institut de Mécanique des Fluides de Lille, 5 Bd Paul Painlevé, 59045 Lille Cedex, France, E-mail: [email protected]

The evolution of the trailing vortices behind a free flying aircraft model has been investigated experimentally by means of digital particle image velocimetry (PIV). The measurements took place in the catapult facility of ONERA, Lille, where the near field as well as the very far field of the wake flow can be studied in a ground-based frame of reference. Tracer particles added to the flow were illuminated by a pulsed laser light sheet in a plane crosswise to the flight path of the model. Two CCD cameras recorded neighbouring areas of the flow field simultaneously. Image recording was set off by the launching model crossing a photoelectric barrier in order to survey defined planes behind the model. The resulting velocity and vorticity fields give quantitative information on the structure and trajectory of the trailing vortices.

1. INTRODUCTION The minimum distance between successive air-liners landing limits the passenger throughput of an airport and depends on the persistence and strength of the aircraft’s trailing vortices. The latter increases with the aircraft’s weight, so that in the case of wide-bodied aircraft the advantage of a higher passenger capacity may be partly lost by the delayed landing of the following air-liner. For this reason the understanding of wake vortex formation, evolution and, if possible, accelerated decay is of great commercial interest. The goal of the current European research programme1 WAVENC (WAke Vortex evolution and wake vortex ENCounter) is to improve the physical understanding of wake vortex evolution up to the far-wake region, including the influence of meteorological conditions on the farfield characteristics of the wake, and to develop practical calculation methods for the aerodynamic forces on an aircraft in a wake-vortex disturbed flow field. Within this framework, optical whole-field measurements have been carried out in the catapult facility of ONERA, Lille, in order to investigate chronologically the structure and motion of trailing vortices behind a free flying Airbus model in a ground-based frame of reference.

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Brite/EuRam project BE97-4112

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In a first step, the rolling up of the shear layer shed by the aircraft an the formation of the main vortex pair were visualized by smoke and recorded with a CCD camera at a 50 Hz frame rate. The single images revealed the coarse structure of the complete trailing vortex system, and the image series yielded the time-resolved trajectory of single vortices, see Coton et al. [1]. In a second step, the wake flow of the model’s starboard wing was studied in more detail by means of particle image velocimetry (PIV). From the highly resolved particle images, the local distribution of velocity and vorticity could be extracted utilising the cross-correlation method. First results have already been presented by Coton et al. [1] as well. The present paper focuses on the technical aspects of the PIV measurements.

2. EXPERIMENTAL SET-UP 2.1 Catapult Facility and Model The ONERA’s free flight analysis laboratory in Lille consists of a pneumatic driven catapult with a 25 m long launch rail, a 30 m x 9 m x 10 m observation area, small wind tunnels simulating lateral or vertical gusts of wind (not used for the experiments presented here) and a recovery system for a soft “landing”, figure 1. A 1:22 scale model of the Airbus A300 (full wingspan 2.04 m) without any nacelle representation was employed and set into a high lift configuration with inner and outer flap angles of 0° and 15° respectively. The catapult accelerated the model via a trolley to a launch velocity of 23 m/s. Once launched the model flew at a lift coefficient of 1.2 over a distance of about 30 m and afterwards was collected by the recovery system. Some more details of this set-up are also given in the paper of Coton et al. [1]. FIGURE 1. Schematic view of the free flight analysis laboratory

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2.2 Measurement Technique The IMFL provided the light source for particle illumination, i.e. a Q-switched and frequencydoubled Nd:YAG laser system (b.m.industries) with two independent oscillators. The laser system creates double light pulses of 2 x 350 mJ at 532 nm wavelength and with a 10 Hz repetition rate. A combination of cylindrical and spherical lenses formed the laser beam to a light sheet which lit a vertical plane crosswise to the flight path of the model at a distance of about 8 m from the catapult’s dropping section, figure 2. The laser system is triggered externally. All other components of the PIV system, i.e. the seeding generators, the cameras as well as the synchronisation electronics were provided by the DLR. Two aerosol generators produced 1 µm oil droplets by means of so-called Laskin nozzles [2]. The generators were put on the laboratory’s floor ahead of the light sheet. Prior to a launch it took about 15 minutes to create sufficiently uniform and dense seeding of the observation area and about 5 minutes to give the circulating air a chance to rest. The latter was crucially important to avoid the flow field being investigated being altered by convection currents. FIGURE 2. Experimental set-up 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111 000000000000000000000000000000000000000000000000000000000000000000000 111111111111111111111111111111111111111111111111111111111111111111111

testing hall

catapult

photoelectric barrier

dropping of model

mechanical support

model

aerosol generator

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CCD cameras

light sheet optics

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PC acquisition operator room

laser system

Two identical progressive scan CCD cameras (PCO Computer Optics) with full frame interline transfer sensors were placed below the catapult on the starboard side of the model in order to take pictures of neighbouring areas of the flow field simultaneously. The cameras viewed in the flight direction. The CCD sensors have a dynamic range of 12 bits and resolve the image plane with 1280 x 1024 pixels each. These cameras are able to take two full frames in quick succession (≈ 1 µs). Due to the large amount of image data to be read out (5.2 Mbytes per double frame), the frame rate is limited to 4 Hz. The cameras can be triggered externally. In general, the particles were imaged by a 180 mm lens (Zeiss) with an f-number 2.8 and at a fixed working distance of about 8.5 m. In this case the cameras had a 40 x 32 cm2 field of view each. For the sake of a higher resolution, the field of view of a single camera was sometimes reduced to 23 x 18 cm2 by use of a 300 mm lens, also set to f# 2.8.

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A delay generator triggered and synchronised the laser system and the cameras. Within the dropping section of the catapult the model passes a photoelectric barrier which starts the measurement. The delay generator sends a 6 s sequence of trigger signals to the laser system and the cameras. FIGURE 3. Planes of measurement at different times of image recording (wsp. = wingspans)

catapult (25 m)

5.2 m light sheet wavenc

first image at t = 300 ms after launch

flight path

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dropping section model wavenc

second image at t = 600 ms

2.7 wsp.

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plane of 2. image (model’s frame of reference)

wavenc

third image at t = 900 ms

6.2 wsp.

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plane of 3. image

plane of 2. image 3.5 wsp. wavenc

9.7 wsp.

fourth image at t = 1200 ms

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During this time the image recording runs and each camera takes a series of double images of the trailing vortices in successive, equally spaced planes relative to the model, figure 3. As the repetition rate of the laser is 10 Hz, the cameras can make use of every third laser double pulse only, resulting in a frame rate of 3.3 Hz. Consequently, the distance between two successive planes corresponds to 3.5 wingspans. In this way the near, middle and far field of the model’s wake flow up to about 70 wingspans could be observed. Taking into account that vorticity and the maximum velocity close to the vortex core gradually decrease during the 6 s lasting repetitive measurement, the separation time between the two consecutive light pulses of the laser system were increased linearly in order to take advantage of the full dynamic range of particle image displacement. FIGURE 4. Picture of the calibration grid (on the left) and inverted particle images (on the right) taken with the two CCD cameras and a 180 mm lens, observation area: two times 40 x 32 cm2, (A) upper camera - (B) lower camera

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The seeding system, the cameras as well as the focus adjustment of the lenses were operated via remote control. A separate external source triggered the laser system with a repetition rate of 10 Hz before and after a measurement in order to keep the thermal condition of the oscillators stable. The trigger line was switched automatically to the delay generator when starting the measurement and reconnected to the external source when terminating. Details of the PIV system operation are described by Dieterle et al. [3]. FIGURE 5. Particles in the light sheet plane and the model’s starboard wing with slat and outer flap before crossing the light sheet (b/w-background) taken with the upper camera and a 100 mm lens 300 ms after launch according to the first scenario in figure 3 (air at rest, remaining seeding inhomogeneity is visible), velocity field (coloured) of the wing tip vortex taken with the upper camera and a 180 mm lens 600 ms after launch (second scenario in figure 3) in the same reference frame

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The image data were transferred to the acquisition PCs via optical fibre and evaluated by means of digital cross-correlation, see Raffel et al. [4]. Calculation of particle image displacement took place in interrogation windows of 32 x 32 pixels. 50 % overlapping of neighbouring windows yielded almost 5000 displacement vectors per camera. In order to convert the displacement data into velocity data, the magnification of the imaging system was determined by means of a calibration grid placed and recorded in the light sheet plane prior to a launch, figure 4 on the left.

3. RESULTS Figure 4 shows on the right the two single-exposed first frames of the two cameras taken with a 180 mm lens 600 ms after launch, i.e. the second scenario in figure 3 or the first image after the model crossed the light sheet. The particle images are inverted for the sake of a better presentation in this print. The upper camera (A) depicts the anticlockwise rotating wing tip vortex. Its core is bare of particles. The camera’s second frame (not depicted in figure 4) was taken 350 µs later resulting in a maximum particle image displacement of almost 10 pixels. FIGURE 6. Chronology of vorticity fields behind the model’s starboard wing beginning with the 2nd image (i.e. 2.7 wingspans behind the model, compare figure 3) up to the 13th image

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The local distribution of velocity calculated from the particle images of figure 4 (A) and of the corresponding second frame is presented in figure 5. Besides the main vortex the plot reveals two additional vortex cores close to the centre of the wing tip vortex. These cores belong to vortices generated by the outside edges of the slat and the outer landing flap respectively and are going to merge. All fields of velocity (and vorticity) were measured in a groundbased frame of reference. In order to transpose them into a model’s frame of reference, the model’s wing was captured with the first image taken 300 ms after launch according to the first scenario in figure 3. In figure 5, the velocity plot is superimposed on the first image of a series taken with the upper camera and a 100 mm lens. The wing as well as the slat, the outer landing flap and their outside edges are clearly visible. Apparently, the vortex system initially moves down and inwards. Later on and up to a distance of about 16 wingspans behind the model, the vortex amalgam moves outwards before remaining in a nearly fixed lateral position, figure 6. In this figure, the chronology of vorticity fields in terms of wingspans behind the model is depicted for another launch. For a reliable calculation of vorticity, an accurate measurement of velocity by exploiting the full dynamic range of image displacement is required. In the case of figure 6, the separation time between the two consecutive light pulses starts with 350 µs and goes gradually up to 830 µs, so that the maximum image displacement is always of about 10 pixels, even in planes beyond 30 wingspans behind the model, where the decay of the trailing vortices is in progress and the core region of the amalgam leaves the lower camera’s field of view.

4. CONCLUSIONS The near, middle and far field in the wake of a free flying Airbus A300 model have been investigated chronologically in a catapult facility using the PIV method. The measurement technique was adapted to the particular requirements of a long-lasting, unsteady “single event”. For example, the full dynamic range of image displacement could be exploited by a dynamic adaptation of light pulse separation. The resulting velocity and vorticity fields provided detailed information on the structure and evolution of the trailing vortices.

REFERENCES [1] Coton P., Monnier J.C., Stuff R., Dieterle L., Schneider G.: Characterisation of the wake far field from high lift configurations of the Airbus A300 by PIV measurements in the ONERA/Lille flight analysis laboratory; AAAF-DGLR Symposium on Large Aircraft Operational Challenge: Wake Vortices – Aerodynamics and Noise Effects, St. Louis (France), January 21-22, 1999 [2] Echols W.H., Young J.A.: Studies of portable air-operated aerosol generators, NLR (Naval Research Laboratory) Report 5929, Washington 1963 [3] Dieterle L., Ehrenfried K., Stuff R., Schneider G., Coton P., Monnier J.C., Lozier J.F.: Quantitative flow field measurements in a catapult facility using particle image velocimetry; Proc. of the 18th Int. Congress on Instrumentation in Aerospace Simulation Facilities (ICIASF), Toulouse, June 14-17, 1999 [4] Raffel M., Willert C., Kompenhans J.: Particle Image Velocimetry - A Practical Guide; Springer Verlag, Berlin 1998 8 of 8