New optical tools for fluid mechanics

Transcription

1 Sddhangt, Vol. 23, Parts 5 & 6, Oct & Dec 1998, pp Indian Academy of Sciences New optical tools for fluid mechanics G E A MEIER DLR-Institute for Fluid Mechanics, Bunsenstrasse 10, D G6ttingen, Germany G.E.A.Meir@dlr.de Abstract. Optical flow measurement is the most important non-intrusive technique in fluid mechanics. A second advantage of some new optical measurement techniques is the high resolution in space and time. As a third advantage, flow visualisation also has a special importance in fluid mechanics. The unsteady three-dimensional flow fields need a visual display of experimental as well as theoretical results. Optical visualisation techniques often lead to completely new insights in flow phenomena especially in experiments. Some discoveries which have been made this way are mentioned here. Keywords. Optical tools; fluid mechanics; laser doppler velocimetry; particle image velocimetry; pressure sensitive paint. 1. Long range laser doppler velocimetry as a tool for flow diagnostics Non-intrusive flow measurement is always a difficult problem, particularly in large wind tunnels, especially for three-dimensional configurations. Therefore, for instance, the flow in the gap regions of slats or flaps of airfoils is a very delicate and difficult measurement problem. Especially for this purpose we have designed a long range Laser Doppler Velocimeter (LDV) (Btitefisch 1989; Seelhorst et al 1995), which has a maximum measurement distance of five metres and is able to measure three components of velocity simultaneously (figure 1). With the help of this instrument we have performed measurements at DNW (Deutsch- Niederl~indische Windkanfile - German-Dutch Wind Tunnels), where the distance from the point of measurement to a wing model was about 5 m. The wake of a slat in operation was measured with respect to all the velocity components. From these velocity measurements the fluctuation velocities can also be deduced. A typical result shows in the upper part, the configuration, a separated flow with the internal vortices and the point of reattachment. An estimate of the velocity fluctuations and the curl, and of the turbulent quantities in the flow over the airfoil calculated from the data show interesting features (figure 2). Another example of an interesting application is in the use of the LDV in an airport near the engine of a transport aircraft. The purpose of this test was to investigate the structure and the velocity field of the jet from a running engine. The velocities in the jet are, contrary 557

2 Point JJ Mono-mode fibre rransm~ng optocs (476,5nm)... ~m.~ (514,5nm),_aset Transmitter ~ox Vlulti-rnode ~bre Receiving- II optics J Photomuff~plier Transmitting optics (488nm) 5m Figure 1. Schematic diagram of a 3D-LDV set-up (DLR).

3 ~J Yigure 2. Y~ W~ ~V' Velocities and fluctuations al the sial of a win~-seclion (DLR). J i i J W~ RJW ~ ~jt~p Iv YSW~ t.a,1 t.a,% P~

4 560 G E A Meier Seeding Measuring Station iii ~ :! i~i~,:i~ Velocity Field downstream of a A320 Engine CF 56 (Three Components LDV-Measurement) ::i,: ~: : ~il i, ',~ Figure 3. Jet flow measurement of an aircraft engine (DLR).

5 New optical tools for fluid mechanics 561 to expectation, asymmetric with a clear preference for high speeds in the upper and lower parts of the jet. Also a certain skewness of the jet flow can be observed. This example shows that the new measurement techniques often result in increased knowledge about the performance of existing devices (figure 3). 2. Flow structures measured by particle image velocimetry Particle Image Velocimetry (PIV) (Raffel & Kompenhans 1994; Raffel et al 1995, 1998) was formally only developed as a tool for simultaneous flow measurement in a light sheet, later also led to a method of flow visualisation in our applications. The principal set-up for PIV illuminates the particles twice with the help of a laser so that by the registration of the two particle images the flow velocity and the flow direction can be determined from recordings on photographic material or CCD-cameras (figure 4). The instantaneous velocity map of the w)rtices downstream of a high lift configuration of an airplane shows not only the velocity vectors in the instantaneous structure of the flow field but also, by the colours of the arrows, the magnitude of the velocity, so that a really clear impression of the complicated wake is achieved (figure 5). In the case of a transonic flow field around an airfoil, one can get by PIV an impression of the shape of tile supersonic flow regime in a real test and also of the local turbulence in the flow field. The time resolution of this measurement technique is extremely high and the sequences of recordings can also provide the time history of unsteady flows (figure 6). Another use of particle image velocimetry can be made in boundary layers where the velocity fields can be measured. The special experimental arrangement is shown in figure 7. Light sheet optics. Mirror Light sheet Flow with tracer particles First light pulse at t 1 Second light pulse at t 2 ~ II Imaging optics Illuminated Flow direction ~ t Image plane Figure 4. Principal set-up of particle image velocimetry (DLR).

6 562 G E A Meier ' ;/,2;_ m/s E >.- -Q ' ' ' 0.g.... i,. i i I i i i L.... I, i i i i I i i I X [m] Figure 5. PIV measurements in the wake of an airplane-model (DLR). Landing configuration at 60 m/s free stream velocity, measurement plane 0.93 m behind wing tip, observation distance: 8.0 m. The light sheet is passing at a distance of less than a millimetre above the flat plate through the particle seeded boundary layer. The different instability modes of the transition can be exactly measured this way (figure 8). In addition, as a surprising advantage, the flow structure can be made visible by a certain unexpected diffusing effect of the tracer particles by the shear of the boundary layer flow, 3. Surface pressure measurement with pressure sensitive paint As a very recent development, the application of Pressure Sensitive Paint (PSP) (Engler & Klein 1997) led to some extraordinary results in our wind tunnel tests with models of re-entry vehicles in the supersonic flow velocity regime. The surface of the model is coated with a number of special layers of paint, one of which contains a fluorescent substance

7 New optical tools for fluid mechanics Figure 6. Velocity field of a transonic airfoil flow measured by PIV (DLR) (NACA0012, tx 5':', Ma = 0.75) ,, i... ='~i~'~ii:~ CCD Camera i I till lit Seeding!,::!~:::: i ~?i~ ~: PC with ~,.Frame Grabber", ~_ 35 mm Camera~-~ ~ Screens i Amplifier... ~,"~ ~-=-~-~,~,. /U= t ~...,~ ~..., ~ t, y 'q ' ~- -, ~ -'~ Slot f PC with Horn drivers I i/ X".Z~ "j i,,signalgenerator"-board "~... / -' /~ _./ // f_ Traversing!.. /X,/ = ~ ~'~, with Hot Wire i ~ Lase.,._t / ~ "-,.- -.,..! Optic /~ /~/'! Particle. Data acquisition!... L-.-._Z--_:-;_~ I Light_she_et... Figure 7. Set-up of a P1V installation for boundary layer measurement (DLR).

8 564 G E A Meier plane parallel to the wall distance to the wall, d: 0.6 mm size: 180 x 120 mm Oblique-Typ tf,rhl d'~rlt U= H -Typ K-Typ Figure 8. PIV measurements in a boundary layer tot various transition scenarios (DLR). Figure 9. Pressure distribution on the upper side of the HYTEX-model with the aosolute pressure curve Pabs along the cursor line (DLR).

9 : :4! New optical too&for fluid mechanics l c o Figure 10. Contour plot of pressure on the HYTEX-rnodel measured by PSP (DLR). with optical activity depending on the partial pressure of oxygen. Hence, after illumination with a special exciting light source the pressure dependent fluorescence can be recorded by a CCD-camera of high sensitivity. From these recordings, pressure distributions on the surface of the model can be achieved by a complicated process of calibration and evaluation. Pressure distribution for a re-entry model at a high angle of attack in false colours is obtained from the fluorescence intensity pattern (figure 9). How detailed this information is can be seen from the extracted pressure values along a line which crosses the model at the marked position. A surprising finding on these maps of pressure was a periodic structure at the rear end of the side wings which indicated special oscillatory behaviour of the streamwise vortices. This periodic oscillation is even more pronounced in another evaluation of this interesting picture in a type of contour plot (figure 10). Here one can see how accurately the method gives details of the pressure distribution and also the cellular structure of the flow in the central part of the model. By certain techniques of pixelwise calibration, the accuracy of the pressure measurement is at present better than an order of 1%. To obtain such an amount of information on surface pressure by a classical approach, the model will have to be equipped with about 100,000 separate pressure transducers. A comparison between classical measurement with pressure transducers and PSP is shown in figure 11. The PSP measurement shows in the upper part of the figure, three-dimensionality of the flow, which is hard to detect by traditional transducer measurement.

10 566 G E A Meier.2 i. i., i 20 single flashes with 25p, s,pulse durat~)n ~n "~ e kulite measurement Ma= 0.80 _,/~" 0.0 ~ kulite measurement Ma= 0.82,1" / ' I ' ' x/c 1.0 Cp-distribution for Ma =0.82, ~ = 0.24, Figure 11. Averaged PSP measurement of a wing section in comparison with transduce results (DLR).

11 4. Conclusions New optical tools for fluid mechanics 567 Besides accurate information about the flow fields, with incomparably high resolution for all possible details, optical flow diagnostics and imaging often gives more information than expected. Because of the richness in details, information on minor or side effects, which are often overlooked with classical integrating or pointwise measuring methods, can be seen easily by an experimenter with moderate experience. Some evidence for this statement has hopefully been provided by the examples given in this article, taken from the work at the Fluid Mechanics Institute in G6ttingen. References BiJtelisch K-A 1989 Three component laser Doppler anemometry in large wind tunnels. Prog;: Aerosl~ac'e Sci. 26: Engler R H, Klein C 1997a First results using the new DLR PSP system - Intensity and lifetime measurements. Proceedings "'Wind Tunnels and Wind Tunnel Test Techniques" (London: R. Aeronaut. Soc.) pp Engler R H, Klein C 1997b DLR PSP System - Intensity and lifetime measurements. Int. Congr on lnstrum, in Aer~sp. Shmdation Facilities (I 7.1CIASF '97 Re 'otzl) (Monterey. CA) (IEEE Publication 97CH36121 ) pp Raffel M, Kompcnhans J 1994 Theoretical and experimental aspects of image shifting by means of a rotating mirror system for particle image vclocimetry. Measurenwnt, Science and Technology 6: Raffel M, Gharib M, Ronneberger O, Kompenhans J 1995 Feasibility study of three-dimensional PIV by correlating images ot" particles within parallel light sheet planes. Exit. Fluids 19:69-77 Raffel M, Willert C, Kompenhans J 1998 Particle image velocimetit: A practical guide. In series: ~-perimentalfluid mechanics (eds) R J Adrian et al (Berlin: Springer-Verlag) Seelhorst U, Btitelisch K-A, Weiland M Component LDV for near wall measurements on a multi-element high lilt airfoil configuration. Int. Congr. on lnstrum, in Aerosp. Simulation ~i~cilities ( 15. ICIASF '95 Record) (Wright-Patterson Air Force Base, OH) pp (IEEE Publication 95CH3482-7