25 years of PIV development for application in aeronautical test facilities

Transcription

1 25 years of PIV development for application in aeronautical test facilities Jürgen Kompenhans and team Department Experimental Methods Institute of Aerodynamics and Flow Technology German Aerospace Center (DLR)

2 Contents Symposium Objectives Organization Early PIV development at DLR for application in aeronautical test facilities ( ) J. Kompenhans > 2

3 Objectives of symposium Commemorate the 25th anniversary of the first PIV measurements in a wind tunnel at DLR in Göttingen, performed in cooperation between University of Oldenburg and DLR Provide an overview of the major milestones at the development of the general PIV technique for application in aerodynamics and in Europe Provide an overview of the PIV development in other areas and in other countries, to a certain extent Find out whether experiences made at the development of PIV are of more general interest, e.g. as object of studies of experts in the field of history of technology (Falk Rieß and Andreas Junk, University of Oldenburg) Take this symposium as opportunity to provide an instrument to collect documents of the development of PIV on a broader basis than possible via a symposium and, in particular, such documents of the early development of PIV, which are not easily (electronically) available today J. Kompenhans > 3

4 Wiki-like web portal: The History of Particle Image Velocimetry Information about its use: presentation of H. Frahnert today J. Kompenhans > 4

5 Organization of symposium J. Kompenhans > 5

6 Organization of symposium J. Kompenhans > 6

7 Organization of symposium J. Kompenhans > 7

8 Early PIV development at DLR for application in aeronautical test facilities ( ) First steps PIV equipment PIV results Problems still existing in 1990 Progress today J. Kompenhans > 8

9 First steps: Environment in 1984 Communication & electronics No internet, no , no fax (use increasing) Mainly main frame computers and dedicated electronics for measurement equipment Desktop computers beginning to enter the market, but expensive and < 1 MB memory Optical techniques well advanced For fluid mechanics: Traditional techniques such as schlieren techniques and interferometry Laser light sheet Laser Doppler velocimetry (with dedicated electronics) Strong background in flow visualization in Göttingen (Ludwig Prandtl) J. Kompenhans > 9

10 Separating Flow on a Knife-Edge 20 Hz frame rate aluminum flakes on water Prandtl, Tietjens, Müller IWF, MPIDS J. Kompenhans > 10

11 First steps: Activities in 1984 At DLR Göttingen: New quantitative methods needed for observation of unsteady flows for given applications within the objectives of our institute Such activities strongly supported by the new director of the institute Prof. Hans Hornung Which new method to engage in?: Speckle methods known from measurement of discplacement of basilare membran (acoustics) Use of speckle methods to determine flow velocity in fluids, but doubts whether technique will be transferable to aerodynamics Participation in VKI Lecture Series Digital Image Processing in Fluid Dynamics Lecture by Roland Meynart and Louis Lourenco Laser Speckle Velocimetry in fluid dynamics applications one application: jet flow in air J. Kompenhans > 11

12 First steps: Activities in 1984 DLR Göttingen and University of Oldenburg: Klaus Hinsch, University of Oldenburg, was looking for a wind tunnel to explore the potential of the speckle method to determine the degree of turbulence Oldenburg equipment was moved to Göttingen (incl. 10 J ruby laser) After some problems had been solved, recordings of good quality have been obtained (lecture by Klaus Hinsch) DLR Göttingen: Karl-Joachim Ebeling from the University of Göttingen suggested, that, if coherence is not required, we should buy a Nd:YAG laser instead of a ruby laser: 10 Hz repetition rate instead of 0.02 Hz Visible green light (Nd:YAG: λ = 532 nm) instead of dark red light (ruby: λ = 694 nm) Photographic material (KODAK Technical Pan 2415) of higher sensitivity for green light Feasibility test on same set up with Nd:YAG laser borrowed from JK: Much easier alignment Recordings of tracer particles of same quality as with ruby laser Based on VKI experience and on experience gained at test in our wind tunnel, the decision was made to go for speckle methods The institute director Prof. Hans Hornung provided budget for the laser J. Kompenhans > 12

13 First steps: Nd:YAG laser DLR Göttingen and JK (laser manufacturer): Disadvantage of Nd:YAG laser (single oscillator) max. and min. time separation of two illumination pulses generated out of one oscillator during one illumination of the flash lamp (200 µs) is much less than with ruby laser Difficult to obtain two stable pulses with nearly same energy Not adequate for use in aerodynamics with request to be able to measure in low speed boundary layer flows ( t = 200 µs) as well in high speed transonic flows ( t = 3 µs) JK proposed to build a double oscillator Nd:YAG laser in October 1984 (lecture by R. Kelnberger) JK HY200 Special system was delivered to DLR in summer 1985 DLR PIV system (illumination, recording and evaluation) in operation since 1986 J. Kompenhans > 13

14 PIV system Seeding: Laskin nozzle, oil droplets = 1 µm, as for LDV Illumination: Nd:YAG laser, 2 x 70 mj, λ = 532 nm, f r = 10 Hz, t pulse = 16 ns Recording: 35 mm photo camera, KODAK Technical Pan 2415, max 36 PIV recordings for one measurement, time consuming focussing process Evaluation: Young s fringes method, autocorrelation optical/digital Fourier transformation (due to lack of computer power), up to 80 s per Interrogation Window, even with dedicated image processing computer (Kontron IPS, with 8 bit array processor), 50 x 24 grid points = 26 hours per PIV recording Seeding Focussing Evaluation J. Kompenhans > 14

15 PIV system: Double oscillator Nd:YAG laser Delay electronics HY200 Special (still working today) Power supply Laser head Scheme Optical set-up Hardware J. Kompenhans > 15

16 PIV results: jet flow Jet flow, nozzle dia. = 15 mm, U j = 180 m/s, t = 3 µs, internal seeding No external seeding No ambiguity removal 2D flow field, (u x 0.8 U j ; u y ) 1986 J. Kompenhans > 16

17 PIV results: Grid turbulence Tu HW = 0.04 % Low turbulence wind tunnel Tu HW = 1.1 % PDFs of laminar and turbulent flow - Hot-wire - PIV 2D flow field, 1.3 m downstream of grid, (u U, v) Tu HW = 1.4 % 1987 J. Kompenhans > 17

18 PIV results: Transonic flow with shock High speed suck-down wind tunnel Run time 20 s Homogeneous seeding at high flow velocities Condensation Reflections close to wall Strong velocity gradients 2D flow field, (u U, v) 1989 J. Kompenhans > 18

19 Problems still existing in 1990 Photographic recording: No online check of quality of recording possible Wet film processing: long time needed to assess quality of recording (few hours) and to obtain quantitative results Perspective error: only two components of velocity vector can be measured Evaluation: Need to place both illuminations on the same frame (picture) due to small time delays required in aerodynamics resulted in necessity to use autocorrelation for the evaluation process the problem of ambiguity (which image of a tracer particle is due to first and which is due to second pulse) Powerful and homogeneous seeding for large and high speed wind tunnels J. Kompenhans > 19

20 Progress: today These problems have been solved in the meantime Modern CCD sensors allow to place each illumination on a different frame Use of cross correlation algorithms removes ambiguity Sophisticated peak finders and windows deformation techniques lead to better spatial resolution and increased number of vectors per PIV recording Stereoscopic PIV allows measurement of all three components of the velocity vector After 25 years of development and experiences gained at the most different applications PIV became a standard measurement technique in aerodynamics for application in large industrial test facilities at DLR projects, national and international projects and contracts, in particular to understand unsteady flow phenomena and to provide experimental data for validation of numerical codes at DLR J. Kompenhans > 20

21 The Prandtl Movie subjected to PIV Processing Evaluation of Prandtl s movie with specially adapted PIV software by Chris Willert 2D 2C(t) vorticity Spatial jitter due to movie recording not yet removed J. Kompenhans > 21

22 J. Kompenhans > 22