1. F/1 Laboratory studies of the form and oscillations towards breakup of falling raindrops in the Mainz Wind Tunnel Miklós Szakáll, Subir K. Mitra, Karoline Diehl, Stephan Borrmann

2. Objectives Measurement of the time averaged axis ratio of oscillating millimetre size drops with equivalent diameters (Do) above 3 mm Evaluation of the time averaged axis ratio of oscillating drops with equivalent diameters lying between 1.1 mm and 3 mm Experimental determination of the oscillation frequency of all millimetre size drops Comparison of the experimentally obtained dynamic average axis ratios against results of various drop deformation models Evaluation of the dynamic time average form of millimetre size drops after inducing oscillations through collisions

3. Mainz Vertical Wind Tunnel Hauptkammer Netze Strömungsgleichrichter Kontraktionssektion Beobachtungssektion Vakuumpumpe Schalldüse Partikelfilter Heizung/Kühlung Befeuchtung/Trocknung

4. The camera MotionPro X-3 Redlake Inc. 2000 fps @ 1280x512 resolution 12x12 µm pixel size 10-bit CMOS monochrom sensor 4 GB memory (~3 sec) Ethernet interface, analog VideoOut

5. Sample Video I D 0 = 660 µm f = 8; 1:1.4 400 fps

6. Sample Video II D 0 = 6.5 mm f = 8; 1:2 1000 fps

7. Axis ratio

8. Binary evaluated picture Region of interest Raw picture Data evaluation

9. Data sample f=1/Δt b/a

10. Oscillation frequencies Nelson and Gokhale, 1972: J.Geophys.Res. 77, 2724 Feng and Beard, 1991: J. Atmos. Sci. 48, 1856

11. Axis ratios Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

12. Axis ratios – selected drops Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

13. Axis ratios – selected drops Chuang and Beard, 1990: J.Atmos.Sci. 47, 1374

14. Case study I a b a and b are in opposite phase

15. Case study II b a b/a FFT of a, b, b/a

16. Drop width (a)

17. Drop height (b)

18. Oscillation amplitude of drop width

19. Data evaluation interpretation of the data model calculations image processing development Measurements at different turbulence levels, eg. laminar flow smaller drops (1 mm < D 0 < 3 mm) Next steps…