Abstracts - KTH Mechanics

Investigations of turbulence statistics in the laboratory model of an
atmospheric cloud
P. M. Korczyk a, F. Lusseyran b, T. A. Kowalewski a, Sz. P. Malinowski c
Interaction between small-scale turbulence and cloud particles is a key issue in
warm rain formation mechanism, which is an important meteorological and
climatological challenge 1,2. We investigate this interaction observing motion of cloud
droplets in a glass walled chamber 1m deep, 1m wide and 1.8m high, described
elsewhere 3. Cloudy air containing water droplets enters the main chamber forming the
negatively buoyant, turbulent plume, mixing with unsaturated air in the main chamber
and producing additional turbulence by evaporative cooling of droplets. The plume is
illuminated with a 1.2mm thick sheet of laser light, forming the vertical cross-section
through the central part of the chamber. Images of flow are recorded by a highresolution
CCD cameras placed outside the chamber.
Long sequences of images are collected to evaluate small scale turbulence statistics
of the flow. The spatial flow characteristics are obtained using two- and threecomponent
(Stereo-PIV) setup and double pulsed 35mJ Nd:YAG laser. The setup
permits to record only few image pairs per second. Therefore, for temporal turbulence
statistics long sequences of images (1000 and more) are acquired using high-speed
CCD camera (PCO1200HS) and CW 5W Argon laser for illumination.
Droplets visualized in the chamber differ from typical PIV images with uniformly
distributed tracers. Chaotic dynamics of mixing observed in the experiments makes
images of flow complex and not easy to process. Therefore, standard PIV algorithms
have to be used with care to avoid artefacts in the velocity field. Four different PIV
evaluation methods are applied and tested on our experimental data: ILA OFS PIV
algorithm 4, two in house developed multi-scale dynamic widow and image deformation
based PIV codes, and Optical Flow evaluation method 5. Statistical quantities and
decompositions of vector fields are evaluated to check advantages of different
evaluation methods. It was found that image deformation approach and in some sense
equivalent Optical Flow methodology are the most reliable for our purpose.
The retrieved turbulence statistics exhibit significant anisotropy in accordance with
preliminary numerical findings 6. Three components PIV measurements confirm
enhanced fluctuation amplitude for vertical velocity component. An attempt is made
to correlate droplets distribution statistics and the measured velocity fields.
a IPPT PAN, Department of Mechanics & Physics of Fluids, PL 00-049 Warszawa, Poland.
b LIMSI-CNRS, 91403 Orsay, France.
c Warsaw University, Institute of Geophysics, Warszawa, Poland.
1 Vailancourt and Yau, Bull. Am. Meteorol. Soc. 81, 285 (2000).
2 Shaw, Annu. Rev. Fluid Mech. 35, 183 (2003).
3 Malinowski et al., J. Atmos. Oceanic Technol. 15, 1060 (1998).
4 ILA GmbH Optical Flow Systems, Germany.
5 Quenot et al., Exp Fluids, 25, 177 (1998).
6 Andrejczuk et al., J. Atmos Sci., 61, 1726 (2004).
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