16 Dec 2004

Catapult Details - Bremen Drop Tower

Located below the drop tower, a chamber of 11 m depth contains the catapult system. With this recently installed feature ZARM is able to perform parabolic flights within the drop tower and to meet scientists' demands on extending the microgravity time to more than 9 seconds.
The drop capsule is accelerated by a pneumatic piston driven by the pressure difference between the vacuum inside the drop tube and the pressure inside the tanks. The acceleration level is adjusted by means of a servo hydraulic braking system controlling the piston velocity. This catapult system is able to accelerate capsule masses from 300 kg up to 500 kg to a speed of 48 ms-1 within 0.28 seconds.
The experiment capsule is placed directly on the pneumatic piston. The piston must withstand all static and dynamic forces acting during the catapult process.

Funded Project

PRODEX is financing the project of Prof. Vedernikov (ULB, Brussels) to investigate aerosol particle motion in presence of temperature and concentration variations and uses the tower in its original configuration.

The project is focused on the investigation of the basic microscopic mechanisms of interaction between aerosol pollutant particles and hydrometeors (drops and snowflakes), particularly, attraction or repulsion forces arising between the particles in presence of temperature and concentration gradients, or phoretic forces.

Phoretic forces are of central interest in the scavenging of aerosol particles in clouds, when droplets and/or ice crystals grow or evaporate, and below cloud, during the fall of hydrometeors. During processes of evaporation or condensation of droplets or ice crystals temperature and water vapour gradient occur, and this determines movement of aerosol particles.

Professor Vedernikov and his experiment team

Theoretical and experimental analysis of the thermophoresis and diffusiophoresis in the transition region of Knudsen number (most interesting for the atmospheric processes) show high mismatching of the data. All these experiments were performed in the laboratory conditions where gravity highly perturbed these phenomena. As a matter of fact, in normal gravity it is not possible to study the phoretic effects alone, as in the case of thermophoresis particles move due to gravitation force and natural convection, and in the case of diffusiophoresis there is a continuous renewal of the vapor concentration. Only two groups from Japan and the Netherlands performed few experiments in microgravity conditions.

Their results, however, did not give the decisive conclusion most probably due to the low number of the experimental data. In 2000 and 2001 the joint team from the Microgravity Research Center (MRC, Université Libre de Bruxelles, Belgium) and the Institute for Science of the Atmosphere and Climate (ISAC-CNR, Bolgna, Italy) performed a series of experiments in the conditions of reduced gravity of the parabolic flights. In order to overcome the problem of low particle number we used the digital holographic velocimetry, which allowed increasing the observation volume by the decimal order of magnitude as compared to the traditional optical methods. The shortcoming of these experiments was in the presence of the velocity component lateral to the temperature gradient, which normally should be equal to zero and that should be attributed to relatively high microgravity perturbation in the parabolic flights where the residual gravity is about 10^-2g and that is not constant during microgravity period. In these conditions it is completely impossible to make accurate experiments with relatively large particles and droplets (> 10-30 micrometers) as their velocity induced by the residual acceleration is higher than the velocity due to the most of the phoretic forces under investigation.

The later problem is of no importance in the conditions of the drop tower where all the microgravity perturbations are of the order of 10^-6g, being negligible for the phenomena under investigation.

Advantages of making experiments in the drop tower

• High stable quality of microgravity - the main and overwhelming advantage.
  
  Microgravity experiments are separated in time (2-3 drops per day) so that there is time to make preliminary evaluation of the results and make necessary corrections or developments in case of necessity. Contrary to the drop tower, 30 microgravity periods in parabolic flights follow one after another in about 1 minute so that there is no practical opportunity to react.

• Convenient working conditions: extended working area with easy access to electrical power mains, water, and gas lines.

Technical support from the ZARM support company (assistance in assembling, cabling, tests, etc.; capsule control computer programming).

Possibility to use local university services and facilities including machinery works.

Nice working ambience in dealing with all the staff of the ZARM support center.

Main objectives:

• Measurement of the thermophoretic and diffusiophoretic velocities for single non-reacting particles in the region of intermediate Kn number for different temperature conductivities of aerosol particles
• Observation of the aerosol particle motion around the suspended evaporating water drop, which simulate the interaction between the pollutant particles and the hydrometeors
• Tests of the validity of the experimental procedures and configuration (most important being the stabilization of the phoretic motion after transition from 1 to 0 gravity and relaxation of the particles after injection for the chemojet motion)
• Test experimental procedures and instrumentation for the ICAPS project, like digital holographic velocimeter, and aerosol generators

MRC team
Andrei Vedernikov
Frank Dubois
Christophe Minetti
Patric Queeckers
Jean Claude Legros

ISAC-CNR Team
Franco Prodi
Gianni Santachiara
Stefania Travaini
Marcello Tercon