Bremen Drop Tower Research Articles

Application of high performance LTS SQUID systems in gravitational experiments

The design of a Galilean-type free-fall experiment to test Einstein's equivalence principle at an improved level of sensitivity is described. Two test bodies of different material fall down from a height of 109 m inside of an evacuated drop tube (Drop Tower Facility Bremen, Germany). Their possible relative displacement is measured using LTS DC SQUID based position sensors. Experiences and results of this experiment meet directly the current STEP project of NASA/ESA to test the Equivalence Principle in space at a level of one part in 10/sup 18/. Several issues of the developed measuring system are discussed, above all, the performance of two types of SQUID position sensors. Furthermore some recent results of free fall tests of components of the measuring system at the Bremen Drop Tower are presented.

Low noise LTS SQUIDs for application in gravitational experiments

The Weak Principle of Equivalence (WEP) is proved up to now with an accuracy of 1/spl times/10/sup -12/. Short range free fall experiments attained an accuracy of 5/spl times/10/sup -10/ only. Microgravity facilities such as the Drop Tower Bremen enables long range free fall experiments which may improve the accuracy of free fall tests to <10/sup -12/ using high performance SQUID measuring technique. In this review the application of LTS DC SQUIDs with Nb-NbO/sub x/-Pb/In/Au window-type Josephson junctions for gravitational experiments is described. Under optimal conditions a noise limited sensitivity of 1.4/spl times/10/sup -12/ m//spl radic/(Hz) for the position detector was achieved corresponding to a flux noise of the SQUID system of 2/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/(Hz). The contribution gives the design of the DC SQUID position detector and summarizes recent results of its application in gravitational experiments.

Testing the weak equivalence principle at the Bremen drop tower: report on recent developments

The weak equivalence principle (WEP) has currently been shown to be valid to an accuracy of . Free-fall experiments over short distances have attained an accuracy of only . Microgravity facilities such as the `Bremen drop tower' enable long-distance free-fall experiments which may improve the accuracy to .

Thermocapillary Bubble Migration at High Reynolds and Marangoni Numbers under Low Gravity

The thermocapillary migration of single bubbles at high Reynolds and Marangoni numbers has been investigated. Experiments on the transient behavior of moving bubbles with Marangoni numbers up to Mg = 2500 were carried out in the Bremen drop tower. After a long heating period, a sufficiently linear temperature gradient was established with paraffin liquids. For 4.74 s of strongly reduced gravity the speed of the bubble migration was observed with a video camera. The temperature field was recorded by Pt100 temperature gauges and by differential interferometry. Numerical calculations of the steady-state flow were performed and were found to be in good correspondence with the experimental data. The calculations for large Marangoni numbers show a slight but important correction of previous results. The specific advantages of using a drop tower for such experiments result in multiple experimental data on transient and quasisteady thermocapillary bubble migration, showing much better statistical reliability than previous microgravity experiments.

Characteriation of spherical hydrocarbon fuel flames: Laser diagnosis of the chemical structure through the oh radical

For the first time, spherical diffusion flames of n -decane, n -heptane, and methanol established around fuel-drenched porous spheres were investigated by applying the two-dimensional laser-induced predissociation fluorescence method to measure the OH radical under microgravity conditions. Focus of this work was laid on the development of a UV-laser diagnostic system attached to the Bremen drop tower for the characterization of combustion under buoyant-free conditions and on its applicability to the diffusion zone of hydrocarbon droplet flames. The experiments carried out on the previously mentioned fuels showed capabilities and limitations of this method applied on liquid fuels. While it is relatively straightforward to investigate the chemical structure of methanol droplet flames because of the absence of Mie scattering soot particles and of nonresonant fluorescing species, it is difficult for larger hydrocarbons such as n -hepatane and n -decane. It is considered that with the development of a luminous zone, strong broadband absorption by fuel vapor and intermediates larger than methane in connection with high laser pulserepetion rates may cause interactions of the input energy with the chemical kinetics of the combustion process.

Measurement of the thermal conductivity of fluids with low viscosity under reduced gravity conditions using the transient hot-wire technique

Under normal gravitational conditions the measurement of the thermal conductivity λ is often strongly influenced by thermal convection. Heat transfer by convection superposes the heat transfer due to thermal conduction. Systems in which the gravity effects are compensated, e.g. under microgravity conditions, offer decisive advantages because natural convection is strongly suppressed. In this paper an apparatus is described enabling accurate measurements in fluids with low viscosity without any gravitational influence. Comparative measurements carried out in the ‘Drop Tower Bremen’ using isopropanol, ethanol and cyclohexane are presented and compared to data available in the literature.