arXiv subjects

Jürgen Blum

Publications and source records attributed to Jürgen Blum.

59 records · Page 4Linked to original sources

arXiv2009

The Flow Of Granular Matter Under Reduced-Gravity Conditions

To gain a better understanding of the surfaces of planets and small bodies in the solar system, the flow behavior of granular material for various gravity levels is of utmost interest. We performed a set of reduced-gravity measurements to analyze the flow behavior of granular matter with a quasi-2D hourglass under coarse-vacuum conditions and with a tilting avalanche box. We used the Bremen drop tower and a small centrifuge to achieve residual-gravity levels between 0.01 g and 0.3 g. Both experiments were carried out with basalt and glass grains as well as with two kinds of ordinary sand. For the hourglass experiments, the volume flow through the orifice, the repose and friction angles, and the flow behavior of the particles close to the surface were determined. In the avalanche-box experiment, we measured the duration of the avalanche, the maximum slope angle as well as the width of the avalanche as a function of the gravity level.

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arXiv2009

Laboratory studies of ice-particle collisions in Saturn's dense rings

In this work, we report on microgravity studies of particle ensembles simulating ice-particle collisions in Saturn's dense main rings. We have developed an experimental method to study the energy dissipation in a many-body system consisting of approx. one hundred cm-sized glass spheres. The temporal development of the mean particle velocity, ranging from ~10 cm/s (at the beginning) to ~0.35 cm/s (after 9s of experiment duration), can be explained by a constant coefficient of restitution of 0.64. A comparison to values obtained for pure water-ice bodies shows that future cryogenic ice-collision experiments can achieve collision velocities of ~0.1 cm/s, and thus will very well simulate the conditions in Saturn's main rings.

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arXiv2009

Towards a Dynamical Collision Model of Highly Porous Dust Aggregates

In the recent years we have performed various experiments on the collision dynamics of highly porous dust aggregates and although we now have a comprehensive picture of the micromechanics of those aggregates, the macroscopic understanding is still lacking. We are therefore developing a mechanical model to describe dust aggregate collisions with macroscopic parameters like tensile strength, compressive strength and shear strength. For one well defined dust sample material, the tensile and compressive strength were measured in a static experiment and implemented in a Smoothed Particle Hydrodynamics (SPH) code. A laboratory experiment was designed to compare the laboratory results with the results of the SPH simulation. In this experiment, a mm-sized glass bead is dropped into a cm-sized dust aggregate with the previously measured strength parameters. We determine the deceleration of the glass bead by high-speed imaging and the compression of the dust aggregate by x-ray micro-tomography. The measured penetration depth, stopping time and compaction under the glass bead are utilized to calibrate and test the SPH code. We find that the statically measured compressive strength curve is only applicable if we adjust it to the dynamic situation with a 'softness' parameter. After determining this parameter, the SPH code is capable of reproducing experimental results, which have not been used for the calibration before.

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arXiv2009

The Physics of Protoplanetesimal Dust Agglomerates. III. Compaction in Multiple Collisions

To study the evolution of protoplanetary dust aggregates, we performed experiments with up to 2600 collisions between single, highly-porous dust aggregates and a solid plate. The dust aggregates consisted of spherical SiO$_2$ grains with 1.5$μ$m diameter and had an initial volume filling factor (the volume fraction of material) of $ϕ_0=0.15$. The aggregates were put onto a vibrating baseplate and, thus, performed multiple collisions with the plate at a mean velocity of 0.2 m s$^{-1}$. The dust aggregates were observed by a high-speed camera to measure their size which apparently decreased over time as a measure for their compaction. After 1000 collisions the volume filling factor was increased by a factor of two, while after $\sim2000$ collisions it converged to an equilibrium of $ϕ\approx0.36$. In few experiments the aggregate fragmented, although the collision velocity was well below the canonical fragmentation threshold of $\sim1$ m s$^{-1}$. The compaction of the aggregate has an influence on the surface-to-mass ratio and thereby the dynamic behavior and relative velocities of dust aggregates in the protoplanetary nebula. Moreover, macroscopic material parameters, namely the tensile strength, shear strength, and compressive strength, are altered by the compaction of the aggregates, which has an influence on their further collisional behavior. The occurrence of fragmentation requires a reassessment of the fragmentation threshold velocity.

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arXiv2007

The Physics of Protoplanetesimal Dust Agglomerates II. Low Velocity Collision Properties

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity mm-sized dust aggregates into 2.5 cm-sized high-porosity dust aggregates can be studied. The dust aggregates consisted of micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from ~0.1 m/s to ~3 m/s and impact angles were almost randomly distributed. We also used "molded" target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii. The experiments showed that impacts into the highest-porosity targets almost always led to sticking, whereas for the less porous dust aggregates, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the "molded" target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume and could derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account non-central impacts, impact compression, the influence of the local radius of curvature on the collisional outcome and the possible mass transfer between target and projectile agglomerates in non-sticking collisions.

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