Newton vs Stokes : competing forces in granular matter

Granular materials can be encountered everywhere and are very important in industry. The first step in understanding these materials is normally to focus upon gravity and mechanical contact forces only. While this is a valid approximation when the particles are large, for smaller particles also the ambient air has a pronounced influence and must be taken into account. In this thesis we focus on particles where the “Stokesian” forces (drag, air pressure) and the “Newtonian” forces (from collisions and gravity) compete for dominance. Two examples that illustrate this in a particularly clear manner are the formation and coarsening of Faraday heaps and the appearance of regular and inverse Chladni patterns on a resonating plate. We study these systems by a hybrid Granular Dynamics – Computational Fluid Dynamics numerical code and compare the results with experiments. The simulations enable us to provide detailed explanations for these phenomena

Interplay of air and sand: Faraday heaping unravelled

We report on numerical simulations of a vibrated granular bed including the effect of the ambient air, generating the famous Faraday heaps known from experiment. A detailed analysis of the forces shows that the heaps are formed and stabilized by the airflow through the bed while the gap between bed and vibrating bottom is growing, confirming the pressure gradient mechanism found experimentally by Thomas and Squires [Phys. Rev. Lett. 81, 574 (1998)], with the addition that the airflow is partly generated by isobars running parallel to the surface of the granular bed. Importantly, the simulations also explain the heaping instability of the initially flat surface and the experimentally observed coarsening of a number of small heaps into a larger one

An experimental study towards the practical application of closed-loop flat-plate pulsating heat pipes

The thermal performance of a flat-plate closed-loop pulsating heat pipe (PHP) is experimentally obtained. The PHP is manufactured by means of CNC-milling and vacuum brazing of a stainless steel 316L bottom plate and lid. Each channel of the PHP has a 2×2 mm2 square cross section. In total 12 channels (6 turns) fit in the 50×200 mm2 effective area of the PHP. During the experimental investigation, the power input is increased from 20W to 100W through a 50×50 mm2 evaporator section, while cooling is performed through a 50×50 mm2 condenser section with the use of a Thermo-Electric Cooler (TEC). The PHP is charged with methanol with 40% filling ratio. The thermal resistance is obtained for different inclination angles. It is observed that the 6-turn device operates well in vertical orientation. It however does not operate horizontally. Moreover, experiments have shown that the operating orientation is between 15-30°. The overall thermal resistance was determined at 0.48 K/W for a 100 W power input in the vertical evaporator-down orientation

Storage and discharge of a granular fluid

Experiments and computational simulations are carried out to study the behavior of a granular column in a silo whose walls are able to vibrate horizontally. The column is brought to a steady fluidized state and it behaves similar to a hydrostatic system. We study the dynamics of the granular discharge through openings at the bottom of the silo in order to search for a Torricelli-like behavior. We show that the flow rate scales with the wall induced shear rate, and at high rates, the granular bed indeed discharges similar to a viscous fluid

Solving the Thermal Challenge in Power-Dense CubeSats with Water Heat Pipes

This paper describes the results of a project researching the application of water heat pipes in CubeSats. Heat pipes are proposed to solve for the increase in CubeSat power density, being one of the main thermal challenges appearing in high-performance missions. Commercial off the shelve water heat pipes have been tested and a proof-of-concept design has been made showing the flexibility of heat pipe integration. Thermal tests reflecting a common hot- and cold case experienced in low-Earth orbit, have been carried out. These tests have proven that the water heat pipe is capable of keeping a single component generating a continuous heat dissipation of 10W, within a reasonable temperature range and successfully start-up from a frozen state before temperature limits are breached. The outcome of this research has shown that water heat pipes can be the thermal solution for high performance CubeSat missions

Scaling behavior of coarsening Faraday heaps

When a layer of sand is vertically shaken, the surface spontaneously breaks up in a landscape of small conical “Faraday heaps,” which merge into larger ones on an ever increasing time scale. We propose a model for the heap dynamics and show analytically that the mean lifetime of the transient state with N heaps scales as N −2 . When there is an abundance of sand, such that the vibrating plate always remains completely covered, this means that the average diameter of the heaps grows as t 1/2 . Otherwise, when the sand is less plentiful and parts of the plate get depleted during the coarsening process, the average diameter of the heaps grows more slowly, namely as t 1/3 . This result compares well with experimental observations

Inversion of Chladni patterns by tuning the vibrational acceleration\ud

Inverse Chladni patterns, i.e., grains collecting at the antinodes of a resonating plate, are traditionally believed to occur only when the particles are small enough to be carried along by the ambient air. We now show—theoretically and numerically—that air currents are not the only mechanism leading to inverse patterns: When the acceleration of the resonating plate does not exceed g, particles will always roll to the antinodes, irrespective of their size, even in the absence of air. We also explain why this effect has hitherto escaped detection in standard Chladni experiment