Magnetism: the Driving Force of Order in CoPt. A First-Principles Study

CoPt or FePt equiatomic alloys order according to the tetragonal L10 structure which favors their strong magnetic anisotropy. Conversely magnetism can influence chemical ordering. We present here {\it ab initio} calculations of the stability of the L10 and L12 structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters

Het droogproces van slib uit het Delftse Hout

Om baggerslib te kunnen hergebruiken is het belangrijk dat bekend is hoe lang het duurt totdat het gedroogd is. Om dit te kunnen testen is van het baggerslib uit het Delftse Hout een monster genomen van zowel fijn als grof slib. De hoofdvraag hiervan was: hoe snel gaat het dit droogproces en hoe hangt dat af van de samenstelling, in het bijzonder de korrelgrootte en het gehalte organische stof? De verwachting was dat het fijne slib een hoger vochtgehalte heeft dan het grove slib en dat slib met organisch materiaal meer water zou kunnen vasthouden dan slib zonder organisch materiaal. Van dit slib zijn het vochtgehalte, de plasticiteitsgrenzen, het gehalte organische stof en de korrelgrootteverdeling bepaald. Verder is van een deel van het fijne materiaal het organische materiaal verwijderd en zijn opnieuw de plasticiteitsgrenzen bepaald. Om de droging van het slib te kunnen bepalen is zeven weken later opnieuw een monstername gedaan, waarbij het vochtgehalte en het gehalte organische stof zijn bepaald. De conclusie hiervan is dat het droogproces inderdaad afhangt van de korrelgrootteverdeling en van de aanwezigheid van organisch materiaal en dat de verwachtingen klopten. Bij het fijne slib is de vochtafname 2,55 mm per dag.Technische AardwetenschappenGeoscience & EngineeringCivil Engineering and Geoscience

A new physics-based method for estimating the excess turbulence downstream of a structure

Erosion and sedimentation are natural processes that occur in natural flows.If the erosion is a threat for a structure, a bed protection is necessary to stop this process. A bed protection usually consists of relatively large stones.For calculating the required dimeter of the stones several formulae are available, of which the formula of Shields is the most well-known. In most of these formulae the required dimeter depends on the maximum flow velocity.Almost all flows in hydraulic engineering are turbulent. This means that the velocities are not constant in time, but fluctuating. Due to this irregular nature, the flows are described in a statistical way with a mean velocity and a standard deviation. This standard deviation is called turbulence. In a uniform flow, the amount of turbulent remains constant, but it increases rapidly just behind a hydraulic structure and decreases gradually further downstream. The situation that will be dealt with in this research is that of the Backward Facing Step. The turbulence is often represented as the amount of turbulent energy.If the relative turbulence is known, the maximum velocity can be calculated and from that follows the required stone diameter. Knowing the standard deviation of the velocities is therefore very important, since having too small stones can destroy the bed protection and too large stones are more expensive and their placement could lead to practical problems.Voortman (2013) came up with a new method to predict the turbulent energy, based on the energy cascade. He assumed that the dissipation rate of the turbulent energy depends on the amount of energy itself. Hoeve (2015) concluded, based on earlier experiments, that the method might work for the increase of turbulence, but that not enough data was available for calibrating the method for predicting the decrease of turbulence further downstream.For this reason, twelve new experiments were done in the Laboratory for Fluid Mechanics of the Delft University of Technology. The experiments consisted of a step and a certain combination of water depth, flow velocity and bed roughness behind the step. In these experiments, the flow velocities were measured at various locations and levels, so that the mean velocity and standard deviations at various locations are known.In combination with the measured water depths, the head levels and turbulent energy could be calculated at each location and from that the change in head level and turbulent energy could be calculated.The measurement data was obtained in both the deceleration zone and behind the reattachment point. After a careful analysis, it turned out that it is possible to describe the dissipation of the turbulence with an exponential function, as suggested by Voortman (2013).With the obtained data it should be possible in the near future to find a better link between the generation and dissipation of turbulent energy as well, leading to the creation of a new turbulence method and resulting in better bed protections.<br/

The use of Elastocoast in breakwater research

In this report will be explained how breakwaters can be made with the use of Elastocoast, a sort of glue. This makes it possible to fix the individual rocks, and allows repetitive tests possible with exactly the same layer properties. We made six samples of breakwater rock layers, made with Elastocoast and stones, which can be placed and tested in the wave flume. For doing tests it is important to know the properties of the breakwater, such as the grain size distribution, the porosity and the permeability. The permeability and porosity tests were performed on smaller parts than the slabs to be used in the model breakwater. After making the little samples we made the large ones, on the same way, for use in the wave flume. For the testing of the permeability we used a construction in which we could let water flow through the samples. This report shows the results of our tests, so these results can be used for further purposes, when other people use these breakwater samples.Hydraulic EngineeringCivil Engineering and Geoscience