Modelling waving crops in a wind tunnel

Analysis of movie films of a field of barley, combined with observations of the motions of individual plants, show that single stalks oscillate at a well-defined natural frequency even when stimulated by turbulent winds. Treating single stalks as resonant cantilevers allows the use of standard engineering methods to determine their elastic properties. Armed with these values, the application of similarity analysis to the equation of motion of a single stalk leads to criteria for aeroelastic modelling of wheat plants in the wind tunnel. A representative value for the spacing of stalks in a small section of model wheat field was calculated by referring to published data on momentum absorption in a variety of real and model canopies. Preliminary measurements of first and second moments of velocity in the model appear to confirm the importance of including elastic properties in wind-tunnel simulations of airflow in flexible crops.     

Reactivity and point defects of double oxides with emphasis on simple silicates

Heterogeneous solid-state reactions in quasibinary oxide systems are analyzed. As long as local thermodynamic equilibrium prevails, diffusion processes through the reaction product are rate-controlling. The diffusion coefficients are governed by point-defect concentrations. Point-defect thermodynamics allow calculation of relative point-defect concentrations as a function of the relevant thermodynamic variables, if the disorder type of the crystalline product is known. Disorder types in ternary ionic crystals are introduced. On this basis, several reactions leading to simple silicates of the form A ₂ BO ₄ are discussed in terms of ion mobilities and Gibbs energies of formation, and the possible reaction mechanisms are analyzed. Finally some remarks are included on the influence of the gas atmosphere on the reaction rate, on powder reactions and on phase boundary-controlled reaction rates.     

Magmatische Würfelquarze in Rhyolithen (Quarzkeratophyren) des Rheinischen Schiefergebirges

Zusammenfassung Nach einer kurzen Erläuterung der Erkennungsmöglichkeit einfacher Kristallformen zur Unterscheidung von Hoch-und Tiefquarz wird das Auftreten von eindeutigen Würfelquarzen (Rhomboeder), und damit von Tiefquarzen, in Rhyolithen (Quarzkeratophyren) des südlichen Rheinischen Schiefergebirges (Bundesrepublik Deutschland) beschrieben. Diese Quarzrhomboeder-führenden Vulkanite lassen sich petrographisch wie auch chemisch von den übrigen Rhyolithtypen jenes Gebietes unterscheiden. Eine eindeutige genetische Aussage zur Bildung dieser Tiefquarze in einem Vulkanit ist nicht zu machen, es werden dazu zwei alternative Modelle vorgestellt.

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Pseudo-cubic quartz crystals of magmatic origin in rhyolites (quartz keratophyres) from the Rheinisches schiefergebirge

Summary Recognition of simple crystal forms for the distinction between high-and low-quartz is briefly outlined. Pseudo-cubic crystals of low-quartz are described from rhyolitic rocks (quartz keratophyres) of the southern Rheinisch Mountains (Federal Republic of Germany). These volcanics, carrying idiomorphic quartz rhombohedrons, can be distinguished by petrographic as well as by chemical means from the other rhyolites in that area. However, there is no decisive genetic solution to the origin of low-quartz in those volcanic rocks. Two alternative models are proposed.

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Mit 6 Abbildungen