Desert varnish of pristine sandstone and petroglyph surfaces from Takabart Kabort (Naturalistic Bubaline Art School) and Alamas (Tanzina Art School) can be well classified by their (SiO2+Al2O3):MnO2, Al2O3:SiO2, and P2O5:CaO ratios. Specific ratios are due to the occurrence of clay minerals like illite, kaolinite, smectite, and feldspar, quartz, carbonates like calcite and dolomite, manganese oxyhydroxides, and apatite. Their occurrence corresponds to the local origin and composition of the primary aeolian material.
In general, the analyzed desert varnish shows lamination patterns characterized by alternating MnO2-rich and -poor layers (25 wt% MnO2) at rather constant iron oxyhydroxide content (6 wt% Fe2O3). Varnish on non-engraved surfaces exhibits three MnO2-rich layers, whereas varnish-coated petroglyphs reveal minor lamination patterns corresponding to the dating of petroglyphs by rock art. The older Naturalistic Bubaline Art School petroglyphs (about 6–4 ka BP) and the younger Tazina Art School petroglyphs (about 3.8–3 ka BP) contain only two and one MnO2-rich layer, respectively. It is assumed that the occurrence of such microlaminations is caused by climate changes in North Africa. Three humid periods are discerned from the Terminal Pleistocene to Holocene in the literature. Such periods are suitable to induce manganese accumulation by biotic and abiotic processes. Accordingly, the distinct lamination patterns gained from this study verify the dating of petroglyphs by rock art. From another point of view, classification of the above petroglyphs may be provided by analyses of microlaminations independently on cultural historical aspects. 相似文献
This report focuses on active experiments in space as well as laboratory experiments which are directly related to space phenomena. The active experiments involve particle releases in the CRESS and AMPTE missions. Unexpected results such as the motion of the plasma triggered several interesting computer simulations and a laboratory experiment which will be reviewed. Critical Ionization Phenomena will be discussed in this context. A recent release over Arecibo was used to create a plasma lens to focus radiation from the HF heater beam. The higher power triggered several nonlinear processes. Advances in plasma sources and diagnostics have made it possible to scale many laboratory experiments. Discussed here are experiments on Alfvén waves generated by a localized source, 3D magnetic field line reconnection, three dimensional magnetized double layers and simulations of a tethered satellite. 相似文献
A numerical investigation of the bottom pressure and wave elevation generated by a planing hull in finite-depth water is presented. While the existing literature addresses the free-surface deformation and pressure field at the seafloor independently, this work proposes a direct comparison between the two hydrodynamic quantities. The dependence of the pressure disturbances at the ocean floor from the waves generated at the free-surface by a planing hull is studied for several values of both the depth and hull Froude numbers. The methodology employed is Smoothed Particle Hydrodynamics (SPH), a numerical technique based on the discretization of the continuum fields of hydrodynamics through mesh-less particles. The SPH code herein chosen is initially validated against experimental data for transom-stern flow. Subsequently, numerical simulations are presented for a planing hull in high-speed regimes. The results show a direct correlation between surface wave dynamics and hydrodynamic pressure disturbances at the seafloor as the value of the Froude number is varied. This is assessed by studying the inverse dependence of the low-pressure wake angle with the Froude number and by comparison of SPH results with similar works in the cited literature. 相似文献
The object of this study was to investigate the wave resistance component for high-speed catamarans. Two methods were applied: the slender-body theory proposed by Michell [Philos. Mag. 45(5) (1898) 106] and a 3D method used by Shipflow™ (FLOWTECH, Shipflow™ 2.4, User Manual, 1988) software.Results were obtained for different types of twin hulls and attention was given to the effects of catamaran hull spacing.The study also included the effects of shallow water on the wave resistance component.Special attention was given to the height of waves generated by the craft to ascertain effects on river banks. 相似文献