Association of Molybdenum Ionic Species with Alumina Surface

The adsorption isotherms at 25, 45, and 65 degrees C of molybdenum solutions of concentration ranges between 10(-3) and 3x10(-2) M(Mo) (pH 4-5) on different alumina samples are investigated. The analysis is conducted using a modified Frumkin isotherm which takes a more realistic account of the lateral interaction between adsorbed species and considers that the adsorption takes place on the most basic OH groups on the surface of alumina. The results are discussed in view of the difference in solutions speciation, and the changes in the pH of the remaining supernatant solutions. The solution temperature, PZC of the used aluminas, the configuration of the basic OH groups on their surface, and the pore structure have been shown to intervene effectively. Copyright 2000 Academic Press.     

Persistent Non-Homogeneous Features in Periodic Channel-Flow Simulations

Time-resolved simulations of simple shear flows, such as boundary layers and channel flows, are often used as precursor simulations that provide the inflow-boundary conditions for simulations of turbulent flows in and around more complex geometries. For both the precursor and main simulations, the accuracy of the calculated mean flow relies on the simulations being run for long enough to contain the full spectrum of turbulent processes, resulting in a physically valid statistical representation. The time scale needed to achieve convergence of statistics from fundamental studies of simple shear flows is based on data that is averaged in spatial directions in which the flow geometry is invariant—i.e. directions in which homogeneity is expected to be the limiting case. This paper reports and discusses features that represent significant departures from spatial homogeneity of the flow in such a direction, that persist on this time scale, thereby limiting the spatial uniformity of a simulated turbulent inflow. The persistence and size of the features is quantified. A range of simulations for different combinations of domain dimensions and flow parameters has been performed with two independent codes (DNS and LES) to explore how the persistence and size are controlled. While no definitive physical mechanism has been identified, it is suggested that the features may be related to experimental observations of persistent structures in wall-bounded flows.