Turbulence modeling for Francis turbine water passages simulation

The applications of Computational Fluid Dynamics, CFD, to hydraulic machines life require the ability to handle turbulent flows and to take into account the effects of turbulence on the mean flow. Nowadays, Direct Numerical Simulation, DNS, is still not a good candidate for hydraulic machines simulations due to an expensive computational time consuming. Large Eddy Simulation, LES, even, is of the same category of DNS, could be an alternative whereby only the small scale turbulent fluctuations are modeled and the larger scale fluctuations are computed directly. Nevertheless, the Reynolds-Averaged Navier-Stokes, RANS, model have become the widespread standard base for numerous hydraulic machine design procedures. However, for many applications involving wall-bounded flows and attached boundary layers, various hybrid combinations of LES and RANS are being considered, such as Detached Eddy Simulation, DES, whereby the RANS approximation is kept in the regions where the boundary layers are attached to the solid walls. Furthermore, the accuracy of CFD simulations is highly dependent on the grid quality, in terms of grid uniformity in complex configurations. Moreover any successful structured and unstructured CFD codes have to offer a wide range to the variety of classic RANS model to hybrid complex model. The aim of this study is to compare the behavior of turbulent simulations for both structured and unstructured grids topology with two different CFD codes which used the same Francis turbine. Hence, the study is intended to outline the encountered discrepancy for predicting the wake of turbine blades by using either the standard k-ε model, or the standard k-ε model or the SST shear stress model in a steady CFD simulation. Finally, comparisons are made with experimental data from the EPFL Laboratory for Hydraulic Machines reduced scale model measurements

Extending Erlang for Safe Mobile Code Execution

This paper discusses extensions to the functional language Erlang which provide a secure execution environment for remotely sourced code. This is in contrast to much existing work which has focused on securing procedural languages. Using a language such as Erlang provides a high degree of inherent run-time safety, which means effort can be focused on providing a suitable degree of system safety. We found that the main changes needed were the use of unforgeable (capability) references with access rights to control the use of system resources; the provision of a hierarchy of execution nodes to provide custom views of the resources available and to impose utilisation limits; and support for remote module loading. We then discuss prototype implementations of these changes, used to evaluate their utility and impact on visibility for the users of the language, and mention work in progress using this foundation to specify safety policies by filtering messages to server processes...