Back-flow ripples in troughs downstream of unit bars: Formation, preservation and value for interpreting flow conditions

Back-flow ripples are bedforms created within the lee-side eddy of a larger bedform with migration directions opposed or oblique to that of the host bedform. In the flume experiments described in this article, back-flow ripples formed in the trough downstream of a unit bar and changed with mean flow velocity; varying from small incipient back-flow ripples at low velocities, to well-formed back-flow ripples with greater velocity, to rapidly migrating transient back-flow ripples formed at the greatest velocities tested. In these experiments back-flow ripples formed at much lower mean back-flow velocities than predicted from previously published descriptions. This lower threshold mean back-flow velocity is attributed to the pattern of velocity variation within the lee-side eddy of the host bedform. The back-flow velocity variations are attributed to vortex shedding from the separation zone, wake flapping and increases in the size of, and turbulent intensity within, the flow separation eddy controlled by the passage of superimposed bedforms approaching the crest of the bar. Short duration high velocity packets, whatever their cause, may form back-flow ripples if they exceed the minimum bed shear stress for ripple generation for long enough or, if much faster, may wash them out. Variation in back-flow ripple cross-lamination has been observed in the rock record and, by comparison with flume observations, the preserved back-flow ripple morphology may be useful for interpreting formative flow and sediment transport dynamics

Design of a high-power density Ljungstrom turbine using potassium as a working fluid

The ability to generate large quantities of high-quality power in space will be necessary to meet the needs of many proposed future space programs. The Pacific Northwest Laboratory is studying an advanced multi-megawatt space power system employing a liquid metal Rankine power cycle. This paper examines more closely one component of the system, the power turbine. The turbine design selected for this system is a counter-rotating radial-outflow machine developed in the early twentieth century by two brothers, Fredrik and Birger Ljungstroem turbine was selected because it provides a compact, high-power-density turbine with balanced rotational inertia and is tolerant of moisture in the working fluid. In commercial operation, Ljungstroem turbines have demonstrated excellent rapid start capabilities and good overall efficiency. Moreover, the disadvantages that have hindered its use in conventional power plants are tied to the steam's very large change in specific volume. These disadvantages are circumvented in a machine using potassium for a working fluid. A preliminary design study indicates that high-power turbines, using potassium as a working fluid, are feasible for the Ljungstroem turbine, and that Ljungstroem turbines of 200 MW and greater could easily fit into the cargo bay of the space shuttle. 10 refs., 5 figs. 3 tabs

Eating and drinking habits of young London-based Irish men: a qualitative study.

This qualitative study is based on interviews with young Irish men living in London about their diets and their views on healthy eating. The data were analysed using combined thematic and discourse analysis. Interviewees gave various reasons for not adopting healthy eating habits, including the cost of healthy foods, their lack of time and ability to cook, and their prioritisation of drinking. Views about the status of different foods also affected their eating habits: they considered red meat, for instance, as ‘masculine’, while lighter foods associated with healthy diets were considered ‘feminine’