Global instability of low-density jets

The global stability of laminar axisymmetric low-density jets is investigated in the low Mach number approximation. The linear modal dynamics is found to be characterised by two features: a stable arc branch of eigenmodes and an isolated eigenmode. Both features are studied in detail, revealing that, whereas the former is highly sensitive to numerical domain size and its existence can be linked to spurious feedback from the outflow boundary, the latter is the physical eigenmode that is responsible for the appearance of self-sustained oscillations in low-density jets observed in experiments at low Mach numbers. In contrast to previous local spatio-temporal stability analyses, the present global analysis permits, for the first time, the determination of the critical conditions for the onset of global instability, as well the frequency of the associated oscillations, without additional hypotheses, yielding predictions in fair agreement with previous experimental observations. It is shown that under the conditions of those experiments, viscosity variation with composition, as well as buoyancy, only have a small effect on the onset of instability

Comparison of wave power extraction by a compact array of small buoys and by a large buoy

International audienceWave energy converters are usually designed to achieve maximum efficiency by impedance matching with the incoming waves. One of the simplest design is a buoy attached to an energy absorber (power-takeoff devise). For maximum efficiency, the buoy must be large enough in order to resonate at the peak frequency of the incident sea, and the extraction rate is the same as the rate of radiation damping. With one or few degrees of freedom the resonance peak(s) is(are) usually band-limited. The latter feature is common to all wave-power absorbers based on impedance matching. Inspired by the FO3 system being developed in Norway by Fred Olsen and ABB Associates, and the Manchester Bobber from UK, the authors examine theoretically power extraction by a compact array of small buoys, that do not resonate but possess many degrees of freedom. It is shown that such systems have certain advantages over a single large buoy. © 2010 © The Institution of Engineering and Technology