a b s t r a c tA suggested approach to low head hydropower at run-of-river dams and tidal estuary sites is to include an intermediate air transmission stage where water pressure is converted into air pressure. Several studies have been carried out to investigate the hydraulic performance of such system called siphonic turbine. However, their overoptimistic assessment of the performance of the system is not applicable to actual low head sites.This paper investigates the performance of a siphonic turbine considered to be installed in Grevelingen Brouwersdam in the Netherlands. Based on available experimental results on airewater ow in downward sloping pipes, a one-dimensional numerical hydrodynamic model is developed to nd optimised combinations of air and water ow rates. The generated power of an air turbine, connected to the siphon, is calculated and the efficiency is determined for several hydraulic and geometrical conditions. A maximum efficiency of 7.2% is observed, which is way below the estimated values in previous studies. The total generated power is about 4 MW for a total water discharge of 4500 m 3 /s and an available head of 1.25 m. This relatively low performance has to be kept in mind when identifying the best available technology for low head sites.

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The transition from the laminar to the turbulent regime in linearly stable shear flows, for example, pipe and plane Couette flows, occurs abruptly with no precursor. The evolution of turbulent spots has been studied to better understand the dynamics of this transition and the onset of turbulence. These studies have mostly focused on pipe flows for which a finite lifetime of spots was proven. The same conclusion was drawn in the only available study performed in a Taylor Couette setup. Here, the spot lifetime is measured in a different size TC setup. It is shown that the lifetime is indeed finite and also very sensitive to boundary conditions, but not much to perturbation mechanisms. A scaling approach is provided which suggests in addition to the Reynolds number, the aspect and radius ratios are influential parameters on the lifetime. It is found that the spot size varies during its lifetime and increases with the Reynolds number that confirms the rise in turbulence proliferation by approaching the transitional point.