Publication
The dissipation of energy of flood discharges from water releasing structures of dams is often done by plunging jets diffusing in water and impacting on the riverbed downstream. The construction of expensive concrete structures for energy dissipation can be avoided but the assessment of the scour evolution is mandatory for dam safety. The scour growth rate and shape depend on the riverbed geology. The geometry of scour may influence the turbulent flow pattern in the pool, the dynamic loadings acting on the rock interface, and the pressures propagating inside rock joints. Up to present, dynamic impact pressures at the pool bottom have been investigated mainly in pools with flat bottom and are therefore described as function of the pool depth and the characteristics of the jets only. To approach the conditions found in practice, non-flat plunge pools and turbulent two-phase jets are investigated in this research work. This fundamental investigation focuses on the interaction between the development of plunging jets in the water and the geometry of the plunge pool. The influence of laterally confining jet diffusion is investigated by means of experimental work in near prototype conditions, in terms of jet velocities and air entrainment in the pool. Different pool geometries typical of prototype conditions are tested and compared with a reference pool with flat bottom. Pressure measurements at the jet outlet, at the pool bottom and inside a closed-end fissure are presented. The main emphasis of the text is on the analysis and the description of physical processes. The integration of the findings in existing scour estimation models is discussed. The thickening of the water cushion downstream, artificially or by scour, is investigated for fully controlled jet issuance conditions. The dissipation of jet energy is estimated based on measurements of mean impact pressures and is compared with results from an analytical model. The developed model features jet diffusion in limited-depth pools and is tentatively applied to turbulent two-phase jets. Agreement is good in the early stages of scour and in deep flat pools. For pool depths about the jet core development length (i.e. transitional pools), analytical estimates are quite sensitive to the initial assumptions on the centreline velocity decay, dimensions of the impinging zone and pool aeration. The findings highlight the limitations of existing empirical laws in representing the diffusion of turbulent two-phase jets in pools with flat bottom. Turbulent impact pressures are also investigated for increasing pool depths. Based on an evaluation of high-order statistical moments and autocorrelation functions of pressure fluctuations at stagnation, jet development conditions at impact are distinguished in core and developed impact conditions. Core impact conditions are typical of shallow pools and generate negatively asymmetric distributions at stagnation. The end of core development is associated with highly i
Alain Nussbaumer, Heikki Tapani Remes, Abinab Niraula