Author(s): Erik Bollaert
Linked Author(s): Erik Bollaert
Keywords: Plunge pool bottom pressures; Spectral density curves; Transient excitation; Transient two-phase flow; Resonance frequencies; Scour in fissured rock
Abstract: Plunge pool bottom pressures, created by high velocity jet impact, are of considerable influence on the scouring process downstream of spillways of dams. In function of the ratio of pool depth to jet impact diameter Y/Dj, core impact or developed jet impact occurs in the pool. Spatial mapping of plunge pool turbulence characteristics has been performed by means of mean and fluctuating dynamic pressure coefficients. It was found that plunging jets are capable to create transient pressure conditions at the bottom of the pool that generate oscillatory and even resonant pressure wave phenomena inside underlying rock fissures. Experimental pressure measurements at high jet velocities in artificially created 1D and 2D rock fissures have been performed. The spectral density curves of the pool bottom pressures indicated non-negligible spectral energy beyond the macroturbulent flow range (> 25 Hz). Pressure records made inside the rock fissures were analysed in the time, frequency and Strouhal domains and revealed the appearance of violent transient flow phenomena, such as oscillations, resonance conditions, and even weak shock waves. These phenomena are characterized by maximum spectral energy at resonance frequencies corresponding to the Helmholtz-resonator model for open or closed-end systems. It was found that these resonance frequencies rapidly grow with jet velocity in case of jet core impact, while developed jet impact is characterized by a much slower increase. The latter is related to the capability of air bubble release during pressure drops and thus considerable increase of the air-water compressibility. Further experiments on differently shaped open and closed fissures will allow numerical simulation of unsteady two-phase flow conditions.
Year: 2001