Pressure Fluctuations beneath Submerged Jump

Wall pressure fluctuations are measured beneath an hydraulic jump that is completely submerged. Two efflux velocities, 3.3 m per s and 2.6 m per s, are used; the corresponding Froude numbers are 9.8 and 7.7, respectively. For a distance of 42 times the efflux depth the intensity of the fluctuations is four to seven times that beneath a turbulent boundary layer. It declines only slowly with downstream distance. Inertial scaling brings together the spectra for the velocities at higher frequencies. Evidence is presented for the contribution to the fluctuations due to thee unsteadiness of the mass of water in which the jet spreads. The inviscid instability of the submerged hydraulic jump is investigated. There is agreement between the dominant frequency of pressure fluctuations beneath the submerged hydraulic jump and the frequency of the most unstable disturbances predicted by the theory.