Analysis of minimal and maximal pressures, uncertainty and spectral density of fluctuating pressures beneath classical hydraulic jumps

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Abstract
Knowledge of extreme pressures and fluctuations within the stilling basins is of the utmost importance, which may cause potential severe damages. It is complicated to measure the fluctuating pressures of hydraulic jumps in the real-scale structures. Therefore, little information is available on pressure fluctuations in the literature. In this paper, minimal and maximal pressures are analyzed on the flat bed of a stilling basin downstream of an Ogee spillway. Attention has been focused on dimensionless pressures related to the low and high cumulative probabilities of occurrence (P*0.1 and P*99.9%), respectively. The results are presented based on laboratory-scale experiments. These parameters for the relatively high Froude numbers have not been investigated. The total standard uncertainty for the dimensionless mean pressures (P*m) is obtained around 1.87%. Spectral density analysis showed that the dominant frequency in the classical hydraulic jumps is about 4 HZ. Low-frequency of pressure fluctuations indicates the existence of large-scale vortices. In the zone near the spillway toe, P*0.1% reached to negative values around – 0.3. The maximum values of pressure coefficients, namely |CP0.1%|max and CP99.9%max, were achieved around 0.19 and 0.24, respectively. New original expressions were proposed for P*0.1 and P*99.9%, which are useful for estimating extreme pressures.

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