Experimental Observations on Impact Velocity and Entrapped Air for Standing Wave Impacts on Vertical Hydraulic Structures with Overhangs

Abstract

This study focusses on increasing the understanding on vertical hydraulic structures with relatively short overhangs subjected to standing wave impacts. To this end, the impact velocity and the entrapped air are studied in detail, given their influence on the impulsive loading characteristics and consequently on the structural dynamic response. This study is based on regular wave laboratory experimental data obtained for relatively short overhangs with respect to the wave length and with respect to the overhang height. The laboratory tests illustrate the complex wave hydrodynamics before the wave impacts, influenced by the incident wave conditions and structural characteristics. Regarding the impact velocity, the experimental measurements with a wall wave gauge in the tests without overhangs show that the maximum upward velocities deviate from linear wave theory between +5.5% and +13.0%, while the zero-crossing upward velocities deviate from linear wave theory between +1.9% and +7.0%. The zero-crossing upward velocities estimated from third order wave theory deviate from the linear wave theory between +1.8% and +4.7%. In the tests with overhangs, the maximum upward velocity below the overhang estimated by camera recording measurements deviates from linear wave theory between −11.8% and +13.4%. It was also found that when considering the experimental impact velocity from camera recordings in the tests with overhangs, the mean effective bounce-back factor β deviates relatively little from when linear wave theory is used (≈1%), while the uncertainty described by the standard deviation increases significantly (≈35%). Regarding the entrapped air, it is shown that the interaction between incident wave parameters and structural configurations leads to a large variation in the entrapped air area, up to a factor of 5.7 for shorter overhangs and a factor of 9.5 for longer overhangs. This variability in entrapped air characteristics leads to significant effects on the loading on the structure, as observed by the variability on pressure measurements. The experimental results showed increasing impact durations and increasing effective bounce-back factor β in the tests with increasing entrapped air dimensions. This study highlights the importance of the details of the impact velocity and entrapped air for load estimations during the design of vertical hydraulic structures exposed to standing wave impacts. This is particularly important for thin structures such as steel gates which are susceptible to a dynamic behaviour under such impulsive loads.

Standing wave impacts on vertical hydraulic structures with overhangs for varying wave fields and configurations

“…In addition, a reflection coefficient of cr = 1 is used in this method, as the incident wave is not influenced by the presence of the overhang during the period T /2 prior to the wave impact. The results presented in de Almeida and Hofland (2020b) showed that the wave surface position/velocity estimated with this method is in agreement with the laboratory measurements. A reflection coefficient of cr = 1 leads to a total wave amplitude at the wall equal to the incident wave height (A w = H).…”

“…This can be observed in the results of impact duration (see Table A2 for details) and also visually in Figures 5 and 6. As highlighted in de Almeida and Hofland (2020b), this is explained by the fact that a longer overhang is directly related to larger air entrapments and thus directly related to longer impact durations (Mitsuyasu, 1966). In addition, it is also observed that lower water levels are strongly related to shorter impact durations in tests with shorter overhangs.…”

Experimental Study on the Probability of Different Wave Impact Types on a Vertical Wall with Horizontal Slab by Separation of Quasi-static Wave Impacts

“…As shown in Section 3.4, the presence of the vertical wall could significantly enhance the wave impacts on the slab, which was due to the interactions between the wave-induced flow accelerations and the entrapped air within the relative confined space formed from the slab and vertical wall. However, different from the impulsive wave impact processes and the peak impulsive pressure values that were greatly affected by the entrapped air [46], the quasi-static processes seemed to be not significantly sensitive to the entrapped air [33], and thus the selected cut-off frequency representing the upper limitation of low-pass filtering from the horizontal plate tests could be reasonably adopted in the vertical wall with overhangs, particularly under the similar incident wave conditions. In addition, the cut-off frequency of 7 Hz in this study was used for measured wave pressure low-pass filtering, and was thus of course at the model scale (about 1/10 of the wave period).…”

“…For such hybrid structures, the roles of wave conditions and structural geometry on wave loads have been studied to a lesser extent. Recently, De Almeida and Hofland investigated how the roles that variable wave fields and entrapped air play in controlling the standing wave impacts on vertical hydraulic structures with overhangs [32][33][34]. Based on physical model tests, Huang and Chen [35,36] proposed that the maximum wave impact on the horizontal cantilever slab occurred when the structural clearance is approximately 0.2 times the incoming significant wave height, and further presented the relationships between the wave impact rising time, the wave impact frequency, and the wave load.…”

Pulsed LED line light for large-scale PIV—development and use in wave load measurements

“…where the horizontal overhang is located), discrepancies between the two methods are stronger. A main reason for these discrepancies is the presence of air pockets below the overhang during wave impacts, which was studied in more detail by [31].…”