Author(s): Biruk S. Belay; Xiaoyang Shen; Mario Oertel

Linked Author(s): Mario Oertel

Keywords: Free-surface; Fluctuation; Piano Key Weir; Power-Spectrum; Experimental modelling

Abstract: In recent years, progressive research has been conducted on piano key weirs to explore possible technical aspects that enhance their performance. This special type of non-linear weir is considered to have merits in terms of overall hydraulic performance for head discharge control applications, both in open channel flows and dam-top-integrated projects. With the aim of expanding the current knowledge of non-linear weir hydraulics, this study experimentally investigates the free-surface fluctuation downstream of two different scaled piano key weirs. Laboratory experiments that comprised multi-point flow depth measurements using ultrasonic sensors were performed for multiple discharges. Scenarios without and with the presence of weir models were investigated focusing on the downstream free surface fluctuation in a distance ranging from 15 to 20 times the weir height. Overall, results show that the local water surface downstream of a piano key weir manifests nonnegligible spatially periodic undulations which extend farther downstream and intensify with increasing inflow as well as model size. For purposes such as energy dissipation estimation, where the downstream boundary condition is required, a single point measurement of local water depth may not provide an accurate representation of the downstream boundary condition. In order to obtain the actual mean value of free surface profiles, multi-points measurements over a distance that exceeds half of the free surface undulation wavelength is recommended based upon derived data. In addition, to mitigate the effect of propagating undular surface waves, flow depth data should be collected at locations where the flow has reached a stable condition. Based on experimental data, 15 times weir height was observed to be an adequate length to obtain a reasonably stable free surface for geometrically similar models with weir height up to 300 mm and a flume with free outflow conditions.

DOI: https://doi.org/10.3850/978-90-833476-1-5_iahr40wc-p0365-cd

Year: 2023