Author(s): Yuki Kajikawa; Takashi Wada; Masamitsu Kuroiwa; Hiroshi Miwa

Linked Author(s): Yuki Kajikawa, Takashi Wada, Hiroshi Miwa

Keywords: Cylinder; Local scour; Supercritical flow; Hydraulic experiment; Numerical simulation

Abstract: Numerous studies on local scour around a cylinder installed in a uniform flow have been carried out. However, most of them are for the phenomenon under subcritical flow condition, and there are very few studies on the local scour under supercritical flow condition. Therefore, the purpose of this study is to clarify the characteristics of local scour around a cylinder in the supercritical flow regime. In this study, hydraulic experiments were conducted with different cylinder diameters and the channel slopes, and numerical simulations for the experiments were carried out using a two-dimensional (2D) shallow water flow model with modification to the calculation of the tractive force around the cylinder. First, hydraulic experiments were conducted using a 0.4 m wide, 7.0 m long, and 0.4 m deep straight channel with a flat bed of gravel with an averaged grain size of 5.21 mm. A single cylinder was installed in the center of the channel. The flow discharge was set at a constant rate of 0.0165 m3/s. The experiments were conducted for each combination of two cylinder diameters (0.06 m and 0.09 m) and three channel slopes (1/50, 1/75, and 1/100). The Froude number ranged from 0.95 to 1.30 approximately. The results showed that the larger the Froude number, the faster the maximum scour depth reached dynamic equilibrium stage, and the smaller the maximum scour depth and scour area around the cylinder. In the downstream area of the cylinder, the larger the Froude number, the larger the gravel deposition area. In addition, scouring was observed on the back of the cylinder to the same extent as on the front. This phenomenon has been rarely observed under subcritical flow condition. Next, numerical simulations were carried out for the experiments using a 2D shallow water flow model. In the model, only bed load with uniform particle size was considered, and the critical tractive force was corrected for the steep gradient of the channel. In order to take into account the increase in tractive force due to the formation of horseshoe vortices in front of the cylinder, a correction using the pressure gradient in the stream-wise direction of the depth-averaged flow velocity was added to the calculation of the tractive force. Consequently, the 2D shallow water flow model was able to reproduce the scouring situation in front of the cylinder, although not as well as the experimental results. In addition, as with the experimental results, the model was able to reproduce the phenomenon that the larger the Froude number, the faster the time required for the maximum scour depth to reach the dynamic equilibrium stage.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022862

Year: 2022