• Analytical and experimental investigations of dam-break flows in triangular channels with wet-bed conditions

      Wang, B.; Liu, X.; Zhang, J.; Guo, Yakun; Chen, Y.; Peng, Y.; Liu, W.; Yang, S.; Zhang, F. (2020-05-19)
      Based on the method of characteristics, an analytical solution for the one-dimensional shallow-water equations is developed to simulate the instantaneous dam-break flows propagating down a triangular wet bed channel in this study. The internal relationships between the hydraulic properties associated with the dam-break flow are investigated through the comparisons with the well-known analytical solutions for rectangular channels. Meanwhile, laboratory experiments are conducted in a prismatic, horizontal and smooth flume with a triangular cross-section. The non-intrusive digital image processing is applied for obtaining water surface profiles and stage hydrographs. Results show that the dam-break flow propagation depends on the dimensionless parameter defined as the ratio of initial tailwater depth over reservoir head. has significant effect on the dam-break wave in the downstream flooded area. For , the water surface profiles in the reservoir for different at a given time remains similar. For ≥ 0.5, extra negative waves occur in the reservoir, leading to the water surface undulations. Undular bores are generated at the dam site and propagate downstream. Time evolution of dam-break flows under three different reservoir heads is similar for the same . The inception of water surface profile change is earlier when the reservoir head is larger. The analytical model shows satisfactory agreement with the experimental results though some errors exist between the analytical solution and measurements due to the formation of extra negative waves, jet and undular bores. The similarities and discrepancies between the hydraulics in the triangular and rectangular channels are identified analytically in terms of the profiles of water depth, velocity, discharge, bore height and wave-front celerity with . The presented solution could be applied to predict the effect of wet bed condition on the dam-break wave in triangular channels, while laboratory measurement data could be used for validating analytical and numerical models.
    • Gate-opening criterion for generating dam-break flow in non-rectangular wet bed channels

      Yang, S.; Wang, B.; Guo, Yakun; Zhang, J.; Chen, Y. (2020-11)
      A sudden dam failure is usually simulated by the rapid removal of a gate in laboratory tests and numerical simulations. The gate-opening time is often determined according to the Lauber and Hager instantaneous collapse criterion (referred to as Lauber-Hager criterion) established for a rectangular open channel with a dry bed. However, this criterion is not suitable for non-rectangular channels or initial wet-bed conditions. In this study, the effect of the gate-opening time on the wave evolution is investigated by using the large eddy simulation (LES) model. The instantaneous dam break, namely the dam break without a gate, is simulated for comparison. A gate-opening criterion for generating dam-break flow in non-rectangular wet bed channel is proposed in this study, which can be used as an extension of the Lauber-Hager criterion and provides a more comprehensive and reasonable estimate of the gate opening time.
    • Investigation on scour protection of submarine piggyback pipeline

      Yang, S.; Shi, B.; Guo, Yakun; Yang, L. (2019-06-15)
      This paper presents the results of laboratory experiments and numerical simulations to investigate the effect of different piggyback pipeline configuration on the morphology of local seabed scour subject to steady currents. Piggyback pipeline configuration investigated includes the commonly used piggyback pipeline, namely a small pipe attached on the top of large pipe and new form of piggyback pipeline proposed in this study in which a small pipe is attached to the large pipe on the upstream and downstream side, respectively. Pressure gradient, drag coefficient, lift coefficient and scour extent around pipelines are measured and analyzed for a range of pipelines and current conditions. Results show that the vortex strength downstream of the commonly used piggyback pipeline is larger than that for a single as well as the new piggyback pipeline under the same condition. This new type piggyback pipeline can effectively reduce the depth and width of the scour hole. In particular, when the ratio of the small pipe diameter over the large pipe diameter is greater than 0.3, little scour under this new type piggyback pipeline occurs for the test conditions. The bed topography downstream of the pipe has also been altered to favor the backfill.
    • Investigation on scour scale of piggyback pipeline under wave conditions

      Yang, S.; Shi, B.; Guo, Yakun (2019-06-15)
      Laboratory experiments are presented to investigate the effect of different piggyback pipeline configurations on the morphology of local scour under wave conditions. Scour depth and width around the pipelines under regular and irregular waves are measured and analyzed for a range of pipeline and wave conditions; such as the spacing between two pipes (G), gap between the main pipe and seabed (e), pipe diameter (D), wave height (H) and period (T). Experimental results reveal that both the scour depth and width around piggyback pipeline is much larger than those around single pipe under the same wave conditions. Scour depth increases with the increase of the Keulegan-Carpenter (KC) number and decreases with increase of G and e. When e exceeds 0.5D, scour depth tends to approach 0.When spacing G is greater than 0.4D, the destabilization from small pipe to large one is greatly reduced, resulting in scour depth around piggyback pipeline being close to that around single pipe. Similar to scour depth, scour width broadens with the increase of KC number increasing and decreases with the increase of G. Experiments also show that the effect of e on scour depth is greater than that of G under the same test conditions, while their impact on scour width is opposite. Furthermore, scour width under irregular waves is extended slightly compared with regular wave for otherwise the identical conditions.
    • Numerical investigation of the influence of the small pipeline on local scour morphology around the piggyback pipeline

      Yang, S.; Guo, Yakun; Shi, B.; Yu, G.; Yang, L.; Zhang, M. (2021-11-15)
      This paper presents the results from a numerical simulation study to investigate the effect of the position angle (α) of small pipeline on the local scour and the hydrodynamic force around the piggyback pipeline in steady current conditions. Results show that the local scour depth around the piggyback pipeline increases first and then decreases with the increase of α. The scour depth and width reach the maximum values as the small pipe locates at the top of the large pipeline (i.e. α = 90°). The scour around the piggyback pipeline is accelerated when α ranges between 30° and 165°, while for α = 0°–30° and 165°–180°, the local scour around the piggyback pipeline is inhibited. Furthermore, the small pipe placed in front of the large pipe has slightly larger effect on the scour hole morphology than that when it is placed behind the large pipe. The drag force coefficient increases first and reaches the maximum value at α = 75°, and then decreases with the increase of α. Eventually the drag force coefficient approaches roughly a constant. The lift force coefficient is approximately a V-shaped with the variation of α and has the maximum value at α = 90°.