In scour simulation models, a sand slide step is necessary to treat the bed areas where the slope exceeds the angle of repose. This exceedance is due to the fact that the Exner equation does not take into consideration the angle of repose. Methods for sand slide should conserve mass, be based on physics, and give a unique final bed configuration. However, there is no existing method that satisfies all these requirements. Most previous methods use either a modified bed-load transport rate based on bed slope or purely geometric corrections. This work proposes a new method that solves a slope-limited diffusion equation. The diffusivity is conditioned upon bed slope. The method is based on the physics of sand slide and its diffusive nature. Special numerical schemes, linear gradient scheme (LGS) and Gauss nonlinear gradient scheme (GNGS), were tested for the evaluation of bed elevation gradient, which is used in the conditional function for diffusivity. LGS gives severe bed distortion due to the time lag for sand particles to slide in the diagonal direction of mesh. GNGS greatly alleviates this problem by using an extended stencil. The new sand-slide method was implemented in a 3D scour model and tested with two cases. Results show that the new sand slide method, in conjunction with the GNGS scheme or the combination of GNGS and LGS, produces efficient, physically correct, and mesh-independent results. The simulated scour hole development compares well with the experiment.
Abstract. Step-pool systems are common bedforms in mountain streams and have been utilized in river restoration projects around the world. Step-pool units exhibit highly non-uniform hydraulic characteristics which have been reported to closely interact with the morphological evolution and stability of step-pool features. However, detailed information of the three-dimensional hydraulics for step-pool morphology has been scarce due to the difficulty of measurement. To fill in this knowledge gap, we established a hybrid model based on the technologies of Structure from Motion (SfM) and computational fluid dynamics (CFD). The model used 3D reconstructions of bed surfaces with an artificial step-pool unit built by natural stones at six flow rates as inputs for CFD simulations. The hybrid model succeeded in providing high-resolution visualization of 3D flow structures for the step-pool unit. The results illustrate the segmentation of flow regimes below the step, i.e., the integral jump at the water surface, streaky wake vortexes near the bed, and high-speed jets in between. The highly non-uniform distribution of turbulence energy in the pool has been revealed and two energy dissipaters with comparable capacity are found to co-exist in the pool. Pool scour development under flow increase leads to the expansion of the jump and wake vortexes but this increase stops for the jump at high flows close to the critical condition for step-pool failure. The micro-bedforms as grain clusters developed on the negative slope affect the local hydraulics significantly but this influence is suppressed at pool bottom. The drag forces on the step stones increase with discharge before the highest flow is used while the lift force has a larger magnitude and wider varying range. Our results highlight the feasibility and great potential of the hybrid model approach combining physical and numerical modeling in investigating the complex flow characteristics of step-pool morphology.
Abstract Micro-bubbles have attracted much attention in wastewater treatment due to their strong stability, long residence time in water, high mass transfer efficiency, and high Zeta potential [1-2]. Among the commonly used bubble generation methods, the ultrasonic method, Venturi method, and micro-dispersed method are the most popular methods [3–4]. To improve the efficiency of bubble generation and enhance the degree of bubble fineness, this study proposed a new Venturi tube bubble generator, mainly by adding multiple throat pipes based on the traditional Venturi tube to increase the liquid flow rate at the throat and increase the shearing area, thereby improving the bubble generation rate. This study investigated the fluid flow field distribution characteristics of the two-throat cascaded structure bubble generator based on the CFD method and analyzed the distribution of fluid velocity field, pressure field, trajectory, and turbulent kinetic energy in the bubble generator. The research results show that compared with the traditional Venturi bubble generator, the new generator not only significantly improves the bubble generation efficiency but also achieves better bubble fineness, which will provide more efficient and stable performance in real-world applications.
Large wood (LW) has been widely used in river restoration projects due to its ecological benefit. 3D modeling has rarely been conducted. In this study, a 3D modeling procedure is proposed and a new 3D model is developed. The model incorporates advanced CFD techniques and is easy to apply. An engineered log jam (ELJ) is constructed and an experiment is conducted. The data are used to demonstrate the procedure and validate the CFD model. A comparison of the predicted and measured velocity shows that agreement is good with the Nash-Sutcliffe coefficient about 0.4 and index of agreement about 0.85. The model is further applied to a curved channel with ELJ on the outer bank, demonstrating the use of the 3D model to guide the proper representation of ELJs in a 2D model. The 3D model may be used to reveal complex flow patterns generated by LW for ecological benefit analysis.
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