Journal of Hydrodynamics最新文献

In winter, rivers in cold regions often experience flood disasters resulted from ice jams or ice dams. Investigations of the variation of ice jam thickness and water level during an ice jammed period are not only a practical need for ice prevention to avoid disaster and plan water resource, but also essential for the development of any mathematical model for predicting the evolution of ice jam. So far, some equations based on the energy equation have been proposed to describe the relationship between ice jam thickness and water level. However, in the derivation of these equations, the local head loss coefficient at the ice jam head and the riverbed slope factor were neglected. Obviously, those reported equations cannot be used to preciously describe the flow energy equation with ice jams and accurately calculate the ice jam thickness and water level. In the present study, a more comprehensive theoretical model for hydraulic calculation of ice jam thickness has been derived by considering important and essential factors including riverbed slope and local head loss coefficient at the ice jam head. Furthermore, based on the data collected from laboratory experiments of ice jam accumulation, the local head loss coefficient at the ice jam head has been calculated, and the empirical equation for calculating the local head loss coefficient has been established by considering flow Froude number and the ratio of ice discharge to flow discharge. The results of this study not only provide a new reference for calculating ice jam thickness and water level, but also present a theoretical basis for accurate CFD simulation of ice jams.

To investigate the energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole, we utilized a nanosecond resolution photography system based on a Q-switched Nd: YAG laser and conventional industrial camera to carefully observe the transient process of bubble collapse near the rigid wall with a gas-containing hole. We analyzed the generation of collapse microjets and the emission of collapse shock waves. We found that the cavitation bubble near the rigid wall with a gas-containing hole collapsed at different times and space, and produced various types of shock waves. Based on the far field pressure information of the shock waves measured by hydrophone, the energy of the shock waves generated by the bubble collapse near the rigid wall with a gas-containing hole is calculated for the first time. The results show that the ratio of collapse shock wave energy to bubble energy is approximately between 0.7 and 0.8.

Due to vegetation drag and vegetation-generated turbulence, bedload transport in vegetated channels is more complicated than that in nonvegetated channels. It is challenging to obtain accurate predictions of bedload transport in vegetated channels. Previous studies generally used rigid circular cylinders to simulate vegetation, and the impact of plant morphology on bedload transport was typically ignored; these methods deviate from natural scenarios, resulting in prediction errors in transport rates of more than an order of magnitude. This study measured bedload transport rates inside P. australis, A. calamus and T. latifolia canopies and in arrays of rigid cylinders for comparison. The impact of plant morphology on bedload transport in vegetated channels was examined. Inside the canopies of natural morphology, the primary factor driving bedload transport is the near-bed turbulent kinetic energy (TKE), which consists of both bed-generated and vegetation-generated turbulence. A method was proposed to predict the near-bed TKE inside canopies with natural morphology. For the same solid volume fraction of plants, the transport rate inside canopies with a natural morphology is greater than or equal to that within an array of rigid cylinders, depending on the plant shape. This finding indicates that plant morphology has a significant impact on transport rates in vegetated regions and cannot be ignored, which is typical in practice. Four classic bedload transport equations (the Meyer-Peter-Müller, Einstein, Engelund and Dou equations), which are suitable for bare channels (no vegetation), were modified in terms of the near-bed TKE. The predicted near-bed TKE was inserted into these four equations to predict the transport rate in canopies with natural morphology. A comparison of the predictions indicated that the Meyer-Peter-Müller equation had the highest accuracy in predicting the transport rate in vegetated landscapes.

Accurate estimation of the drag forces generated by vegetation stems is crucial for the comprehensive assessment of the impact of aquatic vegetation on hydrodynamic processes in aquatic environments. The coupling relationship between vegetation layer flow velocity and vegetation drag makes precise prediction of submerged vegetation drag forces particularly challenging. The present study utilized published data on submerged vegetation drag force measurements and employed a genetic programming (GP) algorithm, a machine learning technique, to establish the connection between submerged vegetation drag forces and flow and vegetation parameters. When using the bulk velocity, U, as the reference velocity scale to define the drag coefficient, C_d, and stem Reynolds number, the GP runs revealed that the drag coefficient of submerged vegetation is related to submergence ratio (H*), aspect ratio (d*), blockage ratio (ψ*), and vegetation density (λ). The relation between vegetation stem drag forces and flow velocity is implicitly embedded in the definition of C_d. Comparisons with experimental drag force measurements indicate that using the bulk velocity as the reference velocity, as opposed to using the vegetation layer average velocity, U_v, eliminates the need for complex iterative processes to estimate U_v and avoids introducing additional errors associated with U_v estimation. This approach significantly enhances the model’s predictive capabilities and results in a simpler and more user-friendly formula expression.

The objective of this paper is to investigate the turbulent flow structures around the submarine model and evaluate the effect of the yaw angle on the turbulent flow characteristics. The large eddy simulation based on the boundary data immersion method is used to investigate. The computational domain consists of 1.2×10⁸ uniformly distributed Cartesian orthogonal grid nodes to capture the basic flow characteristics around the model. The pressure coefficient, friction coefficient and wake velocity distribution are in good agreement with the experimental data. Three different types of vortex structures were mainly captured around the model, including horseshoe vortex, sail tip vortex and crossflow separation vortex. With the increase of the yaw angle, the asymmetry of the horseshoe vortex and the tip vortex gradually increases, and the vortex strength of the vortex leg on the windward of the horseshoe vortex and the vortex strength of the tip vortex also increase gradually. For the crossflow separation vortex, the flow separation zone gradually expands and migrates downstream with the increase of the yaw angle.

In this paper, the coupled sloshing and motion characteristics of a cylindrical floating production storage offloading (CFPSO) are numerically investigated by means of computational fluid dynamics (CFD) tool. The simulations are performed using an in-house CFD solver naoe-FOAM-SJTU which is developed based on OpenFOAM. The active wave generating-absorbing boundary condition (GABC) is utilized for wave generation and absorption. The stabilized k-omega SST turbulence model are used to avoid excessive eddy viscosity near the free surface. CFPSO with and without partially filled liquid tanks in regular waves with different wave periods are simulated and vertical planar motions such as surge, heave and pitch response amplitude operators (RAOs) are compared. Forces due to liquid sloshing and wave loads are extracted and analyzed. The free surface motions inside liquid tanks in one wave period presented to explain the motion characteristics.

Plunging breaking waves play an important role in the exchange of heat, momentum, and mass between the atmosphere and ocean. In this paper, a series of direct numerical simulations is conducted to investigate the fragmentation process of the ingested main cavity in plunging breaking waves. The two-phase Navier-Stokes equations are solved using the finite-volume method based on adaptive refinement meshes. The free surface is captured using a geometrical volume of fluid method. Both 2-D, 3-D simulations are conducted. Instantaneous flow fields at different stages of wave breaking are presented and quantitative analysis for bubbles is performed. The 2-D instantaneous vorticity field and local velocity field are visualized to discuss the general flow characteristics during the fragmentation process. Then a 2-D parametric study is conducted to investigate the differences in the flow characteristics during the fragmentation process under different wave parameters including initial wave steepness (ε), Bond number (Bo), and Reynolds number (Re). 3-D vortex structures are shown to further investigate the mechanisms behind the differences in the flow characteristics. The bubble size distributions under two different initial wave steepness are also discussed with their relationship to the fragmentation process of the ingested main cavity. This research offers a significant understanding of the distinct procedures and fundamental dynamics involved in wave breaking, enhancing our comprehension of this intricate event.

The wake induced vibration (WIV) of a one- and two-degree-of-freedom (1DOF, 2DOF) downstream wave-cone cylinder (WCC) behind a stationary equal-size upstream wave-cone cylinder in the staggered arrangement is numerically investigated at subcritical Reynolds number of 3 900 by using shear stressed transfer (SST) k - ω turbulence model. The streamwise pitch ratios (P / D_m) vary from 4 to 6 with a fixed incident angle α = 8°. Experimental measurements were also performed for the validation of the present numerical models. It is found that the largest vibration amplitude in crossflow direction occurred at P / D_m = 4, U_r =8 with small difference of streamwise vibration at P / D_m = 4, 6. Different from single wavy-cone cylinder (SWCC), the downstream flexible one of a pair staggered WCCs got larger vibration amplitude during phase switching stage instead of in-phase stage. The upstream wake will suppress the triple frequency of main frequency in the power spectra density (PSD) functions of Cl but stimulate the double one of that. An intriguing vibration mechanism happened in all 2DOF cases where the trajectory of downstream WCC is a significant ellipse rather than a figure of 8. The transformation of phase switching and the variation of the main frequency of drag coefficient (Cd) can be explained by the vortex-shedding modes of downstream WCC