Chunling Wang

Ventilation is paramount in sanitary and stormwater sewer systems to mitigate odor problems and avert pressure surges. Existing numerical models have constraints in practical applications in actual sewer systems due to insufficient airflow modeling or suitability only for steady-state conditions. This research endeavors to formulate a mathematical model capable of accurately simulating various operational conditions of sewer systems under the natural ventilation condition. The dynamic water flow is modeled using a shock-capturing MacCormack scheme. The dynamic airflow model amalgamates energy and momentum equations, circumventing laborious pressure iteration computations. This model utilizes friction coefficients at interfaces to enhance the description of the momentum exchange in the airflow and provide a logical explanation for air pressure. A systematic analysis indicates that this model can be easily adapted to include complex boundary conditions, facilitating its use for modeling airflow in real sewer networks. Furthermore, this research uncovers a direct correlation between the air-to-water flow rate ratio and the filling ratio under natural ventilation conditions, and an empirical formula encapsulating this relationship is derived. This finding offers insights for practical engineering applications.

Large-amplitude internal waves (IWs) with strong lateral shear forces can cause destructive effects on marine engineering structures. In this study, a large eddy simulation (LES) method was employed to simulate the generation and propagation of IWs in a three-dimensional numerical wave tank, and the pressure distribution, flow field characteristics, and force behaviors of two tandem cylinders under the coupling effect of the IWs and slope terrain were studied. The influence mechanism of the normalized value of center-to-center spacing (L) and the diameter of the cylinder (D), i.e., (L/D), on the strength of the vortex disturbance between cylinders was studied by comparing the simulation results of two tandem cylinders with those of a single cylinder (SC) to further explore the mechanical response characteristics of the upstream cylinder (P₁) and downstream cylinder (P₂). The simulation results showed that the vortex interaction between cylinders is the critical factor that affects the forces acting on the cylinders. The strength of the vortex disturbance could be distinguished by the dimensionless critical center-to-center spacing between cylinders (L_c/D = 3.0). When L/D ≤ L_c/D, the vortex disturbance was severe, causing P₁ and P₂ to experience significant horizontal positive forces and negative forces, respectively. In the case of L/D > L_c/D, the forces acting on both cylinders gradually returned to those on a single cylinder.

In this study, the evolution of transient pipe flow along the wet bed is numerically investigated. In the investigation, the shear stress transport k-ω model is used and the volume of fluid method is employed to track the surface of air and water. Two key parameters in the flow as upstream head H and initial water depth h in the pipe are examined. It is found that the bottom stress is significantly affected by the two parameters. The upstream head H determines the magnitude of the shear force, and the downstream water depth in the pipe affects the stability of the shear force. The boundary layer separation and flow pattern are the essential causes of shear instability. By analyzing the simulation results, an empirical equation with the average flow velocity is obtained to estimate the overflow capacity of the pipe by just the upstream water level and the depth of the wet bed.

In this study, a three-dimensional internal wave (IW)—cylinder—terrain coupled numerical model is established. Based on the large-eddy simulation (LES) method, the IW mechanical characteristics of the cylinder and the flow field evolution around the cylinder over different types of terrains are explored. The similarities and differences in the mechanical characteristics of the cylinders in the environments with and without terrains are compared. The research results show that, when the IWs propagate over terrain, the waveform structures are prone to continuous changes. The intense reverse alternating flow of the upper and the lower water, bounded by the pycnocline, results in huge IWs forces differences between the case without terrains and the cases with terrains. In the case without terrains, the maximum horizontal resultant force on the cylinder is positive, while the resultant forces are negative in the cases with terrain. Compared with the case without terrain, the shallow-water effect caused by the combined action of the terrain and the IWs enhances the flow field strength, making the lower parts of the cylinder suffer larger horizontal forces in the opposite direction to the IW direction. Moreover, the additional vortices produced by the interaction between the IWs and the terrain causes a more complex flow field around the cylinder and the greater forces on the cylinder.

In this paper, internal solitary waves (ISWs) propagating over shelf topographies are studied based on numerical experiments conducted according to the lock-release method. The wave generation, propagation, and interactions with slope-shelf topography are investigated in a wave tank. The primary features of the propagation of ISWs on the slope are assessed. Shear and advection instabilities are observed in the current simulations in some cases with 1.29 < B_s < 1.75 (B_s is defined as the ratio of layer water depth on the shelf to the ISW amplitude). The induced density flow contributes to the growth of potential energy by dilution and stripping, primarily through its head, which is one of the factors used to enhance the mixing efficiency. In addition, the obtained results are compared with those of previous experiments conducted by other researchers, while considering the differences in local topography. The comparison reveals that local topography is a reason for the experimental differences of some research studies. ISW breaking on a slope is strongly influenced by the initial flow field of the slope, which may lead to differences in the prediction of the breaking point by using boundary layer separation as a criterion. As the incident wave amplitude increases, the location of the breaking point shifts downward and its magnitude gradually decreases.

The basic wind pressure or the reference wind pressure for structural design varies greatly across complex terrain. Since only a few meteorological stations can provide adequate extreme wind speed records, it is very difficult to appropriately determine the basic wind pressure for a specific site without a long history of meteorological records. To solve this problem, a mesoscale CFD model was developed and optimized based on geographic information data for Taizhou and suitable turbulence models were selected for CFD simulation. The mean extreme wind speed and the corresponding direction at five main weather stations with long observation histories in Taizhou were used as the verification conditions to perform the CFD simulation of the extreme wind field. The maximum wind speeds of the rural areas, cities, and streets of Taizhou were obtained from the results of the mesoscale CFD simulations. Then, the 50-year return period reference wind pressures were calculated and could be used for the wind-resistant structural design of buildings for sites without a long history of meteorological records. The reliability of the results was verified by comparing the simulation results with the observation data at five main stations with a long history.

A large-eddy simulation of a three-dimensional numerical wave flume is used to study the forces on two tandem cylinders in a stratified strong shear internal wave (IW) environment. By analyzing the pressure distribution and the flow field around two cylinders compared with that of a single cylinder, the mechanism for the influence of the center-to-center (CTC) spacing (L), which is normalized by the cylinder diameter (D), i.e., (L/D), between the two tandem cylinders on the vortex disturbance intensity is explored, further revealing the mechanical response characteristics of the upstream (P₁) and downstream (P₂) cylinders. The results show that the vortex between two cylinders is the key factor affecting the pressure resistance of the cylinders in the IWs of the depression environment. The vortex disturbance intensity can be distinguished by a normalized critical CTC spacing (L_c/D): when L/D ≤ L_c/D = 2.5, the disturbance is strong, causing P₁ and P₂ to undergo large forces along and in the opposite direction of the waves, respectively. In addition, the vortex disturbance is more severe in the upper layer than in the lower layer. The correlation between the nondimensional force amplitude (C_Fn-max) and L/D and that between C_Fn-max and the nondimensional IW amplitude (η_o/H) is quantified. In the strong disturbance area (L/D ≤ L_c/D), C_Fn-max has an exponential relationship with (L/D)/(η_o/H) for P₁ and is a power function of (L/D)/(η_o/H) for P₂.