Based on the two-dimensional hydrodynamic model of the finite volume method and structured multigrid, the flow characteristics around a square cylinder with boundary constraint are analysed. The gap ratio G/D (G is the distance from the cylinder to the channel boundary, and D is the side length of the square cylinder) does not change the four flow patterns. Under the laminar vortex street phase, the boundary constraint only reduces the scale of the vortices. The vortex centres are pressed toward the boundaries of the channel, and a low velocity zone is formed near the boundary, but the law of vorticity attenuation along the flow direction is not changed. The flow pattern classification map shows that the boundary constraint increases the Reynolds number required to generate the turbulence flow pattern, and the range of the Reynolds number in the flow pattern of the laminar vortex street has a maximum increase range. The correlation between the time-averaged drag coefficient or the vortex shedding frequency and Reynolds number under different gap ratios indicates that the resistance of the square cylinder and the vortex shedding frequency increase accordingly with the strengthening of the boundary constraint. When G/D < 3.5, the increase is particularly obvious. Meanwhile, the correlation characteristics between the resistance or the vortex shedding frequency of the square cylinder and the Reynolds number are unrelated to the boundary constraint strength.

Semi-enclosed bays subject to intensive thermal discharges present significant challenges in quantifying cumulative thermal effects and assessing thermal environmental carrying capacity. Taking Daya Bay as the study site, this paper develops a two-dimensional hydrodynamic–thermal coupling model using MIKE 21 FM to simulate the spatially integrated temperature rise field induced by four thermal discharge sources under a worst-case hydro-meteorological scenario—characterized by neap tide conditions and a low wind speed of 1.5 m/s. Model performance is rigorously validated against in situ monitoring data and Landsat-derived sea surface temperature observations. Building upon this validated simulation framework, the thermal environmental carrying capacity is quantified via the contour-constraint method, and the spatial compatibility between the simulated temperature-rise contours and designated marine functional zones is systematically evaluated. Results show the following: (1) Intake recirculation significantly reduces the effective thermal load discharged into the bay, and the 1 °C temperature-rise envelope is reduced by 49.2% after intake flow correction. (2) Under the benchmark parameter, the bay-scale thermal environmental capacity utilization rate is 90.7% with approximately 2000 MW residual capacity, and the comprehensive uncertainty range is ±13% to ±17% considering parameter sensitivity. (3) The 1 °C temperature-rise contour overlaps with the core protected area by about 8.7 km², showing potential spatial conflict with ecological zoning. This study offers technical support for optimizing the spatial layout of thermal discharges and implementing total load control in semi-enclosed bays.

To clarify airflow allocation and its driving mechanisms in a complex ventilation system during multi-tunnel flood discharge, a prototype-scale three-dimensional air–water two-phase flow model was developed for the three spillway tunnels and the complete ventilation system of the Baihetan Hydropower Station. The model included the upstream reservoir, three spillway tunnels, multi-level ventilation passages, and downstream channel, and was evaluated against prototype air-discharge measurements and outlet surface-velocity observations. At the normal pool level with all three spillway tunnels fully open, the Y-shaped ventilation tunnels, upper-section ventilation adit, and steep-slope-section ventilation tunnels accounted for 17.1%, 37.0%, and 45.9% of the total air inflow, respectively. Although individual ventilation components showed different inter-tunnel allocation patterns, their combined effects produced similar total air discharges among the three tunnels. Flow separation and vortices in the Y-shaped ventilation tunnels reduced the effective flow area, producing local peak air velocities approximately twice the cross-sectional mean. In the upper-section ventilation adit, axial momentum, branch-turning resistance, and the terminal boundary jointly produced sequential flow diversion that increased from Spillway Tunnel No. 3 to No. 1. Air discharge through each branch in the steep-slope section was jointly controlled by the effective pressure difference and passage resistance. Compared with external independent air supply, intake from the tunnel-roof airspace reduced aerator air discharge and shortened aeration cavities. Although the results are specific to the Baihetan configuration, they highlight the need to evaluate complex spillway-tunnel ventilation systems using their complete connected geometry rather than isolated ventilation branches.