Stepped dropshafts are key structures in urban deep tunnel drainage systems, yet their limits in conveyance capacity remain inadequately quantified. This study identifies an unfavourable hydraulic phenomenon, termed flow impingement, as the governing factor defining their conveyance capacity. Based on theoretical analysis and systematic experiments on seven physical models with varying step geometries, the onset of flow impingement is formulated as a hydraulic constraint governed by a dimensionless flow depth ratio. Results reveal that the flow depth ratio is primarily influenced by geometric parameters, including the relative vertical drop per turn, relative chute width, and relative step height, while showing negligible dependence on the dimensionless discharge. A multivariate regression model was developed to predict the flow depth ratio and was incorporated into the constraint equation. Consequently, a model of practical criterion for estimating the conveyance capacity is established. The proposed model demonstrates good agreement with independent literature data, suggesting its potential applicability in engineering design.

With rising global seafood demand, offshore aquaculture has become a prominent topic of research in recent years. However, high construction costs of offshore aquaculture structures hinder the economic viability for the industry. To address this challenge, a novel concept of fish farming vessel was proposed, considering the lower cost of modification compared to constructing new offshore structures. An aged bulk carrier's side plating was replaced with aquaculture nets, transforming it into a novel type of fish farming vessel with open aquaculture system. A series of wave basin tests were performed on 1:40 scale models of the novel vessel and original bulk carrier, with their results compared and analyzed. White noise waves tests were carried out to determine the response amplitude operators (RAOs) of both vessels, revealing their seakeeping characteristics. Irregular wave tests were subsequently conducted to evaluate the dynamic motion response of the vessels under environmental conditions representative of a potential offshore site near Guangdong Province, China. The results demonstrated that the novel fish farming vessel have superior seakeeping performance compared to the bulk carrier, particularly in roll and heave motions. The replacement of side plating reduces the lateral forces and moments acting on the vessel, thereby decreasing its response induced by lateral loads. The standard deviation of heave motion in irregular waves is reduced by over 30%, while results of roll motion is reduced by more than 50%. This study provides significant benefits to the cost-effective development of offshore aquaculture.

The deep tunnel stormwater system, consisting of dropshafts and underground tunnels, is used to alleviate or prevent urban water problems associated with extreme rainfall events. The stepped dropshaft can transport surface runoff to the tunnels with high energy dissipation, low risk of cavitation and good exhaust performance, which well meets the requirement of the deep tunnel stormwater system. In the present study, the characteristics of the standing wave were investigated by experiments and numerical simulations, including the peak, trough and length of the standing wave. The flow regimes were divided into the nappe flow, the transition flow and the skimming flow with the increase of discharge, in which the standing wave mainly occurs on the external wall under the nappe flow and the transition flow. Influences of inflow discharge and dropshaft geometries were analyzed, including step rotation angle, relative step height and dropshaft curvature. The relations of the characteristics of standing wave with these effect factors were obtained. The maximum discharge capacity of the stepped dropshaft was established by considering that the standing wave just reached the above steps, which could be useful for the design and safe operation of the stepped dropshaft.

This paper presents the results of a hydrodynamic experimental study of the hydraulic characteristics of flow in different vertical slot fishway (VSF) designs with L-shaped, polygonal, and semicircular baffles. A biological experiment was conducted using endemic fish to analyze the efficiency of the upstream passage of different VSF designs. Based on the results, a significant difference in water depth, slot velocity, turbulence kinetic energy, and efficiency of the upstream passage was observed. The L-shaped baffle had better energy dissipation; however, the fishway with a polygonal baffle had better efficiency of the upstream passage. The results indicated that the present VSFs are all fish-friendly for Chinese endemic fish, but VSFs with polygonal baffles may be a better choice.

In this paper, a series of model tests were carried out in wind tunnel flume to test the formation and evolution characteristics of wind waves in small water area and its influencing factors and studied the corresponding law of the wind wave run-ups. The results showed that the characteristics of wind waves formed by different wind speed levels were not the same. Small amplitude fluctuations were always formed in the test water area under the condition of low wind speed. And when the wind speed got higher, the characteristics of the wind wave will change to low frequency and large amplitude wave with the increase of blowing distance. Furthermore, the characteristic parameters of the wind waves will not be significantly affected when the water depth is not too shallow. The wind wave run-ups induced by the vertical component of wind wave increased with the increasing wind speeds. After the full development of wind wave, the water depth would not have a significant impact on the wind wave run-ups. Based on this, the calculation formula of wind wave run-up was put forward. Compared with the experimental data, the accuracy was good.

提出在不改变分岔角角度的同时,通过设置直线倒角,以应对分岔角应力集中问题.通过数值模拟方法,研究了设置倒角的卜形岔管水力特性,具体对比分析了不同结构参数的圆弧形、直线倒角体型岔管汇流的水流流态、泄流能力和压强特性.结果表明,直线倒角体型岔管流场平顺,主流流向与支管轴线的一致性较好,而圆弧形倒角体型岔管的流场顺畅性相对较差;对于相同的倒角半径,直线倒角体型岔管的泄流能力优于圆弧形倒角体型岔管,且倒角半径越大,对泄流能力的影响越大,与无倒角时相比,直线倒角与圆弧形倒角体型岔管泄流量最大降幅分别为0.5％、6.8％;直线倒角体型岔管压强分布特征与无倒角体型岔管较为接近,但圆弧形倒角体型岔管与之有明显区别,存在局部低压区.

集鱼槽的布置及体型直接影响着集诱鱼水流的水力学特性,是整个集诱鱼系统设计的关键.为分析集鱼槽的布置及体型对诱鱼水流的影响,采用RNG κ-ε紊流模型和VOF模型对某水电站集诱鱼系统不同参数进行三维数值模拟,并通过物理模型试验验证数值模拟结果的合理性.结果表明,集鱼槽适合布置在尾水隧洞下游,集鱼槽宽度对诱鱼水流影响较小,而导墙长度和集鱼槽布置角度是影响诱鱼流速的关键性因素,同时集鱼槽布置角度还会对诱鱼口的紊动能场产生一定影响.

As the economic and social development, the sewage amount is becoming larger and larger. Moreover, in recent years, extreme weather events appear frequently, leading to severe urban flooding and then considerable economic damages. The urban deep-tunnel drainage system is an effective solution, which could transfer the stormwater down into the deep tunnels and storm it there, then pumped it into the rivers afterwards with a relatively small discharge. Clearly, it can effectively lower and delay the flood peak. However, during the operation of the urban deep-tunnei drainage system, the air may go into the system, especially the deep tunnels, which would be harmful to the safety of the system. In this paper, we systematically investigated the process of the air bubbles through physical model tests and found out that falling jet and swirling in the baffle drop shafts were the two main ways that could result in the air bubbles entering the deep tunnels. And improving the design of the diving wall in inflow shafts and separating the bends and the shafts in space were both effective ways to decrease the air entering the deep tunnel. In addition, increasing the water depth in the shaft could also largely decrease or even avoid the air bubbles getting into the deep tunnels.

Aiming to achieve energy dissipation and prevention of cavitation erosion, a kind of dropshaft in urban drainage systems called the helical-step dropshaft is introduced in this paper. It dissipates flow energy by means of step geometry and prevents cavitation erosion through air entrainment. To verify its availability, the hydraulic characteristics of the helical-step dropshaft were experimentally investigated, including the flow regimes, the efficiency of energy dissipation, characteristics of air entrainment and pressure distribution. The results demonstrate that, even for a large discharge, flow can be discharged smoothly and steadily, and a high energy-dissipation rate of over 87% is achieved. There are three distinct flow regimes observed in the dropshaft, namely nappe flow, mixed flow and skimming flow. Moreover, there is no less than 1.6% air concentration and a reasonable pressure distribution on the step surface. This study provides an attractive alternative for the design of drop structures.

基于某大型水电站导流隧洞,对出口段负压问题进行了模型试验和数值模拟研究,试验结果表明长距离导流隧洞存在大范围负压.通过对长距离导流隧洞建立了三维数值模型,采用CFD方法对其三维流场进行了仿真计算,数值模拟结果和物理模型试验吻合较好,验证了数值模拟的合理性.据此进一步模拟了不同特征流量下导流隧洞出口段的负压情况,模拟结果表明导流隧洞在各流量工况下负压范围和最大负压值均大于有压泄水隧洞自由泄流情况下的负压范围和最大负压值,最大负压出现位置也存在一定差异.对此模拟了几种典型压坡方式以及出口段轴线偏转对出口段负压的影响,提出了改善导流隧洞大范围负压的合理措施,同时分析了各压坡方式以及出口体型对导流隧洞泄流能力的影响,为类似工程优化设计提供科学依据.

The bitumen-aggregate interface is regarded as a significant weak link in asphalt mixtures. Therefore, the fundamental understanding of adhesion between bitumen and aggregate is crucial for designing and preserving asphalt pavement. By far, the molecular dynamics simulation method has notably attracted interest in the characterization of the bitumen-aggregate system. However, the effort towards a reasonable simulation is still ongoing. This study investigated nine cases of bitumen-aggregate systems using molecular dynamic simulations. Specifically, three bitumen binders and three mineral types were considered, respectively. In addition, mineral surface properties and some simulation details were discussed, aiming to obtain a reasonable simulation. The results indicated that polar molecules in bitumen played a vital role in the adhesion between bitumen and aggregates. The mineral surface properties in terms of mineral cleavage surface, surface atomic charge, and surface hydroxylation significantly influenced the simulation results. Accordingly, some simulation tricks were recommended for molecular dynamic simulation of bitumen-aggregate systems: (1) Geometry optimization for the mineral model is not necessary; (2) A vacuum slab with 0 angstrom should be added on the cleaved surface; (3) Chemical bonds should be deleted after the construction of the bitumen-aggregate system; (4) The Ewald and Atom-based methods are recommended for the energy summation of electrostatic energy and Van der Waals energy, respectively. (C) 2021 Elsevier Ltd. All rights reserved.

A stop-log gate, installed in water intake of hydropower project, has become an effective facility in achieving selective withdrawal and temperature control for the sake of benefiting downstream ecosystems. Hence, it is of great importance to comprehensively explore the water intake hydraulics with the gate, not limited to some specific case studies. This study deals, through laboratory experiments and numerical simulations, with flow features of such a gate-functioned intake. The physical model test is used to validate the numerical simulation. Subsequently, a series of numerical cases considering different hydraulic and geometric conditions are performed to help look into the behaviors. Particular attention is paid to the flow regimes, head loss and flow velocity distributions. The results showcase the effect of the gate on the intake flow regime, and in terms of head loss and flow velocity distribution, the influences of the upstream water head, intake chamber width and withdrawal depth are revealed in detail. An empirical expression, with regard to the coefficient of head loss, is derived and validated by data from the available literature. Moreover, it is found that the maximum velocity at trash rack section is dependent exclusively on the relative withdrawal depth and always occurs at a certain height range above the gate. These results may provide a meaningful reference for the research of water intake with similar situations.

For the hydraulic design of a stepped channel, it is crucial to estimate the limit of flow regimes. Step flow in the quasi-uniform region of a stepped channel is analyzed theoretically in this paper. The present work adopts a flow-regime criterion and some assumptions from a method from the literature, a specific energy equation is established, and a weak assumption from the literature is modified to determine the flow conditions in and out of the control volume (CV). A pragmatic method is therefore provided to predict the onset of skimming flow in the quasi-uniform region, and its accuracy is examined through comparison with data from two experimental tests and previous studies.

Standing waves occur frequently at the inlet due to the change of the flow direction from an approach channel to the dropshaft. The performance of the standing wave, characterized by the relative height, the location and the extent, is theoretically and experimentally investigated in the present paper. It is shown that the height of the standing wave decreases with the increases of the approach flow Froude number and the sub-channel number in the inlet, but increases with the increase of the curvature of the dropshaft. The errors of the expressions for the relative height, the location and the extent of the standing wave, are 9.7%, 7.8% and 13.1%, respectively, as compared with the experimental data.