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The present study expresses the turbulent flow characteristics through a 90° pipe bend using a numerical ‎method by determining the solutions for Reynolds Averaged Navier-Stokes (RANS) expression using the k-ω ‎‎(SST) turbulence model. For that purpose, numerical analysis has been carried out by solving RANS equations ‎using ANSYS FLUENT 16.2, considering incompressible fluid in turbulent flow conditions. Simulations have ‎been carried out for three different Reynolds number ranging from 1×105 to 10×105 at three different bend ‎curvature ratios (Rc/D = 1, 1.5, and 2). Pipe bends with guide vane are generally used where flow separation ‎and space problem makes an issue in mechanical design. The presence of guide vane inside the bend ‎positively suppressed the flow separation and presence of cross-flow which can cause the engine to run off ‎design, thus reducing the engine efficiency. So, to observe the effect of guide vane and its position on ‎turbulence characteristics, four different positions of guide vane inside the bend are considered in the present ‎study. At first, an analysis was led to make sure that the results obtained from the present numerical model are ‎reliable and in line with previous results obtained from similar published experiments and numerical work. ‎Research has been conducted to find out the impact of Reynolds number, bend curvature ratio and position of ‎guide vane on different turbulence characteristics namely; turbulent kinetic energy, turbulent intensity, and ‎wall shear stress at bend outlet position. In general, the turbulent intensity is found larger for the lower bend ‎curvature ratio at the inner wall curvature side. Results for turbulent kinetic energy have similarities in results ‎with turbulent intensity. Significantly, the wall shear stress represented a strong dependency on the ‎circumferential angle at the bend outlet cross-section, and curvature ratio rather than Reynolds number and ‎guide vane positions‎‎.

Incipient wave breaking on a vertical circular pile is simulated with a Reynolds stress–$\\omega$ turbulence model. Comparison of results simulated with a stabilized two-equation turbulence model, as well as no turbulence model, demonstrates that the breaking point and the peak force on a vertical cylinder due to incipient breaking should not be affected by the turbulence closure model, provided that it is stable and the simulations are converged. Notably, the present results show that the build-up to peak force induced by incipient wave breaking can be accurately predicted without any turbulence closure model. However, for the prediction of the secondary load cycle (SLC), proper turbulence modelling is required, as this process involves both turbulence production and lee-side flow separation. The Reynolds stress–$\\omega$ model is demonstrated to predict the SLC more accurately than a stabilized two-equation $k$–$\\omega$ turbulence model, as the flow separation points and vorticity field are better predicted. Some existing studies indicate that the generation of the SLC does not necessarily result from flow separation, but is rather due to the suction force. The present work finds that the occurrence and point of flow separation significantly affect the magnitude of the suction force, which hence affects the SLC prediction significantly. For waves breaking on a vertical pile, proper turbulence modelling is therefore essential for accurate SLC predictions. (In the above, $k$ is the turbulent kinetic energy density and $\\omega$ is the specific dissipation rate.)

The presence of gap flow has a significant impact on the stable and efficient operation of hydraulic turbines. This study used a numerical simulation method using the SST k-ω turbulence model to investigate the multi-scale flow characteristics inside the components of a large Francis turbine containing gap flow under different operating conditions. The results indicate that, at low head, localized vortices in the runner area and non-uniform flow velocity in the draft tube cone section primarily cause reduced turbine efficiency. At high head, the vortex zone within the draft tube similarly contributes to decreased turbine efficiency. Additionally, the elevated head causes an increase in flow velocity within the gaps, which increases hydraulic losses.

One of the latest OWC concepts that is expected to improve OWC performance and lessen the sloshing phenomenon is the Helmholtz resonator OWC. This study uses the Reynold Averaged Navier-Stokes (RANS) based CFD method with the k-ω SST turbulence model. 2D modeling is performed under transient conditions by applying the Volume of Fluid (VoF) technique to model the sloshing phenomenon in the OWC. According to the simulation result on input waves with sloshing characteristics (T = 1.08 s, H = 0.13 m, H/L = 0.073) the Helmholtz resonator OWC ( S c = 0.05 m, e = 0.01 m, e = 0.05 m) produces better performance with ξ by 17%, C r by 7% and is able to prevent energy loss ( C d ) up to 9.4% compared to the conventional OWC ( S c = 0.05 m, e = 0.05 m, e = 0.01 m). From the results of the OWC airspace velocity contours, it is found that the conventional OWC has a larger and faster vortex shedding than the Helmholtz resonator OWC. In this case, the Helmholtz resonator OWC in waves with sloshing characteristics can reduce the effect of turbulence-shedding vortex caused by waves with sloshing characteristics much better than the OWC.

As supporting equipment for deep-sea oil and gas resource exploitation, how to improve the transmission performance and stability of the impeller of the mixed-flow pump (MFP) has become a key technical problem. This paper focuses on studying the impeller blade of the MFP. Based on the blade optimization technology, four different tandem misaligned blade (TMB) angle schemes of α =0°,15°,45°,75° are proposed. The SST k-ω turbulence model is used to simulate the different blade angles under the pure liquid condition. The pressure pulsation of the MFP and the blade load distribution are studied. This result show that the TMB structure increases the pressure fluctuation inside the MFP. The pressure coefficient inside the impeller exhibits a more consistent pattern at an angle of α =45° compared to other angles, and the internal flow field of the MFP exhibits most stable. From this analysis of blade load distribution, TMBs have a greater effect on the pressure surface of the front blade of the MFP. When α =15°, the best pressurization performance of the MFP. This study may serve as a benchmark for subsequent iterations of optimizing the MFP’s model and hydraulic design.

This study’s primary goal is to investigate how various airfoils’ maximum camber positions affect hydrofoil cavitation performance. Through numerical simulation, the cavitation low properties of hydrofoils with various maximum camber positions are compared. The accuracy of the modi ied turbulence viscosity and SST k-ω turbulence model on the temporal and spatial evolution characteristics of cavitation near the hydrofoil is evaluated by combining it with the model test. Analysis is done on the cavitation low ield of four airfoils at two distinct design angles of attack (+4° and +6°) with varying maximum camber locations (fmax = 35%C, 40%C, 50%C, and 60%C). The indings indicate that at 35%C, the hydrofoil’s maximum camber position has improved cavitation performance. The hydrofoil’s cloud cavitation evolution time is shorter than that of the original hydrofoil, and during the same time period, more cavitation is generated. The lift-to-drag ratio and lift coef icient of the cavitation low ield are signi icantly improved at both angles of attack. At the same time, the vorticity distribution and entropy generation distribution can be effectively reduced under the design angle of attack and high angle of attack cavitation, and the hydraulic loss in the cavitation low ield can be reduced. This research can serve as a guide for optimizing the hydrofoil’s cavitation performance and designing the impeller of the axial low pump that follows.

The vane in the injector can reduce the influence of the secondary flow caused by the injector, thereby prolonging the durability and ensuring the stable and economic operation under high head conditions. In this paper, the numerical simulation of water-air two-phase flow is carried out for the whole flow channel of the injector and injector of the Pelton turbine with a design head of 1200m. The 3D model of different shape vanes is constructed by controlling the thickness, length and radius of the vane. The SST k-ω turbulence model is used to analyse the influence of the shape of the vane on the downstream velocity distribution, jet shape and pressure pulsation. The circumferential velocity non-uniformity in the jet core area caused by the injector can be controlled by increasing the length of vane, while the smaller curvature radius of trailing edge may cause downstream periodic pressure pulsation increase. The research results can provide reference for the hydraulic design and performance optimization of the injector of the high head Pelton turbine.

In this study, the key issue of turbulence model applicability in the 3D numerical simulation of the whole system of hydraulic machinery is addressed. Taking the pump-turbine unit with complete hydraulic units as the research object (covering the upstream diversion pipeline, the ball valve structure, the reversible pump-turbine body, and the downstream branch pipeline), a comparative study on the influence of turbulence model selection on the simulation accuracy of multiple conditions is carried out systematically. By constructing a high-precision three-dimensional numerical model of the whole flow channel (using ANSYS Fluent solver, applying the multi-reference system model to deal with the dynamic and static interference effects of the runner, with the total number of meshes up to 7,310,000, and verified by mesh-independence), a comparative analysis was carried out on the k-ε standard model, k-ω base model, RNG k-ε modified model, and the SST k-ω shear stress transport model for a wide range of prediction under the operating conditions of the guide vane in the range of 18 to 27 degrees. The prediction performance of the SST k-ω shear stress transport model in a wide range of working conditions. The numerical simulation results show that the SST k-ω turbulence model has the best agreement with the experimental data in terms of unit efficiency and power prediction, and the overall trend is consistent with some deviation in the values. Therefore, this study concludes that the SST k-ω turbulence model should be preferentially adopted in the simulation of transient processes in the whole system of the pump turbine (e.g., load dumping condition, guide vane opening and closing process) to ensure the characterisation accuracy of static and dynamic interference effects and rotating stall phenomenon, and at the same time, attention should be paid to combining with the twisting model to enhance the prediction reliability of the part-loaded condition.

When a trans-media vehicle impacts the water surface at a small angle, it will bounce and take off and then re-enter the water, which is called near-water-surface Skipping Motion. The study of this kind of movement is of great significance to the high maneuvering penetration of ships or aircraft near sea level. This paper adopted the method of numerical simulation, selected the SST k-ω turbulence model, combining the N-S equation and the six-degree-of-freedom (6DOF) algorithm, and used the overall dynamic grid technology to calculate the vehicle entry condition at speed of 50m/s, 100m/s, 150m/s &10°, 20°, 30° angle of attack, and the near-water-surface skipping motion characteristics of the vehicle were explored, which provided a reference for studying the high maneuvering motion of offshore plane trans-media vehicles.

This study investigates the thermal-hydraulic characteristics on the shell side of the Helical Once-Through Steam Generator (HOTSG) in a Lead-cooled Fast Reactor (LFR), with a specific focus on the influence of the axial pitch-to-diameter ratio — a key structural parameter — on heat transfer and flow performance. A three-dimensional, detailed geometric model was developed and experimentally validated. CFD numerical methods were employed, incorporating the SST k-ω turbulence model along with a Prandtl number modification model suitable for liquid metals. Systematic simulations were conducted to analyze the flow and heat transfer processes of the HOTSG under various operating conditions at pitch-to-diameter ratios of 1.5, 1.8, and 2.1. The results indicate that as the pitch-to-diameter ratio increases, both the heat transfer capacity and pressure drop initially rise and then gradually stabilize. Flow field analysis reveals that a larger pitch-to-diameter ratio enhances fluid mixing and increases vortical randomness, which contributes to improved heat transfer but also leads to higher pressure losses.