Analysis of the flow field of tropical cyclones in the mature stage by the Liutex methods, such as the Omega Liutex and the Liutex core line, is reported. This is the first known study analyzing tropical cyclones with the Liutex method. Liutex has the potential to enable greater understanding of tropical cyclone dynamics and to improve forecasts of their track, intensity, and structure.
The fact that the staggered impeller of a double-suction centrifugal pump can effectively suppress pressure fluctuations has been proved by engineering practice, but the flow mechanism behind it is still not fully understood. In this study, numerical simulations with a proof experiment were conducted, and the vortex dynamics analyses were performed using the newly developed rigid vorticity (Liutex) theory. The following valuable results are obtained: (1) In terms of the intuitive vortex structure, each blade of the impeller induces a trailing vortex rope with a strong rigid vorticity, which gradually evolves inside the volute casing with the rotation of the impeller. The trailing vortex ropes of the symmetric impeller are symmetrically distributed, while those of the staggered impeller present a staggered distribution, and the latter corresponds to a relatively lower rigid vorticity. (2) In terms of the correlation between the vortex and the pressure, the high rigid vorticity zone corresponds to the low-pressure zone. For a fixed point in the volute casing, there is a major “falling-rising” fluctuation in pressure as the symmetric vortex ropes transit it simultaneously, and a minor “falling-rising” fluctuation in pressure as the staggered vortex ropes transit it successively, corresponding to a lower peak-to-peak value of the pressure fluctuations. (3) In terms of the relation between the vortex and the velocity, the vortex ropes induced by the left and right impellers are counter-rotating and develop along the radial direction. This pattern results in high-speed zones at the middle part of the cross-section of the volute casing, both in the streamwise and radial directions, and contributes to velocity fluctuations due to the evolving vortex rope. However, the staggered distribution of vortex ropes can weaken the coupling of vortex pairs, thereby causing lower velocity and pressure pulsations, but can make the main frequency twice that of the symmetric impeller. This study enriches our physical knowledge by revealing the vortex dynamics mechanism of the staggered impeller of a double-suction centrifugal pump to suppress pressure fluctuations.
This study experimentally examines the coupling response of an S-shaped flexible riser exposed to internal gas-liquid two-phase flow and external shear current using the optical non-intrusive measurement. The vibration tests were conducted with the internal fluid transported at a constant velocity of vi = 0.9 m/s with three representative gas-liquid ratios (Qg/Ql) and the depth-averaged reduced velocity of external current (Urm) ranging from 9.32 to 23.19. The riser top was hanged underneath an oscillating cylindrical platform. The out-of-plane response is enhanced in the presence of external current, while the in-plane response is suppressed due to the more even distribution of liquid slugs. The response competition among the riser, buoyancy module and platform is quantified in terms of dominant frequencies and interaction lengths, which are sensitive to Qg/Ql, Urm. The shift of zero value among the coupling length, affecting length and affected length indicates the switching of dominant role. In general, the response competition in the in-plane direction is more intense than that in the out-of-plane direction.
To clarify the influence of the hydrofoil characteristic thickness on the distribution characteristics and mechanisms of clearance cavitation erosion risk, a large eddy simulation (LES) is conducted to study the clearance cavitating flow around NACA0012 and NACA0024 hydrofoils under identical conditions. The study predicts cavitation erosion risk using three methods: The erosive power method (EPM), the improved gray level method (IGLM) and the energy conservation method (ECM). The numerical results are in good agreement with the experiment data and the ECM is applied due to its simplicity in parameter adjustment and low sensitivity. The results indicate that the characteristic thickness significantly influences the flow field, leading to variations in the position and intensity of cavitation collapse, ultimately resulting in notable differences in cavitation erosion risk distribution. The high cavitation erosion risk region on the clearance surface of NACA0012 is concentrated around the midsection, while it is concentrated in the upstream region for the NACA0024, with a lower frequency of extreme events. Tip separation vortex (TSV) cavitation is the main cause of the differences in cavitation erosion risk distribution. On the clearance surface of the NACA0012, TSV cavitation primarily collapses in the central region, whereas for the NACA0024 hydrofoil, TSV cavitation occurs only in the upstream region of the clearance surface and exhibits more stability. The differences in vorticity distribution near the clearance surface partially influence the distribution of TSV cavitation, thereby affecting the characteristics of cavitation erosion risk distribution. The larger characteristic thickness of the NACA0024 reduces the effects of the stretching term and the baroclinic torque term, weakening the effect of vorticity on TSV cavitation, resulting in more stable patterns of the TSV cavitation.
Superhydrophobic drag reduction technique has garnered significant attention in recent years due to its outstanding performance in reducing frictional drag, presenting promising application potential in areas such as marine vessels and pipeline transportation. This paper provides a comprehensive review of the wetting theory and slip theory associated with superhydrophobic surfaces, and systematically summarizes the mechanisms of turbulence drag reduction from the perspective of coherent turbulent structures. The research on the combination of superhydrophobic and other drag reduction techniques to improve the drag reduction effect is summarized, focusing on the combined drag reduction of superhydrophobic-riblet. The stability and recoverability of the gas layers on superhydrophobic surfaces are critical for advancing this technology toward practical engineering applications. Consequently, this review places particular emphasis on recent research progress concerning the enhancement and restoration of the underwater gas-liquid interface stability on superhydrophobic surfaces.
Rogue waves pose a significant threat to the safety of ships and offshore structures, making it crucial to understand their physical mechanisms, such as spatial-temporal focusing, which can lead to their formation. This study investigates three-dimensional focused waves using the newly developed deep-water high-level Green-Naghdi (HLGN) model. Through numerical simulations, we evaluate the selection of the involved wave numbers within the HLGN model and present the algorithm for the three-dimensional implementation. Validation of the model is conducted through numerical reproduction of the three-dimensional focused waves considered in other’s laboratory measurements. The simulated wave profiles and velocity fields are compared with experimental data, demonstrating strong agreement. Discussion is provided about the robustness and accuracy of the HLGN model in simulating three-dimensional focused waves under deep-water conditions.
To clarify the internal flow field characteristics of cavity vortex in the sediment transport pipe (STP) of the desilting channel with a swirling flow generator (DCSFG), this study adopted a method combining model test, numerical simulation, and theoretical analysis to investigate flow field characteristics such as water flow regime, cavity morphology, pressure, flow velocity and vorticity, analyze the distribution of combined vortex indexes and radial pressure difference of cavity vortex, and discuss the motion feature differences between the combined vortex in the cavity vortex and the ideal combined vortex. The results show that large eddy simulation (LES) exhibits higher accuracy than the Realizable k -ε model, the distribution of combined vortex n values along typical cross-sections inside the STP ranges from −0.901 to 0.913 radially, indicating quasi-forced vortex motion on the inner side of the vortex area and quasi-free vortex motion on the outer side, the theoretical values of radial pressure difference align well with the simulation results, with a maximum relative error of 15%, confirming that the flow characteristics of the vortex are in accordance with the motion features of combined vortex, the distribution of radial pressure, tangential velocity, and vorticity in the cavity vortex conform to the distribution pattern of ideal combined vortex, whereas significant differences exist in terms of fluid force conditions, structural composition, and generation mechanism. The research findings may provide reference for further analyzing the sediment transport mechanism in the cavity vortex and for the practical engineering design and application of the DCSFG.
To enhance the gas-liquid mixed transport performance of the first-stage centrifugal impeller of the multistage side-channel pump, a diagonal perforation oriented towards the exit is fabricated in the front shroud of the impeller. Based on the Euler-Euler non-homogeneous model and the SST k -ω turbulence model, the gas-liquid two-phase unsteady numerical simulation of the internal flow under various inlet gas volume fraction (IGVF) is conducted, the reliability of the simulation is verified through comparison with experiments. The results indicate that under the circumstances of high flowrate and high IGVF, the perforation design of the front shroud can increase the head of the centrifugal impeller by 4%–7% while the efficiency is slightly decreased under gas-liquid two phase flow. According to the internal flow analysis and Liutex vortex identification, the high-pressure and high-speed fluid in the front pump chamber is introduced into the impeller through the front shroud perforation, smashing and dispersing the originally aggregated bubble groups in the flow channel, causing the average pressure in the impeller to rise after the perforation, increasing the number and intensity of vortexes, significantly reducing the number and the accumulation area of bubbles, greatly reducing the air volume fraction of the impeller. The bubble blockage phenomenon in the flow channel is observably improved, and the gas-liquid mixed transport capacity of the centrifugal impeller is significantly enhanced, providing a theoretical basis for the optimization design of the gas-liquid two-phase flow of vane pumps.
With the advancements in computer technology, the simulation-based design (SBD) technology has emerged as a highly effective method for hull form optimization. The SBD approach often employs various methods to evaluate the hydrodynamic performance of the sample ships. Although the surrogate model is applied to SBD method to replace time-consuming evaluation, many high-fidelity data are typically required to guarantee the accuracy of the surrogate model, resulting in significant computational costs. To improve the optimization efficiency and reduce computational burdens, we propose a novel hull form optimization framework utilizing the multi-fidelity deep neural network (MFDNN), leveraging multi-source data fusion and transfer learning. This framework constructs an accurate multi-fidelity surrogate model which correlates design parameters with hydrodynamic performance of the hull by blending data with different fidelity. Besides, computational fluid dynamics (CFD) evaluations based on viscous flow are served as the high-fidelity model, while potential-theory evaluations represent the low-fidelity model. Then, this framework is validated using mathematical functions to prove its practicability in optimization. Finally, the optimization design of the resistance of the DTMB-5415 ship is carried out. Our findings demonstrate that this framework can take into account both efficiency and accuracy, which is preferable in optimization tasks. The optimized hull form obtained by the framework has better resistance performance.
This paper aims to elucidate the vortex evolution characteristics generated by the tongue of the semi-spiral suction chamber and its influence on the cavitation of the pump. Based on the turbulent viscosity correction model, the internal flow of a centrifugal pump with a specific speed of 160 was simulated, and experimental data verified the simulation. This study focuses on analyzing the conditions of large flow rate, high-efficiency, and partial flow rate. The results show that the tongue will induce a tongue-induced vortex. The tongue-induced vortex extends from the tongue region to the impeller region, and its shape is curved and slender. The shape and volume of the tongue-induced vortex are related to the flow rate. The vortex’s shape is blurred and small in the partial flow rate. There is a complete and obvious curved slender vortex in the high-efficiency zone. In large flow conditions, the vortex’s shape is consistent with the high-efficiency zone and the volume is larger. The vortex’s strength is positively correlated with the circulation of the inlet, which is in the suction chamber. The tongue-induced vortex affects the distribution position of the low-pressure zone on the blade, thereby promoting the leading edge cavitation.
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