We have investigated the flow behavior around a hydrofoil bubble generator in a high-speed channel flow up to 10 m/s for the first time to understand the air entrainment characteristics of this ...hydrofoil device caused purely by the inertia effect of water flow. This study is motivated by our previous success in achieving ship drag reduction for full-scale vessels (Kumagai et al., 2015). Depending on the water flow velocity U and the angle of attack of the hydrofoil α, three different major patterns were identified: continuous generation of dispersed bubbles, intermittent formation of an air cavity, and steady formation of a super air cavity that stretched behind the hydrofoil. The transition from the dispersed bubble state to the air cavity formation regime is explained theoretically using the equation of bubble motion coupled with a potential flow analysis. The air volume flow rate increased linearly with U, and its slope was steeper in the dispersed bubble state than in the air cavity formation regime. The highest air flow rate was realized at U = 9 m/s, which corresponds to the nominal air layer thickness at ta = 4.1 mm, and reaches the feasible range for realization of bubbly drag reduction for a long ship hull.
•Air entrainment behavior by a hydrofoil bubble generator for ship drag reduction in a flow up to 10 m/s was investigated.•Three different modes of air entrainment were identified, which depend on flow velocity and angle of attack of the hydrofoil.•The characteristics of air entrainment modes is described by the equation of bubble motion using a potential flow analysis.•The air flow rate obtained by the hydrofoil device reaches the feasible range for bubbly drag reduction of a long ship hull.
Tidal energy was one of the renewable and sustainable energy forms under rapid development in the past decades. In tidal energy conversion, the hydraulic machinery is the key work component. However, ...the tip leakage vortex and cavitation have negative influence on the energy performance and operation stability of hydraulic machinery. In the present work, a C groove method is developed to improve the energy performance and suppress the tip leakage vortex and cavitation around a NACA0009 hydrofoil in tidal energy conversion. The optimization mechanism of the C groove on the energy improvement and vortex suppression is that the groove can suppress the primary tip leakage vortex (PTLV) by the groove jet impingement, and suppress the secondary tip leakage vortex (STLV) by the groove channel flow. The numerical simulations under non-cavitation and cavitation conditions are conducted to investigate the effect of C groove with different groove depths. The results demonstrate that the C groove can improve the lift coefficient by a maximum rise of 1.04% under 50% δ, and improve the lift-drag ratio by a maximum rise of 2.85% under 25% δ, respectively. The C groove most effectively reduces the TLV volume and swirling strength under 50% δ. Meanwhile, the C groove makes the vortex core shrink and far away from the foil surface, and most effectively suppresses the TLV cavitation under 50% δ. Therefore, the groove depth of 50% δ is recommended as the optimal depth on suppressing the TLV cavitation.
•C groove is developed to suppress vortex and cavitation.•C groove effectively suppresses TLV cavitation and improves energy performance.•Influence of C groove depth from 25% δ (tip clearance size) to 100% δ is investigated.•25% δ improves energy performance by a maximum of 2.85%.•50% δ presents a comprehensive optimal effect on suppressing TLV and cavitation.
This study proposes a cost-effective mixed-learning method for the cavitation behavior on hydrofoils, aiming to predict the global pressure field on the hydrofoil surface with sparse placed pressure ...sensors and the variation of the cavity outline captured by the high-speed cameras. The method integrates CS (compressed sensing) and deep learning techniques. Firstly, within the framework of CS, the Particle Swarm Optimization (PSO) algorithm is utilized to identify the best measuring points and achieve the initial reconstruction accordingly. Subsequently, in combination with the phase information, the final reconstructions are obtained within the deep learning framework. Through validation, the proposed mixed-learning model demonstrates significantly improved reconstruction performance compared to a single source of pressure information and an unoptimized measurement point position and significantly reduces the number of sensors. This provides a novel and effective approach for accurately predicting the pressure field on hydrofoil surfaces. Furthermore, the study evaluates the robustness concerning hydraulic sensor measurement errors, sensor placement deviations, missing measuring points, reconstruction performance under various cases and hydrofoil surface regions, and the contribution of optimized measuring points to the global field. Results show that the mixed-learning model exhibits robustness against typical measurement errors, position deviations of hydraulic sensors and missing measuring points on the suction side. Additionally, a positive correlation exists between the average pressure gradient under different conditions and regions and the reconstruction error, with the pressure side region significantly contributing to global field reconstruction. These findings underscore the mixed model's superiority in predicting pressure fields on hydrofoils and offer guidance for rational sensor installation.
Jupiter and Saturn formed in a few million years (ref. 1) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ∼100,000 years ...(ref. 2). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.
In this paper, we numerically investigate the influence of unsteady partial cavitation on the fluid–structure interaction of a freely vibrating hydrofoil section at high Reynolds numbers. We consider ...an elastically-mounted NACA66 hydrofoil section that is free to vibrate in the transverse flow direction. The fluid–structure interaction of this system is of interest in characterizing the cavitation-induced vibration and noise of marine propellers. The coupled cavitation dynamics and the fluid–structure system are solved using a body-fitted variational framework based on homogeneous mixture-based cavitation with a hybrid URANS-LES turbulence modeling. Specifically, we explore the occurrence of large-amplitude vibrations during unsteady partial cavitating conditions that are absent in the non-cavitating flow configuration. We examine a frequency lock-in phenomenon as the main source of sustained large-amplitude vibration whereby the unsteady lift force locks into a sub-harmonic of the hydrofoil’s natural frequency. We determine the origin of this flow unsteadiness in the vicinity of the trailing edge of the hydrofoil through the interplay between the growing cavity and the adverse pressure gradient. We systematically analyze the impact of flow-induced structural vibration on the combined vortex and cavity synchronization.
•Unified variational framework for cavitating flow and fluid–structure interaction.•Lock-in mechanism of freely oscillating hydrofoil with cavitating flow.•Vibration frequencies synchronize at sub-harmonic of natural frequency.•Unsteady features of cavity dynamics drive fluctuations in lift forces.•Full cavity growth–detachment–collapse cycle with frequency lock-in.
•The jet can diminish the kinetic energy of the re-entrant flow, reducing cavitation pulsation frequency.•The movement and rebound of vortex cavitation can lead to significant surface erosion.•The ...EPM approach can predict the risk of cavitation erosion resulting from cloud cavity collapse.•PU coatings exhibit excellent resistance to cavitation erosion.
The unstable detachment of cloud cavitation has a profound impact on the safe operation of hydraulic machinery. This study aims to elucidate the relationship between cavitation structures and cavitation erosion, with particular attention to how slits affect the periodic characteristics of unstable cloud cavitation. The results indicate that the interaction between the high-speed jet from the slit and the main flow generates leakage vortex cavitation, which subsequently merges with the main cavitation zone, resulting in complex cavitation phenomena. The high-speed jet from the slit markedly diminishes the momentum of the re-entrant jet, consequently suppressing the complete detachment of the attached cavitation. This leads to secondary development of the attached cavitation and reduces the primary frequency of cavitation pulsations by 48.5 %. Additionally, the detached cloud cavitation is smaller in scale and collapses more rapidly. Furthermore, this study performed cavitation erosion experiments on hydrofoil made from three different materials, finding that PU-coated material demonstrates considerable resistance to cavitation erosion. Notably, vortex cavitation collapse and rebound near the slit can cause significant erosion on the surface of the hydrofoil.
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The specifics of how galaxies form from, and are fuelled by, gas from the intergalactic medium remain uncertain. Hydrodynamic simulations suggest that 'cold accretion flows'--relatively cool ...(temperatures of the order of 10(4) kelvin), unshocked gas streaming along filaments of the cosmic web into dark-matter halos--are important. These flows are thought to deposit gas and angular momentum into the circumgalactic medium, creating disk- or ring-like structures that eventually coalesce into galaxies that form at filamentary intersections. Recently, a large and luminous filament, consistent with such a cold accretion flow, was discovered near the quasi-stellar object QSO UM287 at redshift 2.279 using narrow-band imaging. Unfortunately, imaging is not sufficient to constrain the physical characteristics of the filament, to determine its kinematics, to explain how it is linked to nearby sources, or to account for its unusual brightness, more than a factor of ten above what is expected for a filament. Here we report a two-dimensional spectroscopic investigation of the emitting structure. We find that the brightest emission region is an extended rotating hydrogen disk with a velocity profile that is characteristic of gas in a dark-matter halo with a mass of 10(13) solar masses. This giant protogalactic disk appears to be connected to a quiescent filament that may extend beyond the virial radius of the halo. The geometry is strongly suggestive of a cold accretion flow.
Improved hydrodynamic design of a Horizontal Axis Tidal Turbine (HATT) blade is key to increasing the efficiency and annual power production of the turbine. One of the crucial stages in hydrodynamic ...design is the selection of the 2D cross-section (hydrofoil) of the blade. Selecting the hydrofoils for a blade design that results in superior turbine characteristics for a given flow condition is tedious. In this study instead of choosing hydrofoils for a given flow condition, a base hydrofoil geometry is optimized to obtain a new hydrofoil that has superior characteristics for the given flow conditions. Hydrofoils were optimized for the flow conditions prevalent in South East Asia. Optimization was performed until the desired objectives were met while satisfying a set of constraints. Maximizing lift-to-drag ratio and lift coefficient of the hydrofoil was set as objectives (non-conflicting) while avoiding cavitation during turbine operation was one of the constraints. OpenMDAO and NSGAII were used to set up and solve the multi-objective optimization problem respectively to generate four optimized hydrofoils. Harp_opt was used to design a 1 m rotor HATT blade using the optimized hydrofoils which exhibited better performance than another 1 m rotor HATT blade designed with NREL hydrofoils as 2D sections.
•Optimized hydrofoils were generated for tidal turbines using NSGAII.•Low Reynolds number flow in tropical conditions of SE Asia is considered.•Maximizing lift-to-drag ratio improved power production of the turbine.•Hydrofoils were optimized to avoid cavitation during turbine operation.
Planetary systems are born in the disks of gas, dust and rocky fragments that surround newly formed stars. Solid content assembles into ever-larger rocky fragments that eventually become planetary ...embryos. These then continue their growth by accreting leftover material in the disk. Concurrently, tidal effects in the disk cause a radial drift in the embryo orbits, a process known as migration. Fast inward migration is predicted by theory for embryos smaller than three to five Earth masses. With only inward migration, these embryos can only rarely become giant planets located at Earth's distance from the Sun and beyond, in contrast with observations. Here we report that asymmetries in the temperature rise associated with accreting infalling material produce a force (which gives rise to an effect that we call 'heating torque') that counteracts inward migration. This provides a channel for the formation of giant planets and also explains the strong planet-metallicity correlation found between the incidence of giant planets and the heavy-element abundance of the host stars.
The interaction between the unsteady cavitating flow and hydrodynamic performance around a pitching Clark-Y hydrofoil is investigated experimentally and numerically. The experiments were conducted in ...the looped cavitation tunnel, and the cavitation patterns are documented by two high-speed digital cameras, and the moment of hydrofoil is measured by the moment sensor. The pitching hydrofoil is controlled from α+ = 10° to α+ = 15° firstly, and goes back to α− = 5° from α+ = 15°, then goes back to α+ = 10° from α− = 5° at Re = 4.4∗105. The upstream velocity U∞ and the cavitation number σ is fixed at 6.3 m/s and 1.38, respectively. And the pitching rate is α˙=40∘/s,α˙∗=0.086. The numerical investigations were performed by solving the incompressible UNRANS equations via the commercial code CFX using the Merkle cavitation model, the coupled k-ω SST turbulence model and γ-Reθ transition model. The predicted cavity patterns and moment coefficients agree well with the experimental results. The results showed there are two distinct cavitation patterns (Multi-scale cloud cavitation and Traveling sheet cavitation). For the Multi-scale cloud cavitation phase (α+ = 10°-α- = 10°), the re-entrant flow is the main factor on the stability of cavitating flow structures, which is responsible for different shedding patterns. According to the breaking position and re-entrant flow thickness, this stage is divided into three different patterns of the cavity development and shedding. For the Traveling sheet cavitation phase (α− = 10°-α+ = 10°), the shedding of cavity mainly results from the interaction of the re-entrant flow and the fluctuation of the gas liquid interface, thus leading to the irregular breaking points. The cavitating flow structure of different phases were further investigated using the LESs, which will be help to identify the dynamic behavior of flow structures effectively.
•Two different cavitation patterns in the pitching process are investigated experimentally and numerically.•Cavitating flow stability is investigated for the multi-scale cloud cavitation phase with three cavity patterns.•Cavity shedding irregularity is further analyzed for the traveling sheet cavitation phase.
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