Journal of Hydrodynamics最新文献

The study examines the cluster effect of arrays of square cylinders in smooth and turbulent inflows, focusing on vortex shedding and wake dynamics. Several clusters in varying arrangements (2×2, 3×3 and 4×4) and spacings were investigated using high-fidelity large-eddy simulations (LESs). The obtained data reveal that the shedding frequency of the dominant vortex is primarily influenced by the solid portion of the cluster’s cross-section. From this observation, an effective cluster size, which mainly consists of the solid portion with a minor adjustment for the cluster porosity, is proposed in the study. The Strouhal number (Sr) based on the effective cluster size closely resembles that of an isolated square cylinder, suggesting that the characteristic length is the effective cluster width, but not the geometric cluster size. For turbulent inflows with an integral length scale exceeding the cluster size, Sr decreases markedly compared to smooth inflow cases, indicating a significant influence of large-scale turbulence on cluster dynamics.

Operation of pump-turbines in the S-shaped region is characterized by amplified pressure pulsations and grid synchronization challenges, often leading to operational instability and compromised grid compatibility. This study employs dynamic mode decomposition (DMD) method to extract coherent flow structures correlated with dominant pressure pulsation frequencies in the S-shaped region, while experiments have been conducted to validate numerical simulations. Results show that the DMD method can effectively identify characteristic frequencies of complex flows in the S-shaped region: low-frequency pressure pulsations in the vaneless space originate from circumferential transmission component of the water ring over time, which is called the water ring pulsation component. During operation condition transitions, this pulsation component first intensify then attenuate as it moves outward along the guide vanes. The dominant draft tube pressure pulsation frequency driven by vortex rope dynamics induces morphological vortex rope transformations via DMD mode shifts during operational transitions. Runner pressure pulsations within the S-shaped region predominantly stem from rotating stall propagating counter to the runner’s rotation. These findings advance the understanding of S-shaped region flow instabilities, linking component-specific flow structures to global pressure pulsation characteristics, and thereby providing critical insights for operational stability enhancement.

Cone valves can meet the needs of large-flow water conveyance systems well, while cavitation inside caused by large flow rate is becoming more and more prominent. The large eddy simulation (LES) approach combined with the Schnerr-Sauer cavitation model was conducted to investigate the cavitating flow in the cone valve. The results show that the cavitation structure inside the cone valve can be divided into relatively stable attached cavitation and shear cavitation. Shear cavitation mostly occurs in quasi-streamwise vortices and elongated circumferential vortices subjected to strong stretching. The vorticity transport equation was utilized to discuss the interaction between shear cavitation and vortex further. The vortex stretching term indicates that the circumferential vortex has a tendency to break along the axis and becomes more three-dimensional when it moves downstream. Shear cavitation will be formed in the broken vortex. The cavity growth leads to a rapid increase in the vortex dilatation term. The baroclinic term has a maximum value when the shear cavitation collapses. Evolution of shear cavitation could increase turbulent pulsation, with the generation of strong turbulent kinetic energy. Turbulence and shear cavitation interact and form a complex flow field structure. In addition, it is found that the second-order conductance of cavitation volume to time is mainly responsible for the pressure pulsation inside, while the geometric parameters of the cone valve on pressure pulsation cannot be ignored as well. Therefore, an improved one-dimensional model is proposed, which verifies the influence of the evolution process of shear cavitation and geometric parameters of the cone valve. The results are helpful in understanding the hydrodynamic and cavitation characteristics inside the cone valve.

The expansion of large-scale ports necessitates a delicate balance between operational efficiency and environmental preservation. Our study tackles this issue by exploring sustainable water exchange optimization in intricate port systems. Focusing on Dalian Port’s Taiping Bay as our research site, we integrate ecological principles into port development and apply a tidal flow model to assess the effects of diverse channel designs on water dynamics. A key advancement of our work is the refinement of the “retention factor” concept and the adaptation of the concentration-water exchange rate equation. This innovative approach allows for a more precise evaluation of water exchange capabilities amidst complex environmental scenarios. Our findings reveal that channel width plays a pivotal role in water exchange efficiency, with a notable plateau effect beyond a threshold that achieves a 90% exchange rate. These insights underscore the potential for design optimization that aligns with ecological sustainability, offering a roadmap for future port developments that prioritize minimal ecological disruption.

Bell-mouth shafts equipped with middle piers are widely used in pumped storage power stations in recent years. Predicting the discharge coefficient of bell-mouth shafts with middle piers is of critical importance. This study integrates experimental observations and numerical simulations to investigate the discharge behavior of those bell-mouth shafts. The findings indicate that the discharge coefficient is primarily governed by the relative weir head through the inlets and the contraction ratio of the weir orifice of the inlets. An empirical equation is further proposed to estimate the discharge coefficient with the relative error of less than 10%.

This study proposes a novel partitioned decomposition approach based on the improved single-phase level-set model, spectral wave explicit Navier-Stokes equations (SWENSE) model, and the implicit-inner-iteration motion-solving method for accurate and efficient solution of wave-ship-sloshing interaction. A hybrid functional-decomposition method is employed to accurately and efficiently solve the wave-ship interaction in the external domain, while using the original viscous method for simulating sloshing in the internal domain. The issues of non-conservation and boundary conditions for free surface motion in the original level-set model are addressed for the improved single-phase level-set model. The ability of this approach to predict sloshing, and seakeeping performance with/without sloshing is evaluated by comparing numerical results with experimental data. Further, the effects of sloshing, different DOFs (degrees of freedoms), wavelengths, wave heights and wave directions on ship motions and added resistance are investigated. Findings include confirmation that the proposed level-set method is reliable for predicting sloshing behaviors, and that the partitioned functional-decomposition approach is superior for modeling wave-ship-sloshing interactions. Surge motions are found to have a significant influence on sloshing behaviors and added resistance, causing peak frequency offsets and considerable changes in peak values. Nonlinear characteristics in seakeeping performance of ships with sloshing effects result from the combined effects of nonlinear wave-ship interaction and nonlinear sloshing behaviors. The effects of sloshing on ship motions and added resistance stem from the interaction between sloshing-induced forces/moments and corresponding motions, as well as changes in the natural characteristics of ships due to the presence of the free surface of the inner tank.

Experimental measurements for the motion responses of the moored barge alongside a bottom-mounted platform in beam sea are carried out, by which the behavior of motion responses of the barge and the forces on hawsers and fenders are considered. The sway and roll motions are the most important responses in six degrees-of-freedom. The seaward motion responses are mainly held by the hawsers; while the landward motion responses are mainly astricted by the fenders. Correspondingly, the variation tendencies between the tension forces on the hawsers, the compressed deformation and forces of the fenders, and the sway and roll motion responses of the barge generally have the similar behavior. The nonlinear characteristic in this work is mainly from the force-deformation relationship of fenders. With the increase of incident wave amplitudes, the generally decreased normalized amplitudes can be observed, including the tension forces of hawsers, the compressed deformation and the forces of the fenders. More relative kinetic energy is absorbed by the fenders under larger incident wave amplitudes, which is the essential reason for this phenomenon. The dependence of normalized sway/roll motion responses with incident wave amplitudes is more complex. The normalized sway and roll motion amplitudes around the natural roll periods significantly increase with the increase of incident wave amplitudes. This might be the most noteworthy phenomenon in practical engineering.

As the incidence of heart failure continues to rise globally, the demand for interventional blood pumps for heart failure treatment has increased. However, the use of these pumps may lead to infection issues. Given the limited research on blood pump infections, methods for evaluating the risk of infection are crucial. In this study, first, an infection risk assessment model for blood pumps was constructed to quantify the relationships among the loss rate of leukocytes, shear strain rate, and temperature. A computational fluid dynamics (CFD) simulation model was subsequently developed to perform fluid–thermal coupling simulations of the internal flow field, temperature field, and index of infection of the blood pump. Furthermore, the impeller design was optimized via an orthogonal experimental design and regression analysis to reduce the risk of infection. Finally, in vitro experiments were conducted to validate the accuracy of the simulation results. Numerical simulations demonstrated that the impeller design significantly influenced the risk of infection. The optimized impeller design (with a larger proximal fillet radius and a smaller blood outlet axial length) reduced the risk of infection. The trends observed in the in vitro experimental results were consistent with the simulation values. The impeller optimization design method proposed in this study effectively reduces the risk of infection in interventional blood pumps, providing a theoretical basis and practical guidance for the design and risk assessment of interventional blood pumps.

Wall-resolved large-eddy simulation (WRLES) and wall-modeled large-eddy simulation (WMLES) of turbulent channel flow at Re_τ ≈ 1000 are performed to explore their capabilities in predicting the space-time correlations of near-wall velocity and pressure fluctuations. Our findings indicate that both WRLES and WMLES can effectively capture the essential features in the wavenumber-frequency spectra. Doppler shifts due to the convection velocity and Doppler broadening caused by the random sweeping effects of large-scale eddies are observed in the wavenumber-frequency spectra. Both methods predict the near-wall velocity fluctuations with reasonable accuracy, but predicting wall pressure fluctuations seems to be more challenging for the current wall modeling approach. Without resolving the viscous sublayer, WMLES slightly overestimates the spectral levels of wall pressure fluctuations in low-wavenumber (or frequency) region. In addition, the two-dimensional spatial spectra indicate that isotropic subgrid-scale models may struggle to accurately capture the anisotropy of small-scale eddies near the wall.

Lateral-intake multiple forebays are widely used in urban pumping stations, but there is still insufficient understanding of their hydraulic characteristics, making it difficult to provide scientifically based guidance for determining optimal start-up combinations, which are critical to pumping station efficiency. This study numerically investigated the hydraulic characteristics of lateral-intake multiple forebays under different start-up combinations and, for the first time, applied entropy production theory to analyze the causes and locations of hydraulic loss. The lateral-intake causes undesired flow patterns, including helical mainstream and flow recirculation in the forebays, and mainstream one-sided concentration and backflow in the sumps. These undesired flow patterns become more significant with reduced standby pumps and increased inflow Froude numbers, directly degrading flow uniformity and increasing total hydraulic loss. Keeping the downstream pumps in operation for one-pump and two-pump standby conditions helps to improve forebay flow uniformity and reduce total hydraulic loss. Entropy production method revealed that turbulence dissipation is the main cause of total hydraulic loss in the forebays, accounting for over 90%, which is induced by helical mainstream and flow recirculation and impingement, occurring near the forebay inlets, sump piers and suction pipe mouths. By comprehensively evaluating flow patterns, uniformity, and hydraulic loss, the optimal start-up combination for different standby pump requirements was concluded as operating the downstream pumps while keeping the upstream pumps on standby. This provides a theoretical basis for investigating hydraulic characteristics and optimizing pump operation in urban pumping station forebays.