Journal of Hydrodynamics最新文献

In open channels, floating vegetation canopies (e.g., E. crassipes) reduce the flow velocity within canopies and increase the velocity in the free-flow region below canopies. The variation in the flow structure below the canopies impacts both sediment resuspension and deposition onto the channel bed. Combining the random displacement model with the sediment deposition probability, a new model was proposed for predicting the longitudinal distribution of sediment deposition below floating vegetation canopies. The deposition distributions were measured below floating canopies of model vegetation (rigid cylinders) and real vegetation (E. crassipes). The measurements agreed well with the predictions of the proposed deposition model. With the use of the proposed model, the impacts of floating E. crassipes canopies on the deposition distribution were examined for different upstream mean flow velocities (U₀), relative flow depths (h_g/H), vegetation densities (a) and sediment sizes (d_s). The results suggested that net deposition below floating canopies decreased with increasing U₀ and a and with decreasing h_g/H. Under the same flow and vegetation conditions, smaller sediment particles were more likely to become resuspended, resulting in less net deposition below the floating canopies. These results can improve the fundamental understanding of vegetation-based river management and ecological restoration.

This study investigates cavitating swirling flow in a diffuser, i.e., a simplified model of a Francis turbine draft tube, using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) applied to velocity and pressure field data. The interaction between vortex rope precession and cavitation surge under varying swirl and cavitation numbers is analyzed. The modal analysis results depicted the coherent structures correlated to the vortex rope precession near the diffuser inlet and the diffuser outlet, and cavitation surge in the diffuser. The POD analysis accurately revealed the flow features in the diffuser: The conical structure represents the flow diffusion with vortex rope precession and the reverse core indicates the backflow in the diffuser for the averaged flow, and the double helical structure near the diffuser inlet for the representative flow oscillation. The typical coherent structures obtained by the DMD for the cavitating swirling flow in the diffuser are the double helical structure concentrated near the diffuser inlet. The double helical structure also appears near the diffuser outlet where the breakdown of vortex rope occurs and the flow oscillation slows down. Once cavitation occurs, the mode induced by cavitation surge and its corresponding coherent structure may change according to the operating condition. The flow oscillation can be changed from the double helical mode to the axial oscillation caused by cavitation surge named breathing mode if cavitation surge becomes strong enough at a small cavitation number or large swirl number.

Research on the high maneuverability of fish swimming primarily involves addressing the batch processing of large experimental data, specifically how to simultaneously capture and rapidly process deformation-displacement information of fish bodies and related flow fields. The primary objective of this study is to integrate high-speed photography technology with deep learning methods to propose a set of data processing methods suitable for extracting fish swimming characteristic parameters. For the rapid movements of zebrafish (millisecond-level motion), this study utilized a high-speed camera for image acquisition, obtaining batches of swimming fish images and fluorescence particle information in the flow field. The geometric reconstruction of zebrafish under high-speed swimming was achieved by introducing deep learning algorithms and refining the U-Net model. To tackle the challenges of complex fish swimming scenes, we utilized a novel residual connection approach (path modification) and constructed a hybrid function model (module enhancement), resulting in a new neural network model tailored for zebrafish swimming image processing: Mod-UNet. Through testing, the improved Mod-UNet model effectively eliminated interference from fluorescence particles in the flow field on the extraction of fish body contours, achieving an overall IoU coefficient of 93%. The processing results demonstrated a kind of consistency compared to results obtained with traditional methods by previous researchers. By calculating the geometric morphology of zebrafish, we further derived the kinematic characteristics of zebrafish. Simultaneously, by applying cross-correlation algorithms to calculate the positions of fluorescence particles, the velocity characteristics of the flow field were obtained. The λ_ci method and the Ω method were used to identify vortex structures, providing the evolution patterns of corresponding flow field characteristic parameters. The experimental data processing method proposed in this paper provides technical support for establishing a zebrafish swimming information database.

Cloud cavitation that forms around a two-dimensional hydrofoil may exhibit three-dimensional characteristics. This study investigates the spanwise variations of cloud-cavitating flows through comprehensive analyses of both numerical results and experimental snapshots. Experiments were conducted in the cavitation tunnel, utilizing high-speed cameras to record the evolution of cavitation, while the cavitating flow of the same configuration was simulated using detached eddy simulation (DES). The three-dimensional shedding phenomena of cloud cavitation, characterized by spanwise variations, are observed in both numerical and experimental results, impacting on the oscillation of hydrodynamic forces. According to the investigation on the spatial-temporal evolution of flow field, two distinct patterns in terms of spanwise shedding of cloud cavitation, namely complete and incomplete shedding, are identified.

This paper employs the high-order-spectral-computational fluid dynamics (HOS-CFD) method to analyze the motion responses of a moored container ship at three positions in a rogue wave: before, at, and after its maximum wave height. These three positions display during the nonlinear evolution of the rogue wave. Numerical results are validated against physical wave tank experiments, where the rogue wave is accurately reproduced using the HOS method. The numerical results of position-dependent hydrodynamic responses in the rogue wave show that the maximum heave and surge motions do not occur at the location of maximum wave height. The heave motion peak appears before the location, the surge motion peak happens afterward and the pitch motion peak is at the location. Wavelet transform analysis is adopted to explain this situation. Scattering wave field analyses are carried out to show the different scattering wave types around the ship during the evolution of the rogue wave.

To enhance the efficiency of wind energy harvesters, aerodynamic modifications to bluff bodies prove highly effective. This study introduces two innovative galloping piezoelectric energy harvesters (GPEHs) equipped with two symmetrical splitters on a cuboid bluff body: GPEH with upstream splitters (GPEH-US) and GPEH with downstream splitters (GPEH-DS). Wind tunnel experiments evaluated the impact of splitter angle and length on energy harvesting performance across varying wind speeds. The results indicate that larger splitter angles and shorter lengths are more favorable for energy harvesting in GPEH-US. The optimal configuration, determined as GPEH-US with α = 90° , L = 0.4D , reduces the threshold wind speed, expands the effective wind speed range for energy harvesting, and increases maximum voltage and power output by over 99%, 301%, respectively, compared with conventional GPEH. Conversely, GPEH-DS are less effective for energy harvesting but demonstrate potential in vibration control applications. Computational fluid dynamics (CFD) simulations using the OpenFOAM toolbox qualitatively elucidate the physical mechanisms driving these results. A larger splitter angle enables secondary small-scale vortices (SV) to absorb more energy, accelerates boundary layer separation, intensifies and disorderly vortex shedding, enhances aerodynamic instability, and improves energy harvesting performance.

Addressing the issue of excessive cavitation pressure fluctuation on the propeller behind a catamaran, numerical simulation is conducted to assess the quality of the wake flow and to numerically predict the pressure fluctuation induced by the propeller cavitation. Additionally, the interaction between the wake vortex field and the propeller is investigated, revealing the presence of propeller-hull vortex. To improve the propeller's inflow quality, the impact of vortex generators on the wake flow and pressure fluctuation is numerically simulated. Then, numerical simulations are conducted to compare the cavitation pressure fluctuation of optimized propeller design, evaluating the effects of vortex generators at different spatial locations and angles of attack, to determine the optimal vortex generator scheme. A more comprehensive integrated control plan for the wake flow and the cavitation pressure fluctuation of the propeller behind the catamaran is finally formed.

During strong unsteady flow processes such as cavitation initiation and collapse, the volume changes generated by the materials transformation of cavitation phase transition seriously lag behind the volume evolution formed by the flow process. The phase transition and hydrodynamics are in a non-equilibrium state. A cavitation model that can describe such non-equilibrium phenomena is needed in numerical simulations of cavitation flow. The paper starts from the molecular dynamics’ principle of phase change of matter, and based on the Maxwell velocity distribution form of molecular thermal motion, elaborates on the formation process of Hertz Knudsen formula for material exchange at the interface between liquid and vapor. On this basis, using the evolution equation of gas nucleus number density in water and the compressible state equation of vapor, a non-equilibrium cavitation model for phase transition and hydrodynamics is established. The simulation results of a vapor bubble collapse process in the non-equilibrium cavitation model show different behavior from the simulation results of the equilibrium cavitation model. The simulation results of the equilibrium cavitation model show that the vapor bubble collapses once and completely disappear, while the simulation results of the non-equilibrium cavitation model show multiple collapses and rebound, which is agreement with the experimental results of the vapor bubble collapse.

The random displacement model (RDM) can efficiently simulate particle transport processes, which are difficult to observe, incorporating stochastic and hydraulic parameters. In recent decades, it has been used in many domains, including environments, hydraulics, and ecology. However, the results exhibit significant uncertainties arising from the model resolution, hydrodynamic accuracy, intrinsic characteristics of particles, and boundary conditions. The objective of the present study is to comprehensively interpret the RDM from theory to application, and emphasize essential considerations for users in different domains. The study also provides several application strategies for the model, based on several practical RDM cases. Determining the turbulent diffusivity and velocity profiles in complex flow field is a critical step to precisely simulate particle movement. Furthermore, the physical and biological properties of passive and active particles require fundamental investigation to extend the applicability of the model. Existing studies suggest that flexibly coupling the RDM with other numerical models customized to the characteristics of distinct problems will substantially expand the utility of the RDM and could yield innovative approaches for addressing previously intractable issues.

Tip vortex cavitation (TVC) is a critical phenomenon in propeller and turbine machinery. While much of the existing research on TVC has focused on its inception, the mechanisms driving its continuous growth remain under-explored. In this study, we propose a comprehensive theoretical model that integrates both gas diffusion and free nuclei entrainment to better understand the slow growth of tip vortex cavity. The efficacy of this model is validated by comparing its predicted temporal evolution of cavity size with experimental data, under both nuclei-depleted and large nuclei-injection conditions. Additionally, the model is used to further examine the individual effects of nuclei content and size on tip vortex cavity growth. Results reveal that, in sub-saturated nuclei flow, two critical equilibrium values for cavity size are identified: one determined by the balance of dissolved gases inside the cavity and the surrounding fluid, and the other by the balance between dissolved gases inside the cavity and the surrounding gas nuclei. The cavity stability size gradually shifts from the first to the second critical value as the gas nuclei content increases. However, since the model does not consider the destabilization mechanism of the cavity, the cavity may destabilize before reaching the second critical value. Meanwhile, the cavity growth rate increases significantly with increasing gas nuclei size. This work not only provides a comprehensive explanation for the experimental observations, but also provides new insights into the hysteresis phenomenon observed in TVC.