Acta Mechanica Sinica最新文献

In this study, we perform particle-resolved simulations of settling spheroidal particles, considering oblate and prolate spheroids and spheres, and investigate the shape effect on the particle dynamics in suspensions with volume fraction 1% and 5%. We first examine the single-point statistics of the translational and rotational motion of the settling particles. The horizontal velocity has a symmetrical distribution with standard deviation dependent on the particle shape. The greater horizontal velocity fluctuations of the non-spherical particles, compared to that of spheres, are attributed to the horizontal drift of settling spheroids with oblique orientations induced by the fluid-particle and particle-particle interactions. The fluctuation of particle vertical velocity, instead, is skewed under the effect of wake-induced hydrodynamic interactions. Further, we explore the particle pair statistics, which demonstrate the formation of column-like particle micro-structures for the lowest volume fraction considered. This clustering is more pronounced for spheroidal particles than spheres, due to the stronger attractions among vertically-aligned settling spheroids. Moreover, the particle pair statistics are directly related to the collision rate among the dispersed particles. The local accumulation of oblate/prolate spheroids serves as the major mechanism to promote the particle-particle collisions in dilute suspensions.

An unsteady numerical simulation is conducted to examine the dynamic runback characteristics of a water film flow driven by a boundary layer airflow over a solid surface pertinent to the dynamic glaze ice accretion process over aircraft wing surfaces. The multiphase flow simulation results of the wind-driven water runback (WDWR) flow are compared quantitatively with the experimental results in terms of the time-dependent variations of the water film thickness profiles and evolution of the front contact point of the runback water film flow. The underlying mechanism of the intermittent water runback behavior is elucidated by analyzing the time evolution of the airflow velocity and vorticity fields above the runback water film flow over the solid surface. To the best knowledge of the authors, the work presented here is the first successful attempt to numerically examine the transient runback characteristics of WDWR flows. It serves as an excellent benchmark case for the development of best practices to model the important micro-physical processes responsible for the transient water transport over aircraft wing surfaces.

Noncohesive particle clusters are identified and tracked in turbulent flows to determine the breakdown and time evolution of cluster statistics and their implications for interscale mass transfer, which has connections to the classical turbulent energy cascade and its mass cascade counterpart running in parallel. In particular, the formation and dynamics of sediment and larvae clusters are of interest to coral larvae settlement in coastal regions and particularly the resilience of green-gray coastal protection solutions. Analogous cluster behavior is relevant to cloud microphysics and precipitation initiation, radiation transport and light transmission through colloids and suspensions, heat and mass transfer in particle-laden flows, and viral and pollutant transmission. Following a comparison between various clustering techniques, we adopt a density-based cluster identification algorithm based on its simplicity and efficiency, where particles are clustered based on the number of neighboring particles in their individual spheres of influence. We establish parallels with lattice-based percolation theory, as evident in the power-law scaling of the cluster size distribution near the percolation threshold. The degree of discontinuity of the phase transition associated with this percolation threshold is observed to broaden with larger Stokes numbers and thereby large-scale clustering. The sensitivity of our findings to the employed clustering algorithm is discussed. A novel cluster tracking algorithm is deployed to determine the interscale transfer rate along the particle-number phase-space dimension via accounting of cluster breakup and merger events, extending previous work on the bubble breakup cascade beneath surface breaking waves. Our findings shed light on the interaction between particle clusters and their carrier turbulent flows, with an eye toward transport models incorporating cluster characteristics and dynamics.

Arteriovenous fistula (AVF) calcification is a common complication in hemodialysis patients that leads to AVF dysfunction and decreases the AVF survival, but the mechanisms of AVF calcification, especially the role of hemodynamic changes in AVF calcification have not been fully investigated. In this study, a computational fluid dynamics (CFD) model was carried out based on AVF, at the distal anastomosis of the cephalic vein and radial artery, generated from a patient-specific computed tomography (CT) angiography and Doppler ultrasound image. Hemodynamic factors were considered to explore the mechanisms responsible for the initiation and progression of AVF calcification. Five stages in one cardiac cycle were chosen to be studied for the velocity field, pressure, time-averaged wall shear stress (TAWSS), and oscillatory shear index (OSI). Blood pressure was higher in the arteriovenous anastomosis, and variations of great amplitude of pressure were examined during the cardiac cycle. Blood pressure, transient shear stress, TAWSS, and OSI were higher in the arteriovenous anastomosis and at the bottom of expanded outflow vein, and these sites were highly consistent with the calcified areas shown on CT angiography. On the contrary, no calcification was found in sites where streamline was stable, blood pressure did not change dramatically, as well as TAWSS and OSI were lower. It was shown that AVF calcification was correlated with hemodynamic changes, which may contribute to further understanding the mechanisms of AVF calcification and providing scientific evidence to inform the optimization of surgical strategies and the development of personalized interventional measures in clinical contexts.

Thermal fluctuations have been found to significantly influence the dissipation range of turbulence, an effect beyond the scope of the classical Navier-Stokes equations. In this study, we investigate their impact on turbulent channel flow by numerically solving the fluctuating hydrodynamic equations. Simulation results confirm theoretical predictions that the energy spectrum, dominated by thermal fluctuations, follows a k² power law. When thermal fluctuations reach sufficient intensity, they disrupt the dominant turbulent structures responsible for most of the kinetic energy, leading to a reduction in large-scale spectral energy. Additionally, thermal fluctuations increase wall skin friction by modifying mean velocity profiles. The injected energy amplifies Reynolds normal stresses while maintaining the magnitude of Reynolds shear stress. Furthermore, thermal fluctuations enhance the symmetry and homogeneity of velocity fluctuations while reducing their intermittency. Despite these effects, the balance between kinetic energy production and dissipation, including both turbulent and thermal contributions, remains preserved.

Due to the high critical current density J_c, Nb₃Sn becomes the promising candidate for future high-field magnets. Unfortunately, especially at high fields, mechanical loads such as Lorentz force and thermal stress can lead to damages, critical property degradations, and even quench. It has long plagued high-J_c Nb₃Sn wire at the core of high-field magnets, and seriously threaten applications of Nb₃Sn magnets. In this paper, we introduce a multiscale nonlinear mechanical model coupled with progressive damage effects, thermal, and electromagnetic fields to simulate the multi-physics behaviors of superconducting magnets. This model is validated by conducting comparisons with uniaxial tensile experiments of multi-filamentary Nb₃Sn wires and further measurements conducted on Nb₃Sn solenoid magnets. Leveraging this model, we investigate the nonlinear mechanical response of Nb₃Sn solenoid during preloading, cooling down, and current ramping process. There exist obvious mechanical property deteriorations caused by filament damages and plastic deformation of Copper matrix at high fields. Remarkably, mechanical reaction results in significant degradation of quench current threshold, and changes the quench propagation path. The current margin of Nb₃Sn solenoid of FECR (first 4th generation electron cyclotron resonance ion source) magnet versus transport current with consideration of strain sensitivities of J_c has been illustrated. These findings pave the way for analysis of elastoplastic damage behaviors and quench characteristics of superconducting solenoid magnet wounded by multi-filamentary wires.

In practical engineering applications of polymer drag reduction (DR), the environment is often complex and dynamic. Factors such as water depth and flow velocity could interfere with the constant polymer releasing state, resulting in the occurrence of fluctuation in the polymer releasing process. An experimental study was conducted to evaluate the polymer DR performance when periodically released into turbulent channel flow in square and triangular waveform patterns. The polymer diffusion and the velocity profile were obtained using particle image velocimetry and planar laser-induced fluorescence measurements to further analyze the DR mechanism. For the square wave pattern with abrupt changes within a period, the existing rising and declining phases in DR and near-wall polymer concentration curves as a function of time always resulted in poor tracking performance. Although increasing the slow-release period shifted the peak point towards a plateau stage and elevated the peak DR, the gradual decline in trough DR and the extended duration of the decline still weakened the average DR efficiency over one period. In other words, the DR effects became increasingly weaker compared to those achieved by continuously releasing methods due to the extended period of ineffectiveness. Increasing the slow-release rate and the duty cycle could improve the average DR efficiency, but this was still inferior to the continuously releasing method because of the aforementioned poor tracking performance. Conversely, the DR and near-wall polymer concentration curves exhibited significant tracking performance in response to the triangular wave pattern with gradual change characteristics. Increasing the period enlarged and then stabilized the peak DR, while diminishing and stabilizing the trough DR. Thus, the average DR effect remained unchanged, and always comparable to the continuously releasing method. Increasing the slow-release rate promoted and then stabilized both the peak and trough DRs. The increasing trend of the overall DR efficiency versus slow-release rate nearly overlapped with that achieved by the continuously releasing method. This indicated that releasing polymer by triangular waves could not only provide equivalent DR to the continuously slow-release method but also offer greater environmental robustness. It is conjectured that other gradient wave patterns for periodically releasing polymer solution may also result in improved DR performance, potentially surpassing the efficiency of the continuous releasing method. This study offers valuable insights into optimizing polymer release strategies for the purpose of enhancing DR in external flows.

In sediment transport on a mobile erodible bed, near-bed particles tend to preferentially locate under specific flow regions and form an uneven bedform. These moving and mobile particles can significantly modulate turbulence at various scales, from inertial large-scale motions to small viscous motions. This study analyzes the particle-resolved direct numerical simulation data of particle-laden two-phase flow with multilayers of particles in turbulent flows over static and mobile beds. The double-average method is adopted for energy transfer analysis. The result shows that the alternative streaky bedform in the spanwise directions correlates with the streak structures in the near-wall turbulence in the mobile bed case. The energy redistribution and exchange, as well as the dissipation, are analyzed in detail, and an energy transfer diagram is given in the last to summarize the energy transfer processes. In both the static and mobile bed cases, flow energy is introduced into the system via the work performed by volume forces acting on the mean flow. The viscous dissipations in the double mean and form-induced fields are more pronounced in the static bed case, and the work done by the fluid-particle interfacial stress in the double mean and turbulent fields is more pronounced in the mobile bed case. The prominent energy contribution in the form-induced field is the production by the form-induced stress on the mean strain in the mobile bed case. In addition, sediment transport involving a limited number of mobile particles is insufficient to capture the energy transfer processes that occur over the troughs and may intertwine the energy transfer processes over the mobile particles and the fixed particle bed.

Uncertain parameters are widespread in engineering systems. This study investigates the modal analysis of a fluid-conveying pipe subjected to elastic supports with unknown-but-bound parameters. The governing equation for the elastically supported fluid-conveying pipe is transformed into ordinary differential equations using the Galerkin truncation method. The Chebyshev interval approach, integrated with the assumed mode method is then used to investigate the effects of uncertainties of support stiffness, fluid speed, and pipe length on the natural frequencies and mode shapes of the pipe. Additionally, both symmetrical and asymmetrical support stiffnesses are discussed. The accuracy and effectiveness of the Chebyshev interval approach are verified through comparison with the Monte Carlo method. The results reveal that, for the same deviation coefficient, uncertainties in symmetrical support stiffness have a greater impact on the first four natural frequencies than those of the asymmetrical one. There may be significant differences in the sensitivity of natural frequencies and mode shapes of the same order to uncertain parameters. Notably, mode shapes susceptible to uncertain parameters exhibit wider fluctuation intervals near the elastic supports, requiring more attention.

In-situ experiments were carried out using the powder metallurgy superalloy FGH96 to observe the evolution of small fatigue cracks under different thermal and mechanical loading scenarios. Electron backscatter diffraction was subsequently employed to assist in the analysis of how changes in temperature and stress levels influence the growth behavior of small fatigue cracks. The influence mechanisms of crystallographic parameters (Schmid factor and geometric compatibility factor) on small fatigue crack propagation were quantitatively examined. During the study, a temperature-induced transition in the dominant crack propagation mode was observed in FGH96, occurring between 600 and 700 °C. Specifically, the dominant mode changed from transgranular to intergranular propagation. Within the temperature range from room temperature to 600 °C, neither temperature nor applied stress level showed a noticeable effect on the relationship between the Schmid factor of the activated slip system and the maximum Schmid factor. Under these conditions, small fatigue cracks followed a consistent propagation mechanism governed by the Schmid factor. The Schmid factor emerged as a key parameter in controlling the transgranular propagation behavior of small fatigue cracks in FGH96, whereas the role of the geometric compatibility factor appeared to be limited.