Acta Mechanica Sinica最新文献

Intracranial aneurysm (IA) is a prevalent cerebrovascular disease associated with high mortality and disability rates upon rupture. The hemodynamics of IA, which are significantly influenced by geometric parameters, directly impact its rupture. This study focuses on investigating the transient flow characteristics in saccular IA models fabricated using a water droplet-based method, specifically examining the influence of neck widths. Particle image velocimetry technique and numerical simulation were employed to investigate the dynamic evolution of flow structures within three IA models. The results reveal that neck width (W) has a substantial effect on flow characteristics in the neck region, subsequently impacting the deep flow inside the sac. Three distinct patterns were observed during flow evolution inside the sac: for W = 2 mm, two vortices occur and then disappear with relatively low average flow velocity; for W = 4 mm, enhanced effects of a high-speed jet result in periodic pulsatile flow velocity distribution while maintaining stable vortex core position; for W = 6 mm, significant changes in flow velocity occur due to size expansion and intensity increase of vortices. These findings demonstrate that neck widths play a complex role in influencing transient flow characteristics within IAs. Overall, this research contributes to further understanding transient flow behaviors in IAs.

It is commonly accepted that the formation of oil and gas reservoirs in deep-buried strata is almost impossible due to the huge compaction of in-situ crustal stresses. Nevertheless, recent hydrocarbon explorations in the Tarim Basin have discovered reservoirs at depths exceeding 8 km. The reservoirs exhibit a strong correlation to the strata’s faults and large fractures, yet the precise underlying mechanical mechanism remains obscure. To illuminate how the faults may facilitate the existence of such deep-buried reservoirs, we consider three ideal scenarios involving unconventional hole-crack interactions under remote biaxial compression. Our focus is on the stress concentration of the hole, influenced by the long main cracks. Closed-form compressive stress solutions are obtained based on our simple theoretical models, showing that long cracks significantly reduce the stress concentration of nearby holes. We quantify the reducing effect of the cracks’ angle, surface friction, and pressure on the maximum shear and von Mises stresses around a hole, combining with finite element analysis results. The stress shielding effect is qualitatively consistent with the available experimental observations that the deep-buried caves are often located near the faults and large fractures in carbonate strata. Our results will be beneficial for future exploration of superdeep petroleum reservoirs.

Recently, significant progress has been made in conceptually describing the dynamic aspects of coarse particle entrainment, which has been explored experimentally for open channel flows. The aim of this study is to extend the application of energy criterion to the low mobility aeolian transport of solids (including both natural sediment and anthropogenic debris such as plastics), ranging from incomplete (rocking) to full (rolling) entrainments. This is achieved by linking particle movements to energetic flow events, which are defined as flow structures with the ability to work on particles, setting them into motion. It is hypothesized that such events should impart sufficient energy to the particles, above a certain threshold value. The concept’s validity is demonstrated experimentally, using a wind tunnel and laser distance sensor to capture the dynamics of an individual target particle, exposed on a rough bed surface. Measurements are acquired at a high spatiotemporal resolution, and synchronously with the instantaneous air velocity at an appropriate distance upwind of the target particle, using a hot film anemometer. This enables the association of flow events with rocking and rolling entrainments. Furthermore, it is shown that rocking and rolling may have distinct energy thresholds. Estimates of the energy transfer efficiency, normalized by the drag coefficient, range over an order of magnitude (from about 0.001 to 0.0048 for rocking, up to about 0.01, for incipient rolling). The proposed event-based theoretical framework is a novel approach to characterizing the energy imparted from the wind to the soil surface and could have potential implications for modelling intermittent creep transport of coarse particles and related aeolian bedforms.

The increasing accumulation of space debris threatens the integrity and functionality of satellites and complicates orbital operations. This paper constructs an advanced rigid-flexible coupling dynamic model for tethered satellite systems, tailored to enhance space debris management. Utilizing the nodal position finite element method, the model significantly improves the precision of simulating tether dynamics and captures the complex interactions involving satellite and debris attitude dynamics. This advancement allows for detailed examination of potential tether entanglements and provides crucial data for optimizing deorbiting processes. To overcome the limitations of conventional control techniques, a robust adaptive sliding mode control strategy is developed. This approach is specifically designed to manage the unpredictable conditions of the low-Earth orbit and ensure precise satellite attitude control, critical for successful debris removal. Validated through extensive numerical simulations, our model and control strategy demonstrate substantial improvements in operational reliability and safety, significantly enhancing the success rate of deorbiting missions.

The accurate assessment of cardiac motion is crucial for diagnosing and monitoring cardiovascular diseases. In this context, digital volume correlation (DVC) has emerged as a promising technique for tracking cardiac motion from cardiac computed tomography angiographic (CTA) images. This paper presents a comprehensive performance evaluation of the DVC method, specifically focusing on tracking the motion of the left atrium using cardiac CTA data. The study employed a comparative experimental approach while simultaneously optimizing the existing DVC algorithm. Multiple sets of controlled experiments were designed to conduct quantitative analyses on the parameters “radius” and “step”. The results revealed that the optimized DVC algorithm enhanced tracking accuracy within a reasonable computational time. These findings contributed to the understanding of the efficacy and limitations of the DVC algorithm in analyzing heart deformation.

We present the approaches to implementing the (k-{sqrt{k}} L) turbulence model within the framework of the high-order discontinuous Galerkin (DG) method. We use the DG discretization to solve the full Reynolds-averaged Navier-Stokes equations. In order to enhance the robustness of approaches, some effective techniques are designed. The HWENO (Hermite weighted essentially non-oscillatory) limiting strategy is adopted for stabilizing the turbulence model variable k. Modifications have been made to the model equation itself by using the auxiliary variable that is always positive. The 2nd-order derivatives of velocities required in computing the von Karman length scale are evaluated in a way to maintain the compactness of DG methods. Numerical results demonstrate that the approaches have achieved the desirable accuracy for both steady and unsteady turbulent simulations.

This paper studies the bandgap characteristics of a locally resonant metamaterial beam with time delays. The dispersion relations are addressed based on transfer matrix method. The governing equations of motion of the beam in the frequency domain are given according to spectral element method. The amplitude-frequency responses of the forced beam are determined by solving linear algebraic equations. The obtained results show that the time-delayed feedback control has great relationships with the location, width and number of the bandgaps. It is interesting that the time delay can change the direction of the movement of the bandgap and give rise to the generation of multiple bandgaps. The influences of different combinations of control parameters on the bandgap properties are shown, such as broadening effects.

This study numerically investigates the locomotion of active matter over a circular cylinder in a confined microchannel. We consider the effects of cylinder size, swimming Reynolds number on the motion characteristic of three kinds of swimmers. The swimmer’s motion over a cylinder in a microchannel can be classified into seven modes. The cylinder diameter and swimming Reynolds number have no impact on the motion mode of neutral swimmers. When approaching the cylinder, pullers mainly perform periodic motion near the left side of cylinder, the pushers primarily perform periodic motion near the right side of cylinder. The mechanism of the periodic motion is mainly induced by the hydrodynamic interaction between the cylinder, channel walls, and the pressure near the swimmer. As cylinder diameter increases, pushers are more likely to exhibit periodic motion on the surface of cylinder than the pullers. Puller is unable to stabilize on the surface of cylinder at low Reynolds number, it migrates to the right side of cylinder at high Reynolds number, showing a pattern opposite to that observed for pushers. The results provide a possible new path for controlling active matter in microfluidic devices.

A hydrodynamic model is used to study Kelvin-Helmholtz (KH) instability of the interface between two particle-laden inviscid fluids moving with two different uniform mean velocities. Explicit eigen-equation is derived to study the effect of suspended particles on the growth rate of KH instability. For dusty gases with negligible volume fraction of heavy particles and small particle-to-fluid mass ratio, the real and imaginary parts of leading-order asymptotic expression derived by the present model for the growth rate are shown to be identical to the earlier results derived by the classical Saffman model established for dusty gases. Beyond the known results, explicit leading-order asymptotic expressions for the effect of suspended particles on the growth rate are derived for several typical cases of basic interest. It is shown that the suspended particles can decrease or increase the growth rate of KH instability depending on the Stokes numbers of the particles and whether the particles are heavier or lighter than the clean fluid. Compared to the mass density of the clean fluid, our results based on leading-order asymptotic solutions show that heavier particles and lighter particles have opposite effects on the growth rate of KH instability, while the effect of neutrally buoyant particles on the growth rate of KH instability is negligible.

The transformation from multibody models to lumped-parameter models is a crucial aspect of vehicle dynamics research. The velocity transformation method is adopted in this research, and the suspension multibody model is described using only one degree of freedom. It is found that the equivalent mass of the system is time-dependent during the simulation process, as observed in numerical simulations. Further symbolic calculations are conducted to derive the analytical form of the equivalent mass, and the results show that once the static parameters are determined, the equivalent mass of the suspension system is determined solely by the vertical position of the suspension upright, which reveals the kinematics characteristic of the equivalent mass of the suspension system. It is found that the equivalent mass experiences smaller changes when the suspension is compressed from the middle position, but larger changes when the suspension is extended. Furthermore, by comparing the multibody model, the lumped-parameter model with static mass, and the proposed lumped-parameter model considering the kinematics characteristic of the equivalent unsprung mass, the proposed model produces simulation results that more closely match the original multibody model than the model with static mass. The improvements in accuracy can be up to 20% under certain evaluation metrics.