Institute Of Mechanics,Chinese Academy of Sciences
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Direction-enhanced guided wave transducer based on pulse compression technique
No full textUltrasonic guided waves are widely employed for structural health monitoring (SHM) as well as in nondestructive testing and evaluation (NDT&E). The 0th order shear horizontal (SH0) guided wave, known for its simple vibration mode and non-dispersive characteristics, simplifies the interpretation of detection signals. However, traditional transducers that generate and receive the SH0 guided wave bidirectionally pose challenges for defect localization. This paper introduces an electromagnetic acoustic transducer (EMAT) based on the pulse compression technique for directional enhancement. The direction-enhanced EMAT consists of a linear frequency modulation (LFM) excitation signal and periodic permanent magnets (PPMs) arranged in variable spacings and widths. An analytical model is developed to predict the received signals and analyze the pulse compression mechanism of the PPM EMAT. During the transmitting and receiving process, the PPMs align the phase of the broadband excitation signal to obtain a compressed and enhanced received signal. Additionally, the enhanced side can be controlled using different LFM excitation signals. Finally, the directional enhancement is validated through finite element simulations and experimental results. The proposed direction-enhanced guided wave transducer and its design methodology demonstrate significant potential for SHM and NDT&E in plates, shells, and pipelines
On the vibrational excitation of shock-heated air. I. CO/O<sub>2</sub>/Ar mixtures
No full textThis series of papers aims to investigate the vibrational excitation behaviors of shock-heated high-temperature air using the CO rovibrational thermometry. The present part of this series serves as a prerequisite for the work. The time-dependent rovibrational temperatures of CO were measured during the vibrational excitation process of CO/O-2/Ar mixtures within the temperature range of 1680-2970 K and pressures of 1.37-2.43 bar to investigate the vibrational energy transfer between CO and O-2. The state-to-state (StS) approach was employed in the simulations, where the vibrational energy levels of CO and O(2)are treated as pseudo-species. The datasets of vibrational state-specific reactive and inelastic rate coefficients for CO + O-2, O-2+ Ar, and CO + O collisions were generated in this paper. The present StS simulations agreed well with the experimental data, whereas the Schwartz-Slawsky-Herzfeld (SSH) formula predicted faster CO vibrational temperatures. A sensitivity analysis was then performed and highlighted that the vibration-vibration (V-V) energy transfer between CO and O-2(tau(CO-O)(V-V)(2)) is the key item inducing the discrepancies observed between the SSH results and experimental data. Therefore, a new expression was summarized for calculating tau(CO-O)(V-V)(2)based on the present experiments, and it also agreed well with the measured data in the literature at 100-700 K. In addition, the oxidation of CO and the effects of vibrational energy transfer between CO and O(2)were discussed. Since differences in vibrational temperatures between CO and O(2)were observed during the relaxation process, the refined rate of vibrational energy transfer between CO and O(2)paves the way for the investigations in Part II
Dynamic evolution of oil droplet population in non-Newtonian swirling flow field
No full textPolymer flooding is widely applied in offshore oil recovery, but the resulting non-Newtonian, high-viscosity mixtures pose challenges for rapid oil-water separation in compact vane-type swirl separators. To investigate the dynamic evolution of oil droplet clusters in such complex flows and improve separation efficiency, this study developed a coupled simulation model integrating the Eulerian multiphase model, power-law rheological model, population balance model, and Reynolds stress model turbulence model. The model captures the breakup and coalescence behavior of oil droplets in non-Newtonian swirling flow fields under varying polymer viscosity coefficient, rheological index, inlet liquid velocity, and inlet oil-phase concentration. The results show that increasing the polymer viscosity coefficient leads to larger average droplet diameters (D32, D90, D5) across the pipe section, but a gradual decrease in centerline D32. A lower rheological index reduces average D32 and D5, but increases centerline D32 and slightly increases average D90. As inlet liquid velocity rises, average D32 increases, while centerline D32 and D90 first rise then fall, peaking at 1.2 m/s; D5 decreases steadily. Higher inlet oil-phase concentration enhances collision frequency, resulting in an overall increase in droplet sizes. These findings elucidate the interaction between rheology and flow conditions in governing droplet cluster dynamics within swirling flows, thereby enabling the directional optimization of swirl-vane separators for non-Newtonian fluids
Investigation of pore structures and flow transport characteristics of continental shale in Dongying Sag
No full textMultiscale pore-slit structures, non-linear flow and complex lithofacies of continental shale in Dongying Sag present significant challenges for reservoir evaluation and sweet spots identification. This study establishes a comprehensive analytical framework through integrated physical experiments and high-resolution imaging to characterize pore-slit structures and flow dynamics across various lithofacies. Firstly, a novel PSD (pore size distribution) splicing technique is developed by combining multiscale feature extraction and a transformed method, seamlessly uniting nitrogen adsorption data with SEM (scanning electron microscopy) image analysis. Compared with traditional methods, this advanced technique provides a more comprehensive representation of shale pore size distribution and structural characteristics. Three permeability indexes, including the non-Darcy index (Knon-D), anisotropy index (Ka), and slit index (Kf), are proposed to quantify flow behavior based on the cubic-block and matrix permeability experiments. These innovative indexes allow us to explore the complex non-linear flow behaviors, the anisotropic flow properties and pore-slit crossflow characteristics of shale. Comparative analysis reveals distinct pore structure and flow characteristics among different lithofacies: clay-rich shale exhibits high porosity and matrix permeability, while carbonate-rich shale demonstrates stronger nonlinear flow capacity and anisotropic permeability. Finally, the intrinsic relationship between mineral composition, pore structure, and complex flow has been investigated, and the identification charts for porosity and permeability have been established. These findings provide valuable tools for enhanced reservoir evaluation and sweet spot identification in the continental shale formations of Dongying Sag
Influence of elastic stress field on subsurface cracks in Vickers indentation of BK7 glass
No full textIn this work, the elastic stress field dependence of subsurface cracks in silicate glass during indentation is investigated with theoretical and experimental methods. An analytical model for the indentation elastic stress field is proposed to evaluate the evolution of subsurface cracks during Vickers indentation of silicate glass. To validate the proposed model, indentation cracking experiments were performed on BK7 glass with an instrumented indentation machine, and the morphology of subsurface cracks was detected using a laser confocal scanning microscope from the indentation cross sections. The results showed that the median crack, originating at the bottom of the plastic zone, propagated downward straightly, while two distinct lateral crack mechanisms occur adjacent to the elastic-plastic boundary and propagate in different directions. Furthermore, the measured values of the plastic zone radius and median crack depth were compared with the theoretical model results. It was found that the experimental measurements coincide well with the model predictions
Human vibration comfort analysis for a train entering a tunnel
No full textWith the rapid advancement of high-speed railway networks, trains increasingly transition between open lines and tunnels at high speeds. These sudden entries into confined tunnel spaces induce sharp changes in aerodynamic loads, directly affecting passenger comfort. To investigate passenger vibration comfort during highspeed train tunnel entries, this paper proposes an integrated approach combining a train/tunnel aerodynamic model with a train-track-seat-human body coupling model. Among them, the train-track-seat-human coupling model is constructed using multi-rigid-body model, and the accuracy of both models is verified through a vibration test bench, full-scale vehicle experiments, and moving model tests. Based on the two established models, the vibration responses of the car body and the human body under different train/tunnel blockage ratios and operating speeds are obtained. Additionally, the flow field characteristics during high-speed train tunnel entry are studied in detail, providing a theoretical basis for subsequent research on resonance effects between the car body and the human body. The vibration comfort of the human body at different carriage positions is evaluated according to the ISO 2631 standard when the train enters single-and double-track tunnels. The above analysis is crucial for the forward design of passenger ride comfort during high-speed train tunnel entry
Destabilization mechanism of oblique detonation induced by the recirculation zone in a channel flow
No full textThe stability of flow structures is crucial for the combustion efficiency of oblique detonation waves (ODWs). Prior studies have predominantly attributed the destabilization of ODWs to the merging of subsonic regions behind detonation Mach stem. However, the flow structures of ODWs in channels are complex, probably leading to a variety of destabilization mechanisms. This study numerically investigates the ODWs under the influence of viscosity using a detailed chemical reaction model. Results show that the recirculation zone on the lower channel wall plays an important role in the stability of the detonation wave system, which has been ignored in most studies. Specifically, when the secondary reflected shock generated by the lower recirculation zone interacts with the recirculation zone on the upper wall, it triggers the continuous growth of the upper recirculation zone and the formation of an aerodynamic throat. This ultimately leads to flow choking and destabilization of the detonation waves. Based on the above findings, we further evaluate the effectiveness of a moving wedge in regulating the unstable ODWs. It is found that promptly moving the wedge downstream can suppress the upstream movement of the lower recirculation zone, preventing secondary reflected shocks from disrupting the upper recirculation zone. As a result, the unstable detonation wave system is successfully re-stabilized
Numerical Simulation of CO<sub>2</sub>-Water Two-Phase Flow Characteristics in CO<sub>2</sub> Sequestration in Deep-Sea Sediments
No full textCO2 sequestration in deep-sea sediments is an effective approach for mitigating climate change. Understanding multiphase migration during the CO2 injection and long-term containment is critical. In this study, we establish a large-scale three-dimensional numerical model based on the real stratigraphy of the Shenhu area in the South China Sea to investigate the migration patterns of free-phase CO2. First, to reveal the influence of injection parameters on the short-term sequestration efficiency, we investigate the CO2 distribution pattern, saturation distribution, pressure evolution, and seepage velocity under different injection rates. Subsequently, the Weibull distribution theory is introduced to characterize the permeability heterogeneity in order to clarify the effects of heterogeneity on the CO2 replacement. Finally, the flow migration patterns of CO2 in fracture-matrix interactions are investigated. The results indicate that the injection rate dominates the short-term sequestration efficiency. A higher CO2 injection rate enabled the maximum saturation near the wellhead to reach 0.379 and generated a maximum pressure of 17 MPa. Meanwhile, the CO2 migration distance under these conditions is 54.88 m longer than that of the low-injection-rate case. However, this also makes it more difficult for reservoir pressure to return to its original state after stopping injection. The heterogeneity of the permeability significantly alters the migration pathways and saturation distribution of CO2, resulting in a larger area of high-pressure zones but lower saturation near the wellhead while also generating higher peak pressure. The fracture networks can effectively reduce pressure buildup and enhance the level of CO2 saturation during the injection stage by regulating seepage pathways. While in the subsequent free-migration stage, it promotes the redistribution of CO2 and accelerates the recovery of reservoir pressure. We compare the present numerical model with the theoretical model and the experimental study, whose average absolute relative deviations are 7.6% and 3.75%, respectively. This study provides a theoretical basis for the parameter design and reservoir risk assessment of CO2 sequestration projects in the South China Sea
Inherent Tensile Strength and Stretchability of Unentangled Elastomers
No full textElastomers, as three-dimensional cross-linked polymer networks, are essential materials in a wide range of applications. Strength and stretchability are fundamental mechanical properties of elastomers, but how they are determined by the network structure remains inadequately understood. In this work, we combine theoretical analysis with extensive molecular dynamics (MD) simulations to investigate the single-chain mechanics and large-strain mechanical behavior of unentangled elastomers. We show that the rupture of individual polymer chains follows a mechanochemical process governed by an external force-dependent energy barrier E b. We derive the relationship between macroscopic tensile stress, the conformational statistics of network strands, and the force-extension behavior of individual chains for unentangled elastomers. This relationship is corroborated by our MD simulations. The inherent strength sigma inh of elastomers is found to be 1-2 orders of magnitude lower than the theoretical ideal strength sigma is, as only a small fraction of strands bear significant stretching tensions during network rupture. During deformation, strand scission is driven by the straightening and rupture of the shortest paths (SPs) in the polymer network, and the critical stretch ratio lambda d marking the onset of scission can be predicted through topological SP analysis of the undeformed network. Our findings hold for unentangled elastomers with varying chain lengths, network junction functionalities, and fractions of topological defects. This work promotes the fundamental understanding of the strength, damage evolution, and stretchability of polymer networks, and also provides valuable guidance for designing elastomers with tailored strength and stretchability
Abnormal stress rebound after dynamic void coalescence in metallic glasses
No full textUnderstanding the microscopic mechanism of void coalescence is essential for evaluating the accumulation of dynamic damage in structural materials. However, experimental characterization of such a transient process remains extremely challenging. Here, the spatial arrangement of pre-existing voids and the influence of strain rate on dynamic void coalescence in a prototypical metallic glass (MG) are systematically investigated by molecular dynamics under conditions of uniaxial (1D) and triaxial (3D) tensile loading. It is found that, under 1D loading, the void arrangement affects only the stress-strain response, without impacting the growth and coalescence rate of the voids. However, under 3D dynamic loading, temperature around the voids undergoes a significant decrease after void coalescence. From the perspective of atomic packing, the number of mechanically stable atomic Voronoi polyhedra recovers as strain rate goes up. As a result, material experiences abnormal stress rebound after void coalescence due to the unexpected microstructural hardening effect, which is absent in crystalline metals. Meanwhile, the stress rebound strength can be controlled by adjusting void characteristics or material parameters. This unusual stress rebound might find applications for metallic materials under extreme conditions