Institute Of Mechanics,Chinese Academy of Sciences
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The significant contribution of stochastic forcing to nonlinear energy transfer in resolvent analysis
No full textNonlinear energy transfer is represented through eddy viscosity and stochastic forcing within the framework of resolvent analysis. Previous investigations estimate the contribution of eddy-viscosity-enhanced resolvent operator to nonlinear energy transfer. The present article estimates the contribution of stochastic forcing to nonlinear energy transfer and demonstrates that the contribution of stochastic forcing cannot be ignored. These results are achieved by numerically comparing the eddy-viscosity-enhanced resolvent operator and stochastic forcing with nonlinear energy transfer in turbulent channel flows. Furthermore, the numerical results indicate that composite resolvent operators can improve the prediction of nonlinear energy transfer. © 2024 The Author
Modeling of Thermomechanical Energy Pile Behavior with Temperature-Induced Radial Effects
No full textGeothermal energy foundations or thermoactive ground structures present sustainable alternative systems for meeting heating/cooling needs of buildings in different seasons and are also cost-effective in comparison with stand-alone ground source heat pumps. In simultaneously supporting the upper structure and exchanging heat with the surrounding soil, energy piles may suffer from coupled thermomechanical loads and exhibit complex behaviors in stress and deformation. For predicting energy pile responses to the external loads, numerical models have been established with load transfer method considering the deformation and force in the longitudinal direction of the pile. In heating and cooling, the deformation may not only occur along the length of the pile, but also happen in the radial direction, which may lead to a change of ultimate lateral frictional resistance and load transfer characteristics. Very little work has been undertaken to include the radial effects in the numerical model of energy pile behavior. With the theory of geotechnical medium circular hole expansion, the elastic and elastic-plastic solutions of radial stress of energy piles caused by temperature change are derived in this work and the analytical model under joint action of axial-radial temperature effect are implemented into the numerical model with load transfer method. The results of the study indicate that the operation of energy piles not only affects lateral resistance and axial force in the axial direction but also has an impact on radial aspects due to temperature changes. This influence is primarily manifested when the energy piles are heated, resulting in radial expansion. Such expansion strengthens the soil's constraint on the piles, thereby increasing their bearing capacity. The opposite effect occurs when cooling. The numerical calculation method established in this paper effectively captures these phenomena
Unifying linear proportionality between real contact area and load in rough surface contact
No full textA long-standing debate and challenge in contact mechanics is to confirm the linearity between the real contact area and load on rough surfaces as well as its proportionality. Here, we first theoretically prove the linearity between the real contact area and load on rough surfaces by considering an infinite number of surface asperities. The mechanism for such linearity is that the applied force on each "small region" on the rough surface is directly proportional to the area of the region, resulting in a statistical proportionality between the total load and area. This explanation is confirmed via Green's function molecular dynamics (GFMD) simulations. On this basis, we develop a novel framework of surface slope-based multi-asperity contact model. The proportionality between the contact load and area is governed by the elastic property, mean absolute slope, and shape coefficient of the contact surface over the pressed depth. The elastic contacts of single-scale and multiscale rough surfaces are investigated using the developed contact model and GFMD. The shape coefficient of rough surfaces predicted by numerical simulations closely resembles that of surfaces with symmetric parabolic asperities. This work not only sheds light on the physical mechanism underlying the linearity between the contact area and load on rough surfaces but also provides a theoretical foundation for designing and evaluating surface contact and friction performance in micro- and nano-engineering systems
Performance enhancement of ammonium dinitramide (ADN)-based thruster using coaxial dielectric barrier discharge
No full textTo enhance the performance of conventional ADN-based catalytic ignition thrusters, a plasma-assisted thruster was developed employing coaxial cylinder electrodes within a 1N-class thruster configuration. Steady-state ignition tests conducted on the experimental bench, demonstrated high repeatability in an atmospheric environment. High-speed imaging and proper orthogonal decomposition (POD) analysis revealed that plasma reduced the pulsation energy of the first mode from 84.96% to 75.21%, improving flame stability and concentrating the flame in the upstream part of the combustion chamber, without propagating toward the nozzle. Experimental evolution spectra of H2O, NH3, and CO2 were obtained through near-infrared (NIR) and midinfrared (MIR) spectrometers to probe chemical dynamics. The peak radiation intensity of H2O and NH3 in the 1.6-2.4 mu m range was observed to precede by about one second with plasma compared to the catalyst-only case. An increased radiation intensity ratio of H2O/NH3 in the presence of plasma indicated plasma's promotion of H2O production and NH3 consumption. Furthermore, the radiation intensity of CO2 increased by approximately 100-fold in the plasma-assisted case, indicating accelerated chemical reactions and more complete combustion. These findings highlight the potential of plasma-assisted technologies to improve the efficiency of ionic liquid-based thrusters and provide a foundation for future advancements in plasma-assisted propulsion systems
Optimized dynamic similarity models to predict SGS backscatter in 2D decaying turbulence
No full textLarge eddy simulation (LES) of two-dimensional (2D) turbulence is often used in the geostrophic flows. However, some basic dynamics underlying traditional SGS models are absent in 2D turbulence, e.g. the vortex stretching. Hence, this research proposes an optimized dynamic similarity model (DSM) for the SGS stress, which is constructed through the dynamic procedure based on the Germano identity. In addition, a modification is made to the dynamic mixed model (DMM) for the sake of realizability condition. The optimized DSM is justified in comparison with the DMM, through the a priori and a posteriori verifications, in the context of the 2D decaying turbulence with turbulent Reynolds number of Re = 3.7 x 104 and turbulent Mach number of Mt = 0.1. Special attention is paid to the consistency of the verification procedure, so that the filtering operations used in the direct numerical simulation (DNS) and LES are optimally equivalent. The SGS transport phenomena, especially the SGS backscatter, predicted by these two models are studied in detail. In addition, the optimized DSM and the DMM are extended for the modified SGS transport vectors of passive scalars to show their capability in calculating 2D turbulent mixing. The numerical results show the optimized DSM provides larger correlation coefficient, better locality, and stronger SGS backscsatter than the DMM does, and therefore it is more suitable for the LES of 2D turbulence
Rock fracture mechanism of buffer blasting with cushion layer at the borehole bottom
No full textThis study primarily investigates the rock fracture mechanism of bottom cushion layer blasting and explores the effects of the bottom cushion layer on rock fragmentation. It involves analyses of the evolution patterns of blasting stress, characteristics of crack distribution, and rock fracture features in the specimens. First, blasting model experiments were carried out using the dynamic caustics principle to investigate the influence of bottom cushion layers and initiation methods on the integrity of the bottom rock mass. The experimental results indicate that the combined use of bottom cushion layers and inverse initiation effectively protects the integrity of the bottom rock mass. Subsequently, the process of stress wave propagation and dynamic crack propagation in rocks was simulated using the continuum-discontinuum element method (CDEM) and the Landau explosion source model, with varying thicknesses of bottom cushion layers. The numerical simulation results indicate that with increasing cushion thickness, the absorption of energy generated by the explosion becomes more pronounced, resulting in fewer cracks in the bottom rock mass. This illustrates the positive role of the cushion layer in protecting the integrity of the bottom rock mass
Rock fracture mechanism of buffer blasting with cushion layer at the borehole bottom
No full textThis study primarily investigates the rock fracture mechanism of bottom cushion layer blasting and explores the effects of the bottom cushion layer on rock fragmentation. It involves analyses of the evolution patterns of blasting stress, characteristics of crack distribution, and rock fracture features in the specimens. First, blasting model experiments were carried out using the dynamic caustics principle to investigate the influence of bottom cushion layers and initiation methods on the integrity of the bottom rock mass. The experimental results indicate that the combined use of bottom cushion layers and inverse initiation effectively protects the integrity of the bottom rock mass. Subsequently, the process of stress wave propagation and dynamic crack propagation in rocks was simulated using the continuum-discontinuum element method (CDEM) and the Landau explosion source model, with varying thicknesses of bottom cushion layers. The numerical simulation results indicate that with increasing cushion thickness, the absorption of energy generated by the explosion becomes more pronounced, resulting in fewer cracks in the bottom rock mass. This illustrates the positive role of the cushion layer in protecting the integrity of the bottom rock mass
Unified model for adhesive contact between solid surfaces at micro/nano-scale
No full textBecause of the huge specific surface area at the micro/nano scale, inter-surface adhesion and surface effects play a critical role in the behavior of solid-to-solid contact. The inter- surface adhesion originates from the intermolecular traction between two surfaces, while the surface effects, including residual surface stress and surface elasticity, result from the physical discrepancy between the surface atoms and their bulk counterparts. Despite the importance of both effects, theoretically modeling them together is still a challenging open issue because of the nonlinear coupling nature in between. This study is dedicated to the development of a unified theoretical framework with consideration of both inter-surface adhesion and surface effects based on the Gurtin-Murdoch surface elasticity theory. The two effects are integrated into a self- consistent equation concerning surface gaps and interactions, and a novel regularization method is proposed to address the oscillation and singularity of the equation. It is demonstrated that an adhesive contact problem with surface effects can be decomposed into two fundamental issues. One addresses the classical problem without considering residual surface stress or surface elasticity, and the other focuses solely on residual surface stress. Theoretical predictions show that the surface effects suppress or even eliminate the surface deformation and jumping instability during contact, effectively stiffening the solid surfaces. Three types of pull-off force transitions with surface effects are obtained, forming continuous bridges among the rigid (Bradley), soft (JKR), and liquid-like (Young-Dupre) limits. The adhesion transitions considering surface effects in this work are universal, and the existing limits or transitions can be regarded as special cases of this work. Our study provides a further understanding of the adhesive contact between micro/nano solids and may be instructive for practical applications where inter-surface adhesion and surface effects are dominant, such as nanoindentation, micro-electro-mechanical systems, and microelectronics
Dynamics of solitary waves on a ferrofluid jet: the Hamiltonian framework
No full textThe stability and dynamics of solitary waves propagating along the surface of an inviscid ferrofluid jet in the absence of gravity are investigated analytically and numerically. For the axisymmetric geometry, the problem is shown to be a conservative system with total energy as the Hamiltonian; however, one of the canonical variables differs from those in the classic water-wave problem in the Cartesian coordinate system. The Dirichlet-Neumann operator appearing in the kinetic energy is then expanded as a Taylor series, described in homogeneous powers of the surface displacement. Based on the further analysis of the Dirichlet-Neumann operator, a systematic procedure is proposed to derive reduced model equations of multiple scales in various asymptotic limits from the full Euler equations in the Hamiltonian/Lagrangian framework. Particularly, a fully dispersive model arising from retaining terms valid up to the quartic order in the series expansion of the kinetic energy, which results in quadratic and cubic algebraic nonlinearities in Hamilton's equations and henceforth is abbreviated as the cubic full-dispersion model, is proposed. By comparing bifurcation curves and wave profiles of various types of axisymmetric solitary waves among different model equations, the cubic full-dispersion model is found to agree well with the full Euler equations, even for waves of considerably large amplitudes. The stability properties of axisymmetric solitary waves subjected to longitudinal disturbances are verified with the newly proposed model. Our analytical results, consistent with Saffman's theory, indicate that in the axisymmetric cylindrical system, the stability exchange subjected to superharmonic perturbations also occurs at the stationary point of the speed-energy bifurcation curve. A series of numerical experiments for the stability and dynamics of solitary waves are performed via the numerical time integration of the model equation, and collision interactions between stable solitary waves show non-elastic features
Intrinsic characteristics of grain boundary elimination induced by plastic deformation in front of intergranular microcracks in bcc iron
No full textAdditive grain boundary (GB) engineering holds significant potential for developing materials and structures with excellent mechanical properties by precisely controlling GB structure. The GBs that can be eliminated by plastic behavior activities prior to crack cleavage are ideal special ones for resisting intergranular fracture. Through molecular dynamics simulation, this work constructs special boundaries and studies the intrinsic characteristics of GB elimination. The results show that GB elimination phenomenon significantly depends on crack growth direction and GB plane. The classical theory developed by Rice fails to identify the mechanisms of two dependent characteristics. According to shear forces on atoms at crack tip, this work finds that the dependence of GB elimination on crack growth direction is attributed to the change of atomic slip characteristics. GB elimination occurs in specific growth directions where atomic slip is driven by the system of (112)[111]. By considering T stress effect, GB elimination and its dependence on GB plane are well explained. The dependence of GB elimination on GB plane is attributed to the complex changes in critical stress intensity factors for twinning formation, perfect dislocation nucleation, and cleavage. GB elimination occurs on specific GBs where T stress makes the critical stress intensity factors for twinning and dislocation nucleation significantly lower than that for cleavage. The identified intrinsic characteristics of GB elimination provide references for GB design