力学学报

NUMERICAL SIMULATION AND MECHANISM ANALYSIS OF HYPERSONIC ROUGHNESS INDUCED TRANSITION

Zhu Dehua, Yuan Xiangjiang, Shen Qing, Chen Lin

Physical mechanisms of hypersonic boundary layer transition induced by diamond roughness element are investigated by means of direct numerical simulation (DNS) from the aspects of topological structure stability and hydrodynamic stability. Topological structure stability theory reveals the existence of unstable connection orbit between saddle-saddle points in front of the roughness element and unstable connection orbit between saddle-node-saddle points behind it. Consequently, unsteady and asymmetric structures are formed owing to the influence of disturbance. The hydrodynamic stability analysis shows that high-frequency disturbing waves are invoked by the diamond roughness element and large-scale vortices tend to break up along the development of disturbance. The effects of both instabilities coexist in the flow field. In addition, comparisons are made between different roughness types (cylindrical, ramp, diamond), analyzed transition mechanism of different roughness types, support design of hypersonic forced-transition device in the theory.

2015, 47(3): 381-388. doi: 10.6052/0459-1879-14-217

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EXPERIMENTAL STUDY ON JUDGMENT METHOD OF COMBUSTION MODE ON DUAL-MODE SCRAMJET

Li Fei, Wang Zhipu, Yu Xilong, Gu Hongbin, Chen Lihong, Zhang Xinyu

For a dual-mode scramjet engine, different combustion modes present different flame-stability mechanisms and different flow characteristics. Besides, thrust varies remarkably when mode transition occurs. Therefore, it is extremely important to discriminate combustion mode accurately, so as to capture flame location, measure the distribution of heat release and further optimize combustor design (i.e., configuration and fuel supplying). Since there is no effective experimental method to estimate combustion mode, a new judgment method is proposed in this paper and validation experiments were implemented in a direct-connected scramjet test facility. Multi-diagnostics, including wall pressure, high frame-rate schlieren, CH^* chemiluminescence imaging, and TDLAS (tunable diode laser absorption spectroscopy), were used in these experiments. Distributions of temperature, velocity, Mach number and heat release were obtained simultaneously. These data can be used to discriminate combustion mode and relate different flow/combustion characteristic swith different combustion modes.

2015, 47(3): 389-397. doi: 10.6052/0459-1879-14-251

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EXPERIMENTAL STUDY ON SELF-DISSIPATION-WAKE WING BASED ON INTERACTIVE INSTABILITY

Bao Feng, Yang Jinwen, Liu Jinsheng, Jiang Jianhua, Wang Junwei, He Yi

Aircraft wake vortex is an inherent co-existing phenomenon connecting to the lift-generating mechanism. It exhibits in the form of tri-dimensional vortex generated by limited wings at the wing tip, which would introduce great hazard to following flight and threaten to the flight safety. Based on a simplified rectangular airfoil, two rectangle-plate flaps were attached onto the airfoil to construct a counter rotating four-vortex wake system to induce the Rayleigh-Ludwig instability of the wake, resulting in a premature breakdown of the wake vortex. The research was performed in Fluid Mechanics Lab in Xiamen University, which equipped with a towing tank and a Particle Image Velocimetry system. Under different experimental conditions, in terms of towing speed and attack angle, the wake vortex development of the test model, both with and without flaps, as well as the circulation analysis, were acquired. The study demonstrated that the decrease in circulation was 35% to 45% in 45 wingspans when flaps were introduced, whereas the counterpart of the baseline airfoil, without flaps, was only 0% to 10%, which revealed the application possibility of Rayleigh-Ludwig instability in alleviating the wake vortex.

2015, 47(3): 398-405. doi: 10.6052/0459-1879-14-301

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HYPERSONIC WIND TUNNEL FREE-FLIGHT TEST WITH BIPLANAR OPTICAL SYSTEM ON THE NON-SPINNING BLUNT CONE

Jiang Zenghui, Song Wei, Chen Nong

To investigate the dynamic stability of the non-spinning blunt cone, wind-tunnel free-flight tests with biplanar optical system, were conducted at Mach 6 in the hypersonic wind tunnel. The tests show that coning motion appears for all the models,and in the range of viewing area the amplitude of angle of sideslip is larger than that of angle of attack, though only angle of attack is preset. For all the models, the amplitude of both angles of attack and sideslip is no more than 10 degree, while damping moment shows obvious nonlinearity and static moment exhibits approximate linearity. One of the models shows the tendency of limit planar motion or limit coning motion with small amplitude of angle of attack, others all tend to limit coning motion. End cover has little effect on the angular motion of the models, while the protuberance symmetrically placed on the model tail can affect the transformation of amplitude of angular motion, and the fluctuation of angular motion amplitude is obviously smaller for models with strake.

2015, 47(3): 406-413. doi: 10.6052/0459-1879-14-332

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RESEARCH OF THE CONTROL EFFICIENCY OF LIFT INCREASE AND DRAG REDUCTION BASE ON FLOW AROUND HYDROFOIL CONTROLLED BY LORENTZ FORCE

Chen Yaohui, Li Baoming, Pan Xuchao, Liu Yixin

Lorentz force can control the flow of low-conduction fluid effectively,increasing lift and reducing drag, suppressing flow separation; however, the problem of control efficiency is the main bottleneck to restrict its application. In order to enhance its control efficiency, numerical simulation research had been carried out base on the flow control around hydrofoil using Lorentz force. Energy consumption of Lorentz force control had deduced on the basis of the law of conservation of energy; the amount of saving energy had calculated by using the lift and drag of hydrofoil. The control efficiency of the Lorentz force is defined as the ratio between saved power and used power. The control efficiency of lift increase and drag reduction under different working conditions had been studied. The results had shown that the control efficiency of Lorentz force index decrease with the increase of inflow velocity, and increase with the control time increase. To increase the input energy of Lorentz force actuator can enhance the control effect of Lorentz force, but cannot increase the control efficiency distinctly.

2015, 47(3): 414-421. doi: 10.6052/0459-1879-14-346

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STUDY ON TRANSITION TO CHAOS OF THERMOCAPILLARY CONVECTIONIN A RECTANGULAR LIQUID POOL

Jiang Huan, Duan Li, Kang Qi

This paper mainly does the experimental research about transition to chaos of thermocapillary convection in the rectangular pool. In the experiment, we observed that the transition process of thermocapillary convection in the rectangular pool has different phases of steady, regular oscillation and irregular oscillation. For different Prandtl numbers of silicone oil in different aspect ratios, there are different transition routes to chaos. When Prandtl number of silicone oil is less than or equal to 16 (1cSt) or Prandtl number equal to 25 (1.5cSt) with aspect ratios of 26, thermocapillary convection transition follows quasi-periodic bifurcation routes to chaos as the temperature increases. But when silicone oil of Prandtl number is greater than or equal to 25, it mainly follows period-doubling bifurcation routes to chaos. Sometimes two types of bifurcation would be accompanied by emergence of tangent bifurcation. In the experiment, we observed the distribution of the temperature field of liquid surface by using thermal imaging cameras, and found some phenomena of the surface fluctuation and convective cell oscillation.

2015, 47(3): 422-429. doi: 10.6052/0459-1879-14-296

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THE INFLUENCE OF GRAVITY AND CONTACT ANGLE ON THE LIQUID FLOW IN THE SURFACE TENSION TANK

Liu Zhaomiao, Zhao Tingting, Shen Feng

The flow behavior inside the vane surface tension tank is studied by using CFD-VOF method. Gravitational acceleration and contact angle are taken into consideration as control parameters. It is shown that when the contact angle is 10° and the gravitational acceleration is less than 10^-2g₀, the fluid reaches the top of the tank along the outer vanes. Moreover, the flow along the outer vanes is little influenced by the decrease of the gravitational acceleration when the gravitational acceleration is less than 10^-3g₀. The fluid velocity near the outer vanes decays gradually during the flow process, and the initial velocity depends on gravitational acceleration, the less gravitational acceleration, the larger initial velocity. Furthermore, the height of the flow climbing is impacted significantly by the contact angle. As the contact angle decreases the time of the fluid climbing to the top of the tank increased, and when the contact angle is greater than 45°, the fluid near the outer vanes cannot reach the top of tank. Ultimately, the relationship between the length of the climbing flow (L(t)) with the contact angle (θ) is also analyzed. It is observed that the increase of contact angle leads to the decreases of slope of the L(t) and the initial velocity of the fluid near the vanes.

2015, 47(3): 430-440. doi: 10.6052/0459-1879-14-255

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APPLICATION OF A CIP-BASED NUMERICAL SIMULATION OF FLOW PAST AN IN-LINE FORCED OSCILLATING CIRCULAR CYLINDER

Zhao Xizeng, Fu Yingnan, Zhang Dake, Cheng Du

In this paper, A CIP-ZJU (Constrained Interpolation Profile method in Zhejiang University) model is developed to study a forced oscillating circular cylinder at low KC (Keulegan-Carpenter) in static fluid and flow past an in-line forced oscillating circular at Reynolds number of 200. The model was established in the Cartesian coordinate system, using the CIP method as the base flow solver to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow with liquid and solid phases solved simultaneously. An Immersed boundary method was used to deal with the boundary of solid body. Computations were compared with available results and good agreements were obtained, validating the reliability of the method to solve the problems of complex flow.

2015, 47(3): 441-450. doi: 10.6052/0459-1879-14-387

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MICROMORPHIC MODEL OF GRAPHENE-LIKE TWO-DIMENSIONAL ATOMIC CRYSTALS

Yang Gang, Zhang Bin

A novel mechanical model is proposed for graphene-like two-dimensional (2D) atomic crystals based on micromorphic continuum theory, in which the macro-displacement and micro-deformation of a basal element in the Bravais cell of finite size are considered. The governing equations of the model are derived from the basic equations of micromorphic theory in global coordinates. For the Bravais cell of graphene-like crystals containing two atoms, the secular equations of phonon dispersions are then obtained in micromorphic form by analyzing the relations between the vibrational modes of phonons and the independent degrees of freedom of the basal elements, and are further simplified according to the properties of phonon dispersion of 2D crystals, thus the constitutive equations of the model are conformed. Finally, the material constants are determined respectively by fitting the data of the in-plane phonon dispersion relations of graphene and monolayer hexagonal boron nitride with the simplified expressions. The obtained equivalent Young's modulus and Possion' ratio of graphene are 1.05 TPa and 0.197 respectively, and those of monolayer hexagonal boron nitride are 0.766 TPa and 0.225 respectively; both show good agreements with available experimental values.

2015, 47(3): 451-457. doi: 10.6052/0459-1879-14-282

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CONTINUUM DAMAGE MECHANICS MODEL FOR LOW-VELOCITY IMPACT DAMAGE ANALYSIS OF COMPOSITE LAMINATES

Li Nian, Chen Puhui

A three-dimensional anisotropic continuum damage mechanics (CDM) model, including damage characterization, damage initiation criterion and damage evolution law, was presented to analyze low-velocity impact damage of composite laminates. With considering affects of the fracture plane angle, material constitutive relation of damage states in the fracture plane coordinates was established by introducing damage state variable matrix in the material principal coordinates. The onset of damage was evaluated by the Puck criterion and the evolution of damage was controlled by equivalent strain on the fracture plane. Based on the viewpoint of strain energy release, the material was assumed to exhibit linear strain-softening behavior. Fiber fracture (FF) and inter-fiber fracture (IFF) were simulated within the lamina. Due to the experimental evidence which indicated that multiple intralaminar cracks developed in a composite laminate and coalesced in to one interface crack (delamination) under impact load, a intralaminar matrix crack density parameter at saturation was introduced to scale interlaminar delamination. The relevant low velocity damage response parameters of the laminates [0₃/45/-45]_S and [45/0/-45/90]_4S were predicted with the proposed model at various impact energies. A good agreement was achieved with experimentally obtained data and showed the validity of CDM model in this paper. Furthermore, results for different mesh-densities indicated that the approach by introducing a characteristic length of the element could alleviate the mesh dependency at the stage of material damage evolution.

2015, 47(3): 458-470. doi: 10.6052/0459-1879-14-169

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MULTIPLE CRACK PROPAGATION BASED ON THE NUMERICAL MANIFOLD METHOD

Xu Dongdong, Zheng Hong, Yang Yongtao, Wu Aiqing

The numerical manifold method based on the two covers (mathematical cover and physical cover) and contact loop have been used to solve the continuum and discontinuum problems in a unified way. The mathematical cover is not enforced to coincide with the cracks when dealing with the discontinuous problem, facilitating the simulation of failure in rock mass. An enriched numerical manifold method is developed by adding the enriched displacement functions used for simulating the stress singularity to the physical patches around the crack tip. And on this base, a new algorithm of the multiple crack propagation is proposed; the overall response of the material is given during the crack propagation. Numerical examples with NMM for the classic linear elastic fracture mechanics problems are presented, suggesting that the proposed procedure is accurate and efficient.

2015, 47(3): 471-481. doi: 10.6052/0459-1879-14-347

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THE INCREMENTAL ENRICHED FINITE ELEMENT METHOD FOR FRACTURE ANALYSIS OF VISCOELASTIC INTERFACE

Yang Junhui, Meng Shangyang, Lei Yongjun

The incremental enriched finite element method (FEM) is developed for viscoelastic interface crack problems. According to the basic displacements fields of elastic interface crack tip, the crack tip displacement fields of viscoelastic interface crack are derived through the correspondence principle and an approximate Laplace inverse transform method. By incorporating the displacement expressions to the common element, the displacement models of enriched and transition elements are obtained, and then the incremental formulations of the enriched FEM are deduced. The fracture parameters, stress intensity factors and strain energy release rate can be obtained through solving the finite element equation. The enriched finite element model of typical plane problems containing viscoelastic interface crack are constructed, and the fracture parameter solutions show that the presented method is quite accurate for both elastic/viscoelastic and viscoelastic/viscoelastic interface crack and can reflect the creep and relaxation characteristics. It can be applied to the fracture analysis of viscoelastic interface crack.

2015, 47(3): 482-492. doi: 10.6052/0459-1879-14-391

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REVIEW SUGGESTION TO TECHNICAL PAPER SURFACE ACOUSTIC WAVE PROPAGATION IN THE PMN-PT lAYER/ELASTIC SUBSTRATE

Kong Yanping, Liu Jinxi

Surface acoustic wave propagation in the layered PMN-PT/elastic substrate is studied in this paper. The surface of the piezoelectric layer is mechanically free, and the electric boundary conditions are electrically open and shorted. The electro-elastic fields satisfying the governing equations and the boundary conditions are derived, and the dispersion equation of the elastic wave in the structure are obtained. The influence of the polarization direction of the PMN-PT on dispersion relations and the electromechanical coupling coefficients are analyzed. The variation curves of the elastic displacement and electric potential with the depth direction of the layer piezoelectric/elastic half-space structure are plotted when the PMN-PT is poled along two different directions. The obtained results may be useful for the application of the high-frequency surface acoustic devices.

2015, 47(3): 493-502. doi: 10.6052/0459-1879-14-299

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EXPERIMENTAL VERIFICATION OF CHAOTIC ATTITUDE MOTIONOF TETHERED SPACECRAFT SYSTEM

Pang Zhaojun, Jin Dongping

The paper studies chaotic motions of a tethered spacecraft by utilizing the ground-based experimental system. Based on the dynamics similarity principle, the dynamic equivalent model between the on-orbit tethered spacecraft and its ground experiment system are obtained. The space dynamics environment of the tethered spacecraft is simulated with the help of thrust forces and torque of momentum wheel on satellite simulator. Numerical simulations of the on-orbit tethered spacecraft show that the pitch motion of the tether is period or quasi-period one, while the attitude motion of the mother spacecraft behaves chaotic motion. The numerical results are verified based on the ground-based experimental system. The experiment shows that the torque of momentum wheel with a negative damping is able to suppress the chaotic motion.

2015, 47(3): 503-512. doi: 10.6052/0459-1879-14-308

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INVESTIGATION INTO IWAN MODEL BASED ON THE SIX-PARAMETER NON-UNIFORM DENSITY FUNCTION

Li Yikun, Hao Zhiming, Zhang Dingguo

Iwan model, which is commonly applied to describe the properties of energy dissipation and damping occurred in joints, becomes a meaningful tool for understanding both static and dynamic nonlinearity mechanisms of jointed structures. This paper presents a six-parameter Iwan model based on the non-uniform density function which contains an abridged power-law distribution and two impulsive functions. The analytic expressions of the six-parameter Iwan model including the backbone curve, unloading curves and energy dissipations during both micro-slipping and macro-slipping are deduced. Discussions on the effects of model parameters to energy dissipation are also performed. Compared to those existing Iwan models, the presented one is able to give a better description on the experimental phenomena of jointed structures.

2015, 47(3): 513-520. doi: 10.6052/0459-1879-14-375

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EXPERIMENTAL STUDY ON THE STATIC MECHANICAL PROPERTIES OF HYDRATE-BEARING SILTY-CLAY IN THE SOUTH CHINA SEA

Shi Yaohong, Zhang Xuhui, Lu Xiaobing, Wang Shuyun, Wang Ailan

By using an integrated experimental apparatus for syntheses of hydrate-bearing sediment (HBS) and tri-axial mechanical tests, a series of static tests were conducted on the silty-clay containing tetra-hydro-furan (THF) hydrate. The samples were prepared with the silty-clay obtained from the South China Sea where gas hydrate has been found. The stress-strain curves before and after hydrate dissociation and the strength parameters were obtained. It is shown that the hydrate-bearing silty-clay behaves as elastic-plastic failure and obvious strain hardening. The failure stress (the stress difference of the maximum and minimum stresses), internal friction angle and cohesion increase with the rise of hydrate saturation. The internal friction angle increases a little bit with the rise of hydrate saturation, while the other parameters increase rapidly once the hydrate saturation is over 25%, which is agreed well with the change of hydrate occurrence form in the pore of HBS. The dissociation of hydrate can cause the sediment's strength decrease to be 1/4 of the initial value. The strengths of the sediments after hydrate dissociation at different initial hydrate saturations are almost the same and larger than that of the sediment initially without hydrate.

2015, 47(3): 521-528. doi: 10.6052/0459-1879-14-424

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FLOW VISUALIZATIONS ON KELVIN-HELMHOLTZ-LIKE ROLLER STRUCTURES IN TURBULENT BOUNDARY LAYER OVER RIBLETS

Yang Shaoqiong, Kwing-So Choi, Jiang Nan

Riblets perform a drag-reducing effect on the turbulent boundary layers (TBLs) in a certain range of their peak-to-peak spacings, s⁺. However, the drag reduction produced by the riblets breaks down after s⁺ increasing beyond its optimum value, and eventually the drag reduction becomes an increase case. García-Mayoral and Jiménez (2011) believed that the breakdown is due to the onset of a Kelvin-Helmholtz instability in the TBL over riblets, which introduces a kind of Kelvin-Helmholtz-like roller structures. In the present study, these spanwise rollers, for the first time in experiments, were clearly observed by the smoke flow visualization technique. And their conceptual model was outlined lastly as well.

2015, 47(3): 529-533. doi: 10.6052/0459-1879-14-345

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A TWO-STEP MESH DEFORMATION STRATEGY BASED ON RADIAL BASIS FUNCTION

Liu Zhongyu, Zhang Mingfeng, Nie Xueyuan, Yang Guowei

Mesh deformation method based on radial basis function (RBF) is a reliable technique to be used to obtain high-quality deformed mesh for arbitrary topology. Reduced control points method can greatly improve the efficiency of the mesh deformation, while there exists the problem of large deformation error on surface and inverted boundary layer mesh. In this paper a new mesh deformation method with viscous boundary layer mesh is developed. The method selects two groups of control points from the object surface, a set of control points to roughly calculate grid position and deformation error, and the second set of control points to get a more accurate deformed mesh with interpolation deformation error. Mesh deformation of NLR-7301 with flap and DLR-F6 wing-body-pylon-nacelle configuration is presented with the viscous mesh deformation method. Results show that the method can greatly reduce deformation error on surface and effectively avoid boundary layer mesh inverted.

2015, 47(3): 534-538. doi: 10.6052/0459-1879-14-280

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DYNAMIC ANALYSIS ON A CIRCULAR INCLUSION IN A RADIALLY INHOMOGENEOUS MEDIUM

Yang Zailin, Hei Baoping, Yang Qinyou

Based on the complex function theory, scattering by elastic waves around a homogeneous circular inclusion buried in a radially inhomogeneous elastic medium is investigated in the paper. The inhomogeneity of the medium is assumed that the density depends on the radial distance as a power-law function and the shear modulus is constant. Inhomogeneous wave equation with variable coefficient is converted to the standard Helmholtz equation by using the coordinate transformation. The expressions of displacement and stress fields in complex coordinate are presented due to the existing of both the inhomogeneous base and homogeneous inclusion. The dynamic stress concentration factor (DSCF) around the inclusion is illustrated numerically by examples. Results show that wave number and shear modulus ratios between the base and inclusion, wave number of the base, inhomogeneous parameter have great influence on the dynamic stress concentration.

2015, 47(3): 539-543. doi: 10.6052/0459-1879-14-204

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THE INTRODUCTION OF APPLICATION PROJECTS ON MECHANICS IN 2015

Zhan Shige, Zhang Panfeng, Xu Xianghong, Sun Zhongkui

The paper introduced the applications for NSFC programs on mechanics in 2015. The statistics of application projects for General Programs, Young Scientists Fund, Fund for Less Developed Regions, Key Programs, Excellent Young Scientists Fund, National Science Fund for Distinguished Young Scholars, and Joint Research Fund for Overseas Chinese Scholars and Scholars in Hong Kong and Macao are presented and compared with applications in 2014.

2015, 47(3): 544-545. doi: 10.6052/0459-1879-15-161

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