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1.
基于DELFT3D模型研究了清澜-八门湾潟湖的水位分布和潮汐波内部结构,以及由于人类开垦引起的海岸线变化(以1962年、1985年和2008年为例)对潟湖水动力特性的影响。结果表明,清澜-八门湾潟湖潮汐是由多种分潮耦合而成的复杂驻波,其中K_1,O_1,M_2,S_2和M_4分潮的影响最大。由湾外向湾内传输,由于湾内红树林和浅滩引起的底部损耗增加,M_2,S_2,K_1和O_1分潮幅值减弱;M_4分潮幅值增强,表现出明显的浅水增幅效应;M_2和S_2分潮相位在文教河和文昌江领域表现出明显的干湿效应。不同年代海岸线的研究表明,1985年和2008年间,人类复垦导致潟湖及其潮汐汊道附近的红树林和滩涂区域严重破坏,海岸线缩减,引发了水位降低、纳潮量减少和潟湖潮汐汊道底摩擦弱化,从而削弱了干湿、潮呛和浅水效应。后果提示持续的人类复垦活动将会引发清澜-八门湾潮汐水动力环境的进一步恶化,可能导致未来发生更大的自然灾害。  相似文献   

2.
In this paper, we proposed a new cr tenon or mixea-moae brittle fracture, i.e., the strain energy criterion, which can be stated as (K/KⅠc)2 +(K/KⅠc)2+(K/KⅡc)2=1. This criterion is shown to be in good agreement with known experimental data.In this paper, an experimental criterion:(K/KⅠc)m+(K/KⅡc)n=1, 1≤nm≤2.is also proposed.  相似文献   

3.
There are N domains Dj(j=0,1,...,N-1) of different physical parameters in the whole space and their interfaces Sj,i+1 are non-horizontally smooth curved surfaces. The following boundary problem is called Hclinholiz boundary problem:V2H(1)+KjH(1)=0 (j=0,1,…,N-1)(H(0)-H(1))=δ(S) (δ(S):generalized function)(H1-Hi+1)=0 (j=0,1,…,N-2)The analytical solution of the above problem is given in this paper.  相似文献   

4.
轮轨滚动接触下,钢轨表面会产生典型的鱼钩形剥离掉块,其形成机理目前暂未明确。为了探究轮轨滚动接触下钢轨表面裂纹扩展机理,基于最大周向拉应力准则,建立轮轨滚动接触疲劳计算模型,提出裂尖扩展路径预测方法,并对不同初始角度裂纹的扩展路径进行预测。结果表明,钢轨表面微裂纹为I-II复合型裂纹,随着裂纹长度增加,KI先增加后减小,而KII呈现持续增加的变化趋势。当裂纹尖端扩展至一定深度时,裂尖扩展的驱动力由KIKII转变,此时裂尖在KII的驱动下发生约70°的转折,倾向于向上扩展而导致剥离掉块。钢轨表面萌生的微裂初始角度对剥离掉块的路径影响较为明显,但对剥离掉块深度影响较小。本文研究成果可为钢轨表面打磨维修规程的制定提供理论依据。  相似文献   

5.
Let X be a convex metric space with the property that every decreasing sequence of nonenply dosed subsets of X with diameters tending to has menemptyintersection. This paper proved that if T is a mapping of a elosed conver nonempty subset K of X into itself satisfying the inequality:d(Tx,Ty)≤ad(x,t)+b{d(x,Tx)+d(y,Ty)}+c{d(x,Tx)+d(y,Ty)}for all x,y in K,where 0≤a<1,b≥0,c≥0,a+c≠0 and a+2b+3c≤1, then T has a unique fixed point in K.  相似文献   

6.
细观结构是认知土石体力学行为本质的关键科学问题。本文通过引入物理学和数学等方法,借助颗粒物质力学理论,从几何排列与接触力的空间分布来定量刻画水平固结与山前坡地堆积两种典型环境下土石体的细观结构特征,并建立其与抗剪强度指标的关联。研究表明,(1)两种环境的土石体在细观结构上存在较大差异。在几何排列上,水平固结环境下的土石体具有长程无序和短程有序的特点,坡地堆沉积环境下的土石体表现出了无序的无定形结构;在接触力与单位接触向量的空间分布上,两者较为相似,绝大多数接触力以小于均值接触力的形式存在,其概率密度曲线Pf)呈幂函数衰减;90%以上接触方位角集中在40°~160°和220°~340°范围内。(2)基于径向分布函数、接触力概率密度和单位接触向量分别定义细观结构的特征量KaKsKo,发现三个特征量的增大对内摩擦角呈线性促进作用,对黏聚力呈非线性削弱作用。  相似文献   

7.
根据有限元成果的滑动面切向应力达到可用极限值的抗滑稳定极限平衡概念,建立投影方向极值原理单元滑面应力矢量极限平衡抗滑稳定计算理论模型,揭示不平衡虚拟耗散力、耗散能和极值条件方程的力学本质联系。根据投影极值方向平衡状态时的垂直方向还存在不平衡耗散力,提出复杂滑裂面极限平衡状态接近程度系数λ及矢量安全度系数Kλ评价抗滑稳定性能的最佳路径。通过边坡考题和坝基深层抗滑稳定算例的势能极值规律存在性和极值点的吻合度,证明了理论模型的合理可靠性。有限元应力矢量理论模型的建立及其安全度系数Kλ是极限平衡抗滑稳定计算理论发展的重要基础理论突破和全新理论研究成果。  相似文献   

8.
Fe-Mo-石墨和Fe-Mo-Ni-石墨的高温摩擦磨损行为   总被引:3,自引:3,他引:0  
采用粉末冶金技术制备了Fe-Mo-石墨材料(FM)和Fe-Mo-Ni-石墨材料(FMN),分析了其显微组织及组成,并研究了其在室温、320 ℃和450 ℃下的摩擦磨损性能.结果表明:FM 和FMN两种复合材料主要含有铁素体、珠光体、Fe3Mo金属间化合物、Mo2C、Fe2MoC以及少量石墨,各相分布均匀.FMN较FM珠光体数量要少, Ni元素的加入在一定程度上抑制了珠光体的生成;室温条件下,富含石墨的摩擦层发挥了减摩作用,而在高温条件下,复合材料中的石墨与对偶表面生成的Fe氧化物Fe2O3+Fe3O4+FeO·Cr2O3对其减摩降磨至关重要.  相似文献   

9.
基于态型近场动力学理论进行复合陶瓷板淬火破坏过程力学建模与分析。引入热膨胀项反映材料的热致变形,基于物质点对的断裂定义损伤,构建三维非局部常规态型近场动力学热弹脆性模型,结合多速率显式积分法计算热-力耦合,实现热冲击荷载作用下陶瓷板起裂和裂纹扩展全过程的模拟。通过模拟典型淬火Al2O3陶瓷板的破坏过程,并与实验和其他数值结果对比,验证了该模型和方法。使用该模型研究了含不同α-Al2O3纤维掺杂比率的Al2O3复合陶瓷板淬火破坏过程,结果表明,断裂韧性的提高显著减少了裂纹数量并延后起裂时间;含5%α-Al2O3纤维掺杂比率的复合陶瓷材料综合力学性能最优,能够有效提升构件抵抗热冲击荷载能力。  相似文献   

10.
为研究柱状颗粒在线性剪切流场中的运动状态和受力情况,本文以颗粒长径比为2,颗粒之间的初始距离ΔSPy=4D为例,基于直接力浸入边界法数值模拟了双柱状颗粒在三维线性剪切流场中的运动过程。根据模拟结果分析了柱状颗粒周围流场参数分布,在考虑壁面对颗粒的影响和颗粒之间相互影响的条件下,研究了颗粒的受力和运动的变化,探索了流体曳力导致柱状颗粒迁移和转动的规律。研究结果表明,双柱状颗粒在线性剪切流场中易向速度大的流体区域运动;前后两颗粒运动状态和轨迹不同,颗粒之间距离较近时,曳力会产生较大的波动;只有当颗粒在壁面附近时,滞后颗粒才能追上领先颗粒,两颗粒发生牵引、翻滚和分离过程。  相似文献   

11.
For a central crack in a piezoelectric plate, the mode-I stress intensity factor (KI), electric displacement intensity factor (KD), energy release rates (GGM) and energy density factor (S) are obtained from the finite element results. For the impermeable crack, the numerical results of KI and KD are coupled; this error is contrary to the uncoupled analytical solutions. The error has little effect on the total energy release rate G and energy density factor S, but in some cases, large errors in the mechanical energy release rate GM are observed. G is global while SED is local. Also G is negative which defies physics where energy cannot be created while crack attempts to extend as implied by G. Computations should be made for the J-integral and also show that J becomes negative. What this shows is that the global fracture energy criterion is not suitable to address the local release of energy because it includes the overall energy which are irrelevant to fracture initiation being a local behavior. In addition, the case study shows that the energy density theory is the better fracture criterion for the piezoelectric material. According to the results of S, it retards the crack growth when the external electric field and piezoelectric poling are on opposite directions. This conclusion agrees with analytical and experimental evidence in the past references.  相似文献   

12.
Summary A method is presented for the direct stress analysis of orthotropic skew bridge slabs.The method of analysis employs the Principle of Least Work, in conjunction with the assumption that the stress resultants may be expressed as Fourier series in the chordwise co-ordinate, the coefficients being functions of the spanwise position only. A system of oblique co-ordinates is used to simplify the analysis.Nomenclature O(x, y) system of orthogonal co-ordinates - O(u, v) system of oblique co-ordinates - angle between axes Ou, Ov - , non-dimensional oblique co-ordinates - l oblique span of slab - L right span of slab - c transverse slab width - t slab thickness - r i c/(2il) - M x , M y , M xy bending and twisting moments per unit length - M u , M v , M uv oblique bending and twisting moments - S x , S y shear forces per unit length - normal load intensity - p oblique normal load intensity - E x , E y E xy , G xy elastic moduli defining stress-strain relationships in orthotropic plates - A ij strain energy coefficients - F i , G i bending moment functions - R i , S i twisting moment functions - C 1, C 2, C 3 statical bending and twisting moment constants - D operator d/d - S u , S v oblique shear forces per unit length  相似文献   

13.
Schweizer et al. [J Rheol 48(6):1345–1363, 2004] showed nonlinear step shear rate data for a polystyrene melt (M w=200 kg/mol, M w/M n=1.06, T=175°C). For different rheometers, cone angles, and sample sizes, the delayed normal force rise observed therein relative to a compliance-free reference N 1 (from a thermodynamically consistent reptation model) is shown to depend on rheometer compliance characterized by the instrument stiffness K A. K A can be obtained from mapping N 1 on the measured N 1,meas. or directly from mechanical contact measurement with a mismatch of 20–30%. The ranking of the stiffnesses found is K A(RMS 800)>K A(MCR 300)>K A(ARES LR2). Once K A is known, N 1,meas.-data can be corrected by solving the ill-posed Volterra equation involved in it. The correction shown for experiments with the 0.15-rad cone angle gives very good results. The characteristic decay time of the normal force after cessation of flow scales linearly with the axial response time t a calculated from K A, cone angle, and sample radius. The torque decay time is practically independent of t a.Extended Version of a paper presented at the 2nd Annual European Rheology Conference in Grenoble, France, April 21–25, 2005.  相似文献   

14.
The flow behaviour of various polymer solutions of non-hydrolyzed polyacrylamide, hydrolyzed polyacrylamide, polyox and Xanthan was investigated in a plexiglass column having a succession of enlargements and constrictions, and compared with the flow behaviour and mechanical degradation of a solution of non-hydrolyzed polyacrylamide in a packed column of non-consolidated sand. The flow behaviour of this solution was found to be very similar in both the sand pack and plexiglass pore.Apart from the Xanthan solution, all other polymer solutions showed a viscoelastic behaviour in the plexiglass pore. The onset of viscoelastic behaviour, which has previously been defined using the shear rate ( ), stretch rate ( s ) and Ellis number (E 1), could be more precisely evaluated using a modified stretch rate (S G). The pressure losses across the plexiglass pore for different polymer solutions of the same type were found to follow a unique curve provided the suggested group (S G) was used, a situation which was not achieved with the other rheological parameters.The multipass mechanical degradation of the non-hydrolized polyacrylamide was tested through the sand pack against the suggested group (S G) and Maerker's group (M a). It was found that the loss of the solution viscoelasticity due to multipass mechanical degradation was better represented usingS G thanM a. A cross-sectional area (cm2) - C * critical concentration of polymer (ppm) - d plexiglass pore enlargement diameter - D average sand grain diameter (cm) - e equivalent width for the plexiglass pore - E 1 Ellis number (a Deborah number) - F R resistance factor - F Ri resistance factor at the first pass - h height of the flow path of the plexiglass pore - K power-law constant - K h,K w effective permeability to hydrocarbon and water, respectively (10–8 cm2) - M a Maerker's group for a given porosity (s–1) - M ai value ofM a at the first pass - N D Deborah number - n power-law index - Q flow rate (cm3/s) - R capillary radius (cm) - R g radius of gyration - S G suggested group of rheological parameters representing a modified maximum stretch rate (s–1) - S Gi value ofS G at the first pass - T R,t characteristic time for the fluid (s) - t s residence time (s) - V 0 superficial velocity (cm/s) - V mean velocity of flow through a porous medium (cm/s) - average axial velocity in the enlargement section of the plexiglass pore (cm/s) - V 1,V 2 maximum velocity at a plexiglass enlargement neck and centre - [] intrincis viscosity - viscosity (mPa s) - r relative viscosity (ratio of the viscosity of the polymer solution to that of the solvent) - shear rate (s–1) - s stretch rate (s–1) - characteristic time for the polymer solution (s)  相似文献   

15.
We study the onset of time dependent Marangoni-Bénard convection in binary mixtures subject to Soret effect by numerical computation of linear instability thresholds in infinite fluid layers and two-dimensional boxes. The calculations are done for positive Marangoni numbers (Ma > 0) and negative Marangoni Soret parameters S M = –(D S γ c )/(Dγ T ) where D S and D are the Soret and mass diffusion coefficients, respectively, and γ T , γ c are the first derivatives of the surface tension with respect to temperature and concentration. Our purpose is to understand why for particular choices of Prandtl and Schmidt numbers, the increase of the stabilizing solutal contribution leads to a decrease of the critical temperature difference, a phenomenon already reported by Chen & Chen [5] and Skarda et al. [12] For various choices of Prandtl and Schmidt numbers we analyze the evolution of the critical Marangoni number Ma c , critical wavenumber k c and angular frequency ω c with S M and compute the corresponding eigenvectors. We next propose a physical mechanism which explains how the stabilizing solutal contribution acts as a catalyst for overstability. Finally, we extend our results to two dimensional boxes of small aspect ratio.  相似文献   

16.
Supersonic biplanes can possibly achieve low-boom and low-drag supersonic flights. In the present study, aerodynamic analysis and design of two-dimensional (2-D) biplanes were investigated with the help of computational fluid dynamics (CFD) tools. By utilizing an inverse-design method, a 2-D biplane configuration with lower wave drag than the single flat-plate airfoil at sufficient lift conditions (C l > 0.14) was designed at its design Mach number (M  = 1.7). In general, although, supersonic biplanes show superior aerodynamic characteristics at their design Mach numbers, unfortunately, they are characterized by poor performances under their off-design conditions. Flow choking occurs at high subsonic speeds, and continues to Mach numbers greater than the design Mach number in the acceleration stage due to flow hysteresis. Hinged slats and flaps were applied as high-lift devices to avoid the flow choking and concomitant hysteresis problems, and were also used as actual high-lift devices under take-off and landing conditions. As further improvement, morphing and Fowler motion were considered. Finally, a series of 2-D biplane configurations from take-off (and landing) to cruise conditions were studied by applying the slats and flaps, as well as morphing mechanisms, to our inversely designed biplane.   相似文献   

17.
The effects of capillary number Ca and viscosity ratio M on non-uniqueness of the relative permeability (RP) – wetting saturation (Sw) relationships of steady-state immiscible two-phase flow in the heterogeneous porous media are studied by a developed lattice Boltzmann method (LBM). In this work, it is suggested that the non-uniqueness of capillary number Ca and viscosity ratio M influencing RP–Sw curves is due to the presence of the heterogeneity of porous media. For the different Ca and M, the RP–Sw curves in the homogenous and heterogeneous porous media with the same porosity are discussed in detail. It is found that (1) the pore-size distributions in porous media have significant effect on the RPs of both wetting phase (WP) and non-wetting phase (NWP); (2) the heterogeneity of porous media has negative effect on the increment of the RP of NWP caused by the large capillary number. The lubricating effect is strongly dependent on pore-size distributions in porous media; and (3) the large viscosity ratio increases the RP of NWP, while it has little effect on the RP of WP. The effect on the RP of NWP may be magnified by the heterogeneity of porous media.  相似文献   

18.
A mathematical model for blood flow through an elastic artery with multistenosis under the effect of a magnetic field in a porous medium is presented. The considered arterial segment is simulated by an anisotropically elastic cylindrical tube filled with a viscous incompressible electrically conducting fluid representing blood. An artery with mild local narrowing in its lumen forming a stenosis is analyzed. The effects of arterial wall parameters represent viscoelastic stresses along the longitudinal and circumferential directions T t and T θ , respectively. The degree of anisotropy of the vessel wall γ, total mass of the vessel, and surrounding tissues M and contributions of the viscous and elastic constraints to the total tethering C and K respectively on resistance impedance, wall shear stress distribution, and radial and axial velocities are illustrated. Also, the effects of the stenosis shape m, the constant of permeability X, the Hartmann number H α and the maximum height of the stenosis size δ on the fluid flow characteristics are investigated. The results show that the flow is appreciably influenced by surrounding connective tissues of the arterial wall motion, and the degree of anisotropy of the vessel wall plays an important role in determining the material of the artery. Further, the wall shear stress distribution increases with increasing T t and γ while decreases with increasing T θ , M, C, and K. Transmission of the wall shear stress distribution and resistance impedance at the wall surface through a tethered tube are substantially lower than those through a free tube, while the shearing stress distribution at the stenosis throat has inverse characteristic through totally tethered and free tubes. The trapping bolus increases in size toward the line center of the tube as the permeability constant X increases and decreases with the Hartmann number Ha increased. Finally, the trapping bolus appears, gradually in the case of non-symmetric stenosis, and disappears in the case of symmetric stenosis. The size of trapped bolus for the stream lines in a free isotropic tube (i.e., a tube initially unstressed) is smaller than those in a tethered tube.  相似文献   

19.
An empirical study is made on the fatigue crack growth rate in ferrite-martensite dual-phase (FMDP) steel. Particular attention is given to the effect of ferrite content in the range of 24.2% to 41.5% where good fatigue resistance was found at 33.8%. Variations in ferrite content did not affect the crack growth rate da/dN when plotted against the effective stress intensity factor range ΔKeff which was assumed to follow a linear relation with the crack tip stress intensity factor range ΔK. A high ΔKeff corresponds to uniformly distributed small size ferrite and martensite. No other appreciable correlation could be ralated to the microstructure morphology of the FMDP steel. The closure stress intensity factor Kcl, however, is affected by the ferrite content with Kcl/Kmax reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite.Dividing the fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in ΔK and the latter would increase with ΔK depending on the R = σminmax ratio. The same data when correlated with the strain energy density factor range ΔS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter α involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that the da/dN data for different R would collapse onto a single curve with a narrow scatter band when plotted against αΔS.  相似文献   

20.
Oscillatory MHD Couette flow of electrically conducting fluid between two parallel plates in a rotating system in the presence of an inclined magnetic field is considered when the upper plate is held at rest and the lower plate oscillates non-torsionally . An exact solution of the governing equations has been obtained by using Laplace transform technique. Asymptotic behavior of the solution is analyzed for M 2 1, K 2 1 and ω 1 and for large M 2, K 2 and ω. Numerical results of velocities are depicted graphically and the frictional shearing stresses are presented in tables. It is found that a thin boundary layer is formed near the lower plate, for large values of rotation parameter K 2, Hartman number M 2 and frequency parameter ω. The thickness of this boundary layer increases with increase in inclination of the magnetic field with the axis of rotation.  相似文献   

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