Sloshing of liquid propellant in microgravity and its influence on the attitude control system have been studied for several decades. Accurate modeling is necessary, especially, for modern spacecrafts carrying large amounts of liquid propellant. Research progress on this issue is reviewed, including a summary of analytical methods for both linear and nonlinear sloshing, an introduction of numerical techniques such as modal analysis and CFD (computational fluid dynamics) methods, and also a description of experimental approaches and advances. Finally, some critical problems concerning liquid sloshing in microgravity are proposed and discussed.
Earth penetration in geotechnical engineering is difficult to model, largely due to the transient, cou- pled nature of the impact event. The interaction between the penetrator and the target is inherent due to their vastly different material responses. The loading induced on the penetrator can lead to a significant deformation and potentially high shock levels on any onboard components. It is a challenging task to simulate such problems properly. This paper presents typical progresses, and the principles and features of each simulating method are elaborated. In addition, the rationale of the coupled Eulerian-Lagrangian method is discussed in detail. Two examples, the surface footing and the pile penetration, show that the coupled Eulerian-Lagrangian method is more accurate and convenient than other methods for the penetration analysis in geotechnical engineering. The numerical simulation methods of penetration in geotechnical engineering discussed in this paper can be used by related researchers.
The high-cycle and giga-cycle fatigue properties of Ti-6Al-4V for hollow fan blade of aero-engine are investigated. The observations show that the Ti-6Al-4V may have fatigue failure above the 107 cycles. The high-cycle and giga-cycle fatigue data are fitted by three parameter power function life curves. The data obtained by two test methods can be joined smoothly. It is concluded that the effect of frequency on giga-cycle fatigue properties under the test conditions is negligible. The fractograph of specimens is analyzed, which indicates that the giga-cycle specimens might generate internal fractures or surface fractures while there are not defects on the specimen surface, and the high-cycle specimens generate only surface fractures.
This paper studies the implementation and the accuracy of the FADS (flush air data sensing) system applied to the vehicle with blunt fore-bodies. For the 15° blunt fore-bodies at Mach number Ma = 2.04, 3.02, 5.01, angle of attack α is between -5° and 25°, and without the consideration of the angle of sideslip, the FADS model is a compromise between a simple potential flow model and the modified Newtonian flow theory. In this paper, the algorithm for the angle of attack based on the triple algorithm is developed, and the calibration error for the angle of attack is determined by the least-squares curve fitting. The free stream pressure and the dynamic pressure can then be determined by an iteration algorithm. Finally, the Mach number can be computed by using the normal one-dimensional fluid mechanics relationships. Systematic comparisons are conducted and it is shown that the model is reliable and the accuracy is high for the FADS system. The error for the angle of attack is less than 0.1°, the error for the free stream pressure is less than 5%, and the error for the Mach number is less than 0.01.
At the meso-scale, the digital image processing technology is adopted to characterize the heterogene- ity of granite caused by different distributions, sizes and shapes of quartz, feldspar, mica, etc., combining with RFPA-DIP to establish a numerical model of flawed granite. The model can reflect the real meso-scale behaviors of material accurately. The simulation tests of conventional uniaxial compression are carried out, the influences of diverse mineral particle structures and flaws on the meso-fracture behaviors are studied, to reveal the realistic failure process and final failure mode based on different numerical models under the external load. It is shown that the influence of the flaws on the strength of the sample is more significant than that of the transformation of the mineral particle structure. The existence of flaws weakens the influence of the particle morphology on the strength of granite; the flaws and the structures of mineral particles have a direct influence on the initiation and the propagation of crack and the final failure mode of samples, the spatial structure of mineral particles and flaws is the main factor for the complex failure modes of the rock. The inner-microstructure and the flaws of samples influence the crack initiation stress level, and the existence of flaws has a more significant effect on the crack initiation stress.
Based on elastic-plastic fracture mechanics theory, combined with mixed-mode I/II rupture test of UPVC (unplasticised polyvinyl chloride) under multiple loading angles, the composite fracture toughness of UPVC is studied. The critical CTOA (crack-tip-opening angle) (ΨC1) at stable composite crack extension is determined by using the digital correlation image technique, and ΨC1 is used as the fracture toughness parameter to describe the fracture toughness of UPVC. It is shown that: (1) unlike the common metallic materials, the load-carrying capacity of UPVC under mode II loading conditions is the largest, while under mode I loading conditions it is the smallest; (2) the CTOA tends to be stable when the crack extension is steady, so it can be used as the fracture toughness parameter of UPVC.
In order to overcome the shortcomings of the traditional stability safety factor, in this paper, the unload-load response ratio based on coupling of dynamic and displacement is used as the new stability safety factor to evaluate the stability of slope. In view of the connotation and the limitations of the traditional safety factor of slope, the use of the reliability degree to reflect the slope safety degree is proposed by using the reliability analysis method. From the damage mechanics, the parameters of the unload-load response ratio is derived through the damage variable. It is demonstrated that the unload-load response ratio as an expression of reliability can quantitatively characterize the safety degree of slope. The practical application and advantages of the unload-load response ratio are analyzed, the results further show that it is reasonable, practical and effective taking the unload-load response ratio as a new kind of safety factor of slope.
A series of overland flow experiments are conducted, focusing on the effects of the different height flexible vegetation on the hydraulic characteristics of the overland flow. Experimental results show that the 4 cm vegetation shows the best effect on the overland flow while the 15 cm vegetation shows the least effect. The difference of the effects reduces with the increase of the slope. With the same coverage rate, the flow resistance increases and then decreases with the increase of the Reynold's number. That is because the main flow resistance is the bed resistance in case of a small Reynold's number while the vegetation resistance prevails in case of a large Reynold's number. Besides, the revised coefficient ranges from 0.2 to 0.4 on smooth slopes while 0.4 to 0.8 on vegetation slopes, and the revised coefficient increases with the increase of the averaged velocity.
Since the wetting deformation of the rockfill material after soaking is not generated instantaneously but rather in a gradual development process, the time-effect analysis for the wetting deformation is proposed in this paper. The formula for the wetting shear strain component of the rockfill material is derived based on the Prandtl-Reuss flow rule. Then the three-dimensional wetting strain component is obtained by superposition of the wetting volumetric strain component. The relationship between the three-dimensional wetting strain component and the wetting strain component in the uniaxial state is analyzed. It is pointed out that the computational formula of the three-dimensional wetting strain component in literature is not rigorous. In analogy with the formula of the three-dimensional rheological deformation of the rockfill material, the time-effect computational formula of the wetting deformation is derived. Example analysis shows that the settlement of the dam crest produced by the wetting deformation increases with time, and the time for the steady deformation is inversely proportional with the rate of the wetting deformation.
Based on the governing equation of the soil consolidation with consideration of the effect of thermo- osmosis and coupling, a method for solving the consolidation of a cylinder of saturated porous medium with infinite length is established. Fourier, Laplace Transforms and their inverse transformations are used to obtain the analytical solution of the thermal consolidation. Then for the problem of the heat source consolidation of a cylinder with infinite length, approximate solutions of the soil's temperature and the pore pressure around the cylinder under the non-isothermal condition are obtained. The influence of the heat permeability coefficient and the consolidation coefficient of the soil consolidation under the action of the temperature is analyzed.
The DDARF (discontinuous deformation analysis for rock failure) method is improved by using the displacement convergence criterion and a program is developed for the method. Combining the discontinuous deformation analysis (DDA) and the fracture network seepage (FNS) model,a hydro-mechanical coupling model is established. The effects of the deformation of the fractured rock mass on the fracture seepage and the failure characteristics of the rock mass under the hydro-mechanical coupling are studied. It is found that the main fracture plays a role of controlling the distribution of the seepage field. The water head with consideration of the coupling effect is larger than that without considering the coupling effect. Meanwhile, it is shown that the coupling helps the slope deformation and will lead to failure more easily. The program convergence criteria in the DDARF are compared with those of the laboratory tests and the engineering example. The form of the crack propagation does not change before and after the improvement. The results show that the program convergence criteria of the DDARF are effective.
In order to avoid the drawback of BP (back propagation) neural network of too slow velocity convergence, the network structure has to be defined in advance. A cascade-correlation artificial neural network model is adopted to create the relationship between stress and strain of the artificial frozen soil, the consistent stiffness matrix is derived based on this model for the frozen soil, the neural network model is trained by the triaxial test data to replace the traditional finite element constitutive model, and the calculated results of the properties and the moisture content of the frozen soil are compared with the experimental results. It is found that this neural network constitutive model can represent the nonlinear response of the material very well, can improve the numerical analysis results, with very good agreement with the measured results, and the results are closer to the measured results than the BP model with same number of neurons in the hidden layer.