Soil water retention characteristics are the key information required in hydrological modeling. Frac-tal models provide a practical alternative for indirectly estimating soil water retention characteristics frompartic...Soil water retention characteristics are the key information required in hydrological modeling. Frac-tal models provide a practical alternative for indirectly estimating soil water retention characteristics fromparticle-size distribution data. Predictive capabilities of three fractal models, i.e, Tyler-Wheatcraft model,Rieu-Sposito model, and Brooks-Corey model, were fully evaluated in this work using experimental datafrom an international database and literature. Particle-size distribution data were firstly interpolated into20 classes using a van Genuchten-type equation. Fractal dimensions of the tortuous pore wall and the poresurface were then calculated from the detailed particle-size distribution and incorporated as a parameter infractal water retention models. Comparisons between measured and model-estimated water retention cha-racteristics indicated that these three models were applicable to relatively different soil textures and pressurehead ranges. Tyler-Wheatcraft and Brooks-Corey models led to reasonable agreements for both coarse- andmedium-textured soils, while the latter showed applicability to a broader texture range. In contrast, Rieu-Sposito model was more suitable for fine-textured soils. Fractal models produced a better estimation of watercontents at low pressure heads than at high pressure heads.展开更多
To control the growth of space debris in the geostationary earth orbit （GEO）, a novel solution of net capture and tether-tugging reorbiting is proposed. After capture, the tug （i.e., active spacecraft）, tether, ne...To control the growth of space debris in the geostationary earth orbit （GEO）, a novel solution of net capture and tether-tugging reorbiting is proposed. After capture, the tug （i.e., active spacecraft）, tether, net, and target （i.e., GEO debris） constitute a rig- id-flexible coupled tethered combination system （TCS）, and subsequently the system is transported to the graveyard orbit by a thruster equipped on the tug. This paper attempts to study the dynamics of tether-tugging leorbiting after net capture. The net is equivalent to four flexible bridles, and the tug and target are viewed as rigid bodies. A sophisticated mathematical model is developed, taking into account the system orbital motion, relative motion of two spacecraft and spacecraft attitude motion. Given the complexity of the model, the numerical method is adopted to study the system dynamics characteristics. Particular attention is given to the investigation of the possible risks such as tether slack, spacecraft collision, tether rupture, tether-tug intertwist and destabilizing of the rug＇s attitude. The influence of the initial conditions and the magnitudes of the thrust are studied.展开更多
基金Project supported by the National Natural Science Foundation of China (No, 49971041), the National Key Basic Research Support Foundation (NKBRSF) of China (No. G1999011803) the Director Foundation of the Institute of Soil Science, CAS (No. ISSDF0004).
文摘Soil water retention characteristics are the key information required in hydrological modeling. Frac-tal models provide a practical alternative for indirectly estimating soil water retention characteristics fromparticle-size distribution data. Predictive capabilities of three fractal models, i.e, Tyler-Wheatcraft model,Rieu-Sposito model, and Brooks-Corey model, were fully evaluated in this work using experimental datafrom an international database and literature. Particle-size distribution data were firstly interpolated into20 classes using a van Genuchten-type equation. Fractal dimensions of the tortuous pore wall and the poresurface were then calculated from the detailed particle-size distribution and incorporated as a parameter infractal water retention models. Comparisons between measured and model-estimated water retention cha-racteristics indicated that these three models were applicable to relatively different soil textures and pressurehead ranges. Tyler-Wheatcraft and Brooks-Corey models led to reasonable agreements for both coarse- andmedium-textured soils, while the latter showed applicability to a broader texture range. In contrast, Rieu-Sposito model was more suitable for fine-textured soils. Fractal models produced a better estimation of watercontents at low pressure heads than at high pressure heads.
基金supported by the National Natural Science Foundation of China(Grant No.11272345)
文摘To control the growth of space debris in the geostationary earth orbit （GEO）, a novel solution of net capture and tether-tugging reorbiting is proposed. After capture, the tug （i.e., active spacecraft）, tether, net, and target （i.e., GEO debris） constitute a rig- id-flexible coupled tethered combination system （TCS）, and subsequently the system is transported to the graveyard orbit by a thruster equipped on the tug. This paper attempts to study the dynamics of tether-tugging leorbiting after net capture. The net is equivalent to four flexible bridles, and the tug and target are viewed as rigid bodies. A sophisticated mathematical model is developed, taking into account the system orbital motion, relative motion of two spacecraft and spacecraft attitude motion. Given the complexity of the model, the numerical method is adopted to study the system dynamics characteristics. Particular attention is given to the investigation of the possible risks such as tether slack, spacecraft collision, tether rupture, tether-tug intertwist and destabilizing of the rug＇s attitude. The influence of the initial conditions and the magnitudes of the thrust are studied.
