排序方式： 共有33条查询结果，搜索用时 31 毫秒
21.
Riccardo Marson Mauro Pontani Ettore Perozzi Paolo Teofilatto 《Celestial Mechanics and Dynamical Astronomy》2010,106(2):117-142
The possibility of communicating with the far side of the Moon is essential for keeping a continuous radio link with lunar
orbiting spacecraft, as well as with manned or unmanned surface facilities in locations characterized by poor coverage from
Earth. If the exploration and the exploitation of the Moon must be sustainable in the medium/long term, we need to develop
the capability to realize and service such facilities at an affordable cost. Minimizing the spacecraft mass and the number
of launches is a driving parameter to this end. The aim of this study is to show how Space Manifold Dynamics can be profitably
applied in order to launch three small spacecraft onboard the same launch vehicle and send them to different orbits around
the Moon with no significant difference in the Delta-V budgets. Internal manifold transfers are considered to minimize also
the transfer time. The approach used is the following: we used the linearized solution of the equations of motion in the Circular
Restricted Three Body Problem to determine a first–guess state vector associated with the Weak Stability Boundary regions,
either around the collinear Lagrangian point L1 or around the Moon. The resulting vector is then used as initial state in
a numerical backward-integration sequence that outputs a trajectory on a manifold. The dynamical model used in the numerical
integration is four-body and non-circular, i.e. the perturbations of the Sun and the lunar orbital eccentricity are accounted
for. The trajectory found in this way is used as the principal segment of the lunar transfer. After separation, with minor
maneuvers each satellite is injected into different orbits that lead to ballistic capture around the Moon. Finally, one or
more circularization maneuvers are needed in order to achieve the final circular orbits. The whole mission profile, from launch
to insertion into the final lunar orbits, is modeled numerically with the commercial software STK. 相似文献
22.
Remote surface flow observations are crucial for improving the comprehension of hydrological phenomena. A recent advancement in remote hydrological measurements involves the use of drones for generating surface flow‐velocity field maps through large‐scale particle image velocimetry (LSPIV). In this work, we perform a comparative analysis of drone‐based LSPIV with fixed implementations. Quantitative indices are introduced to test the efficiency of the techniques with regards to measurement accuracy, sensitivity to the transit of tracers, and platform mobility. Experimental findings support drone‐based observations in outdoor settings. Specifically, measurements from the aerial platform are more sensitive to the transit of tracers and closer to benchmark values than traditional LSPIV implementations. Future work should aim at improving the stability of the aerial platform and mitigating the effects of tracer scarcity. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Ettore Perozzi Lorenzo Casalino Guido Colasurdo Alessandro Rossi Giovanni B. Valsecchi 《Celestial Mechanics and Dynamical Astronomy》2002,83(1-4):49-62
The authors here propose a mission scenario, aimed at close exploration of a Near-Earth Asteroid, exploiting low-V resonant trajectories. These trajectories allow repeated fly-bys of a chosen target. A selection procedure, which has been used to find some interesting mission opportunities in the 2005-2015 time frame, is described and the corresponding detailed mission profiles are derived, using an indirect optimization method. 相似文献
25.
This paper describes the results of using geophysical techniques to investigate three columns of the Pronaos of the Antonino and Faustina temple ( AD 141) in Rome, Italy; the columns are of cipollino marble which shows alternate sequences of mica and calcite beds. We applied seismic refraction using traditional interpretation, and seismic transmission tomography. The comparison between the results of the refraction study and the 2D and 3D isotropic tomographic analyses suggested anisotropic characteristics for the marble, and this prompted us to perform a further tomographic experiment, taking into account these characteristics of the material. Assuming an elliptical model, the main directions of anisotropy were detected. Two velocity fields corresponding to the main directions of the anisotropy were measured and anomalies such as cracks and fractures were noted. The conjugate-gradient algorithm was used to invert the data. The results of the isotropic and anisotropic models were compared. The correlation between the methods highlighted the characteristics of the marble, i.e. anisotropy, depth of the weathering, fractures and small cracks. The results show that the material of the columns is in reasonable condition, with the exception of a surface area 6 cm to 15 cm deep that we estimate has been weathered for 2000 years, and that has been particularly affected by pollution in the last century. 相似文献
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Lorenzo Perozzi Erwan Gloaguen Bernard Giroux Klaus Holliger 《Computational Geosciences》2016,20(6):1287-1300
We present a two-step stochastic inversion approach for monitoring the distribution of CO2 injected into deep saline aquifers for the typical scenario of one single injection well and a database comprising a common suite of well logs as well as time-lapse vertical seismic profiling (VSP) data. In the first step, we compute several sets of stochastic models of the elastic properties using conventional sequential Gaussian co-simulations (SGCS) representing the considered reservoir before CO2 injection. All realizations within a set of models are then iteratively combined using a modified gradual deformation algorithm aiming at reducing the mismatch between the observed and simulated VSP data. In the second step, these optimal static models then serve as input for a history matching approach using the same modified gradual deformation algorithm for minimizing the mismatch between the observed and simulated VSP data following the injection of CO2. At each gradual deformation step, the injection and migration of CO2 is simulated and the corresponding seismic traces are computed and compared with the observed ones. The proposed stochastic inversion approach has been tested for a realistic, and arguably particularly challenging, synthetic case study mimicking the geological environment of a potential CO2 injection site in the Cambrian-Ordivician sedimentary sequence of the St. Lawrence platform in Southern Québec. The results demonstrate that the proposed two-step reservoir characterization approach is capable of adequately resolving and monitoring the distribution of the injected CO2. This finds its expression in optimized models of P- and S-wave velocities, density, and porosity, which, compared to conventional stochastic reservoir models, exhibit a significantly improved structural similarity with regard to the corresponding reference models. The proposed approach is therefore expected to allow for an optimal injection forecast by using a quantitative assimilation of all available data from the appraisal stage of a CO2 injection site. 相似文献
28.
Ettore Vidotto Rainer Helmig Martin Schneider Barbara Wohlmuth 《Computational Geosciences》2018,22(6):1487-1502
In this paper, we present a fast streamline-based numerical method for the two-phase flow equations in high-rate flooding scenarios for incompressible fluids in heterogeneous and anisotropic porous media. A fractional flow formulation is adopted and a discontinuous Galerkin method (DG) is employed to solve the pressure equation. Capillary effects can be neglected in high-rate flooding scenarios. This allows us to present an improved streamline approach in combination with the one-dimensional front tracking method to solve the transport equation. To handle the high computational costs of the DG approximation, domain decomposition is applied combined with an algebraic multigrid preconditioner to solve the linear system. Special care at the interior interfaces is required and the streamline tracer has to include a dynamic communication strategy. The method is validated in various two- and three-dimensional tests, where comparisons of the solutions in terms of approximation of flow front propagation with standard fully implicit finite-volume methods are provided. 相似文献
29.
Federico Falcini Daniele Iudicone Ettore Salusti 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(3):314-329
We analyze absolute velocities on the continental shelf off Cape Adare, in the western sector of the Ross Sea (Antarctica). Such a velocity field is here inferred by using a novel inverse method of absolute velocity determination, namely the tracer PV method, related to potential vorticities of temperature and salinity. This theoretical choice allows us to directly use in situ temperature and salinity data. Moreover, it avoids high-order derivatives, which can give large uncertainties that affect estimates made using previous approaches. The tracer PV method also allows us to separately estimate the steady and non-diffusive component and the unsteady and diffusive components of the flow. The western sector of the Ross Sea is characterized by a surface layer of Antarctic Surface Water over layers of Low Salinity Shelf Water and High Salinity Shelf Water, flowing northward with average velocities ～6–7 cm/s. At ～200 m depth an intrusion of warmer and saltier Circumpolar Deep Water is also evident in our data. The steady absolute velocities are in good agreement with those obtained from the classical Margules equation, in particular regarding the northward flux of the High Salinity Shelf Water. Furthermore, velocities due to diffusive processes and mesoscale activity are discussed. Finally, a steady “thermal” approximation is discussed; it allows for a qualitative check of the results by means of temperature horizontal sections only. 相似文献
30.
We discuss the usefulness of unstable Earth horseshoe orbits to access interplanetary space, and in particular when low-velocity escape from Earth is involved. The application to a solar stereoscopic mission is described. 相似文献