The internal dynamics of an illuminated dust cloud of finite optical thickness is investigated. The dependence of the radiation pressure on the optical depth makes the individual particles oscillate, in one dimension, around the accelerated centre of gravity of the cloud. The cloud moves as an entity, irrespectively of the velocity dispersion of the particles and their efficiency for radiation pressure. If the optical depth does not change, i.e. if the cloud does not expand laterally, its lifetime is unlimited. A contraction caused by energy dissipation in mechanical collissions between the dust particles is expected. The range of particle sizes which can be transported by such a “coherent cloud” is estimated, as well as the acceleration of the whole cloud. The structure of the cloud in real space and in velocity space is investigated. A comparison with the “striae” observed in the dust tails of great comets shows that the parent clouds of these striae may have been of the kind considered. 相似文献
Based on the partial differential equation governing the effect of atmospheric pressure on water level of confined well, deriving
the boundary condition and considering the seepage water between well and aquifer, the author obtained the analytical solution
of water level change in time domain under the action of an atmospheric pressure history with the Laplace transform method.
This solution is composed of two terms:stable and retarded terms. The stable term is the multiplication of barometric efficiency
and simultaneous atmospheric pressure, and it implies the value of water level after infinite time when the atmospheric pressure
is a constant from the time in question. The retarded term is the transient process due to the time lag of water exchange
between well and aquifer. From the solution, it is obtained that the interference of atmospheric pressure on water level is
the integral superimposition of the contribution of all atmospheric pressure changes before the time in question. So that,
we further found out the response function of pulsive atmospheric pressure history. Calculation shows: (1) The pulsive response
function starts from zero and tends to a steady value, which is proportional to the barometric efficiency, when the time tends
to infinity; (2) The retarded time depends on the mechanical property of aquifer and the radius of well. The larger the seepage
coefficient, the smaller the radius of well and the thicker the aquifer, then the shorter the retarded time gets. This solution
can be used as the theoretical basis for further analysis of the atmospheric effect and practical correcting method in the
future. 相似文献
Crack nucleation has been the subject of important contributions in the last two last decades. However, it seems that few attention has been granted to the case of saturated porous media. This is the question addressed in the present paper which is devoted to nucleation in traction mode. From a physical point of view, nucleation is a sudden phenomenon, so that the material response is both adiabatic and undrained. In the spirit of the variational approach, the nucleated crack is viewed as the final state of a region of space in which the material undergoes a full damage process. In traction mode, the opening of a saturated crack in undrained condition induces a drop of fluid pressure. In case of low fluid compressibility, the presence of the fluid delays the brittle failure usually associated with nucleation, as long as the fluid pressure remains above the saturation vapor pressure. Nucleation is therefore possible only if a partial vaporization of the fluid takes place. 相似文献