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The interactions between perpendicularly crossing tunnels in the Sydney region are investigated using a full three-dimensional (3D) finite element analysis coupled with elasto-plastic material models. Special attention is paid to the effect of subsequent tunnelling on the support system, i.e. the shotcrete lining and rock bolts, of the existing tunnel. The results of the analysis show that in a region such as Sydney, with relatively high horizontal stresses, installation of the new tunnel causes the shotcrete lining of the existing tunnel to be in tension in the side facing towards the tunnel opening and in compression at the crown and invert. The pre-stressed rock bolts are usually tensioned more in the sections closest to the tunnel opening. For this particular study, if a new tunnel is driven perpendicularly beneath an existing tunnel, significant increases are induced in the bending moments in the shotcrete lining at the lateral sides of the existing tunnel and in the axial forces at its crown and invert. The increase in side bending moments causes further tensile cracking but the crown and invert stresses remain within the thresholds for both compressive failure and tensile cracking for shotcrete lining of typical concrete quality. Moreover, the driving of the new tunnel causes the tensile forces in the existing side rock bolts to increase and those in the existing crown rock bolts to decrease. In contrast, if the new tunnel is driven perpendicularly above the existing tunnel, compressive failure of the existing shotcrete lining is induced at the crown of the deeper tunnel for concrete of typical capacity and a significant tensile force increase of the existing rock bolts around the crown. It is concluded that in order to ensure the stability of the existing tunnel, local thickening is needed at the sides of the existing shotcrete lining if the shallow tunnel is installed first and local thickening is needed at the crown if the deep tunnel is installed first. 相似文献
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Benoît Oesterlé 《Boundary-Layer Meteorology》2009,130(1):71-95
In the approaches used to predict the dispersion of discrete particles moving in a turbulent flow, the effects of crossing
trajectories due to gravity (or any other external force field) are generally accounted for by modifying the integral time
scales according to the well-known analysis of Csanady (J Atmos Sci 20:201–208, 1963). Here, an alternative theoretical analysis
of the time correlation of the fluid velocity fluctuations along a particle trajectory is presented and applied in a turbulent
shear flow. The study is carried out in the frame of three-dimensional Langevin-type stochastic models, where the main unknowns
are the drift tensor components rather than the conventional integral time scales of the fluid seen by the particles. Starting
from a model for the space-time velocity covariance tensor of the turbulence under the assumption of homogeneous shear flow,
the various components of the time correlation tensor of the fluid seen are expressed in the asymptotic case of large mean
relative velocity (between the particles and the flow) compared to the particle velocity fluctuations. In order to provide
comparison with the generally used expressions arising from isotropic turbulence assumption, we examine also the conventional
integral time scales of the fluid seen in the directions parallel and perpendicular to the mean relative velocity. The most
prominent deviations from isotropic turbulence are observed when the external force field is in the direction of the mean
velocity gradient: in this case the loss of correlation in the mean flow direction is significantly lower than expected in
a uniform flow, an observation that is in qualitative agreement with the few available data. 相似文献
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Renzo L. Ricca 《地球物理与天体物理流体动力学》2013,107(4):385-402
In this article we present a review of some of the author's most recent results in topological magnetohydrodynamics (MHD), with an eye to possible applications to astrophysical flows and solar coronal structures. First, we briefly review basic work on magnetic helicity and linking numbers, and fundamental relations with magnetic energy and average crossing numbers of magnetic systems in ideal conditions. In the case of magnetic knots, we focus on the relation between their groundstate energy and topology, discussing the energy spectrum of tight knots in terms of ropelength. We compare this spectrum with the one given by considering the bending energy of such idealized knots, showing that curvature information provides a rather good indicator of magnetic energy contents. For loose knots far from equilibrium we show that inflexional states determine the transition to braid form. New lower bounds for tight knots and braids are then established. We conclude with results on energy-complexity relations for systems in presence of dissipation. 相似文献