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Cytoplasmic flows are widely emerging as key functional players in development. In early Drosophila embryos, flows drive the spreading of nuclei across the embryo. Here, we combine hydrodynamic modeling with quantitative imaging to develop a two-fluid model that features an active actomyosin gel and a passive viscous cytosol. Gel contractility is controlled by the cell cycle oscillator, the two fluids being coupled by friction. In addition to recapitulating experimental flow patterns, our model explains observations that remained elusive and makes a series of predictions. First, the model captures the vorticity of cytosolic flows, which highlights deviations from Stokes' flow that were observed experimentally but remained unexplained. Second, the model reveals strong differences in the gel and cytosol motion. In particular, a micron-sized boundary layer is predicted close to the cortex, where the gel slides tangentially while the cytosolic flow cannot slip. Third, the model unveils a mechanism that stabilizes the spreading of nuclei with respect to perturbations of their initial positions. This self-correcting mechanism is argued to be functionally important for proper nuclear spreading. Fourth, we use our model to analyze the effects of flows on the transport of the morphogen Bicoid and the establishment of its gradients. Finally, the model predicts that the flow strength should be reduced if the shape of the domain is more round, which is experimentally confirmed in Drosophila mutants. Thus, our two-fluid model explains flows and nuclear positioning in early Drosophila, while making predictions that suggest novel future experiments. [ABSTRACT FROM AUTHOR]

To address the transfer connection problem for urban rail transit (URT) and bus bridging in the case of unscheduled operation disruption, an integrated optimization model for bus bridging timetables and vehicle scheduling is proposed that considers the surge characteristics of transfer passenger flows. First, we analyze the transfer connections between the URT system and the bus bridging system and introduce the concept of passenger tolerance to determine whether a connection is maintained. A bilevel programming model is formulated, in which the upper level addresses timetable optimization with the aim of minimizing the passenger waiting time and the number of transfer failures, and the lower level addresses vehicle scheduling with the aim of minimizing bus operation cost. To solve the proposed model, an improved simulated annealing (SA) algorithm is developed. Finally, a case study of the Shanghai Rail Transit Line 10 is analyzed. The results show that, compared with the even headway timetable, the proposed model results in a 13.7% reduction in total cost, a 14.1% reduction in passenger waiting time, and a 54.9% reduction in the number of failed transfers. [ABSTRACT FROM AUTHOR]

This paper investigates the phase angles of the vibrations and hydrodynamic forces by the model testing of a flexible riser's vortex-induced vibration (VIV) under uniform flow. The VIV displacement of the riser is derived from the measured strains in the cross-flow (CF) and inline (IL) directions. Then, the hydrodynamic forces are obtained by the dynamic equation of an Euler-Bernoulli beam based on the results of VIV displacement. The characteristics of the phase angle of displacement and the hydrodynamic forces are analyzed. The results show that the phase angles of displacement and the hydrodynamic forces are almost identical at different cross sections of the riser under uniform flow. Moreover, within two adjacent vibration nodes in IL direction, the phase angle almost kept constant, while had a 180 deg change at the two sides of each vibration node. When the reduced velocity varies from 5.25 to 7.5, the phase angles of displacement derived from the flexible riser's VIV are 45 deg larger than those from the rigid cylinder's self-excited vibration. [ABSTRACT FROM AUTHOR]