Influence of High-Velocity Blood Flow on Right-to-Left Shunt in Patients with Patent Foramen Ovale during the Valsalva Maneuver

In this study, we investigated the changes in the right-to-left shunt (RLS) of the patent foramen ovale (PFO) at different phases of the Valsalva maneuver and analyzed the possible mechanisms. The study population consisted of 57 patients with symptoms highly suggestive of a PFO. These patients had been diagnosed with apsychia, migraine with aura, cerebral infarction, transient ischemic attack (TIA), and cerebral ischemia with unknown cause. Routine echocardiography was performed in all patients to rule out a cardiac malformation. Contrast-transcranial Doppler (c-TCD) and contrast-enhanced transthoracic echocardiography (c-TTE) were used to visualize and quantify the RLS. The standard apical four chamber view was used to observe the changes of E peak, A peak, and velocity-time integral (VTI) ratio of tricuspid blood flow during the strain phase and release phase of the Valsalva maneuver. Paired t-test was used to compare E peak, A peak, and VTI ratio of tricuspid blood flow during the different phases. The right-to-left shunt across the PFO (PFO-RLS) was graded in the two phases and compared by Kruskal-Wallis test. Compared with the strain phase of the Valsalva maneuver, the parameters of E, A, and VTI in diastolic period in patients with PFO-RLS at the release phase were significantly increased [54.30 ± 13.65 cm/s vs 100.35 ± 21.11 cm/s, 42.21 ± 12.32 cm/s vs 57.30 ± 18.88 cm/s, 10.34 ± 3.27 cm/s vs 19.58 ± 4.56 cm/s, respectively], and the difference was statistically significant. The positive consequence of PFO-RLS, as diagnosed by c-TTE with the Valsalva maneuver at the release phase of the Valsalva maneuver, was significantly higher than that at the strain phase of the Valsalva maneuver. At the beginning of release phase of the Valsalva maneuver, decreased intrathoracic pressure led to increased venous backflow into the right atrium. Thus, high-velocity blood flow rapidly pushed the PFO open, which resulted in a significant increase in the PFO-RLS. Therefore, the increase of the PFO-RLS during the Valsalva maneuver is caused by the impact of high-velocity blood flow the PFO.

Effects of Rigid Vegetation on the Turbulence Characteristics in Sediment-Laden Flows

The effects of rigid vegetation on the turbulence characteristics were experimentally studied in the interior water flume. An ADV was used to determine the three dimensional turbulent velocities in clear water flow without vegetation, sediment-laden flow without vegetation, sediment-laden flow with submerged vegetation and sediment-laden flow with non-submerged vegetation. By experimental and theoretical analysis, the effects of rigid vegetation on the distribution of averaged velocities, turbulence intensities and Reynolds stress were summarized. In sediment-laden flow with submerged vegetation, the averaged stream wise velocities above the top of vegetation fit well with the log distribution low. The three-dimensional turbulence intensities increase from the bottom until they reach the maximum at the top of the vegetation. The method to calculate the shear velocity with the maximum of the Reynolds stress is recommended. In sediment-laden flow with non-submerged vegetation, the turbulence problems cannot be explained by theory of bed shear flow. The average velocities, turbulence intensities and Reynolds stress approximate uniformly distributed along vertical direction.

THE STUDY ON THREE-DIMENSIONAL MATHEMATICAL MODEL OF RIVER BED EROSION FOR WATER-SEDIMENT TWO-PHASE FLOW

Based on the tensor analysis of water-sediment two-phase how, the basic model equations for clear water flow and sediment-laden flow are deduced in the general curve coordinates for natural water variable-density turbulent how. Furthermore, corresponding boundary conditions are also presented in connection with the composition and movement of non-uniform bed material. The theoretical results are applied to the calculation of the float open caisson in the construction period and good results are obtained.

Water demand for ecosystem protection in rivers with hyper-concentrated sediment-laden flow

Sediment transport is one of the main concerns in a river system with hyper-concentrated flows. Therefore, the water use for sediment transport must be considered in study on the water demand for river ecosystem. The conventional methods for calculating the Minimum Water Demand for River Ecosystem (MWDRE) are not appropriate for rivers with high sediment concentration. This paper studied the MWDRE in wet season, dry season and the whole year under different water-and-sediment conditions in the Lower Yellow River, which is regarded as a typical river with sediment-laden flows. The characteristics of MWDRE in the river are analyzed. Firstly, the water demand for sediment transport (WDST) is much larger than the demands for other riverine functions, the WDST accounts for the absolute majority of the MWDRE. Secondly, in wet season when the WDST is satisfied, not only most of the annual incoming sediment can be transported downstream, but also the water demands for other river functions can be satisfied automatically, so that the MWDRE in wet season is identical to the WDST. Thirdly, in dry season, when the WDST is satisfied, the water demands for other river functions can also be satisfied, but the low sediment transport efficiency results in significant waste of water resources. According to these characteristics and aiming at decreasing sediment deposition in the riverbed and improving the utilization efficiency of water resources, hydrological engineering works can be used to regulate or control flow and sediment so that the sediment incoming in dry season can be accumulated and be transported downstream intensively and thus efficiently in wet season.

SIMULATION OF LOW-CONCENTRATION SEDIMENT-LADEN FLOW BASED ON TWO-PHASE FLOW THEORY

Low concentration sediment-laden flow is usually involved in water conservancy, environmental protection, navigation and so on. In this article, a mathematical model for low-concentration sediment-laden flow was suggested based on the two-phase flow theory, and a solving scheme for the mathematical model in curvilinear grids was worked out. The observed data in the Zhang River in China was used for the ver/fication of the model, and the calculated results of the water level, velocity and river bed deformation are in agreement with the observed ones.

A general two-phase mixture model for sediment-laden flow in open channel

This work extends the sediment-laden mixture model with consideration of the turbulence damping and particle wake effects under the framework of improved efficiency and accuracy.The mixture model consists of the continuity and momentum equations for the sediment-laden mixture,and the continuity equation for the sediment.A theoretical formula is derived for the relative velocity between the water and sediment phases,with consideration of the effects of the pressure gradient,the shear stress and the lift force.A modified expression of the particle wake effect,inducing the local turbulence enhancement around the sediment particle,is employed to improve the turbulent diffusion of the coarse sediment.The k_(m)-ε_(m) model is proposed to close the mixture turbulence,with the turbulence damping effect due to the high sediment concentration expressed by the density-stratification term without an empirical parameter.The k_(m)-ε_(m) turbulence model requires smaller computational work and offers better results than an empirical density-stratification turbulence model in high sediment concentration cases.Consequently,with the proposed mixture model,the sediment transport in the open channel under a wide range of sediment sizes and concentrations can be revealed with the results in good agreement with experimental data for the velocity,the sediment concentration and the turbulent kinetic energy.

Multiple time scales of fluvial processes—theory and applications

Fluvial processes comprise water flow,sediment transport and bed evolution,which normally feature distinct time scales.The time scales of sediment transport and bed deformation relative to the flow essentially measure how fast sediment transport adapts to capacity region in line with local flow scenario and the bed deforms in comparison with the flow,which literally dictates if a capacity based and/or decoupled model is justified.This paper synthesizes the recently developed multiscale theory for sediment-laden flows over erodible bed,with bed load and suspended load transport,respectively.It is unravelled that bed load transport can adapt to capacity sufficiently rapidly even under highly unsteady flows and thus a capacity model is mostly applicable,whereas a non-capacity model is critical for suspended sediment because of the lower rate of adaptation to capacity.Physically coupled modelling is critical for fluvial processes characterized by rapid bed variation.Applications are outlined on very active bed load sediment transported by flash floods and landslide dam break floods.

Soil erosion and sediment transport in the gullied Loess Plateau:Scale effects and their mechanisms

Scale effects exist in the whole process of rainfall-3-runoff-3-soil erosion-3-sediment transport in river basins. The differences of hydrographs and sediment graphs in different positions in a river basin are treated as basic scale effects, which are more complex in the gullied Loess Plateau, a region notorious for high intensity soil erosion and hyper-concentrated sediment-laden flow. The up-scaling method of direct extrapolation that maintains dynamical mechanism effective in large scale application was chosen as the methodology of this paper. Firstly, scale effects of hydrographs and sediment graphs were analyzed by using field data, and key sub-processes and their mechanisms contributing to scale effects were clearly defined. Then, the Digital Yellow River Model that integrates sub-models for the subprocesses was used with high resolution to simulate rainfall-3-runoff-3-soil erosion-3-sediment transport response in Chabagou watershed, and the distributed results representing scale effects were obtained. Finally, analysis on the simulation results was carried out. It was shown that gravitational erosion and hyper-concentrated flow contribute most to the spatial variation of hydrographs and sediment graphs in the spatial scale. Different spatial scale distributions and superposition of different sub-processes are the mechanisms of scale effects.