On the Velocity Defect Law of Open Channel Flow

This paper describes the theoretical procedures for deriving a velocity defect law from the equations of motion and energy for turbulent mean flows in open channels, and the verification of the theoretical results from the experimental data of velocity distribution. Furthermore, by the use of the velocity defect law obtained and the boundary conditions imposed in the region near the wall, the velocity profile of turbulent mean flows in open channels and the mean velocity formula are presented

A first-principles study of tunneling magnetoresistance in Fe/MgAl2O4/Fe(001) magnetic tunnel junctions

We investigated the spin-dependent transport properties of Fe/MgAl2O4/Fe(001) magnetic tunneling junctions (MTJs) on the basis of first-principles calculations of the electronic structures and the ballistic conductance. The calculated tunneling magnetoresistance (TMR) ratio of a Fe/MgAl2O4/Fe(001) MTJ was about 160%, which was much smaller than that of a Fe/MgO/Fe(001) MTJ (1600%) for the same barrier thickness. However, there was an evanescent state with delta 1 symmetry in the energy gap around the Fermi level of normal spinel MgAl2O4, indicating the possibility of a large TMR in Fe/MgAl2O4/Fe(001) MTJs. The small TMR ratio of the Fe/MgAl2O4/Fe(001) MTJ was due to new conductive channels in the minority spin states resulting from a band-folding effect in the two-dimensional (2-D) Brillouin zone of the in-plane wave vector (k//) of the Fe electrode. Since the in-plane cell size of MgAl2O4 is twice that of the primitive in-plane cell size of bcc Fe, the bands in the boundary edges are folded, and minority-spin states coupled with the delta 1 evanescent state in the MgAl2O4 barrier appear at k//=0, which reduces the TMR ratio of the MTJs significantly.Comment: 5 pages, 6 figures, 1 tabl

Flow through Curved Open Channels. Part 1. On Characteristics of Upper Layer in Fully Developed Region

This paper describes the hydraulic characteristics of the upper layer in a fully developed region of curved flow, as the first stage of the analytical approach to the general curved flow. On the basis of the fundamental equations of the upper layer unaffected by the wall shear, the effects of the secondary, radial flow on the main, tangential flow, and the variance of the hydraulic behaviors in curved flow with increase in centerline radius are discussed. Furthermore, the radial distribution of the tangential velocity and the surface profile derived from the present theory are examined in comparison with the experimental results in two kinds of test flumes with a single curved section of central angle 90° and 180°, respectively. Resulting from the present study, it was confirmed that the existence of a fully developed region in the curved open channels is restricted to the latter portion of the curved section, and the experimental verification in this region supports present theory

Experimental Study on Stream Channel Processes in Alluvial Rivers

In order to elucidate fluvial processes of alluvial rivers, experiments were carried out in the 7.5m wide flume of Experimental Facilities for Research of River Disaster in Ujigawa Hydraulic Laboratory of the Disaster Prevention Research Institute. In the beginning of the experiments, the stream channel was widened uniformly in longitudinal direction and the channel bed was raised by depositing eroded sand from side banks, and after a while, signs of alternating bars appeared on the bed. And then, channel widening and bed variation were affected by boundary conditions to have a obvious longitudinal change, though planform of channel was still straight, while alternating bars kept on developing and traveling. But channel processes after that became considerably different from each other, being governed largely by experimental conditions which were classified into three groups, and consequently, three kinds of channel patterns were formed, that is, meadering channel, braiding channel and straight channel with armor coat. In addition, it was indicated that there were not a few common aspects in fluvial processes to every experiment under different conditions, and based on these aspects, a diagram explaining the stream channel processes under fundamental conditions was presented

Studies on the Process of Development of Alternate Bars

Alternate bars play a major role in stream channel processes and are very interesting phenomena in rivers. It has a large influence on the safety of river courses and the use of river water. The mechanism and the process of development of the alternate bar, which may be regarded as one of the most important subjects on the meso scale river bed configuration, are investigated in this paper. One reason for this study is that very few confirmative studies have been carried out by means of detailed experiments. Conducting the experiments under widely changed conditions, particularly that of the channel width, we clarified in detail the characteristic changes of the wave length and the wave height of the alternate bar and the channel bed variations in the development process. The development process of the alternate bar proved to be divided into three phases: in the first phase, the bar edge has become clear and the wave length has almost developed, in the second, the bar increases in the wave height to an equilibrium value and the third is a kind of the equilibrium state of the bar geometry. A longitudinally averaged bed shows a Quonset-hut shaped cross section in the second phase, the height of which increases corresponding to the wave height. This is due to lateral sediment transport caused by the meandering characteristics of flow over the alternate bar. By using the results, we proposed an equation for the development time of the alternate bar, and moreover, experimentally verified the effectiveness of parallel groyne works for the control of the bar geometry

The Widening Process of Straight Stream Channels in Alluvial Rivers

The widening process of straight stream channels is the most fundamental and simple fluvial process in alluvial rivers, though it is hardly observed in natural rivers. Prediction of this process based on the hydraulic consideration is a first step for the elucidation of more complicated fluvial processes in alluvium which relate to extreme river disasters. In this paper, as for the straight channel widening, the movement path of the sand eroded from a side bank and the lateral sediment transport are clarified by tracer tests and changes with time of cross sectional shape in detailed experiments, and by using these results the process of cross sectional variation is simulated under longitudinally uniform condition and problems in the analysis are pointed out. In addition, the widening process under longitudinally varying conditions are dealt with by a extended one-dimensional analysis with the consideration of the initial and the boundary condtions. The applicability of this analysis proves to be high till the beginning of channel meandering mainly caused by well-developed alternating bars, though the analysis has two major points to be improved. One of these points is to find out the equation of the side bank erosion expressing the actual process including the conditions of bank collapse and effects of bed and water level variations, and the other is to give the upstream boundary conditions suitably for the case when an initial ability of sediment transport of the stream channel differs largely from the rate of sand supplying

Laboratory Experiments on Thermally Induced Currents in Lake Biwa

Laboratory simulations of the surface gyres which have been stationally observed in Lake Biwa under the stratified condition were conducted by using a rectangular tank of small size and a distorted topographical model. The experimental gyres were generated by heaters installed in water. Through the rectangular tank experiments, the Rossby number, the vertical Ekman number, the densimetric Froude number and the relative upper layer depth were proved to be similar to those of the lake. The distorted model was built taking the similarities of these parameters into account and was employed under heating conditions corresponding to the annual variation of net surface heat (bouyancy) flux. The process of development and decay of the gyres is discussed in relation to heating stratification, and it is shown that the larger scale currents in lakes such as the gyres in Lake Biwa are formed by heat accumulation due to the solar radiation

Experimental Study on Sedimentation over the Floodplain due to River Embankment Failure

River embankment failure and floodplain sedimentation are common and frequent natural disasters in alluvial rivers. In Bangladesh, for instance, every year hundreds of embankments are damaged by flood water resulting in crop damage due to inundation and sedimentation. To elucidate the process of sedimentation over the vegetated floodplain due to embankment failure, a basic series of laboratory experiments was carried out with different lengths of embankment breach and different heights of model plant in the floodplain. The sediment volume and thickness over the floodplain increased with the increase of the breach length in the embankment. The maximum thickness upto a definite limit attained in the early stage was almost equal to or less than the water depth in the floodplain. Sedimentation was also observed to be greater in the floodplain with the more retardative jute field than in that with the rice field. Volume and area of sedimentation increased significantly with the increase of sediment inflow into the floodplain, namely the flood duration, the discharge and sediment concentration in the river