A large number of advanced studies on fluid mechanics have been made in the field of applied mechanics in civil engineering. It is very important for researchers and engineers to overview the present situation of such advanced studies. The editorial board of the A2 division proposed a special edition entitled “A perspective of advanced studies on fluid mechanics in civil engineering”. This special edition consists of an invited paper and five general papers, which are closely connected with dynamical understanding of various fluid phenomena especially in applied mechanics.
This paper describes the applicability of a numerical ecosystem model that was developed by the authors on the basis of the finite volume community ocean model, i.e., FVCOM. We summarize the inhibition effects of oyster reef on the hypoxic water formation in Ariake Sea, Japan, which was investigated by using the developed ecosystem model. The results showed that oyster reef decreased the concentration of phyto-plankton and particulate organic carbon through its filtration activity, and increased the dissolved oxygen (DO) concentration in the bottom layer over a wide area. In addition, we explain an appropriate discretization method for decreasing the pressure gradient force (PGF) errors associated with the application of the terrain-following (σ) coordinate system to estuarine modeling based on FVCOM. The validations of the standard second-order scheme and the z-levels scheme for the PGF errors, were made under the conditions of horizontally-uniform stratification without external forcing, and a realistic stratification with external forcing. The comparison of the numerical results with field observations showed that the z-level scheme can greatly improve the accuracy of the numerical simulation, suggesting the importance of using the appropriate scheme for the PGF errors even in the estuarine modeling with high grid resolution.
Significant contributions of flow and turbulence to ecology related to aquatic benthic organism has been widely reported. Aquatic vegetation is one of environmental variables that influence turbulence and the ecological condition of rivers. Coherent turbulent motions are generated near the vegetation edge and these large-scale coherent eddies control the vertical exchange of mass and momentum between the over- and within-canopies. However, the effect of the submerged vegetation on the turbulent mixing process has not been fully investigated. Therefore, continuous dye-injection experiments were conducted to evaluate the vertical mass transport in open-channel flow with rigid vegetation models. The combination of PIV (Particle Image Velocimetry) and planar LIF (Laser-induced Fluorescene) technique is applied to reveal mass transfer in the vegetated open-channel flows. Two-dimensional data allow the identification of coherent structures and quantification of the vertical mass transport. The results revealed that the vortex structure lose coherence for flexible vegetation flow as compared to rigid vegetation flow. The values of the turbulence diffusivity become larger for rigid vegetation flow than those for flexible vegetation flow.
It is important for designing river structures and those maintenances to clarify flows downstream from a negative step with hydraulic jump of which patterns depend on downstream water depth. In this study, assuming the case in which the bed is covered with relatively large roughness, an enhanced depth integrated model is developed to calculate various hydraulic jump patterns generated downstream from the negative step. The present method is based on GBVC4-DWL, which is a non-hydrostatic depth integrated model with employing a dynamic rough wall law for bed. The model with the virtual bed slope behind the negative step and the critical slope of wave breaking is validated through the comparisons with the experimental results. Then the effects of equations and terms which compose the present method on the flow downstream of the negative step are investigated with several calculation models including GBVC4-DWL, GBVC3, SBVC3 and 1DC. The comparison shows the role of velocity distribution varied with vorticity production and non-hydrostatic pressure distribution.
This is the first attempt to study the effects of mass density ratio on collision statistics in an isotropic particle-laden turbulent suspension. Previous studies have concentrated on the collisions of either the Lagrangian particles following the fluid motions completely or the inertial particles which is much heavier than the surrounding fluid. The present study is an extension of Yokojima et al. (J. JSCE A2 72(2) I_459 2016), where the influence of mass density ratio on clustering/preferential concentration of monodisperse small particles in a turbulent environment was examined closely by direct simulations of turbulent flows. It has been revealed from the present study that the collision frequency is increased with increase in the specific density, i.e., the ratio of the mass density of the particles on the fluid density, and that the primary factor is the local/instantaneous increase in particle number density owing to the particle clustering. The accuracy of the collision-frequency estimation model based on the radial relative velocity and the radial distribution function is also examined. The model prediction is found to be in excellent agreement with the simulation results.
These days, 2-D numerical simulation are used for river management. However, estimating sediment deposition inside vegetated area of a river is still one of the most important research topics for river management. In this paper, the series of flume experiments would have been taken in order that the way how to estimate shear velocity inside vegetated area is investigated. Firstly, the results of some trial experiments led how to take experiments to measure the flow velocity distributions with PIV. The series of experiments were taken, and the vertical distribution of flow velocity and Reynolds stress inside rigid vegetated area, especially near channel bed, could be measured. These results indicated that the vertical distribution of the flow velocity have three regions : the upper region where the averaged velocity takes a fixed value controlled by vegetated area condition; the region near bed surface where the velocity should be controlled by bed roughness; the transit region between the upper and lower region. The vertical distribution of flow velocity and Reynolds stress inside rigid vegetated area were investigated in detail. These results indicate that Reynolds stress changes linearly in the region near bed surface, and the flow velocity distribution follows the logarithmic law. And also these experimental results indicated that shear velocity are evaluated by the weight of thickness of the region near the bed surface under the condition of hydraulics rough boundary. On the other hand, more investigation of detail velocity distribution on the hydraulics smooth boundary are needed.
Alternate bar is common bed form and closely associate with flood disasters and natural environment, it is important to elucidate its formation condition and formation mechanism. However, the formation mechanism of alternate bar is not well understood. As a factor to that, measurement method for water and bed surfaces when developing alternate bar in either flume experiments and actual rivers may not be established yet. We developed a new simultaneously measurement method for water and bed surfaces in evolution process of alternate bar without stopping supply water. We verified accuracy of the measurement method in fixed bed and movable bed condition. The result showed that accuracy of measured bed shape was several percent of wave height of alternate bars at 1cm square resolution in flowing water. The method can be completed simultaneously measurement in about 10 seconds in experimental flume of 10 meter length.
The purpose of this study is reproduction of cross-sectional buckling behavior in carbon nanotubes (CNTs) under hydrostatic pressure by molecular dynamics (MD) simulations. One of the characteristics of CNTs is that the physical properties change due to the change in the cross-sectional shape under high pressure. Therefore, it is necessary to assess the cross-sectional deformation behavior under high pressure, in order to apply excellent material properties of CNTs to products. In earlier work, we carried out simulations based on the thin cylindrical shell theory to predict buckling behavior in cross sections such as critical pressure and buckling mode. In this study, we analyzed the cross-sectional deformation behavior of CNTs by MD simulations, which can follow more precise behavior, and compared it with cylindrical shell theory. This study reveals that the critical pressure of MD simulations agrees well with results of thin shell theory in single-walled carbon nanotubes. In addition, we confirmed that the thin shell theory is effective as a method to quantitatively evaluate critical pressure in the CNT when the diameter of the inner-most layer is somewhat large.
In the analysis of the thick anisotropic laminated plates using the equivalent single-layer theory, calculation accuracy is deteriorated even by using higher order theory because the influence of the zig-zag displacements appears conspicuously as plate thickness ratio increases. In this study, the zig-zag functions of refined zig-zag theory is improved to be applied to isotropic plates, and the new zig-zag functions for out-of-plane displacement w is developed. This new zig-zag theory is applied to the bending analysis of anisotropic laminated plates, the accuracy of this theory and applicability to thick plate analysis are examined. The effective selecting method of the displacement fields in the zig-zag theory is clarified.
We present a comparison of fracture behavior of reinforced concrete (RC) beam between numerical simulation and experimental measurement. To measure the fracture behavior of concrete, we employ the digital image correlation (DIC) method which is capable of measuring and visualizing the crack propagation in concrete. The FEM with a damage model is applied to simulate the fracture behavior with cracking in concrete. The damage model is based on fracture mechanics for concrete in consideration of cohesive zone in the fracture process. An experimental measurement with the DIC and a numerical simulation with the FEM are performed for 4-point bend test of RC beams with different shear reinforcements. The quantitative comparison of fracture behavior with cracking between the numerical and experimental results offer valuable insight into the validation for the fracture simulation of concrete and reinforced concrete.