Author(s): Jin Kashiwada; Yasuo Nihei

Linked Author(s): Jin Kashiwada, Yasuo Nihei

Keywords: Mode splitting method; Three-dimensional flow simulation; Numerical experiments

Abstract: In recent years, a lot of flood inundation and damage to river structure which caused by the largest floods ever recorded have occurred, due to the effects of climate change. In extreme-high-flow conditions, the complex flow patterns are observed in the compound channel which is composed of main-channel and floodplain, or river-channel and inundation area, are expected to have different damage mechanisms from the ordinal flood conditions. With the development of numerical computation technology, two-dimensional analysis has been widely used to reveal these phenomena. However, it is essential to consider three-dimensional flow in order to evaluate flow phenomena in complex channel and hydrodynamic forces around structures. In general, three-dimensional computation requires a large computational cost and is used only for a limited spatial and temporal range. For this reason, quasi-three-dimensional computation that consider three-dimensional flow by assuming a specific vertical distribution of velocity, are being actively developed and applied. However, its reproducibility of phenomena depends strongly on the formula for vertical distribution of velocity. In this paper, the authors present a new mode splitting method to improve the accuracy and efficiency of river flow simulation with considering three-dimensional flow. In the mode-splitting method, two-dimensional calculations (External mode) and three-dimensional calculations (Internal mode) are performed simultaneously, and the results of each calculations are exchanged in order to reflect the effects of three-dimensional flow into the two-dimensional calculations. In addition, our present method can reduce the frequency of three-dimensional calculation, which enables more efficient computation. On the other hand, in the external mode: in the time step without three-dimensional calculation, it is assumed that the time variation of the three-dimensional flow structure is sufficiently small, and the three-dimensional flow structure at the previous internal mode is still used. In addition, the frequency of the Internal mode may cause a trade-off between accuracy and efficiency. In this study, numerical experiments using a typical virtual channel with three-dimensional flow are conducted to evaluate the basic effectiveness and characteristics of the present method. The accuracy and computational load were evaluated by comparing with a three-dimensional river flow model as a reference, and the results indicate that the present model was able to compute high-accurate results as the three-dimensional model with high efficiency by setting the appropriate frequency of internal mode. In addition to the 2D-3D mode splitting model shown in this paper, the authors have also developed a 1D-2D mode splitting model, and based on these models, we will develop a 1D-2D-3D mode splitting model in the future. Based on these models, we plan to develop a 1D-2D-3D mode splitting model and start constructing a river flow calculation system that realizes arbitrary dimensional calculations with high accuracy and high efficiency for the entire river basin.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221420

Year: 2022