Kei Ito

Crossover between Anomalous Superdiffusion and Normal Diffusion in Oscillating Convection Flows

Anomalous diffusion found in fluid systems is studied. Diffusion constants and mean square displacements are analytically obtained on the basis of the continuous-time random walk (CTRW) velocity model, and the values are compared with those obtained from model simulations employing dissipative dynamics describing oscillating convection flows. Good agreement is obtained.

CFD-based Evaluation of Interfacial Flows

Gas-liquid two-phase flows with interfacial deformations have been studied in various scientific and industrial fields. However, owing to the complexity of interfacial transient behaviors, the full understanding of gas-liquid two-phase flows is extremely difficult. For example, the occurrence condition of gas entrainment (GE) from free surface is not yet clarified even though a number of studies have been conducted by a lot of researchers. Unexpected GE phenomena often cause problems with equipments or troubles in plant operations (e.g. pump failure) and suppression of their occurrences by flow optimization is strongly required from the viewpoints of operation rates and safety. Therefore, the highlevel understanding of interfacial flows is very important and should be achieved appropriately based on mechanistic considerations. In this Chapter, the authors propose two methodologies to evaluate the GE phenomena in fast reactors (FRs) as an example of interfacial flows. One is a CFD-based prediction methodology (Sakai et al., 2008) and the other is a high-precision numerical simulation of interfacial flows. In the CFD-based prediction methodology, a transient numerical simulation is performed on a relatively coarse computational mesh arrangement to evaluate flow patterns in FRs as the first step. Then, a theoretical flow model is applied to the CFD result to specify local vortical flows which may cause the GE phenomena. In this procedure, two GE-related parameters, i.e. the interfacial dent and downward velocity gradient, are utilized as the indicators of the occurrence of the GE phenomena. On the other hand, several numerical algorithms are developed to achieve the high-precision numerical simulation of interfacial flows. In the development, an unstructured mesh scheme is employed because the accurate geometrical modeling of the structural components in a gas-liquid two-phase flow is important to simulate complicated interfacial deformations in the flow. In addition, as an interface-tracking algorithm, a high-precision volume-of-fluid algorithm is newly developed on unstructured meshes. The formulations of momentum and pressure calculations are also discussed and improved to be physically appropriate at gas-liquid interfaces. These two methodologies are applied to the evaluation of the GE phenomena in experiments. As a result, it is confirmed that both methodologies can evaluate the occurrence conditions of the GE phenomena properly.

Extended Self-Similarity in Modulational Intermittency Based on the Continuous-Time Random Walk Approach.

Anomalous diffusion caused by modulational intermittency, which is also known as on-off intermittency, is studied on the basis of the continuous-time random walk (CTRW) approach. There exists a characteristic time scale τ . For the time region t � τ , anomalous subdiffusion is observed, which is followed by normal diffusion for t � τ . Higher-order moments are analytically obtained by use of the saddle-point method, and it is found that they obey scaling relations that are reminiscent of extended self-similarity (ESS) and generalized extended selfsimilarity (GESS) found in turbulent systems. The results are compared with those obtained using the numerical inverse Laplace transform and from model simulations employing a coupled chaotic map. Good agreement between these results is obtained even for lowerorder moments.