Takafumi Kawamura

Numerical simulation method to resolve interactions between bubbles and turbulence

A new computational method for investigating interactions between bubbles and turbulence has been developed. Both liquid and gas phases are treated as incompressible fluids and solved by a finite volume method, while the interface between the phases is resolved by a front-tracking method. The accuracy validation carried out for a problem of a single rising bubble has shown this method is capable of simulating flow around deformed bubbles with relatively small number of grid points. Then the method was applied to a direct numerical simulation of a fully developed turbulent channel flow containing bubbles. Statistics of the friction coefficient and the modulation of turbulence intensity were obtained, and they were in qualitative agreement with experiments.

Effect of Bubble Size on the Microbubble Drag Reduction of a Turbulent Boundary Layer

Three different methods have been investigated for generating microbubbles to control the bubble diameter separately from the main flow velocity. The first two methods achieve this by adjusting the local shear stress where bubbles are generated, while the third method uses foaming of dissolved air to generate very small bubbles. The average diameter of bubbles was successfully controlled by the first two method within the range of 0.5–2 mm for the fixed main flow velocity of U = 3 m/s, while the very small bubbles of 20–40 μm were generated by the third method. The influence of the bubble diameter on the frictional drag reduction was found to be insignificant for the diameter range of 0.5–2 mm, while we also obtained experimental results suggesting that smaller bubbles on the order of 10 μm in diameter can be effective for the drag reduction.Copyright

Numerical simulation of three-dimensional breaking waves

Three-dimensional (3D) wave breaking around bodies of complex geometry has been numerically investigated by use of two types of Navier-Stokes solvers, namely the finite-difference and the finite-volume methods employing rectangular and curvilinear coordinate systems, respectively. Both methods employ the density-function technique to capture the free surface location and can cope with complicated free surface configurations such as breaking waves. The accuracy of the density-function method is examined through the comparison with experimental results, and it is confirmed to be satisfactory when the grid spacing and the time increment are sufficiently small.New computational methods are applied to several problems including 3D breaking waves around ships and wave diffraction around offshore structures. The computed results show good agreement with experimental results indicating that wave breaking phenomena are successfully simulated. The qualitative accuracy, however, could be improved by including the dissipating effect of breaking waves.

Numerical simulation of the flow about self-propelling tanker models

A time-marching CFD simulation is performed for self-propelling ships. The flow about the hull is simulated by the finite-volume method, and the propeller action is approximated as a propeller disk for which the solution is given by a simplified propeller model. The interaction of two flow models is treated in a time-marching procedure converging towards the steady self-propelling condition. This method is applied to five tanker models, and detailed comparisons are made between the simulated results and corresponding experimental results. It is shown that the flow field in the self-propelling condition is qualitatively well reproduced in the simulation, and the estimated thrust deduction factors for the five hull forms agree well with measured ones. However, the effective wake factors are underestimated, since the Reynolds number in the simulations differs from that in the experiment.

Turbulent Drag Reduction Effect by Hydrogen and Oxygen Microbubbles Made by Electrolysis

Turbulent drag reduction by very small hydrogen microbubbles was investigated experimentally. The method for generating microbubbles of 10–60 μm by water electrolysis was established firstly. Experiments were carried out using a circulating water tunnel, and it was observed that the small microbubbles generated by electrolysis can achieve the same drag reduction as the injected air bubbles at much lower void fraction. The distribution of microbubble was examined using the microscope photography. The peak of local void fraction was found to be very close to the wall, while no correlation was found between the average bubble diameter and the distance from the channel wall. The present experimental results suggest that the very small microbubbles produced by electrolysis are 10∼100 times more effective in terms of the drag reduction than large bubbles made by air injection. So it is considered that the diameters of microbubbles play an important role to drag reduction.Copyright