arXiv subjects

Hiroshi Kawamura

Publications and source records attributed to Hiroshi Kawamura.

6 recordsLinked to original sources

arXiv2023

Introduction to Molecular-Scale Understanding of Surface Tension

In this article, microscopic understanding of the surface tension are provided, which needs basic knowledge of thermodynamics, statistical mechanics as well as continuum mechanics. By introducing the intermolecular interaction potential and temperature definition, and by showing conceptual pictures including some results obtained by molecular dynamics simulations, the author hopes that the target readers of undergraduate level students can find fascinating aspects of surface tension as the boundary of macroscopic and microscopic physics.

cond-mat.soft↗

arXiv2014

DNS of heat transfer in a transitional channel flow accompanied by a turbulent puff-like structure

Direct numerical simulations of turbulent heat transfer in fully-developed channel flows have been performed in a range of friction Reynolds number between 60 and 180, based on the friction velocity and the channel half width $δ$, with emphasis on a puff-like structure, large-scale spatial intermittency. For the Reynolds numbers lower than 80 with a large computational domain of 51.2 x 2 x 22.5, the turbulent puff was observed and its significant influences on the momentum and heat transports were found. The spatial structure of the equilibrium puff, or the localized turbulence, was examined with taking account of two different thermal boundary conditions: the uniform heat-flux heating and the constant temperature difference between the walls. It was revealed that there existed a localized strong turbulent region in the form of an oblique band, along which a spanwise secondary flow was induced. In consequence, at the present lowest Reynolds number as low as 60, the flow remained turbulent and the larger Nusselt numbers than those without puff was obtained by the presence of puff.

physics.flu-dyn↗

arXiv2014

An experimental study on turbulent-stripe structure in transitional channel flow

Turbulent stripe, which would occur in turbulent channel flows at transitional Reynolds numbers, was studied experimentally by flow visualization using reflective flake particles. In a range of bulk mean Reynolds number Re = 1700-2000, the turbulent stripe was observed to be inclined at angles of 20-30 degree against the streamwise direction, and its streamwise wave length was about 60 times of the channel half width ($δ$). The longitudinal streaks with the spanwise spacing of 1.6$δ$ were found in the quasi-laminar regions. The critical Reynolds number was Re = 1300. Time traces of the streamwise velocity were measured by laser Doppler velocimetry, revealing similarity to the equilibrium turbulent puff in the transitional pipe flow.

physics.flu-dyn↗

arXiv2014

DNS of turbulent channel flow at very low Reynolds numbers

Direct numerical simulations (DNS) of fully-developed turbulent channel flows for very low Reynolds numbers have been performed with a larger computational box sizes than those of existing DNS. The friction Reynolds number was decreased down to 60, where the friction Reynolds number is based on the friction velocity and the channel half width. When the Reynolds number was decreased to 60 with small computational box size, the flow became laminar. Using a large box, we found that a localized turbulence was observed to sustain in the form of periodic oblique band. This type of locally disordered flow is similar to a equilibrium turbulent puff in a transitional pipe flow. Various turbulence statistics such as turbulence intensities, vorticity fluctuations and Reynolds stresses are provided. Especially, their near-wall asymptotic behavior and budget terms of turbulence kinetic energy were discussed with respect to the Reynolds-number dependence and an influence of the computational box size. Other detailed characteristics associated with the turbulence structures were also presented and discussed.

physics.flu-dyn↗

arXiv2014

Turbulent heat transfer in a channel flow at transitional Reynolds numbers

Direct numerical simulation of a turbulent channel flow with heat transfer was performed at very low Reynolds numbers. Two different thermal boundary conditions were studied, and temperature was considered as a passive scalar. The computations were carried out with huge computational boxes (up to 327.7 x 2 x 128 in the streamwise, wall-normal, and spanwise directions, respectively). The emphases of this paper are to investigate the large-scale structure (puff) in the intermittent-turbulent flow including the scalar fields and to provide the values of the transitional and critical Reynolds numbers, below which the turbulent flow becomes intermittent and laminar, respectively. The statistics, such as the skin friction and the Stanton number, were also examined: they suggest that the puff should be effective in sustaining turbulence and in heat transfer enhancement.

physics.flu-dyn↗

arXiv2012

Liquid bridge pinch off and satelite drop formation under thermocapillary effect in Japanese Experiment Module 'Kibo' aboard the International Space Station

The long-duration fluid physics experiments on a thermocapillary-driven flow have been carried out on the Japanese experiment module 'Kibo' aboard the International Space Station (ISS) since 2008. In these experiments, various aspects of thermocapillary convection in a half-zone (HZ) liquid bridge of high Prandtl number fluid have been examined under the advantages of the long-duration high-quality microgravity environment. This fluid dynamics video introduce a pinch off of liquid bridge of 30 mm in diameter as a part of the on-orbit experiments. The effect of thermocapillary-driven flow on the pinch off and satellite drop formation is examined.

physics.flu-dyn↗