Global distribution of the density of sea water is maintained primarily by polar cooling and tropical heating together with polar precipitation and subtropical evaporation at the sea surface. This thermohaline forcing drives a global convection called the deep ocean general circulation. Carried by the circulation, sea water goes around the world oceans on a time scale as long as 1000 years, the longest time scale of the fluid earth. The deep circulation therefore is supposed to dominate the long-term variability of the climate of our aqua-planet.
Long history of oceanographic study has accumulated a vast amount of hydrographic data and established rather reliable though rough global maps of water properties in deep oceans, such as temperature, salinity, dissolved oxygen, silica, nutrients, and so on. Details have been known only little, however, on how water actually circulates in the world oceans from the surface to the bottom, from polar to tropical regions, and from west to east or vice versa, though recent hydrographic explorations have gradually revealed finer structure of circulation. Much less has been understood about what dynamics is responsible for the present state of deep circulation and distribution of water properties.

View full abstract

Near wakes of a circular cylinder in linearly stratified flows of finite depth are experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 10³-1.2 × 10⁴ and stratification parameters k_d smaller than 2.0. The nondimensional parameter k_d is defined by k_d=Nd/U, where N is the Brunt-Väisälä frequency, d, the cylinder's diameter, and U, the approaching velocity. The study demonstrates that as k_d increases from zero, the vortex shedding from a circular cylinder is progressively strengthened, while the Strouhal number becomes gradually lower than that for a neutral flow. Of particular interest is the phenomenon in which at a certain value of k_d, the enhanced vortex shedding is critically weakened, with a sharp rise in the Strouhal number.

View full abstract

The unsteady flow behaviours at the interface between the main flow and the dead zone area in open channels are investigated both experimentally and numerically. The vortex formation processes in the mixing layer between two flow regions (main flow and dead zone) are reproduced numerically by means of the 2 D plane open channel flow equations in the variable grid system. It will be pointed out through the comparison of the numerical results to the laboratory tests that the velocity fluctuations can be decomposed into the two components due to the seiche and the shear layer instability and that the later component is amplified to the large scale vortex, as shown in the photograph of visualization, when the resonance of both components occurs.

View full abstract

This paper is a study of stability of the cold airflow. For this study I propose a model of cold air flow which is based upon Prandtl's model (1952) and assumes a rigid slip boundary at the top. Explaining the causes of instabilities, this paper follows the conditions proposed by Lindzen et al. (1985, 1986), under which overreflection can occur. And the numerical method adopting the third-order spline interpolation is used.
It is shown that two types of disturbances are found, of which the first one arises mainly from the effect of the inflection point of the basic flow and the second one does mainly by the Rayleigh-Taylor type instability due to the change of the sign of potential temperature gradient. Moreover, the two kinds of waves peculiar to the cold airflow are caused by the latter effect.
The critical Reynolds number Re_c becomes greater as the angle of the slope χ decreases, i. e., as the stratification effect increases. The critical condition for instability is approximately given as Ri_o< 0.023 when 2.5≤χ≤10°, where Ri_o is the overall Richardson number defined by Ri_o= 2cotχ/Re_c.

View full abstract

The wind in subway systems is an airflow which is caused by the unsteady motion of trains in subway tunnels. Though the speed of airflow is sufficiently small compared with the sound speed, the compressibility of air should be taken into account in the case of closed space as like as subway systems. Blockage ratio σ=a/A is a parameter which denotes the effect of the compressibility, where “a” and “A” are cross sectional areas of the train and of the tunnel, respectively. For σ smaller than unity, a variable describing the compressibility is introduced in terms of fluctuations of mass density and of flow velocity. This is unitized to arrive at a set of self-consistent equations which governs the macroscopic behavior of the wind in subway systems. It is shown that the wind can be regarded as an incompressible except for the nearest neighborhood of the moving train in view of a nearly homogeneous flow.
Macroscopic justification is given through the comparison between model experiments and theoretical calculation.

View full abstract