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The turbulent motion of the Earth’s atmosphere and oceans is hugely influenced by the effects of rotation and stratification. These effects alter the nature of turbulence profoundly from that of a homogeneous fluid. In particular, motions are dominantly horizontal, with vertical motions some three to four orders of magnitude smaller than horizontal motions. Moreover, coherent structures — vortices—are highly anisotropic, with vertical scales one to two orders of magnitude smaller than horizontal scales. And, fluid particle motions are doubly constrained: they must remain on (nearly flat) density surfaces and must retain their scalar value of ‘potential vorticity’. These constraints are shown to be powerful, even in flow regimes for which rotation and stratification are not dominant effects.

In order to investigate the formation process of the energy spectrum observed in the atmospheric mesoscales, we perform numerical experiments on forced turbulence in a rotating stratified fluid and examine the energy cascade processes. When the energy injection by the dynamical forcing is concentrated in small scales, upscale energy cascade is expected to form a spectrum. However, our result shows that the upscale cascade is not enough to form this spectrum for the terrestrial parameter range. On the other hand, the spectral slope generated by downscale energy cascade from energy injection in larger scales is close to −2 and is not sensitive to static stability when the Coriolis parameter is greater than the terrestrial angular velocity.

An asymmetry of jet profiles is found between eastward and westward jets which appear spontaneously in two-dimensional β-plane decaying turbulence. Westward jets are narrower and more intense than eastward jets. A theory for this asymmetrization is developed using Rossby wave propagation theory. The theoretical explanation also makes clear the maintenance mechanism of the zonal jets.

We have measured instantaneous velocity profiles in mercury thermal turbulence in a cylindrical cell of 1/2-aspect-ratio. In our previous work, we obtained some intriguing results from the measurement of the vertical velocity profile () along the central axis of the cell. To investigate the velocity field in more detail, we have recently improved the apparatus so that horizontal velocity profiles () and () just below the top plate can be measured as well as (). In this paper, some preliminary results from the measurements of the horizontal profiles are introduced together with the speculation of the shape of the mean flow, and future prospects are presented.

We show numerically, theoretically and experimentally that two co-rotating vertical vortices in a stably stratified fluid are subjected to a new three-dimensional similar to the zigzag instability observed on counter-rotating vortices (Billant & Chomaz, 2000). This zigzag instability induces the formation of thin horizontal layers with a thickness inversely proportional to the Brunt-Väisälä frequency. This three-dimensional instability is believed to make stratified turbulence depart from two-dimensional turbulence since it alters the merging of vortices.

In rotating duct flows, coherent longitudinal vortical structures develop even for very low Reynolds numbers due to the shear-Coriolis instability, where the mean absolute vorticity is close to zero. We investigate the creation mechanism of zero-mean-absolute-vorticity region focusing on the role of the longitudinal vortical structures for the plane-Poiseuille- and plane-Couette-type flows with the system rotation. It is found that the way of the vortex tubes to create zeromean- absolute-vorticity state is different between the two cases. For the rotating plane-Poiseuille-type flow, the generated longitudinal vortex tubes develop the spanwise vorticity around them, whereas for the rotating plane-Couette-type flow, they enhance the spanwise vorticity inside them. However, it is common for the two cases that zero-mean-absolute-vorticity state is created by the action of the coherent longitudinal vortices in the anticyclonic region.

Nonlinear development of streak instability modes is examined up to the turbulent stage experimentally through artificially producing spanwise-periodic low-speed streaks in a laminar boundary layer. The experiment is focused on the subharmonic sinuous mode of instability. The sinusoidal motion of low-speed streaks caused by the streak instability is maintained three or four wavelengths downstream beyond the nonlinear saturation stage of instability, and subsequently breaks down. After the breakdown, near-wall low-speed streaks with lateral spacing of 100 wall units newly develop, and the mean velocity profile starts to exhibit the log-law. It is also found that the interaction between the quasistreamwise vortices developing along the neighboring streaks causes large-scale arch-like vortices to develop in the region away from the wall.

We present an experiment where a stretched vortex is experiencing quasiperiodical turbulent bursts inside a laminar environment. The flow is characterized in the spectral and spatial domain using hot film and Particle Image Velocimetry Measurements. Some comparisons with the Lundgren vortex energy cascade mechanism are proposed.

The present paper describes our recent experimental studies on the supersonic mixing and combustion enhancement using streamwise vortices in spanwiserow configuration. Firing tests verify streamwise vortices of = 3 × 10 to be powerful for supersonic combustion enhancement. Cold-flow hot-wire measurements show that Kolmogorov’s −5/3 power law region appears in the spectrum of -fluctuations in the vortices for above about 10. This is not inconsistent with the minimum Reynolds number for the mixing transition proposed by Dimotakis (2000).

We present results obtained from a direct numerical simulation for a model of incompressible trailing vortices consisting of an array of counter-rotating vortices and a superposed axial velocity in a doubly-periodic domain, infinite in the vertical direction. The Reynolds number based on vortex circulation is 1000. It is found that for suffciently strong axial flow, helical instability modes develop on each vortex. This leads to a decrease in the magnitude of the axial flow and subsequent relaminarization of each vortex. At later times, modes corresponding to the more slowly growing co-operative instability become dominant. These produce their own helical structures followed by the rapid growth of small scales, then vorticity cancellation and decay of the vortex array. In the presence of strong axial flow the helical structure persists and the vortices appear more resistant to the breakdown phenomena than for arrays with no axial flow.