Abo Bibliothek: Guest
Third Symposium on Turbulence and Shear Flow Phenomena
June, 25-27, 2003, International Center, Sendai, Japan

DOI: 10.1615/TSFP3

DIRECT NUMERICAL SIMULATION OF A HOMOGENEOUS THERMAL WIND UNDER DENSITY STRATIFICATION

pages 1097-1102
DOI: 10.1615/TSFP3.1830
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ABSTRAKT

In the Earth's atmosphere, zonal currents with a vertical velocity gradient Su are induced under the action of rotational frequency Ω and horizontal temperature gradient SθH. These currents satisfying the so-called "thermal wind equation" generate the cyclones and anticyclones through the baroclinic instability, and hence have serious effects on the global circulation. The thermal wind is always under the influence of the density stratification. The effects of density stratification is typically represented by the Richardson number, which is defined as
Ri =(N/Su)2, (1)
where N and Su are the buoyancy frequency and the mean shear rate, respectively. At first, we studied the effects of the Richardson number on the thermal wind. With the increase in the Richardson number, the vertical velocity is suppressed and the velocities become two-component. This flow, independent of the Richardson number, becomes vertically collapsed, and the streamwise jets become the dominant flow structure. The pressure gradient becomes almost zero in the streamwise direction, which makes jets homogeneous in this direction.
The critical Richardson number for stabilizing turbulence is strongly dependent on the Coriolis term; the Richardson number needs to increase for stabilizing a shear flow with the large rotational effect. At the sufficiently large Richardson number, however, turbulence becomes vertically collapsed structure as observed in the case of the small Coriolis effect. Interestingly, turbulence cannot be geostrophically balanced between the pressure and the Coriolis terms; the pressure gradient term becomes negligibly small in comparison to the Coriolis term.

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