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  • Author or Editor: S. P. S. Arya x
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J. C. Wyngaard, S. P. S. Arya, and O. R. Coté

Abstract

It is shown that although Coriolis forces cause large production rates of stress in a convective planetary boundary layer, there is a control mechanism, involving mean wind shear which prevents stress levels from becoming large. Higher-order-closure model calculations are presented which show that the stress profiles are essentially linear, regardless of wind direction, providing the geostrophic wind shear vanishes and the wind speed jump across the capping inversion is negligible. It is shown that it will he very difficult to verify these predicted stress profiles experimentally because of averaging time problems. A simple two-layer model is developed which leads to geostrophic drag and heat transfer expressions in fairly good agreement with Wangara data.

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S. P. S. Arya and J. C. Wyngaard
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J. E. Stout, Y-L. Lin, and S. P. S. Arya

Abstract

Trajectories of 500- and 1000-μm diameter particles are calculated as they fall through the spatially varying flow field above sinusoidal terrain for various combinations of atmospheric stability, wind speed, and terrain wavelength. In each case, a set of 20 uniformly spaced particles are released simultaneously above sinusoidal topography and their trajectories are obtained numerically by coupling a linear wave solution for flow over sinusoidal topography with equations for particle motion. The flow field and the associated patterns of deposition are shown to be strongly influenced by atmospheric stratification. For strong stratification, the presence of vertically propagating waves produces relatively concentrated “particle streams.” For less stratified conditions with evanescent waves, little focusing of particle trajectories is apparent. The ability of the atmosphere to focus or concentrate falling particles may ultimately produce regions along the surface with enhanced deposition.

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J. E. Stout, S. P. Arya, and E. L. Genikhovich

Abstract

The effects of nonlinear drag on the motion and settling velocity of heavy particles in a turbulent atmosphere are investigated. The authors approach the problem rather systematically by first considering the response of particles to much simpler fluid motions that are subprocesses of the more complex turbulent field. The authors first consider the motion and time response of particles falling under gravity in still fluid. Then the effects of a sudden gust or step change in relative velocity between a falling particle and its surrounding fluid are investigated. The authors demonstrate that horizontal relative motion produced by a sudden gust tends to reduce the settling velocity of a panicle. In simple oscillating fluids it is shown that the reduction of settling velocity increases with increasing amplitude of fluid oscillation. The authors also explore the effects of oscillation frequency on the settling velocity and show that if the period of fluid oscillation is less than the particle response time, then the settling velocity reduction becomes independent of oscillation frequency. Finally, the authors explore the motion of heavy particles within simulated isotropic turbulence and show that the effect of nonlinear drag is to produce a slowing of particle settling velocity.

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