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Turbulent Boundary Layer

TURBULENT BOUNDARY LAYER In this layer, where flux of momentum decreases linearly with height, solid obstacles, arising at the surface in the laminar boundary layer, project into the wind and cause eddies which break away from the surface and travel downwind. A surface is aerodynamically smooth in conditions when the laminar layer is thick enough to submerge projections from the surface; but if the irregularities project through the laminar layer, the surface is considered rough. As the thickness of the laminar layer depends both on the wind speed and the stability of the atmosphere, it is clear that a particular surface such as a grass sward or a hairy leaf may be aerodynamically smooth under one set of conditions and rough under another. Each surface has a characteristic roughness parameter. Air-flow over calm water may be smooth; but, except at extremely low wind-speeds, flow over land is normally rough and disturbed by surface irregularities which cause turbulence.

Eddies of two types may occur: local or stationary eddies which may arise on both the windward and leeward sides of a bluff obstacle, and eddies which break away and travel with the wind in the obstacle's wake.

The forward velocity of a turbulent wind is thus the net result of a com plex movement; the wind has vertical and lateral components as well as the horizontal movement. Further, vertical and horizontal turbulence may differ in intensity (non-isotropic turbulence).

Occurrence of mechanical or frictional turbulence depends on the wind speed being high enough, and the object large enough, to cause eddying. Whether or not flow will be turbulent can be calculated by the method of Osbert Reynolds, who found that flow is turbulent when the Reynolds number, Here `length' is taken as a characteristic dimension of the object, and kinematic viscosity for air under average surface conditions is o.14 Thus for a leafy bush too cm. high in a wind of too cm. per sec. we have so flow would be expected to be turbulent.

In the turbulent boundary layer, properties such as temperature, amount of water vapour, and wind velocity, change much less rapidly with increasing height than in the laminar boundary layer beneath. Eddies mix the different parts of the layer much more rapidly than do the slow processes of molecular diffusion. Particles can also be carried by eddies upwards and laterally in a manner impossible in the laminar layer. In the turbulent boundary layer the wind velocity, temperature, and amount of water vapour show a change which is linear with the logarithm of the height. In this layer diurnal changes of temperature are less pronounced than in the laminar boundary layer underneath, and diurnal changes decrease still further with increasing height until, at the top of the next layer, they have almost disappeared.

An increase in wind-speed increases the thickness of the turbulent boundary layer both downwards, by thinning the laminar boundary layer, and upwards, by pushing into the transitional layer as turbulence increases. The turbulent boundary layer is thinnest on clear calm nights and thickest on hot sunny days, when it may reach to a height of 15o metres.

The turbulent boundary layer is the part of the atmosphere most familiar to us. While our feet are planted in the violently fluctuating climate at ground-level, our heads, and the weather-recording instruments of the conventional Stevenson's screen, inhabit the relatively equable turbulent layer.

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