Aerodynamic boundary layers

The shearing stress, r, exerted by the wind on the ground entails a downwards flux of momentum. In the aerodynamic boundary layer above the surface, the momentum is transferred by the action of eddy diffusion on the velocity gradient. The friction velocity is defined by w = t/pa and is a measure of the intensity of the turbulent transfer. Near to a rough surface, the production of turbulance by mechanical forces... 

The zones where these gradients occur are often called boundary layers. For example, the aerodynamic boundary layer is the region near a surface where viscous forces predominate. Boundary layers exist with both laminar and turbulent flow and may be either solely laminar or turbulent with a laminar sublayer themselves (Landau and Lifshitz, 1959). 

Since both laminar and turbulent boundary layers contain laminar or viscous layers, it would seem logical that diffusion would primarily take place across these regions. If the boundary layer thickness were known, assuming a linear decrease in concentration, Eq. 10.3 could be used to estimate diffusion current. Unfortunately, the point of uniform velocity is not necessarily the point of uniform concentration. This is because particles, with their large inertia compared to air, can be carried into laminar boundary regions by mixing as well as by diffusion. The value for 8 in Eq. 10.3 will always be less than the equivalent value for the aerodynamic boundary layer thickness, in some cases being only one-tenth or even smaller (Levich, 1962). 

Wu, Y.-L., Davidson, C.I., Dolske, D.A., Sherwood, S.A., 1992. Dry deposition of atmospheric contaminants the relative importance of aerodynamic, boundary layer and surface resistances. Aerosol. Sci. Technol. 16, 65—81. 

Influence of humidity. Surface wetness of compressor blades operating in saturated condition will modify the aerodynamic boundary layer and cause a decrease in performance. 

Heat and mass transfer through the boundary layer flow over the burning surface of propellants dominates the burning process for effechve rocket motor operation. Shock wave formahon at the inlet flow of ducted rockets is an important process for achieving high propulsion performance. Thus, a brief overview of the fundamentals of aerodynamics and heat transfer is provided in Appendices B -D as a prerequisite for the study of pyrodynamics. 

Material transport is usually associated with thermal transport except in situations involving homogeneous phases which can be treated as ideal solutions (L4). For this reason it is necessary to consider the behavior of combined thermal and material transport in turbulent flow. The evaporation of liquids under macroscopic adiabatic conditions is a typical example of such a phenomenon. Under such circumstances the behavior in the boundary layer is similar to that found in the field of aerodynamics in a blowing boundary layer (S4). However, it is not... 

While our primary interest in this text is internal flow, there are certain similarities with the classic aerodynamics-motivated external flows. Broadly speaking, the stagnation flows discussed in Chapter 6 are classified as boundary layers where the outer flow that establishes the stagnation flow has a principal flow direction that is normal to the solid surface. Outside the boundary layer, there is typically an outer region in which viscous effects are negligible. Even in confined flows (e.g., a stagnation-flow chemical-vapor-deposition reactor), it is the existence of an inviscid outer region that is responsible for some of the relatively simple correlations of diffusive behavior in the boundary layer, like heat and mass transfer to the deposition surface. 

This transition has profound effects in all fluid dynamics, and certainly so in aerodynamics. The velocity profile in (he boundary layer becomes fuller neat the surface on account of Ihe higher average kinetic energy of the layer created by turbulent energy exchange from layer lo layer. The effective viscosity is therefore larger in turbulent than laminar flow, ihe turbulent boundary layer thickens more rapidly downstream, the skin friction increases. 

In the simple case of airflow over an aerodynamically smooth surface, with a fully developed boundary layer, the velocity of deposition can be calculated as a function of the diffusivity of the vapour or particle and the air speed. Formulae, developed for mass and heat transfer (Brut-saert, 1982) have been shown to apply to both attached and unattached 212Pb in wind tunnel experiments (Chamberlain, 1966,1968 Chamber-lain etal., 1984). 

All of the many biological transfer processes combine to determine a net surface resistance to transfer. Empirical relationships can be used to infer stomatal resistance from data on photosynthetically active radiation, water stress, temperature, atmospheric humidity and carbon dioxide levels. The resulting net surface resistance has been coupled with mathematical descriptions of aerodynamic and boundary-layer resistances in a "big leaf" model derived on the basis of agricultural and forest meteorology literature (4). At present, the big-leaf model is relatively coarse, permitting application only to areas dominated by maize, soybeans, grass, deciduous trees, and conifers. 

Elsaadawy, E.A. and Britcher, C.P. (2002). The extent of a laminar boundary layer under the influence of a propeller slipstream. AIAA Applied Aerodynamics Conf. paper no. AIAA 2002-2930. 

Kothari, K.M., Peterka, J.A., and Meroney, R.N. (1986) Perturbation analysis and measurements of building wakes in a stably-stratified turbulent boundary layer, Journal of Wind Engineering and Industrial Aerodynamics, 25,49-74. 

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