Fig.I-2
Even though the two-dimensional friciton-inertial cases cannot be studies here in a detail, some information about them must be presented, due to their practical importance. The deep study would start with the Prandtl equation, derived in 1904 for the boundary layer case (Fig.I-2). Its solution, using a clever method of similarity transformation whereby the partial differential equation of the problem is converted to an ordinary equation, was derived by Blasius in 1908. Boundary layer develops on the surface of bodies. It may separate from the surface - but as long as it remains attached, it permits an advantageous division of the flowfield into two region, solved independently by simpler methods. Outside the boundary layer, it is possible to neglect the effect of viscosity altogether - so that it is possible to apply the simple potential flow approach. Since the boundary layer is very thin, its thickness meay be neglected and the nonzero velocity condition of the outer edge of ther boundary layer may be translated to nonzero boundary condition on the surface of the body. The second independently solved region is then the boundary layer itself, which uses as the outer boundary conditions the results of the potential flow solution. Even if the boundary layer separates, its solution may be of interest because it may predict the position of the separation point.
Boundary layers also represent an insulating coating which accounts for most of thermal resistance and its solution is therefore of paramount importance for problems of heat and mass transfer (cooling, heating, drying, ... ) and thus engineering calculations required for design of piston engine cylinders, heat exchangers etc.. This is not handled in the present introduction, being postponed for later stages of the studies.
In many applications in is necessary to study another case from Fig.I-2, the submerged jet. Some of its elementary characteristics will be discussed. This type of shear flow is of importance for applictaions mentioned briefly in chapter [G] - ejectors, fluidic amplifiers. Other important amplications are air jets issuing into ventilated or air-conditioned rooms (properties of these jets determine the temperature diostributions and therefore comfort in the room), burners, pneumatic jet looms,... etc.. Somewhat similar conditions are encountered in wakes formed downstream of bodies exposed to fluid flow. Wakes flows are often complicted by development of vortex structures. Jets issuing along a surface form wall jets . This is encountered in the Coanda effect or in film cooling of components exposed to high temperature flow (e.g. turbine blades). Also the case of the mixing layer at the interface between two flows of different velocity (or of different temperature, composition, etc.) is of practical importance.


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This is page Nr. I02 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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