Boundary Layer Theory

Before the boundary layer flow is discussed, some remarks should be made about the nondimensional variables in the formulas and the units to be used. In mechanics, a system of three units is normally used, like meters, kilograms, and seconds or inches, pounds, and minutes. Once such a system of units is selected, all variables and parameters must be measured in these units; for example, velocities in m/sec or inch/min, forces in Newtons N = m kg/sec2, stresses or pressures in N/m2, and so on. This is not always optimal. In many problems, certain reference values are selected; for example, a characteristic reference length L of the problem and a then transformed in such a way of such a transformation can be characteristic velocity VR of the problem. All equations are that only ratios involving reference values appear. The result achieved in a much simpler way. Instead of the unit system mentioned above, another system with units in closer correlation to the problem to be solved is selected. In problems of fluid mechanics (without considering heat), an adequate system of units can be a characteristic length L, a characteristic velocity VR, and a characteristic density pR of the problem. Once the selection of the unit system is made, all variables and parameters must again be measured in the selected units. For example, a time must then be measured in L/VR and a mass in ϱRL3. Additional examples are given in the following table.