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Unsteady-flow parameters
Nonuniform body motion, which will in general result in an unsteady flow, will have some time scale or frequency associated with the motion. The unsteadiness is typically imposed on the flow via the boundary
conditions, in particular the viscous no-slip condition (1.39) which takes on the following more general form for a body which is moving with local velocity Ubody in some time-varying manner.
V = Ubody(t) (on solid moving surface) (1.49)
Consider for example a sinusoidal body motion
Ubody(t) = Ui sin(wt) (1.50)
where ш is the motion frequency and U1 is some constant. The dimensionless form of (1.49) is
V = U1 sin (Stref t)
where Stref is the Strouhal Number, also called the reduced frequency. This is an additional non-dimensional parameter which would need to be added to Table 1.2 for this unsteady flow case. Unsteady flows will be covered partly in Chapter 6, and in more detail in Chapter 7. The other chapters will focus on steady flows where Ubody = 0.
