**2. Richardson's annular effect in a wind tunnel**

Unsteady pulsating flows occur in many situations that have a practical engineering im‐ portance. These include high- speed pulsating flows in reciprocating piston-driven flows, rotor blade aerodynamics and turbomachinery. They also arise in wind-tunnel flows. When the velocity distribution is measured across the test section of a subsonic wind tunnel that is driven by a high speed fan, it has been observed experimentally that, in addition to the effect of the boundary layer that is expected near the wall, Richardson's annular effect can be demonstrated as well. Indeed, published experimental results from our laboratory have demonstrated that Richardson's annular effect can occur in a wind tunnel (Njock Libii, 2011).

The purpose of the remainder of this chapter is to summarize the theoretical basis of the Ri‐ chardson's annular effect in pipes of circular sections and in rectangular tubes, illustrate its results graphically, and relate them to what happens in a wind tunnel.

First Stokes' second problem is reviewed briefly. The theory of pulsating flows in pipes and ducts is summarized. The anatomy of the shift in the location of the maximum ve‐ locity from the center to points near the wall is presented using series approximations and graphical illustrations.
