The Effect of Free-Stream Forcing on a Turbulent Boundary Layer*

This work investigated the interactions within a zero-pressure gradient turbulent boundary layer in the presence of free-stream external forcing. Controlled generation of energetic large-scale coherent structures was achieved by oscillating a NACA-0010 airfoil section of 8-inch chord length positioned at the entrance to a wind tunnel at mid-section height. This resulted in carefully controlled large-scale structures being introduced by free-stream perturbations. The amplitude and frequency of oscillation were independently varied, and the interactions within the resultant boundary layer were measured by means of a traversing hot-wire probe positioned about 3.0 m downstream of the trailing edge. Measurements were carried out at two friction Reynold’s numbers (Re_τ) of approximately 1770 and 2500, respectively. The free-stream forcing was observed to have penetrated deep into the boundary layer as the flow developed downstream. The resultant energy spectra showed that this introduced scale reached the near-wall region. Hence, this method for generating controlled large-scale structures was found to be an effective technique to generate controllable, large-scale wall shear stress fluctuations.