An unsteady turbulent boundary layer

A low speed parallel flow, whose velocity fluctuates sinusoidally in magnitude about a constant mean, has been produced in a boundary layer wind tunnel. Hot-wire measuring techniques have been developed to permit an investigation of the turbulent boundary layer developing on a flat plate with this free stream condition. The response of the layer at a Reynolds number Rδ * = (Ū_∞ δ^*)|v = 3· 6 × 10³ was measured for free stream fluctuation amplitudes up to 34% of the mean velocity and frequencies ranging from 0 to 48 cycles/sec. Here δ^* = ∫ ^∞ ₀ (1 - Ū Ū_∞ )dy is the boundary displacement thickness, Ū(x, y) ( x , y ) the mean velocity in the boundary layer, Ū_∞ the mean velocity in the free stream, and v the kinematic viscosity. Measurements were made of the mean velocity, the amplitudes of in- and out-of-phase components of the first harmonic of the periodic fluctuations, and the intensity of higher harmonics and turbulence. It was found that non-linear effects, even at the largest fluctuation amplitudes, were so small that they were obscured by experimental errors.