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 $R_{\delta *} = (\overline {U}_\infty \delta^*)|v = 3\cdot 6 \times 10^3$ was measured for free stream fluctuation amplitudes up to 34% of the mean velocity and frequencies ranging from 0 to 48 cycles/sec. Here $\delta^* = \int ^\infty _0 \left(1 - \frac {\overline{U}} {\overline{U}_\infty} \right)dy$ is the boundary displacement thickness, $\overline {U}(x, y)$ (x, y) the mean velocity in the boundary layer, $\overline {U}_\infty$ 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.