Turbulent Boundary Layer Response to Sinusoidal Spanwise Perturbation

In this study, a developing turbulent boundary layer is perturbed with a sinusoidal spanwise mode. The downstream persistence of these modes, and response of the underlying turbulent structure, is measured using hotwire anemometry. The modes are introduced using spanwise fences with a sinusoidally varying height $h$ of a given spanwise wavelength $\Lambda$. The motivation here is to test Townsend’s analysis which suggests that the turbulent boundary layer is receptive to spanwise periodic modes of certain wavelengths $\Lambda$. Hot-wire measurements are performed at different downstream locations and across the span of the introduced perturbation. Preliminary results indicate that high/low momentum pathways will appear behind the peaks/troughs of the perturbations. The persistence of these spanwise heterogeneous patterns are correlated with the ratio $\Lambda/\delta$, where $\delta$ is the boundary layer thickness at the perturbation station. Particularly, the perturbed flow with $\Lambda/\delta\sim2$, exhibits persistent spanwise periodicity up to 70$\delta$ downstream from the perturbation location, whereas the cases with smaller $\Lambda/\delta\sim1$ seem to recover to canonical spanwise homogeneous conditions over shorter downstream distances.