Characterization of the Spatio-Temporal Response of a Turbulent Boundary Layer to Dynamic Roughness

The spatio-temporal response of a smooth-wall turbulent boundary layer to two-dimensional dynamic roughness forcing was investigated using flow structure-resolved particle image velocimetry. While such a flow has been extensively studied through pointwise temporal measurements, we characterize the streamwise-spatial nature of the streamwise and wall-normal velocity components of the synthetic mode excited by the roughness, which had not been previously fully characterized. Investigation of the spanwise variation of the response confirms a broad extent of two-dimensional characteristics in the streamwise and spanwise directions. Spatial amplitude modulation is observed in the synthetic structure and investigated directly through the spatial spectra. A parametric study of the influences of forcing frequency (and roughness height) leads to an empirical relationship between input frequency and resultant streamwise spatial lengthscale, which is proposed to extend the usefulness of dynamic roughness as a well-characterized input by which to probe the structure and dynamics of wall turbulence. In turn, dynamic roughness forcing may be used to study the response of complex fluid systems to these deterministic structures and explore the efficacy of possible flow control strategies in a directed manner