Turbulent boundary layers developing over spanwise heterogeneous surfaces

Turbulent flow over a surface with spanwise heterogeneity induces secondary flow in the form of counter-rotating streamwise roll modes embedded within the boundary layer. In this study, we introduce the secondary flow using surfaces comprised of spanwise alternating smooth and rough strips and investigate the behavior of the secondary flow for various characteristic spanwise wavelengths. Hot-wire anemometry and stereoscopic particle image velocimetry measurements are performed over surfaces with various half-wavelengths Λ (0.32 ≤ Λ/δ ≤ 6.81), where Λ corresponds to the roughness strip width and δ is the spanwise-averaged boundary layer thickness. Preliminarily results indicate that when Λ/δ >> 1, the secondary flow is confined within the interface between smooth and rough strips, with regions far removed from this interface behaving as either homogeneous rough or smooth wall flow. On the other extreme when Λ/δ << 1, the secondary flow appears to be restricted to the near-wall region, while away from the wall, the surface becomes spanwise homogeneous. Our findings further suggest that the roll modes are not a time-invariant feature. In cases where Λ/δ ≈1, we observe that the roll modes strongly meander with an apparent sinous instability.