Following the findings in \cite{wangsawijaya2020}, we re-examine turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various wavelengths $0.32 \leq S/\overline{\delta} 3.63$, where $S$ is width strips and $\overline{\delta}$ spanwise-averaged boundary-layer thickness. The cases induce counter-rotating secondary flows, these are compared to large-scale str...

A sequence of direct simulations is used to study mechanisms for the growth of secondary circulations and turbulence in stratified shear flows. Five cases are examined, of which four deliver Holmboe waves as the primary instability and the fifth generates Kelvin-Helmholtz billows. Secondary circulations range in strength from weak, laminar motions to turbulence that destroys the parent wave. Pr...