Investigation of unsteady secondary flows and largescale turbulence in heterogeneous turbulent boundary layers
Abstract
Following the findings in \cite{wangsawijaya2020}, we reexamine the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various roughness wavelengths $0.32 \leq S/\overline{\delta} \leq 3.63$, where $S$ is the width of the roughness strips and $\overline{\delta}$ is the spanwiseaveraged boundarylayer thickness. The heterogeneous cases induce counterrotating secondary flows, and these are compared to the largescale turbulent structures that occur naturally over the smooth wall. Both appear as meandering elongated high and lowmomentum streaks in the instantaneous flow field. Results suggest that the secondary flows might be spanwiselocked turbulent structures, with $S/\overline{\delta}$ governing the strength of the turbulent structures and possibly the efficacy of the surface in locking the structures in place (most effective when $S/\overline{\delta} \approx 1$). Conditional averages of the fluctuating velocity fields of both spanwise heterogeneous and smooth wall cases result in structures that are strongly reminiscent of the streakvortex instability model. Secondary flows and largescale structures coexist in the limits where either $S/\overline{\delta} \gg 1$ or $S/\overline{\delta} \ll 1$, where the secondary flows scale on $\delta$ or $S$, respectively. When $S/\overline{\delta} \gg 1$, the secondary flows are locked about the roughness transition, while relatively unaltered largescale structures occur further from the transition. In the case where $S/\overline{\delta} \ll 1$, $S$scaled secondary flows are confined close to the surface, coexisting with unaltered larger scale turbulent structures that penetrate much deeper into the layer.
 Publication:

arXiv eprints
 Pub Date:
 October 2021
 arXiv:
 arXiv:2110.02268
 Bibcode:
 2021arXiv211002268D
 Keywords:

 Physics  Fluid Dynamics
 EPrint:
 Currently under review for publication in J. Fluid Mech