Experimental investigation of ridge-induced secondary motions in turbulent channel flows

Many engineering and environmental surfaces exhibit spatial heterogeneity in the spanwise direction and encompass multiple surface length scales. When the dominant spanwise length scale is on the order of the largest flow scales (e.g., the boundary layer thickness or channel half-height, delta), localized delta-scale secondary flows can form. These secondary-flow generating heterogeneous surfaces are typically classified as "ridge-type" (with spanwise variation in surface elevation) and "strip-type" (spanwise variation in skin-friction including through variation in roughness characteristics). Both types of surfaces have been explored in previous studies at high Reynolds numbers using experiments with focus on a variety of characteristics such as mean flow, turbulent statistics and structure, while lower Reynolds number studies in channels have examined influence on skin-friction in addition to flow characteristics and mechanisms. In this work, we augment the previous work through experiments on ridge-type roughness in turbulent channel flows over a large range of Reynolds numbers. Our findings reveal that the global skin friction, as inferred through pressure drop in the channel along the streamwise direction, of these surfaces exhibits a log-linear asymptotic behaviour that is characteristic of a fully-rough surface at sufficiently high Reynolds numbers. Yet, the roughness function where this is observed is "low" compared to other established homogeneous rough surfaces. Moreover, the mean flow, turbulent statistics, and two-point correlation analyses indicate that the secondary flow structure, spatial extent, and magnitude remain largely invariant over the Reynolds number range considered. These findings have important implications on predicting drag of these heterogeneous surfaces as well as developing new models for the flow structure.