Secondary motions above a staggered multi-scale rough wall

Wind tunnel experiments were performed to investigate turbulent flow over an array of heterogeneous roughness elements using stereoscopic particle image velocimetry. Nine streamwise planes, covering one periodic cell of a multi-scale roughness element that is arranged in a staggered pattern, are combined to quantify mean flow features and Reynolds stresses. Dispersive stresses, arising from spatial variations in the temporally averaged mean velocity, are also presented. The results highlight that the roughness elements create a large deficit pathway along the surfaces. Outer scaling of the time-averaged streamwise velocity presents features which are nearly independent of the roughness element type, with parameters of the flow revealing values near those observed in smooth wall boundary layers, such as the wake strength parameter, opposing the earlier work containing aligned patterns. The strength of the secondary motion is most accentuated at the ridges of the roughness, showing that the formation of structures is sensitive to the location investigated in the spanwise direction.