Cross-plane stereo-PIV measurements of a turbulent boundary layer over highly irregular roughness

The characteristics of a turbulent boundary layer overlying a complex roughness topography were explored with stereo particle-image velocimetry measurements in the wall-normal--spanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials containing a broad range of topographical scales arranged in a highly irregular manner. Such roughness is representative of that encountered in a broad range of practical flow systems, such as turbine-blade arrays, heat exchangers and marine vehicle surfaces, for example. Thus, understanding its impact on flow in a controlled laboratory environment is meant to provide a bridge to more fully understanding roughness effects in these practical scenarios.;Low-frame-rate stereo particle image velocimetry (PIV) measurements were conducted in the cross-flow, spanwise-wall-normal, plane at moderate Reynolds number. The single-point turbulence statistics in this plane displayed strong spanwise heterogeneity, in particular spanwise-alternating low- and high-momentum flow pathways in the mean flow marked by enhanced Reynolds stresses and turbulent kinetic energy. The spanwise regions between high- and low-momentum flow pathways were occupied by swirling motions, suggesting the generation and sustainment of turbulent secondary flows due to the spanwise heterogeneity of the complex roughness under consideration.;High-frame-rate stereo PIV measurements were then conducted in the same spanwise-wall-normal plane and at the same Reynolds number to study the turbulent kinetic energy and Reynolds shear stress content of the flow as a function of scale in the presence of this complex roughness. Similar to that observed for the mean and turbulence quantities noted above, frequency spectra of streamwise velocity at fixed wall-normal location also display strong dependence on spanwise position. In particular, the roughness promotes enhanced turbulent kinetic energy content of the large-scale motions and smaller-scale motions. Depending on spanwise location, pre-multiplied spectra highlight significant modification of the energy content of the very large-scale motions due to roughness when compared to smooth-wall flow. Interestingly, spanwise locations where high-momentum pathways reside in the mean flow embody higher turbulent kinetic energy and Reynolds shear stress content at streamwise scales of the very-large-scale motions compared to that observed at spanwise locations of low-momentum pathways.