Turbulent boundary layer flow over multiscale regular roughness

Multiscale rough surfaces have received substantial attention due to their presence in numerous research topics such as atmospheric boundary layers and hydrology. The presence of multiple scales affects both the mean flow and the turbulence when the characteristic length scales of the surfaces are significant compared to the length scales of the flow. Despite the burgeoning interest in this area, little is known about the effect hierarchy of scales on the flow. To this end, a comprehensive experimental study is performed over surfaces where multiscale roughness with self-similar cuboids is used. Starting with large-scale cuboids, their sizes decrease with iterations while their number increases with a power law, with subsequent smaller cuboids being distributed on top as well as in between previous larger cuboids. Wind-tunnel testing using a floating element balance along with cross-plane Stereo-PIV were performed to assess the drag and the flow field over three test cases. Results show that drag increases substantially from two iterations to three iterations of cuboids indicating that the small scales of roughness make a significant contribution to the drag. The effects of scales on the mean flow, turbulence statistics as well as other flow characteristics will be presented.