Roughness density effect on turbulent boundary layer evolution

Abstract

This work aimed to investigate the boundary layer topology over various densities of roughness. For experiments, we used three different surface conditions comprised of rectangular elements with uniform height arranged in staggered rows and covered the entire floor of the wind tunnel. The frontal and plan solidity for each case changed, λf from 0.3 to 1.2 and λp from 1.3 to 4.7, respectively. Also, we used one more surface, without roughness, for comparison. During the experiment, the position of measuring cross-sections varied relative to the inlet, while the free flow velocity was 5 m·s-1. Thus, the corresponding Reynolds number Reτ was from 300 to 1500 (based on boundary layer thickness δ and friction velocity uτ) or Reθ was from 400 to 5500 (based on momentum thickness). To determine the boundary layer topology in streamwise directions, we used a 55P14 miniature hot-wire probe. The obtained data allowed us to estimate patterns of distribution mean velocity profile, streamwise Reynolds stress, and turbulence intensity for different surface cases. In addition, an analysis of the scales of the turbulent flow over different surface types was also carried out. Namely, the integral length scale, Taylor microscale, and dissipation scale were determined. Obtained results show that roughness density composition strongly affects boundary layer characteristics. The obtained results also represent a correlation between the Taylor microscale and the initial position of the log region.