Passive control of near-wall turbulence by means of roughness elements

The present work experimentally investigates the effect of roughness elements onto zero-pressure-gradient turbulent boundary layer. We use an array of spanwisely-aligned cylindrical roughness elements attached on the wall surface to regulate the nearwall low-speed streaky structures. With both qualitative visualization and quantitative measurement, it is found that the regularization only occurs in the region below the height of the roughness element, where the local streamwise turbulent fluctuation intensity can be reduced with about 10%. Moreover, it is also found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence, there is no streak-regularization effect in the buffer region, so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce.