Forcing large-scale coherent streaks in a zero-pressure-gradient turbulent boundary layer

Large-scale coherent streaks are artificially forced in a well-developed turbulent boundary layer at [image omitted] using an array of cylindrical roughness elements. Measures of the velocity field with particle image velocimetry reveal the presence of well-reproducible, streamwise-oriented, steady coherent streaks. We show that the amplitude of these coherent streaks transiently grows in space. The position of the maximum amplitude is proportional to the spanwise wavelength of the streaks and the most amplified spanwise wavelength is of very large scale z approximate to 60. These results are in good agreement with the recent predictions based on the optimal transient growth analysis of turbulent mean flows.