Experimental and theoretical investigation of the non-modal growth of steady streaks in a flat plate boundary layer

Authors: Fransson, J. H. M., Brandt, L.B., Talamelli, A., Cossu, C.
Document Type: Article
Pubstate: Published
Journal: Phys. Fluids
Volume: 16   3627-3638
Year: 2004


An experimental and theoretical investigation aimed at describing the non-modal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable non-modal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.