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F. Gokhan Ergin
Mechanical and Aerospace Engineering Department, Case Western Reserve University Cleveland, Ohio 44106, USA

Meelan M. Choudhari
Computational Modelling and Simulation Branch, NASA Langley Research Center Hampton, Virginia 23681, USA

Paul F. Fischer
Mathematics and Computer Science Division, Argonne National Laboratory Argonne, IL 60439, USA

Anatoli Tumin
Department of Aerospace and Mechanical Engineering, The University of Arizona Tucson, Arizona 85721, USA


Transient growth of linearly stable disturbances is believed to play an important role in the subcritical transition of laminar boundary layers and the self-sustained nature of boundary layer fluctuations in a fully turbulent flow. Prior work on transient growth has focused on identifying the optimum initial disturbances that result in maximum transient growth. This paper addresses the companion issue of receptivity of those disturbances, the mechanism that determines the actual magnitudes of transient growth that are realized in a given physical situation. A synergistic combination of experimental, computational, and theoretical approaches is used to quantify the flow receptivity to surface roughness in a Blasius boundary layer. Results reveal the non-optimality of the transient growth factors involved as well as the sensitive dependence of flow perturbations to the geometric characteristics of the roughness distribution. Direct numerical simulations (DNS) are compared in detail with experimental results, results obtained from linear receptivity theory and optimal disturbance calculations. DNS shows good agreement with the experimental results. Differences between the linear theory and DNS are attributed to nonlinear receptivity mechanisms. Results also support the proposal by Fransson et al. (2004) that disagreement between optimal disturbances and experiments/DNS may be attributed to differences involving the wall normal location of the streamwise vortex initiating the transient growth.