It is well known that surface roughness can have a strong impact on the laminar-turbulent transition process. Whereas for isolated roughness some basic understanding of the physical mechanisms promoting transition has been gathered, the relevant physical mechanisms driving the transition process in the presence of distributed roughness are still far from understood. In this paper the influence of two-dimensional distributed wall roughness on the Blasius boundary layer stability is investigated. Distributed surface roughness is simulated by using an immersed boundary technique. This methodology facilitates the investigation of a large parameter range, regarding size, spacing and shape of the roughness elements, which is relevant for the characterization of distributed wall roughness. Some simulation results are compared to experimental results by Gaster.1 Moreover, it will be demonstrated that these parameters (roughness height, spacing and shape) strongly affect the linear stability characteristics of the boundary layer flow. For example, it was found that small amplitude disturbances experience stronger amplitude growth for sinusoidal roughness elements than for roughness elements with rectangular shape. In addition to investigating the effect of two-dimension roughness on the primary instability we also studied the effect on secondary instabilities.
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