Development of turbulent patch in a subcritical boundary-layer transition

Abstract

In order to clarify the mechanism of wall turbulence generation, we have been working on the subcritical boundary-layer transition triggered by high-intensity hairpin eddies. In this paper, we present our recent results on the subcritical disturbance growth leading to wall turbulence - in particular, on the so-called lateral contamination obtained by periodically exciting hairpin eddies through a wall orifice. In this subcritical transition, which may occur beyond the x-Reynolds number R x = 4–5 × 10 4, wall turbulence develops along a narrow strip extending downstream from the orifice, being caused by the regeneration of hairpin eddies. The half lateral spreading angle of the turbulent patch is found to be about 2° for R x < 1 × 10 5, which is much smaller than the typical value for the lateral growth of a turbulent spot or wedge observed at high Reynolds numbers. With an increase in the Reynolds number, the lateral width of the turbulent patch rapidly increases; that is, the half spreading angle is about 4° for R x = 1.1–1.7 × 10 5 and about 6.5° for R x = 2.0–2.8 × 10 5.