ALBERT

Description: Direct and large-eddy simulations of the interaction between a laminar boundary layer and a von Kármán vortex street behind a circular cylinder are carried out for three values of the Reynolds number based on the cylinder diameter and free-stream velocity: Reed 1155 and 3900. Rapid, bypass-like transition to turbulence is observed in the two higher-Reynolds-number cases. Flow statistics in the transitional and turbulent regions are examined, followed by an investigation of the underlying transition mechanism. Qualitative similarities between wake-induced transition and bypass transition due to free-stream turbulence are discussed and the challenges of predicting boundary-layer transition in this complex environment are pointed out. © 2006 Cambridge University Press.

Description: Direct numerical simulations (DNS) of bypass transition due to high-amplitude free-stream turbulence (FST) are carried out for a flat-plate boundary layer. The computational domain begins upstream of the plate leading edge and extends into the fully turbulent region. Thus, there is no ad hoc treatment to account for the initial ingestion of FST into the laminar boundary layer. We study the effects of both the FST length scale and the disturbance behaviour near the plate leading edge on the details of bypass transition farther downstream. In one set of simulations, the FST parameters are chosen to match the ERCOFTAC benchmark case T3B. The inferred FST integral length scale L11 is significantly larger (RL = UL11/ν = 6580) than that employed in previous simulations of bypass transition (RL≃ 1000). An additional set of simulations was performed at RL = 1081 to compare the transition behaviour in the T3B case with that of a smaller value of FST length scale. The FST length scale is found to have a profound impact on the mechanism of transition. While streamwise streaks (Klebanoff modes) are observed at both values of the FST length scale, they appear to have clear dynamical significance only at the smaller value of RL, where transition is concomitant with streak breakdown. For the T3B case, turbulent spots form upstream of the region where streaks could be detected. Spot precursors are traced to quasi-periodic spanwise structures, first observed as short wavepackets in the wall-normal velocity component inside the boundary layer. These structures are reoriented to become horseshoe vortices, which break down into young turbulent spots. Two of the four spots examined for this case had a downstream-pointing shape, similar to those found in experimental studies of transitional boundary layers. Additionally, our simulations indicate the importance of leading-edge receptivity for the onset of transition. Specifically, higher fluctuations of the vertical velocity at the leading edge of the plate result in higher levels of streamwise Reynolds stress inside the developing boundary layer, facilitating breakdown to turbulence. © 2008 Cambridge University Press.