Elevated free-stream turbulence accelerates the process of laminar-to-turbulent transition by the selective inclusion of low-frequency disturbances into the boundary layer. As these amplify, they form longitudinal streaks that rapidly break-down into localized regions of turbulence that resemble the turbulent spots studied here under low free-stream turbulence. The manner in which free-stream turbulence alters the spot-growth mechanisms is unclear. Because gas-turbine engine environments and turbomachinery applications involve flows with elevated free-stream turbulence levels, this question is important to answer.
Figure 1 illustrates the results of a DNS. The prevalence of longitudinal streaks in the boundary layer are evident. The break-down of the streak into a localized region of turbulence involves the production of small-scale vortical structures via an instability of the longitudinal streaks.
Fig. 1
The results of the DNS indicate that the streak instability is due to an inviscid instability moe that is accelerated by the strong wall-directed motions induced by turbulent eddies in the free-stream. The result is that additional mechanisms are produced that accelerate the spot-growth mechanism that is already present under low free-stream turbulence levels. The turbulent spot that forms is shown in Fig. 2.
Fig. 2