Late-stage transitional boundary-layer structures. Direct numerical simulation and experiment

This paper is devoted to direct comparisons of related, detailed experimental and numerical studies of the non-linear, late stages of laminar-turbulent transition in a boundary layer including flow breakdown and the beginning of flow randomization. Preceding non-linear stages of the transition process are also well documented and compared with previous studies. The experiments were conducted with the help of a hot-wire anemometer. The numerical study was carried out by direct numerical simulation (DNS) of the flow employing the so-called spatial approach. Both the experiments and the DNS were performed at controlled disturbance conditions with an excitation of instability waves in the flat-plate boundary layer. In the two cases, the primary disturbance consists of a time-harmonic, two-dimensional Tollmien-Schlichting wave that has a very weak initial spanwise modulation. Despite somewhat different initial disturbance conditions used in the experiment and simulation, the subsequent flow evolution at late nonlinear stages is found to be practically the same. Detailed qualitative and quantitative comparisons of the instantaneous velocity and vorticity fields are performed for two characteristic stages of the non-linear flow breakdown: (i) "one-spike stage" and (ii) "three-spike stage." The two approaches clearly show in detail the process of development of the A-structure, a periodical formation of ring-like vortices, the evolution of the surrounding flow field, and the beginning of flow randomization. In particular, it is found experimentally and numerically that the ring-like vortices (associated with the well-known spikes) induce some rather intensive positive velocity fluctuations (positive spikes) in the near-wall region which have the same scales as the ring-like vortices and propagate downstream with the same high almost free-stream) speed. The positive spikes form a new NO-shear layer in the near-wall region. In the experiment the induced near-wall perturbations have a significant irregular low-frequency component, These non-periodical motions play an important role in the process of flow randomization and final transition to turbulence that starts under the ring-like vortices in the vicinity of the peak position.