Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
The boundary-layer transition to turbulence has been the subject of research for a long time. The transition process, however, has not yet been fully explained though the final stage of the boundary-layer transition has been explained that some small turbulent sources (turbulent spots) occur suddenly in the boundary layer. As these turbulent spots fill up the boundary layer, they induce a transition to turbulence of a laminar boundary layer. The most important process is the turbulentspot appearance in the transitional boundary layer because a progression to turbulence that is not present in the laminar state is promoted. The process of the transition from the prior state to turbulent spots, however, has not been ascertained. Thus, the mechanism of turbulent-spot appearance has only been explained as the word "breakdown". The computational simulation by Brandt  demonstrates that the breakdown is induced by the interaction of streaks which move laterally and slowly. Meanwhile, we investigate a downstream development of a single hair-pin-type vortex generated by an artificial small jet. From the velocity field measured in detail, this vortex grows and increases in number downstream, and finally the developed vortices constitute a spot. In the initial stage of downstream development where the vortices propagate in the streamwise direction, the velocity perturbations in a spot reiterate the in-phase wave form. In addition, the low- and the high-speed streaks in the spot are elongated straight in streamwise direction. In the transition stage, it is shown that the amplitude in the instantaneous velocity signals in the spot become irregular locally, where the low- and the high-speed streaks distort laterally. Further downstream, it is clarified that the occurrence of the momentum-transfer accompanied with local and temporary ejection movements and sweep movements become irregular in the spot, where the low-speed and the highspeed streaks cross one another and switch their positions with each other in spanwise direction. The appearance of the crossover of the streaks shows the break-up of the spot
structure, i.e., the beginning of its breakdown. A crossover of the streaks produces a new crossover in a chain reaction, so that the transition to turbulence (breakdown) progress rapidly. And finally the spot enter into a turbulent region. The irregularity in the velocity field, showing the other distinct feature of the spot, is occurred owing to the distortion of streaks in spanwise direction and, therefore, the streaks cross one another. Thus, we considered that these characteristics of velocity field are induced by a spanwise flow. In this study, we pay much attention to a spanwise flow and investigate its effect on the boundary-layer transition. From the measurement of the streamwise and spanwise component of velocity using a small X-type hot-wire probe, we found that the spanwise distortion of the velocity field and the irregularity of the velocity perturbation are caused by the spanwise flow in the spot. These results show that the spanwise flow have a critical role in laminar-turbulent transition.