Newsletter 2018.3 Index
Theme : "The Ninth JSME-KSME Thermal and Fluids Engineering Conference (TFEC9)"
Laminar-turbulent transitions of boundary layers are numerically simulated in order to investigate key vortical structures which directly trigger the onset of turbulence in the transition process. A short duration jet is ejected from a square hole into the boundary layer as a localized disturbance. Pairs of flow fields with slightly different jet velocities are prepared. In one of each pair, the flow field becomes turbulent while in the other, the flow field returns back to a laminar state. The key structures are searched by comparing the pairs. As the first flow field, streaky structures are formed using the cuboids installed at wall and a short duration jet is issued beside a low-speed streak changing its velocity. At both jet velocities, primary streamwise vortices are generated triggered by the jet ejection, which do not lead to a boundary layer transition. However, only in the stronger jet case, a secondary streamwise vortex, which is the key structure, appears upstream of the primary streamwise vortex. Following the appearance of the secondary streamwise vortex, new other vortices emerge one after another around the vortex and the flow field soon becomes turbulent. The difference in shapes between the primary streamwise vortex and the secondary one is focused. The secondary vortex has more angle to the flat plate, making it likely to be stretched by the mean velocity gradient of the boundary layer. The angle of the secondary vortex is created because of the swirling flow induced by the primary vortex. In other flow fields, namely when the jet location symmetric to the low speed streak and when there is no streak in the boundary layer, the key vortical structures which share the same feature as the secondary streamwise vortex are found. It is suggested that the streamwise vortex with an angle is playing the key role in starting the laminar-turbulent transition process.
Boundary layer transition, Key structure in transition, Streaky structures, Short duration jet, Numerical simulation
Fig. 1 Time variations of vortical structures. νjet = 0.08 and 0.10 cases compared.
Fig. 2 Side views of vortex A at t = 60 and vortex B at t = 120.( vjet = 0.10 case)
Fig. 3 Velocity vector field with color map of streamwise vorticity in y-z plane at t = 80.
Fig. 4 Key structures in laminar-turbulent transitions.
Symmetric blowing case and no streak case.