Direct numerical simulations are carried out to investigate the role of the turbulent region in a self-sustaining system with a spiral vortex structure in the three-dimensional boundary layer over a rotating disk by solving the full Navier–Stokes equations. Two computational domains with two different azimuthal sizes, $2\unicode[STIX]{x03C0}/68$ and $2\unicode[STIX]{x03C0}/32$, are used to deal with different initially dominant wavenumbers. An artificial disturbance is introduced by short-duration strong suction and blowing on the disk surface. After the flow field reaches a steady state, a turbulent region forms downstream of $Re=640$. The turbulent region is then removed using two methods: a sponge region, and application of a slip condition at the wall. In both cases, the turbulent region disappears, leaving the spiral vortex structure upstream unaffected. The results suggest that the downstream turbulent region is not related to the velocity fluctuations that grow by the global instability. In addition, when the area where the slip condition is applied is changed from $Re>630$ to $Re>610$, the velocity fluctuations decay. The results indicate that the vibration source of the velocity fluctuations which grow by the global instability is located between $Re=611$ and $Re=630$.
The structure at a spanwise edge of spreading turbulent region is studied in a wind tunnel experiment using conditional-sampling and ensemble-averaging techniques. A turbulent region is created by locally tripping the laminar boundary layer using two bi-morph piezoelectric actuators. The turbulent region protrudes into the laminar region at the middle height of the boundary layer. The results show that when the turbulent region spreads, a pair of streamwise vortices exists near the interface on the turbulent side, and the flow induced by the vortex pair pushes the turbulent fluid into the laminar region. An experiment in which a thin flat plate is inserted into the boundary layer reveals that the spanwise spreading of the turbulent region can be effectively suppressed by destroying these vortical structures.
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