Experiments on the Instability and Transition of the Boundary Layer Over a Surface Gap

Surface gaps typically disturb the boundary layer and push the laminar-turbulent transition upstream compared to a smooth and perfect surface. Furthermore, the open-cavity flow introduces new instability mechanisms rather than the Tollmien-Schlichting (T-S) mode. To investigate this topic, the present paper addresses experiments carried out in a low-turbulence wind tunnel. The test model comprises a variable-depth cavity system attached to a flat plate, driven remotely. The disturbance and mean velocity are measured through the hot wire anemometry, whereas a Preston tube was employed to detect the transition location. Three different regimes were observed: T-S mode affected by the gap, concurrence of the T-S and cavity modes, and transition bypassed by the Rossiter mode. A relaminarization-like phenomenon marks the second regime. The turbulence generated inside the gap does not sustain itself downstream of it, rapidly decaying. This regime ends when a sufficiently strong Rossiter mode develops and bypasses the T-S growth.