Self-excited shock layer and laminar separation bubble instability

The theoretical framework underlying global flow instability will be introduced and its application to predict laminar-turbulent transition of flows with multiple inhomogeneous spatial directions from the incompressible to the hypersonic flow regime will be presented. Numerical solution of the multi-dimensional eigenvalue and initial-value problems discretizing the linearized equations of flow motion and respectively governing modal (exponential) and transient (algebraic) growth of small-amplitude perturbations will be discussed. Self-excitation of two- and three-dimensional laminar separation bubbles that form on flight vehicle components, such as adverse pressure gradient boundary layers and incompressible flow over two-/three-dimensional wings will set the scene for the reappearance of the self-excited global mode of laminar separation bubble in shock/laminar boundary layer interactions in supersonic and hypersonic flows over compression ramp, double cone and double wedge configurations. In 2D shock/boundary layer interactions, analysis has shown the intimate connection (through the global mode amplitude function) of instability in the laminar separation zone, the triple point, the slip line and the shock system downstream of separation. In 3D hypersonic flow over a double wedge a previously unknown physical mechanism of amplified shock layer instability has been discovered and shown to synchronize with the self-excited global mode of the laminar separation bubble.