Transient growth has been identified as a possible source of boundary layer transition. Transient growth is initiated as weakly damped vortical disturbances in the boundary layer that redistribute momentum from the freestream into the boundary layer. Even though the vortices are stable and damped by viscosity, the redistribution of streamwise momentum can cause the steady, streamwise velocity disturbance to grow before decaying further downstream. The redistribution of streamwise momentum eventually forms low- and high-speed streamwise streaks. If the streaks are large enough in magnitude, transition can occur through secondary instabilities, or they may modify the behavior or other instability modes. The redistribution of streamwise momentum can be numerically expressed as a collection of linear disturbance modes at different spanwise wavelengths; these disturbance modes are non-orthogonal and can create spatial disturbance growth followed by long-wavelength exponential decay.
At Texas A&M, various students are studying roughness-induced transient growth both theoretically and experimentally. Matthew Kuester recently finished his dissertation research in which he studied transient growth created by distributed roughness patches at low speeds in the KSWT; this built upon the work of Rob Downs. Nicole Sharp recently finished her dissertation, which studied roughness-induced transient growth on cones at hypersonic speeds in the NASA Langley Mach 6 Quiet Tunnel. Jason Monschke is building upon the theoretical work of Nick Denissen by studying the growth and stability of roughness-induced wakes in low- and high- speed boundary layers.