J. Casacuberta, S. Hickel, M. Kotsonis (2024)
Physical Review Fluids 9: 043903. doi: 10.1103/PhysRevFluids.9.043903

A novel mechanism is identified, through which a spanwise-invariant surface feature (a two-dimensional forward-facing step) significantly stabilizes the stationary crossflow instability of a three-dimensional boundary layer. The mechanism is termed here as reverse lift-up effect, inasmuch as it acts reversely to the classic lift-up effect; that is, kinetic energy of an already-existing shear-flow instability is transferred to the underlying laminar flow through the action of cross-stream perturbations.

To characterize corresponding energy-transfer mechanisms, a theoretical framework is presented, which is applicable to generic three-dimensional flows and surface features of arbitrary shape with one invariant spatial direction. Following our analysis, it is found that the lift-up effect dominates the mechanisms of perturbation interaction between a pre-existing stationary crossflow instability and a forward-facing step; whether the classic or a reverse lift-up effect dominates depends, at least, on the step height and the free-stream-flow evolution. The reverse lift-up effect is always localized. Nonetheless, the subsequent slow spatial relaxation of perturbations towards no-step conditions yields a large area of reduced growth rate of the crossflow instability.

The identification of a passive geometry-induced mechanism leading to (primary wavelength) stationary-crossflow perturbation stabilization is a promising finding for flow control research and aircraft design. A significant reduction of the amplitude of stationary crossflow vortices, which drive the process of laminar-turbulent transition in swept-wing flow, may be achieved by appropriate design of surface features. Moreover, the mechanism characterized in this work does not require previous knowledge of the wavelength or the perturbation phase upstream of the surface feature. Therefore, it may be applied successively throughout an aerodynamic surface for an overall enhancement of its underlying benefit without need for active phase calibration.