Stability analysis on the effect of a smooth surface hump on secondary instabilities of crossflow vortices

Abstract

Surface imperfections of aerodynamic surfaces, such as steps, gaps and humps often cause an advancement of the transition front or an immediate laminar breakdown directly in their downstream region. However, recently, Vidales et al. (2025) demonstrated that the use of smooth surface hump can delay crossflow-dominated transition on swept wings. The objective of this work is to demonstrate that linear stability analysis can be employed to study the effect of a hump on the secondary instability development in an incompressible swept-wing boundary layer. A comparison of the instability computations against direct numerical simulation (DNS) results is provided. The geometry under study is a swept-wing model that mimics the experimental setup of Vidales et al. (2025) in the Low Turbulence Tunnel (LTT) at the Delft University of Technology. Two-dimensional Linear Stability Theory (LST-2D) and three-dimensional Parabolized Stability Equations (PSE-3D) are here employed to analyze the impact of the hump geometry and to identify how the secondary instability characteristics change compared to the clean case (without hump). This work adopts the approach used in Ambrosino et al. (2025) to study the evolution of secondary unsteady disturbances in a crossflow-dominated incompressible boundary layer over an infinite swept wing.