3D Laminar Separation Bubbles Induced by Spanwise-Inhomogeneous Pressure Gradients

Characteristics of three-dimensional (3D) laminar separation bubbles (LSBs) are investigated and compared with their two-dimensional (2D) counterparts using direct numerical simulation. Flow separation is induced via suction-blowing profiles applied along the top boundary, and the resultant separation is over a flat plate to avoid geometry-dependent effects commonly present in 3D separation studies. Unlike the canonical spanwise homogeneous suction-blowing profile, the spanwise extent of our profiles are varied using a Gaussian damping to introduce controlled three-dimensionality. Multiple levels of such spanwise inhomogeneity are analyzed. All simulations are performed at Re_δ = 1000. Results reveal distinct differences between 2D and 3D separation bubbles. Strong spanwise recirculation is present in 3D LSBs in addition to the wall-normal one. Instantaneous flow fields show that moderate three-dimensionality can lead to asymmetry in LSB topology. Depending on the degree of 3D, the wake may exhibit either single or dual wakes composed of various types of vortices in the turbulent (due to transition of the laminar separating shear layer) reattachment region. Several unsteady modes are associated with the formation and breakdown of the vortices, as well as the dynamic response of the recirculating flow.