The flow in two three-dimensional (3D) asymmetric diffusers with the same expansion but different aspect ratios was recently measured (Cherry et al.,
). The results revealed complex 3D separation patterns with a severe sensitivity to the geometric variation. The setup served as a test case for two ERCOFTAC workshops (Jakirlić et al.,
) that aimed at assessing the predictive capabilities of various turbulence modeling approaches. Reynolds-Averaged Navier–Stokes (RANS) models based on the eddy-viscosity assumption yielded qualitatively wrong results. These models cannot reproduce secondary vortices (SV) in the inlet duct. Methods that account for SV or even resolve these structures fared better. In particular Large-Eddy Simulation (LES) was able to compute the flow in both diffuser geometries within measurement uncertainty (Schneider et al.,
). The hypothesis that SV have a strong impact on the separation dynamics was further corroborated by recent experiments (Grundmann et al.,
). At the inlet of one of the diffusers, localized (steady and unsteady) perturbations were introduced. The authors conjectured that the forcing generated streamwise vortices and that these SV were responsible for the observed change in pressure recovery by up to 14%. In the present paper, the hypothesis is tested by controlled numerical experiments using LES and manipulation of (mean) SV in the inlet duct for both diffuser geometries.