The Effect of Streamtube Contraction on the Characteristics of a Streamwise Vortex

The ingestion of a vortex by an aero-engine is potentially an area of concern for current and future aircraft-engine configurations. However, there are very little experimental data on the characteristics of a streamwise vortex undergoing ingestion through a contracting streamtube. To address this dearth of knowledge, the ingestion of a streamwise vortex has been studied experimentally using stereoscopic particle image velocimetry (stereo PIV). A subscale model of an aircraft intake has been used to generate a contracting capture streamtube, and an isolated streamwise vortex has been generated upstream of the intake using semispan NACA 0012 and delta wing vortex generators (VGs). A range of contraction ratios, vortex Reynolds numbers, and vortex initial conditions have been examined. Measurements on planes perpendicular to the freestream flow show that the vortex undergoes notable levels of intensification through the contraction streamtube. The characteristics of the vortex are dependent on the streamtube contraction level, the initial aerodynamic characteristics of the vortex, and the trajectory that the vortex follows inside the capture streamtube. Results from inviscid, incompressible vortex filament theory have been compared with the experimental data. At relatively low streamtube contraction ratios this theory provides a good estimate of the vortex characteristics. However, at higher contraction levels, there are notable levels of diffusion, which render the vortex less intense than that anticipated from vortex filament theory.