Vortical flow development in round ducts across scales for engine inlet applications

Turbofan engine performance depends highly on the characteristics and conditions of the inlet flow. Swirl distortions, caused by non-uniformities in flows arising from boundary layer or ground/fuselage vortex ingestion, are of concern and need to be fully understood to guarantee the efficiency and safety of propulsion systems. To investigate a fundamental single-vortex distortion development in a duct at different Reynolds numbers, a StreamVane distortion generating device was designed and experimentally analyzed in a small-scale low-speed wind tunnel (ReD approximate to 500x103) and in a full-scale engine testing rig (ReD approximate to 3x106). Stereoscopic particle image velocimetry was used to measure the three-component velocity fields at discrete measurement planes downstream of the distortion device. Results show that the main features of the secondary flow are generated very similarly in both scales, and that its axial development is also overall very similar and mostly driven by two-dimensional (2D) vortex dynamics. Induced velocities arising from the proximity of the vortex to the duct wall cause the vortex center to convect circumferentially around the duct, in the same sense as the vortex rotation, as it travels downstream. The unsteady flow in the different measurement planes of the wind-tunnel experiment shows the development of local instabilities related to the turbulent development of the vortex. This work shows that the development of this vane-generated, vortex-dominated flow is largely Reynolds number independent for the covered range, so that details of similar duct-bounded flows can be analyzed in depth in small-scale experiments, decreasing development efforts and cost.