Time-resolved particle image velocimetry and pressure sensitive paint measurements of afterbody flow dynamics

In this study, the dynamical behavior of the vortex pair produced in the wake of a cylinder with a slanted afterbody, relevant to cargo aircraft applications, is examined experimentally through time-resolved particle image velocimetry (TR-PIV) and fast-response pressure sensitive paint (PSP). The vortex wandering phenomenon, characterstic of this wake, is measured with state of the art, time-resolved diagnostics at Reynolds numbers ranging from ReD=25000 to 750000 and a slant angle of φ=45°. Spectral proper orthogonal decomposition reveals a consistent and narrow normalized frequency band (0.56<StD<0.64) with significant energetic peaks in both surface pressure and near-field vortex velocity fields across all Reynolds numbers examined. Pressure sensitive paint measurements at free stream velocities ranging from 15 m/s (ReD=150000) to 75 m/s (ReD=750000) reveal a strong unsteady signature of the flow structures that are related to the vortex wandering at these normalized frequencies. These structures consist of traveling pressure waves at the surface that excite the helical mode of the vortex core. These pressure patterns could be detected at free-stream velocities as low as 15 m/s, where the pressure fluctuations are of the order of 20 Pa.