Spectral Analysis of the Transition to Turbulence Downstream a Delta Winglet Pair Vortex Generator in an Airflow Channel

The purpose of this experimental study is to provide a detailed analysis of the transition to turbulence in an airflow channel perturbed by a delta winglet pair vortex generator. Measurements of instantaneous velocity are carried out by a laser Doppler anemometry technique. Experiments are conducted for Reynolds numbers (based on the hydraulic diameter) ranging from Re = 400 to Re=12 000 and at several axial position downstream of the vortex generator. The flow behavior is characterized in each regime by means of a frequency analysis. We identified the inlet conditions corresponding to laminar weakly unsteady, transitional, and fully turbulent flow regimes. Furthermore, we have constructed a correlation between the Strouhal number and the Reynolds number that characterizes the vortex instability mechanism. Finally, under turbulent flow conditions, the dissipation rate was estimated, which showed an increase followed by an exponential decrease with the distance downstream of the vortex generator.