Predicting the Vortex Formation Regimes behind an Obstacle in a Channel in Conditions of Sinusoidal Flow Rate Variation

Two-dimensional optical measurements and spectral clustering were employed to study the effect of forced unsteadiness on the pattern of vortex formation behind a backward-facing step and an array of ribs. The Strouhal number ranged between 0.016 and 0.617, while the relative amplitude of velocity varied from 0.11 to 0.8. Three different unsteady regimes were observed and described in the near wake behind the obstacle, and the criteria characterizing these regimes were estimated. A unified flow map comprising the flows behind backward-facing steps and rib arrays was plotted. The flow map takes into account the variation of parameters. According to the study, the flow regimes can be predicted using only the Strouhal number. Correlation of the estimated boundaries between the flow regimes, the spectrum of natural frequencies, and flapping and shedding frequencies was analyzed. The amplitude of forced velocity pulsation was shown to affect the vortex formation.