Multi-scale proper orthogonal decomposition (mPOD) analysis of vortex evolution and viscous dissipation in a circular-cylinder wake controlled by parallel symmetric jets

This paper describes a novel flow control scheme over circular-cylinder wakes using parallel symmetric jets. The effects of different jet momentum coefficients (C_m) at θ = ±30° on vortex evolution and viscous dissipation in the circular-cylinder wake are experimentally investigated using high-speed particle image velocimetry (PIV) in an open-jet wind tunnel at a Reynolds number of 1.01 × 10⁴. Multiscale proper orthogonal decomposition (mPOD) is used to analyze the evolution of coherent structures and establish the relationship between flow structures and viscous dissipation. At C_m ≤ 0.0089, the vortex formation length is elongated and the scale and intensity of the vortex are suppressed. Small-scale jet vortices are then generated by the interaction between the jet shear layers at C_m = 0.0174, whereupon the turbulent kinetic energy attains a minimum and the Reynolds shear stress in the wake is basically eliminated. At higher values of C_m, band-like jet oscillations appear in the wake, increasing the turbulence intensity. The inhibition effect of the jets on the wake vortex decreases the viscous dissipation in the wake, with optimal viscous dissipation control obtained at C_m = 0.0089. When C_m > 0.0089, the coupling between the jets increases the viscous dissipation of small-scale, high-frequency turbulent structures, and this effect gradually becomes more obvious with increasing C_m.