POD and SPOD analysis of vertical liquid sheet

Modal analysis of thin liquid sheet driven by a harmonic forcing in the lateral velocity component at the inlet section is performed. Proper orthogonal decomposition (POD) and spectral POD (SPOD) techniques have been applied on two-dimensional two-phase numerical simulations data, obtained with the VOF approach, to characterize the main spatial and temporal structures. The investigation is carried out varying the Weber number and the Reynolds number. In supercritical regime (Weber number, > 1) both POD and SPOD techniques yield only leading sinuous modes, featuring a traveling perturbation. Spectral analysis confirms the occurrence of a discontinuity in frequency response between the supercritical and subcritical regimes. In subcritical regime ( < 1) the excitation of a combined sinuous-varicose motion is detected when the system is driven at resonance frequency for relatively high Reynolds numbers.