Hydrodynamic characteristics of harmonically excited thin-film flows : experiments and computations

Abstract

We present new results from the simultaneous application of Planar Laser-Induced Fluorescence (PLIF) and Particle Tracking Velocimetry (PTV), complemented by Direct Numerical Simula- tions (DNSs), aimed at the detailed hydrodynamic characteriza- tion of harmonically excited liquid-film flows. The experimental campaign spans the Reynolds number range Re = 8 − 320, and three Kapitza numbers Ka = 85, 350 and 1800. PLIF was em- ployed in order to generate spatiotemporally resolved film-height data, and PTV to generate two-dimensional (2D) planar velocity- vector maps of the flow-field underneath the wavy interface. By combining the two optical techniques, instantaneous and highly localised flow-rate data were retrieved, based on which the ef- fect of local film topology on the flow-field is studied in detail. Surprisingly, the instantaneous flow rate is found to vary linearly with the instantaneous film-height, while both experimental and numerical flow-rate data are closely approximated by a simple analytical relationship with only minor deviations. This relation- ship, which is reported here for the first time, includes the wave speed c and mean flow-rate Q, both of which can be obtained by simple and inexpensive methods, thus allowing for spatiotempo- rally resolved flow-rate predictions to be made without requiring any knowledge of flow-field information.