Vortex Shedding and Depinning of Wind-Forced Liquid Drops

Water drops adhere to solid substrates but can depin when wind forcing exceeds the adhesion force provided by surface tension. Schmucker and White (2012.DFD.M4.6) measured critical wind forcing limits for high-Reynolds-number airflow forcing and found a critical constant Weber number, We_crit = 8.0 for a range of drop Bond numbers. This work seeks to identify what behavior is associated with We_crit = 8.0 and why wind-forced drops depin when they do. One hypothesis suggests that, at high Reynolds numbers, drops depin when their interface natural frequency matches the frequency of air vortex shedding in the separated drop wake. We investigate whether a resonance between vortex shedding and drop interface oscillations is involved with depinning. We measure the shedding frequencies behind solid protuberances of the same size as typical drops and, separately, water-drop interface frequencies. We compare our measured values under different flow conditions to establish whether shedding and interface resonance are related to depinning.