Vortex Shedding and Interface Oscillations of Wind-Forced Liquid Drops

The conditions that lead to the depinning and runback of wind-forced liquid drops is an important problem in fluid mechanics that relates to aircraft ice accumulation and other situations. Multiple studies have examined wind-forced depinning but none have addressed how airflow and drop interface unsteadiness may be connected to depinning. Motivated by this, water drops ranging from 50 µL to 275 µL were placed on a horizontal, aluminum surface in a miniature wind tunnel and forced nearly to depinning using laminar wind flow. For each experiment, drop interface unsteadiness was captured using a high-speed camera while airflow fluctuations were measured using a hotwire anemometer. A synchronization technique was used in order to simultaneously measure drop oscillation and airflow unsteadiness. The data show that drop oscillation frequencies are independent of airflow speed but that the drop Bond number (the ratio of gravitational forces to surface tension forces) has a significant effect on drop oscillation frequency. Drop oscillation amplitude also increases markedly with freestream velocity. For the drops considered, the Strouhal number that characterizes unsteady airflow frequencies was found to have a range of 0.09 to 0.18. Lastly, a hemisphere versus water drop comparison concluded that the unsteady airflow frequencies is not affected by drop oscillation. These results suggest that while drop interface oscillation amplitude does depend on wind speed, there is not a coupling between drop interface unsteadiness and airflow unsteadiness in the drop wake