The effect of Viscosity on the Interfacial Instability of Liquid-Air Interface Oscillation in a Vertical Pipe

Surface instability has been known to reduce the effectiveness of fluid as medium for energy transfer particularly in a liquid piston or Fluidyne engine. Therefore, understanding how to suppress or influence the presence of instability is crucial for efficient operation liquid piston engines for applications such as energy harvesting. In this study, oscillating surface characteristics of 3 different liquids/oils:VG-22, ISO 10W-40 and ISO 20W-50 were examined. The fluids were oscillated at a frequency of 2 to 24 Hz within a vertical pipe. A piston coupled with permanent magnet exciter was used as a driver of the oscillation. The liquid-air interface (the surface) was then observed via high-speed camera at 1000 fps and 240 fps. Deformation of the liquid-air interface (define here as established modes) occurred at frequencies above 6 Hz. Eight established mode types were observed as the liquid formed unique standing waves patterns at the surface at frequencies varying from 7 – 24 Hz that were dependent on fluid viscosity. It was observed that fluids with lower viscosity and higher Womersley number exhibited deformation at a higher range of frequencies. Therefore, fluids with higher viscosity and lower Womersley number tended to have a better capability of maintaining a stable surface. © 2024 American Institute of Physics Inc.. All rights reserved.