This study reports the identification of a previously uncharacterized type of capillary wave on the moving interface of a Taylor bubble in vertical countercurrent flow. In our experiments, the buoyancy of the Taylor bubble is precisely balanced by a downward laminar water flow (Re = 1400) within a 12.4 mm diameter glass pipe. These previously uncharacterized waves appear at a height one-fourth to one-half of the pipe diameter above the bubble's tail and remain stationary in the laboratory frame, implying that their upward propagation speed exactly matches the downward motion of the interface. The waves persist for 5–10 wavelengths, spanning one-fifth to one-third of the pipe diameter. We measured their wavelength for bubbles of different lengths (1.5–11 pipe diameters) and determined the liquid film thickness at the wave location. From the measured mass flow rate, we calculated the laminar velocity profile in the liquid film and the interface velocity. We have shown that the measured wave propagation speeds align with the interface velocities computed from the film thickness, confirming their capillary nature within experimental uncertainty. Furthermore, our analysis reveals the underlying physics: these capillary waves originate from a newly formed toroidal vortex near the bubble tail that rotates opposite to the primary air vortex inside the bubble, consistent with previous computational fluid dynamics (CFD) simulations and our new observations.

The capability of the standard SST k-ω turbulence model for the prediction of jet impingement cooling characteristics using a coarse mesh is investigated. The discussion is based on a sensitivity study with five computational grids, differing from each other in topology and resolution. The analysis considers a hexagonal configuration of turbulent jets at the inlet Reynolds number equal to 20,000, with the distance between the nozzle and target plates equal to four nozzle diameters. The results of steady RANS simulations are validated against the time-averaged LES results and data from experiments. The mean heat transfer characteristics of turbulent impinging jets have been successfully reproduced with all tested grids, which indicates that for a rather accurate mean heat transfer prediction, it is not necessary to resolve all the small-scale flow features of impinging jets above the target plate.