Free surface instabilities and plunging breaking waves downstream of a bump in a shallow water open channel flume

The present research is motivated to study breaking waves and obtain validation data for two phase computational fluid dynamics (CFD) results. Experiments were conducted in an open channel flume to study the fluid dynamics of plunging wave breaking that were created in a two dimensional open channel using a bottom bump having a fourth order curvature profile and impulsive accelerated flow. In the process of determining the best conditions for wave breaking, free surface span-wise instabilities that were observed in the region downstream of the bump for shallow water depths and low upstream flow speed that inspired the authors to make a separate detailed study of these instabilities.;The present research focuses on two separate studies. The first part aims to derive a critical number that characterizes the onset of the free surface instability. Centrifugal Taylor instability analysis for laminar flow using perturbation method with modified boundary conditions is adopted for theoretical calculations for determining the critical number. Experiments were conducted with two different bump sizes and various water depths to compare and validate the theory. The critical values from the experiments are quite close to the predicted critical Taylor numbers. PIV measurements further downstream captured the centrifugal vortices along with the shear layer secondary structures that exist downstream of the transition distance. However, cross-plane PIV measurements along with theoretical correlations showed that the sizes of the shear layer vortices are much smaller compared to the centrifugal vortices and in most cases the instability originates upstream of their transition distance. While obtaining experimental data for quantifying the free surface instability, a thorough study of open channel flows was also conducted to document the upstream flow.;In the second part an experimental study of the plunging breaking wave is presented along with the comparison between EFD and CFD results. The time evolution of the transient wave and its flow properties are measured using experimental fluid dynamics (EFD): upstream and downstream velocity and flow rates using pitot probes; air-water interface elevation measurements and two dimensional particle image velocimetry in the wave breaking region. CFD wave profiles at various time steps identifies the overall wave breaking process and major events: max wave height, first plunge, oblique splash-up, vertical jet, air entrainment, two repeats of these processes, dissipation and wave swept downstream which is qualitatively validated by EFD results. Both EFD and CFD results showed two subsequent plunging and splash-up events after the first plunge. After the wave breaks, the flow trends in mean velocity and vorticity observed in EFD are very similar to CFD which has more detailed resolutions of plunging, splashing, vertical jet and bubble entrainment. Current studies also revealed the occurrence of chaotic multiple splash-up events after the third plunging that produce span-wise vorticity and turbulence. Generation of a clockwise rotating bump vortex and an anticlockwise rotating span-wise wave breaking vortex that is created from the entrapped air after the breaking which transports turbulence from the trough towards the bulk fluid, were identified as the two important events.