Impact of turbulence and secondary flow on the water surface in partially filled pipes

Large eddy simulations (LESs) of turbulent flow in partially filled pipes at various filling degrees are conducted to investigate the response of the water surface to the turbulence and the secondary flow below it. LESs are validated first using experimental and direct numerical simulation data. At increasing water depth, the magnitude of water surface fluctuations increases with increasing strength of the main secondary flow. Visualizations of the instantaneous water surface and the turbulent flow underneath reveal that thin surface waves are generated by flow meandering in the shallower case, whereas surface waves in the deeper cases are influenced by the vertical velocity fluctuation. Pre-multiplied spectra of the water surface fluctuation, h', provide further evidence of the origin of the surface waves. In the shallow flow, the peak frequency of the h' spectra is consistent with the peak frequency of the u' and v' spectra, while for deeper flows, it agrees more with the w' spectra. Furthermore, the transport patterns of the surface waves are investigated by the wavenumber-frequency spectra. Three types of surface waves are observed in the wavenumber-frequency spectra, i.e., (1) convective waves with phase velocity equaling the surface velocity, (2) irrotational dispersive gravity-capillary waves, and (3) stationary waves caused by secondary currents. (C) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

Automated summary

This study investigates the response of turbulent flow and water surface fluctuations in partially filled pipes through large eddy simulations. The results show that as the water depth increases, the strength of the main secondary flow and the magnitude of water surface fluctuations also increase. Pre-multiplied spectra provide further evidence of the origin of the surface waves. Analysis of the wavenumber-frequency spectra reveals different types of surface waves.