Author(s): Jiayi Wu; Andrew Nichols; Anton Krynkin; Martin Croft

Linked Author(s): Andrew Nichols

Keywords: Partially-filled pipe; Free surface dynamics; Free surface measurement; Turbulent flow; Experimental fluid mechanics

Abstract: The modelling and measurement of the hydraulic flow conditions in partially-filled pipes is of great importance for a broad audience, from academia to industry. Partially filled pipes have many applications in industry, for example, the transport of wastewater in sewer pipes, the transport of food or personal care products, subsurface drainage and in the nuclear and petrochemical industries. Partially-filled pipe flow is an unusual class of open channel flow, the hydrodynamics of which are not fully understood. In particular the mechanism by which flow turbulence gives rise to water surface features (waves) in partially-filled pipes has not been fully explained. The dynamics of free surfaces is an emerging area of interest due to the potential to remotely infer flow properties from the surface behaviour. The present study has developed a unique experimental setup and has collected a large novel dataset. A Digital 3D Image Correlation (DIC) system (Q-400, Dantec Dynamics) was used to measure water surfaces in a partially-filled 290mm diameter pipe, which is effective for all the flows examined, with surface velocity from 0.35-0.64m/s. The water was first dyed white using TiO2 power to reduce the transparency, and the opaque water surface was viewed by two high-resolution digital cameras above the surface from two directions. A stochastic pattern was projected onto the surface by a HD projector, eliminating the need for physical seeding. The 3D fluctuation of the water surface was then obtained via a DIC algorithm. The advantage of this DIC method is that it is a low-cost, high precision seedless technique that is not affected by the high turbulence intensity. The main limitations of this method are that the streamwise particle velocity cannot be obtained (only wave velocity) and that areas of reflections and bubbles in images can result in noisy or missing data. Further study is required to assess applicability in a field environment. In addition to the DIC system, 7 non-equally spaced wave probes were placed downstream of the DIC measurement area to measure surface fluctuations at different streamwise positions, and the surface velocity was measured by timing a floating tracer travelling a known distance. The sub-surface velocity profile was measured by Acoustic Doppler Velocimetry (ADV). The accuracy, reliability and repeatability of data collected from various instruments have been compared and evaluated. A range of analysis methods (for example power spectrum, spatial correlation, frequency-wavenumber, and dispersion relation analyses) have been carried out to explore the behaviour of the turbulent structures in partially-filled pipes, and their role in generating free surface dynamics. This work will shed light on the nature of free surface dynamics in partially-filled pipes, and form an important step towards non-invasive measurement of pipe flow conditions beyond a simple measurement of surface velocity.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022921

Year: 2022