Data on the frequency-wavenumber spectra and dispersion relation of the dynamic water surface in an open channel flow are very scarce. In this work, new data on the frequency-wavenumber spectra were obtained in a rectangular laboratory flume with a rough bottom boundary, over a range of subcritical Froude numbers. These data were used to study the dispersion relation of the surface waves in such shallow turbulent water flows. The results show a complex pattern of surface waves, with a range of scales and velocities. When the mean surface velocity is faster than the minimum phase velocity of gravity-capillary waves, the wave pattern is dominated by stationary waves that interact with the static rough bed. There is a coherent three-dimensional pattern of radially propagating waves with the wavelength approximately equal to the wavelength of the stationary waves. Alongside these waves, there are freely propagating gravity-capillary waves that propagate mainly parallel to the mean flow, both upstream and downstream. In the flow conditions where the mean surface velocity is slower than the minimum phase velocity of gravity-capillary waves, patterns of non-dispersive waves are observed. It is suggested that these waves are forced by turbulence. The results demonstrate that the free surface carries information about the underlying turbulent flow. The knowledge obtained in this study paves the way for the development of novel airborne methods of non-invasive flow monitoring.

The combined length of the sewerage and clean water pipe infrastructure in the UK is estimated to be about 800,000 km. It is prone to failure due to its age and the inadequacies of the current pipe inspection methods. Fibre-optic cable sensing is an attractive way to continuously monitor this infrastructure to detect critical changes. This paper reviews the existing fibre-optic sensor (FOS) technologies to suggest that these technologies have better sensing potential than traditional inspection and performance monitoring methods. This review also discusses the requirements for retrofitting an existing pipeline with an FOS. It also demonstrates that there is a need for further research into methods applicable to non-pressurised pipelines, as there is very little existing literature that focuses on partially filled pipes and pipes with gravity fed flows.

This paper presents a novel method for measuring three-dimensional (3D) water surface dynamics in a partially filled pipe. The study on investigation of the 3D free surface dynamics in partially filled pipes is very limited. The method involves tinting the water white with titanium dioxide so that the water surface appears like a solid surface to image-based measuring systems. This method uses a high-resolution projector to project a stochastic pattern of light onto the water surface and uses two high-resolution cameras to capture the pattern on the deformed water surface. The 3D instantaneous water surface fluctuation can be computed from the images captured by the two cameras using a standard Digital Image Correlation algorithm. It is demonstrated that the surface dynamics parameters of turbulent flow in partially filled pipes, including surface fluctuations and surface velocity, can be measured using the projector, high-resolution cameras and DIC algorithm.

Accurate reconstruction of the surface roughness is of high importance to various areas of science and engineering. One important application of this technology is for remote monitoring of open channel flows through observing its dynamic surface roughness. In this paper a novel airborne acoustic method of roughness reconstruction is proposed and tested with a static rigid rough surface. This method is based on the acoustic holography principle and Kirchhoff approximation which make use of acoustic pressure data collected at multiple receiver points spread along an arch. The Tikhonov regularisation and generalised cross validation technique are used to solve the underdetermined system of equations for the acoustic pressures. The experimental data are collected above a roughness created with a 3D printer. For the given surface, it is shown that the proposed method works well with the various number of receiver positions. In this paper, the tested ratios between the number of surface points at which the surface elevation can be reconstructed and number of receiver positions are 2.5, 5, and 7.5. It is shown that, in a region comparable with the projected size of the main directivity lobe, the method is able to reconstruct the spatial spectrum density of the actual surface elevation with the accuracy of 20%.

Currently, there is no airborne in situ method to reconstruct with high fidelity the instantaneous elevation of a dynamically rough surface of a turbulent flow. This work proposes a holographic method that reconstructs the elevation of a one-dimensional rough water surface from airborne acoustic pressure data. This method can be implemented practically using an array of microphones deployed over a dynamically rough surface or using a single microphone which is traversed above the surface at a speed that is much higher than the phase velocity of the roughness pattern. In this work, the theory is validated using synthetic data calculated with the Kirchhoff approximation and a finite difference time domain method over a number of measured surface roughness patterns. The proposed method is able to reconstruct the surface elevation with a sub-millimeter accuracy and over a representatively large area of the surface. Since it has been previously shown that the surface roughness pattern reflects accurately the underlying hydraulic processes in open channel flow [e.g., Horoshenkov, Nichols, Tait, and Maximov, J. Geophys. Res. 118(3), 1864–1876 (2013)], the proposed method paves the way for the development of non-invasive instrumentation for flow mapping and characterization that are based on the acoustic holography principle.