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A Procedure for Evaluating Shear Stress Frequency Domain Filters Measurements in an Annular Flume Using

Author(s): A. Schellart; F. Clemens; R. Veldkamp; S. Tait

Linked Author(s): Simon Tait

Keywords: Nnular flume; Shear stress; Sewer sediments; Fast Fourier transform; Low-pass filter; Noise reduction

Abstract: Erosion tests in an annular flume with real in-sewer sediments were carried out in a collaborative study with the University of Sheffield and the Delft University of Technology. The annular flume was located in the laboratories of WL|Delft Hydraulics in the Netherlands. The sediment beds were formed in a circular channel which rested within the flume under controlled environmental conditions. The inner sediment channel is supported by vertical wires so that it can move in a horizontal direction in relation to the bottom plate of the annular flume. The channel movement is measured by three cameras. Erosion of the sediment beds was induced by a stepwise increase of the overlying water velocity, thereby increasing the bed shear stress. It was therefore important to estimate the bed shear stress so that the impact of the changing hydraulic conditions on the other measurements TSS, VSS and COD can be determined. Three cameras at different points around the channel record the displacement of the inner sediment channel with a frequency of 2 Hz. The output signal of the cameras shows relatively much more noise with an increasing amplitude at higher velocities. The existing computer program to transfer camera readings to shear stress values was not able to account for this noise. New software was developed using fast Fourier transform and a low-pass filter in the frequency domain to remove undesirable noise and allow reliable measurements of bed shear stress to be made. Shear stress measurements were available for comparison with TSS, VSS and COD data obtained by analysis of collected discrete samples.


Year: 2003

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