Estimating Laboratory Wave Reflection Using Laser Doppler

by Steven A. Hughes, US Army Engineer Waterways, Experiment Station, Vicksburg, United States,

Abstract: Estimates of irregular wave reflection in two-dimensional laboratory wave tanks are typically accomplished using an array of spatially separated wave gauges. This paper presents a frequency domain analysis method that utilizes synoptic time series of either; (1) horizontal water velocity and sea surface elevation collected in a vertical array, or (2) horizontal and vertical water velocities collected at the same location in the water column. The `co-located gauge method' was tested using laboratory measurements obtained using a two-component laser Doppler velocimeter. Estimates of incident and reflected wave spectra compared well to similar estimates derived from spatially separated wave gauges. However, reliable estimates are limited to the range of spectral frequencies having appreciable energy and exhibiting good coherence in the cross-spectrum of the time series signals. Outside this range, improbable results occur. Also in this paper, the co-located gauge reflection analysis method is extended to the quasi-three-dimensional case of unidirectional, long-crested irregular waves being obliquely reflected by a straight reflective surface. This extension is based on the assumption that the oblique angle of wave incidence is known a priori.

Subject Headings: Wave reflection | Irregular waves | Wave tanks | Water management | Lasers | Doppler systems | Wave measurement

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