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No description available.

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We report on different addressing mechanisms for quasi-distributed absorption sensors based on the frequency modulated continuous wave (FMCW) method. The sensor units consist of open-path micro-optic cells constructed from GRIN lenses, each of differing lengths. Guided by initial simulations, two approaches are experimentally investigated and evaluated, namely reference arm addressing and coherence addressing. Reference arm addressing is accomplished by the selection of different length reference arms in a Michelson configuration where each reference arm corresponds to a certain sensing unit. Coherence addressing is achieved by the interferometric mixing of two signals originating from each cell (from the glass/air interfaces). For each method, we show theoretically and experimentally how individual cells can be addressed and the measured signals obtained by suitable choice of cell length, proper modulation of the source and appropriate signal processing. In order to improve sensitivity we present the theoretical analysis of a new scheme based on combining the (FMCW) technique with frequency modulation spectroscopy (FMS). Here we arrange for only one sideband of the rf-modulation to be attenuated by the absorption feature and a new signal, proportional to the absorbance, appears in the output spectrum at a frequency corresponding to the difference between the rf-modulation frequency and the beat frequency of a cell. The method is highly sensitive and applicable to a variety of chemical species with narrow absorption lines, such as in trace gas analysis. We present the mathematical analysis of the proposed method for single and multiple cell systems, using methane detection as an example.

Several studies have previously demonstrated that sediment transported as bed load under a standing wave will be moved from between node and antinode towards the node where it accumulates. The end result is the generation of areas of scour in the bed between node and antinode and areas of sediment accretion around the node. However, these studies have failed to provide an adequate description of the mechanisms which led to this so-called "N-type" response. Consequently it has been the purpose of this study to examine the phenomenon of N-type beach response in some detail. As a first step an experimental programme was conducted in a random wave flume using a model beach and vertical, impermeable wall N-type conditions were produced, observations are made on the mechanisms of sediment transport and flow-field measurements are also reported. The experimental results demonstrate that N-type beach response is the direct result of an asymmetry in the main flow-field caused by the superposition of incident and reflected non-linear waves. A second experimental programme examines the N-type equilibrium profile shape using measurements of profiles generated under a range of wave conditions. An equation defined for a given profile amplitude and profile limits is found to predict the underlying characteristic N-type profile shape very well. Finally, a model of N-type beach response is developed. This model follows a relatively simple "grab-and-dump" concept with coefficients based on the understanding gained of the N-type response processes. It is shown that N-type profiles can be predicted reasonably well for the range of wave conditions used in the experiments here. Outside of this range predictions are less good, suggesting a re-examination of the model coefficients is required.

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