Assessment of a numerical model to reproduce event-scale erosion and deposition distributions in a braided river.

Numerical morphological modeling of braided rivers, using a physics-based approach, is increasingly used as a technique to explore controls on river pattern and, from an applied perspective, to simulate the impact of channel modifications. This paper assesses a depth-averaged nonuniform sediment model (Delft3D) to predict the morphodynamics of a 2.5 km long reach of the braided Rees River, New Zealand, during a single high-flow event. Evaluation of model performance primarily focused upon using high-resolution Digital Elevation Models (DEMs) of Difference, derived from a fusion of terrestrial laser scanning and optical empirical bathymetric mapping, to compare observed and predicted patterns of erosion and deposition and reach-scale sediment budgets. For the calibrated model, this was supplemented with planform metrics (e.g., braiding intensity). Extensive sensitivity analysis of model functions and parameters was executed, including consideration of numerical scheme for bed load component calculations, hydraulics, bed composition, bed load transport and bed slope effects, bank erosion, and frequency of calculations. Total predicted volumes of erosion and deposition corresponded well to those observed. The difference between predicted and observed volumes of erosion was less than the factor of two that characterizes the accuracy of the Gaeuman et al. bed load transport formula. Grain size distributions were best represented using two φ intervals. For unsteady flows, results were sensitive to the morphological time scale factor. The approach of comparing observed and predicted morphological sediment budgets shows the value of using natural experiment data sets for model testing. Sensitivity results are transferable to guide Delft3D applications to other rivers.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis.

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 micro. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 micro/mm.

Phase-based Bragg intragrating distributed strain sensor.

A strain-distribution sensing technique based on the measurement of the phase spectrum of the reflected light from a fiber-optic Bragg grating is described. When a grating is subject to a strain gradient, the grating will experience a chirp and therefore the resonant wavelength will vary along the grating, causing wavelength-dependent penetration depth. Because the group delay for each wavelength component is related to its penetration depth and the resonant wavelength is determined by strain, a measured phase spectrum can then indicate the local strain as a function of location within the grating. This phase-based Bragg grating sensing technique offers a powerful new means for studying some important effects over a few millimeters or centimeters in smart structures.

Bragg intragrating structural sensing.

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.

Practical fiber-optic Bragg grating strain gauge system.

A fiber-optic strain gauge system for use in structural monitoring and smart-structure applications is described. The strain gauge uses a fiber-optic Bragg grating sensor to measure strain and a passive, wavelength demodulation system to determine the wavelength of the narrow-band, backreflected spectrum from the grating sensor. The fiber-optic strain gauge system permits the measurement of both static and dynamic strains with a noise-limited resolution of 0.44 microstrain/√Hz, a measurement dynamic range of 27.8 dB, and a bandwidth of 250 Hz.

Localized dual-wavelength fiber-optic polarimeter for the measurement of structural strain and orientation.

A single-ended, all-fiber polarimetric strain sensor with wide dynamic range and linearized response is described. Linear response is achieved by using a dual-wavelength technique, with modified pseudoheterodyne signal recovery. Single-valued, multiple-fringe phase tracking is obtained by using a binary signal division technique. An average strain sensitivity of 0.050 +/- 0.02 deg microepsilon(-1) cm(-1) is found for sensors that are surface adhered to cantilever beams. The sensor system is applied successfully to the measurement of the local orientation of a 1-m structural beam, with an accuracy of +/-0.02 deg of beam slope. Limitations on the applicability of this sensing technique are discussed.

Thermal apparent-strain sensitivity of surface-adhered, fiber-optic strain gauges.

We have derived, based on established practice in experimental mechanics, an equation for calculating the thermal apparent-strain sensitivity of phase-modulated, surfaceadhered, adhered, fiber-optic strain sensors. This formulation permits the thermal performance of fiber-optic strain gauges to be compared with conventional resistive gauges. This sensitivity for commonly used fiber-optic sensors is summarized.

Fiber-optic interferometric sensor for the detection of acoustic emission within composite materials.

An optical-fiber Michelson interferometric acoustic emission sensor is described. The sensor uses ordinary singlemode fiber and is embedded in the composite material under test. Signals are demodulated through the active homodyne. This system provides a novel approach for material nondestructive evaluation.

Distributed fiber optic sensing based on counterpropagating waves.

The spatially and temporally resolved birefringence of a single-mode optical fiber can be ascertained using backward stimulated Raman scattering. The magnitude of the birefringence is determined from the optical power exchanged between two counterpropagating light pulses. The degree to which a signal pulse is amplified by a pump pulse is governed by their relative states of polarization when they overlap. A novel normalization procedure is proposed that eliminates many of the unknowns. An example of how this technique could be used to evaluate a distributed strain field is provided.

Structurally integrated fiber optic damage assessment system for composite materials.

Progress toward the development of a fiber optic damage assessment system for composite materials is reported. This system, based on the fracture of embedded optical fibers, has been characterized with respect to the orientation and location of the optical fibers in the composite. Together with a special treatment, these parameters have been tailored to yield a system capable of detecting the threshold of damage for various impacted Kevlar/epoxy panels. The technique has been extended to measure the growth of a damage region which could arise from either impact, manufacturing flaws, or static overloading. The mechanism of optical fiber fracture has also been investigated. In addition, the influence of embedded optical fibers on the tensile and compressive strength of the composite material has been studied. Image enhanced backlighting has been shown to be a powerful and convenient method of assessing internal damage to translucent composite materials.

Electron density radial profiles derived from Stark broadening in a sodium plasma produced by laser resonance saturation.

The first free electron density radial profiles of a sodium plasma created by laser resonance saturation are reported. The measurements were based on Stark broadening, of the 4(2)D-3(2)P multiplet and reveal the formation of a conically shaped plasma along the path of the laser pulse, which can be attributed to strong absorption of the laser pulse along the ionization path.

TABLASER: trace (element) analyzer based on laser ablation and selectively excited radiation.

Trace element analysis based on laser ablation and selectively excited radiation (TABLASER) is proposed as a new and reliable microultratrace technique for quantitative in situ element analysis. Measurements of trace quantities of chromium in samples of NBS standard reference steel, doped skim milk powder, and doped flour have been undertaken. A linear 45 degrees slope dependence of signal vs concentration that extends beyond 1% in the case of chromium was observed. Although the present sensitivity limit is in the ppm range, improved overlap between the probing dye laser beam and the wave of atomized material combined with a better design of the optical system could reduce the detection limit of the TABLASER to the ppb range. This would correspond to an absolute detection limit of about 10(-16) g. An important feature of this new technique is its relative freedom from chemical matrix effects, which suggests the possibility of a universal calibration curve for all elements irrespective of the substrate matrix in which they are contained. This technique is also adaptable to multielement analysis.

Laser interaction based on resonance saturation (LIBORS): an alternative to inverse bremsstrahlung for coupling laser energy into a plasma.

Resonance saturation represents an efficient and rapid method of coupling laser energy into a gaseous medium. In the case of a plasma superelastic collision quenching of the laser maintained resonance state population effectively converts the laser beam energy into translational energy of the free electrons. Subsequently, ionization of the laser pumped species rapidly ensues as a result of both the elevated electron temperature and the effective reduction of the ionization energy for those atoms maintained in the resonance state by the laser radiation. This method of coupling laser energy into a plasma has several advantages over inverse bremsstrahlung and could therefore be applicable to several areas of current interest including plasma channel formation for transportation of electron and ion beams, x-ray laser development, laser fusion, negative ion beam production, and the conversion of laser energy to electricity.

Lidar equation analysis allowing for target lifetime, laser pulse duration, and detector integration period.

A detailed study of the lidar equation has been made for both scattering and fluorescent targets. Allowance has been made for the effects of finite excited state lifetime, optical depth, laser pulse duration, detector integration period, and laser pulse shape. Analytical solutions have been obtained, and graphical solutions are also presented to aid in evaluating the magnitude of the correction factor appropriate to several cases of interest.

PROBE: a new technique for measuring the density profile of a specific constituent using counterpropagating laser pulses.

A new approach at attaining density measurements of a specific constituent with spatial resolution using two counterpropagating laser pulses is proposed. This PROBE (Profile Resolution Obtained By Excitation) concept involves exciting the species of interest with one pulse then probing the wake of excited atoms or molecules with a second laser pulse. The lifetime of the excited state, in terms of the time for the laser pulse to cross the region of interest, turns out to be an important parameter in specifying the form of the relation needed to ascertain the profile of the species under investigation. This new technique could find application in several areas, range from remote atmospheric pollution monitoring in the ir to trace species profile evaluation within plasma or chemical reactors.

Physical constraints associated with the development of a laser based on electrochemiluminescence.

A theoretical study of the possibility of attaining laser action from an electrochemiluminescence system has been undertaken. A simple model has been found that is capable of predicting the width of the excited state creation zone in terms of the peak excited state concentration. This relation reveals that diffraction losses are likely to be catastrophic if the concentration of the excited molecules is made sufficient to ensure that the optical gain of the medium exceeds the losses due to the laser cavity. This imposes an extremely severe physical constraint on the development of a laser based on radical-ion annihilation.

Prospects for developing a laser based on electrochemiluminescence.

A new form of laser is proposed in which electrochemical energy in the form of a charge-transfer reaction would be used to create directly a population inversion. The direct formation of either excited-singlet-state molecules or excimers by radical-ion annihilation is suggested as the primary means of attaining a population inversion within the electroactive organic species. 9,10-Diphenylanthracene is proposed as a possible contender for the singlet approach, while 9,10-dimethylanthracene and 9,10-dichloranthracene are suggested for the excimer scheme. An estimate has been made of the conditions necessary to achieve laser action in the case of 9,10-diphenylanthracene.