It is essential to characterize the nonlinearity in scanning probe microscopes (SPMs) in order to acquire spatial measurements with high levels of accuracy. In this paper, a new characterization method is presented that combines a high-resolution image processing technique used by the experimental mechanics community known as Digital Image Correlation (DIC) with digital images from a standard type of SPM known as an atomic force microscope (AFM). The characterization results using this new method match those from the conventional method using micromachined calibration gratings. However, the new method uses the texture of a specimen surface and not a precisely micromachined calibration grating. As a consequence, the new characterization technique is a more direct method for measuring scanning errors that can be conducted in situ when imaging a specimen surface at any scale within the scanning range of the SPM. It also has the advantage of reconstructing the position error curve more continuously with less noise than the conventional method.
This is a speculative work meant to stimulate discussion about the role of subsumability in self-similar software structures for computational simulations. As in natural phenomena, self-similar features in framework structures allow the size and complexity of code to grow without bound and still maintain apparent coherence. As in crystal growth, the coherence may be maintained by the application of a repeated pattern, or patterns may, as in fluid mechanical turbulence, be scaled by size and nested. Examples of these kinds of patterns applied to component systems in particular will be given. Conclusions and questions for discussion will be drawn regarding the applicability of these ideas to component architectures, complexity, and scientific computing.
The dimensionless extinction coefficient, K{sub e}, was measured for soot produced in 2 m JP-8 pool fires. Light extinction and gravimetric sampling measurements were performed simultaneously at 635 and 1310 nm wavelengths at three heights in the flame zone and in the overfire region. Measured average K{sub e} values of 8.4 {+-} 1.2 at 635 nm and 8.7 {+-} 1.1 at 1310 nm in the overfire region agree well with values from 8-10 recently reported for different fuels and flame conditions. The overfire K{sub e} values are also relatively independent of wavelength, in agreement with recent findings for JP-8 soot in smaller flames. K{sub e} was nearly constant at 635 nm for all sampling locations in the large fires. However, at 1310 nm, the overfire K{sub e} was higher than in the flame zone. Chemical analysis of physically sampled soot shows variations in carbon-to-hydrogen (C/H) ratio and polycyclic aromatic hydrocarbon (PAH) concentration that may account for the smaller K{sub e} values measured in the flame zone. Rayleigh-Debye-Gans theory of scattering for polydisperse fractal aggregate (RDG-PFA) was applied to measured aggregate fractal dimensions and found to under-predict the extinction coefficient by 17-30% at 635 nm using commonly accepted refractive indices of soot, and agreed well with the experiments using the more recently published refractive index of 1.99-0.89i. This study represents the first measurements of soot chemistry, morphology, and optical properties in the flame zone of large, fully-turbulent pool fires, and emphasizes the importance of accurate measurements of optical properties both in the flame zone and overfire regions for models of radiative transport and interpretation of laser-based diagnostics of soot volume fraction and temperature.
Proposed for publication in the International Journal of Heat and Fluid Flow.
This paper compares several linear-theory-based models for droplet shattering employed for simulations of spray impingement on flat wall surface or a circular cylinder. Numerical simulations are conducted using a stochastic separated flow (SSF) technique that includes sub-models for droplet dynamics and impact. Results for spray impingement over a flat wall indicate that the linear theory applicable for a single droplet impact over-predicts the number of satellite (or secondary) droplets upon shattering when compared to experimental data. The causes for the observed discrepancies are discussed. Numerical simulation results for spray impingement over for a circular cylinder in cross flow are obtained and discussed.
Transportation of sensitive flight hardware requires information about the expected transportation environment as well as the actual transportation environment during the part's movement--typically vibration with superimposed intermittent shocks. Each data type has different sampling, processing, and specification requirements. Analyzing shock data requires high sampling rates and leads to large file sizes. A barrier to analyzing data has been the vast quantity of information acquired. Previous approaches have focused either on manually separating data or on selectively recording extreme data. The use of an automated approach allows for quickly verifying vibration and shock levels while retaining the robustness of the underlying data set. Further, the automated approach allows the environments engineer to select criteria for shock/vibration sorting, which removes the subjectivity associated with visual differentiation. This automated technique evaluated several vehicles over four different road conditions in the same time that one data set could have been processed using visual discrimination. Automated processing of satellite shipment vibration and shock data is made thoroughly and objectively vs. traditional shock and tilt indicators. The automated technique could also be useful in processing large amounts of on-orbit data for changes in vibration signature.
We compare inexact Newton and coordinate descent optimization methods for improving the quality of a mesh by repositioning the vertices, where the overall quality is measured by the harmonic mean of the mean-ratio metric. The effects of problem size, element size heterogeneity, and various vertex displacement schemes on the performance of these algorithms are assessed for a series of tetrahedral meshes.
Electrical breakdown simulations are carried out for liquids in response to a sub-microsecond ({approx}100-200 ns) voltage pulse. This model builds on our previous analysis and focuses particularly on the polarity effect seen experimentally in point-plane geometries. The flux-corrected transport approach is used for the numerical implementation. Our model adequately explains experimental observations of pre-breakdown current fluctuations, streamer propagation and branching as well as disparities in hold-off voltage and breakdown initiation times between the anode and cathode polarities. It is demonstrated that polarity effects basically arise from the large mobility difference between electrons and ions. The higher electron mobility leads to greater charge smearing and diffusion that impacts the local electric field distributions. Non-linear couplings between the number density, electric field and charge generation rates then collectively affect the formation of ionized channels and their temporal dynamics.
Quantum cascade lasers that operate in the underdeveloped terahertz spectral range (1-10 THz) promise to contribute to applications in sensing, spectroscopy, and imaging. We describe our development of terahertz quantum cascade lasers based on the resonant-phonon depopulation concept and that use low-loss metal-metal waveguides for optical confinement. Two- and three-dimensional finite-element simulations of terahertz metal-metal waveguides are used to demonstrate their high modal confinement even for very narrow ridges. Also, simulations predict high facet reflectivities due to the modal impedance mismatch with free space at the sub-wavelength waveguide aperture of these metal-metal waveguides. Finally, we report the demonstration of a 2.8 THz laser that operates up to 97 K in continuous-wave mode fabricated using a Cu-Cu thermocompression bonding technique.
This paper investigates theoretically the modification of dynamical properties in a semiconductor laser by a strong injected signal. It is found that enhanced relaxation oscillations are governed by the pulsations of the intracavity field and population at frequencies determined by the injected field and cavity resonances. Furthermore, the bandwidth enhancement is associated with the undamping of the injection-induced relaxation oscillation and strong population pulsation effects. There are two limitations to the modulation-bandwidth enhancement: Overdamping of relaxation oscillation and degradation of flat response at low frequencies. The injected-laser rate-equations used in the investigation reproduce the relevant aspects of modulation-bandwidth enhancement found in the experiment on injection-locked vertical-cavity surface-emitting lasers.
A particular inverse design problem is proposed as a benchmark for comparison of five solution techniques used in design of enclosures with radiating sources. The enclosure is three-dimensional and includes some surfaces that are diffuse and others that are specular diffuse. Two aspect ratios are treated. The problem is completely described, and solutions are presented as obtained by the Tikhonov method, truncated singular value decomposition, conjugate gradient regularization, quasi-Newton minimization, and simulated annealing. All of the solutions use a common set of exchange factors computed by Monte Carlo, and smoothed by a constrained maximum likelihood estimation technique that imposes conservation, reciprocity, and non-negativity. Solutions obtained by the various methods are presented and compared, and the relative advantages and disadvantages of these methods are summarized.