20 research outputs found
Flow structure and optical beam propagation in high-Reynolds-number gas-phase shear layers and jets
We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-of-refraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behaviour is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high-Reynolds-number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behaviour, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions
Monte-Carlo study of scaling exponents of rough surfaces and correlated percolation
We calculate the scaling exponents of the two-dimensional correlated
percolation cluster's hull and unscreened perimeter. Correlations are
introduced through an underlying correlated random potential, which is used to
define the state of bonds of a two-dimensional bond percolation model.
Monte-Carlo simulations are run and the values of the scaling exponents are
determined as functions of the Hurst exponent H in the range -0.75 <= H <= 1.
The results confirm the conjectures of earlier studies
Nonlinear Measures for Characterizing Rough Surface Morphologies
We develop a new approach to characterizing the morphology of rough surfaces
based on the analysis of the scaling properties of contour loops, i.e. loops of
constant height. Given a height profile of the surface we perform independent
measurements of the fractal dimension of contour loops, and the exponent that
characterizes their size distribution. Scaling formulas are derived and used to
relate these two geometrical exponents to the roughness exponent of a
self-affine surface, thus providing independent measurements of this important
quantity. Furthermore, we define the scale dependent curvature and demonstrate
that by measuring its third moment departures of the height fluctuations from
Gaussian behavior can be ascertained. These nonlinear measures are used to
characterize the morphology of computer generated Gaussian rough surfaces,
surfaces obtained in numerical simulations of a simple growth model, and
surfaces observed by scanning-tunneling-microscopes. For experimentally
realized surfaces the self-affine scaling is cut off by a correlation length,
and we generalize our theory of contour loops to take this into account.Comment: 39 pages and 18 figures included; comments to
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On the accuracy of PLIF measurements in slender plumes
The purpose of this article was to assess the measurement uncertainty of the planar laser-induced fluorescence (PLIF) method and, as much as possible, to devise corrections for predictable biases. More specifically, we considered the measurement of concentration maps in cross sections parallel to and normal to the axis of a slender plume containing Rhodamine 6G as a passive scalar tracer and transported by a turbulent shear flow. In addition to previously examined sources of error related to PLIF, we also investigated several unexplored ones. First, we demonstrated that errors would arise if the laser sheet thickness was comparable to or larger than the thickness of the instantaneous plume. We then investigated the effect of secondary fluorescence, which was attributed to absorption and re-emission of primary fluorescence by dye both within and outside the laser sheet. We found that, if uncorrected, this effect would contaminate the calibration as well as the instantaneous concentration measurements of the plume, and proposed methods for the correction of these errors and for identifying the instantaneous boundaries of the in-sheet dye regions
Flow Structure and Optical Beam Propagation in High Reynolds Number, Gas-Phase Shear Layers and Jets
We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear-layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-ofrefraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behavior is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high Reynolds number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behavior, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions