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Optical Methods in Flow Measurement
Published in Toru Yoshizawa, Handbook of Optical Metrology, 2015
As the tracer particles pass through the fringe pattern, they scatter light that appears as a blinking signal. The frequency of this on-and-off signal is proportional to the velocity component perpendicular to the planes of the fringes. As shown in Figure 24.4, the scattering light depends on the location of receiving optics for collecting scattering light with respect to the incident light. The light scattered in the forward direction (θ = 0°) has the largest intensity value, about two orders of magnitude larger than the backscatter mode (θ = 180°). When small tracer particles (about 1 μm) are used, the forward-scatter mode is useful, because the light intensity is much stronger than the backscatter mode. However, the backscatter mode has to be employed in the experimental facilities in which back side has space restrictions to locate the receiving optics or a transparent window. The LDV system of backscatter mode is easy to carry out experiments and its optical alignment is not so difficult. However, since the scattering light intensity is weak, the selection of tracer particles and the signal-processing procedure should be done carefully.
Laser Velocimetry
Published in Richard J. Goldstein, Fluid Mechanics Measurements, 2017
If the collecting lens is centered on the axis of the system, e.g., the y axis in Fig. 4.2, the configuration is called coaxial. “Forward scatter” and “back scatter” refer to locations of the collecting aperture that receive light scattered forward from the illuminating beams and scattered backward, respectively. Thus the first dual-beam system in Fig. 4.3 is a coaxial forward-scatter system. While coaxial systems enjoy a certain symmetry, it is often better to use “off-axis” light collection (e.g., not centered on the y axis) to reduce the amount of extraneous light seen by the detector and reduce the size of the measurement volume in the y direction.
Vorticity, backscatter and counter-gradient transport predictions using two-level simulation of turbulent flows
Published in Journal of Turbulence, 2018
The normalised PDF of the fluctuation of the SS dissipation, i.e. ε′ = εL − μ(εL), where εL = −τLijSijL is shown in Figure 15 for the two cases. The SS dissipation appears in the transport equation for the LS (resolved) TKE, and therefore, it can be used to assess the direction of the inter-scale transfer of TKE. A forward-cascade of TKE occurs when εL > 0 and backscatter occurs for εL < 0. In a conventional LES based on eddy viscosity formulation, εL is always positive, thus implying a forward cascade. However, the results shown in Figure 15 indicate that there is presence of both forward and backward transfer of energy even though the mean value of εL is positive for both the cases for all the LS grid sizes as shown in Table 7 indicating a net forward cascade of TKE. With a coarsening of the LS grid up to LS16Δ, the mean value of SS dissipation increases indicating an increase in the amount of forward scatter of TKE in comparison to the backscatter, however, further coarsening leads to a reduction in the mean value indicating increase in contributions from backscatter. For the channel case, with an increase in the distance away from the wall, the mean value of εL decreases as also reported in past study of channel flow [12]. The shape of the PDF shown in Figure 15 in both the cases is similar to that for νLt/ν where a characteristic peak occurs around the mean value with wider tails indicating presence of low probable but large-magnitude positive and negative fluctuations about the mean SS dissipation.
Out-of-plane enhancement in a discrete random halfspace
Published in Waves in Random and Complex Media, 2022
Reid K. McCargar, Roger H. Lang
Figures 3 and 4 plot the EM and acoustic BSCs of 15-wavelength-thick slabs of uniformly distributed, identical, half-wavelength-diameter spheres immediately above the intersection of two uniform halfspaces. In the EM case, the spheres are regarded as perfectly conducting, and the lower halfspace's relative dielectric constant is 8.9−3.5i. Although the scatterers are highly simplified here, similar configurations appear in aerial remote sensing of vegetation over smooth moist soil, for example. In the acoustic case, both the scatterers and the lower halfspace are regarded as perfectly rigid; a similar configuration might arise in underwater acoustic sensing of rocky inhomogeneities entrained in a sediment with similar impedance to the water column, overlaying a layer of rock. In both cases, the zenith angle of the illuminating wave is , the fractional volume occupied by the particles is , and the upper halfspace (which contains the source and observer) is regarded as lossless. These results were obtained from (57), (59), and (60) through (63), using Mie theory for the EM problem (See, e.g. [50].) and its acoustical analog [53] to compute the scatter amplitude for all of the various combinations of and , as well as the forward-scatter amplitude required to compute the mean-wave propagation constant. When pseudo-cyclic mechanisms are included in the calculation a roughly 2 dB enhancement appears (for co-polarized EM and acoustic waves), compared to a prediction that accounts only for the ladder mechanisms under the DBA. Differences between the EM and acoustic results can be attributed to the angular structure of the scatter amplitude and polarization effects, but both clearly exhibit the enhancement.
Computational domain lap model for micro-nano scale radiative heat transfer
Published in Numerical Heat Transfer, Part A: Applications, 2019
Xie Ming, Yang Liu, Ai Qing, Song Kuilong, Tan Heping
The result indicated the single particle’s influence area. From Figure 3, in the spectral area from 2 to 5 µm, the single particle will change the light directions in an area larger than itself; consequently, the near effects are obvious instead of independent scattering, and the forward scatter occupies over 90% of the primary energy.