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A quantification metric is provided to determine the degree to which a particle image velocimetry data set is pixel-locked. The metric is calculated by integrating the histogram equalization transfer function and normalizing by the worst-case scenario to return the percentage pixel-locked. When this metric is calculated for each position in the vector field, it is shown that pixel-locking is non-uniform across the field. Hence, pixel-locking adjustments should be made on a vector-by-vector basis rather than uniformly across a field, although the latter is the common practice. A methodology is provided to compensate for the effects of pixel-locking on a vector-by-vector basis. This includes applying a Gaussian filter directly to the images, processing the images with window deformation, ensuring the vector fields are in pixel displacements, applying histogram equalization calculated at each vector coordinate, and mapping the adjusted vector fields to physical space. 相似文献
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Cinematographic stereoscopic PIV measurements were performed in the far field of an axisymmetric co-flowing turbulent round
jet (Re
T ≈ 150, where Re
T is the Reynolds number based on Taylor micro scale) to resolve small and intermediate scales of turbulence. The time-resolved
three-component PIV measurements were performed in a plane normal to the axis of the jet and the data were converted to quasi-instantaneous
three-dimensional (volumetric) data by using Taylor’s hypothesis. The availability of the quasi-three-dimensional data enabled
the computation of all nine components of the velocity gradient tensor over a volume. The use of Taylor’s hypothesis was validated
by performing a separate set of time-resolved two component “side-view” PIV measurements in a plane along the jet axis. Probability
density distributions of the velocity gradients computed using Taylor’s hypothesis show good agreement with those computed
directly with the spatially resolved data. The overall spatial structure of the gradients computed directly exhibits excellent
similarity with that computed using Taylor’s hypothesis. The accuracy of the velocity gradients computed from the pseudo-volume
was assessed by computing the divergence error in the flow field. The root mean square (rms) of the divergence error relative
to the magnitude of the velocity gradient tensor was found to be 0.25, which is consistent with results based on other gradient
measurement techniques. The velocity gradients, vorticity components and mean dissipation in the self-similar far field of
the jet were found to satisfy the axisymmetric isotropy conditions. The divergence error present in the data is attributed
to the intrinsic uncertainty associated with performing stereoscopic PIV measurements and not to the use of Taylor’s hypothesis.
The divergence error in the data is found to affect areas of low gradient values and manifests as nonphysical values for quantities
like the normalized eigenvalues of the strain-rate tensor. However, the high gradients are less affected by the divergence
error and so it can be inferred that structural features of regions of intense vorticity and dissipation will be faithfully
rendered. 相似文献
3.
R. J. Hearst O. R. H. Buxton B. Ganapathisubramani P. Lavoie 《Experiments in fluids》2012,53(4):925-942
The effect of numerical differentiation is investigated in the context of evaluating fluctuating velocity and scalar quantities in turbulent flows. In particular, 2-point forward-difference and 3-, 5-, 7-, and 9-point centred-difference schemes are investigated. The spectral technique introduced by Wyngaard (in J Sci Instr 1(2):1105–1108, 1968) for homogeneous turbulence is used to quantify the effects of the schemes. Numerical differentiation is shown to attenuate gradient spectra over a range of wavenumbers. The spectral attenuation, which varies with the order of the scheme, results in a reduction in the measured mean-squared gradients. High-order schemes (e.g. 7- or 9-point) are shown to significantly decrease the attenuation at all wavenumbers and as a result produce more accurate gradients. Hot-wire measurements and direct numerical simulations of decaying homogeneous, isotropic turbulence are found to be in good agreement with the predictions of the analysis, which suggests that high-order schemes can be used to improve empirical gradient estimates. The shape of the probability density functions is also found to be sensitive to the choice of numerical differentiation scheme. The effect of numerical differentiation is also discussed with respect to particle image velocimetry (PIV) measurements of a nominally two-dimensional planar mixing layer. It is found that the relatively low signal-to-noise ratio inherent in typical PIV measurements necessitates the use of low-order schemes to prevent excessive noise amplification, which increases with the order of the scheme. The results of the present work demonstrate that high-order numerical differentiation schemes can be employed to more accurately resolve gradients measured at a given resolution provided the measurements have an adequate signal-to-noise ratio. 相似文献
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Experimental Mechanics - This work presents a methodology for reconstructing full-field surface pressure information from deflectometry measurements on a thin plate using the Virtual Fields Method... 相似文献
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Bharathram Ganapathisubramani Ellen K. Longmire Ivan Marusic Stamatios Pothos 《Experiments in fluids》2005,39(2):222-231
Simultaneous dual-plane PIV experiments, which utilized three cameras to measure velocity components in two differentially separated planes, were performed in streamwise-spanwise planes in the log region of a turbulent boundary layer at a moderate Reynolds number (Re 1100). Stereoscopic data were obtained in one plane with two cameras, and standard PIV data were obtained in the other with a single camera. The scattered light from the two planes was separated onto respective cameras by using orthogonal polarizations. The acquired datasets were used in tandem with continuity to compute all 9 velocity gradients, the complete vorticity vector and other invariant quantities. These derived quantities were employed to analyze and interpret the structural characteristics and features of the boundary layer. Sample results of the vorticity vector are consistent with the presence of hairpin-shaped vortices inclined downstream along the streamwise direction. These vortices envelop low speed zones and generate Reynolds shear stress that enhances turbulence production. Computation of inclination angles of individual eddy cores using the vorticity vector suggests that the most probable inclination angle is 35° to the streamwise-spanwise plane with a resulting projected eddy inclination of 43° in the streamwise-wall-normal plane.
相似文献
Ellen K. LongmireEmail: |
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Experimental Mechanics - In this study, pressure distributions were reconstructed from phase-locked surface deformation measurements on a thin plate. Slope changes on the plate surface were induced... 相似文献
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The effect of spatial resolution and experimental noise on the kinematic fine-scale features in shear flow turbulence is investigated
by means of comparing numerical and experimental data. A direct numerical simulation (DNS) of a nominally two-dimensional
planar mixing layer is mean filtered onto a uniform Cartesian grid at four different, progressively coarser, spatial resolutions.
Spatial gradients are then calculated using a simple second-order scheme that is commonly used in experimental studies in
order to make direct comparisons between the numerical and previously obtained experimental data. As expected, consistent
with other studies, it is found that reduction of spatial resolution greatly reduces the frequency of high magnitude velocity
gradients and thereby reduces the intermittency of the scalar analogues to strain (dissipation) and rotation (enstrophy).
There is also an increase in the distances over which dissipation and enstrophy are spatially coherent in physical space as
the resolution is coarsened, although these distances remain a constant number of grid points, suggesting that the data follow
the applied filter. This reduction of intermittency is also observed in the eigenvalues of the strain-rate tensor as spatial
resolution is reduced. The quantity with which these eigenvalues is normalised is shown to be extremely important as fine-scale
quantities, such as the Kolmogorov length scale, are showed to change with different spatial resolution. This leads to a slight
change in the modal values for these eigenvalues when normalised by the local Kolmogorov scale, which is not observed when
they are normalised by large-scale, resolution-independent quantities. The interaction between strain and rotation is examined
by means of the joint probability density function (pdf) between the second and third invariants of the characteristic equation of the velocity gradient tensor, Q and R respectively and by the alignments between the eigenvectors of the strain-rate tensor and the vorticity vector. Gaussian
noise is shown to increase the divergence error of a dataset and subsequently affect both the Q–R joint pdf and the magnitude of the alignment cosines. The experimental datasets are showed to behave qualitatively similarly to the
numerical datasets to which Gaussian noise has been added, confirming the importance of understanding the limitations of coarsely
resolved, noisy experimental data. 相似文献
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