1,721,070 research outputs found
Stability of single image self-calibration for tomographic PIV
No full textIt is commonly assumed that a larger number of cameras is beneficial for the accuracy of multiple cameras system. This tautological assertion is most certainly true for Tomographic PIV and 3D Particle-Tracking-Velocimetry systems, where the search area on each image for the particle corresponding to a 3D trial position is small (typically less than the particle image diameter). On the other hand, when it comes to larger search areas (due to, for example, calibration uncertainties), quite surprisingly increasing the number of cameras might have a detrimental effect. Under some conditions this is the case of the volumetric self-calibration technique [1], in which the residual calibration error and cameras misalignments are corrected by statistically searching matching particles over a search area larger than the expected maximum calibration error. In this work the loss of signal in the self-calibration for systems with 4 or more cameras is discussed. Two readily implementable solutions are provided to reduce this source of error. The algorithms are tested on synthetic and real images
Horizontal convection PIV study
No full textHorizontal convection has received limited attention, despite of its well recognized implication in geophysical flows of relevant interest. E.g., oceans are often modeled as being cooled and heated in a thin surface layer, which is nearly horizontal, due to solar irradiance forcing differentially in latitude and determining strong stable density stratification. The phenomenology is simulated in a plexiglass box, with a piecewise thermal boundary condition on its bottom surface while all the other boundaries are thermally insulated. A Particle Image Velocimetry (PIV) investigation of the natural convection generated by a differential thermal boundary condition along the same horizontal boundary is proposed. A preliminary investigation of the structure of the circulation cell, generated by the hot water rising up in correspondence of the heat flux input zone, and sinking down nearby the cooled zone, is carried out. A strong convection cell, consisting of a rising intense plume (filling the whole box height) and a slow downward average flow, are observed in all the tested configurations
Fast 3D PIV with direct sparse cross-correlations
No full textThe extension of the well-assessed high-accuracy algorithms for two-dimensional-two components particle image velocimetry (PIV) to the case of three-dimensional (3D) data involves a considerable increase of the computational cost. Tomographic PIV is strongly affected by this issue, relying on 3D cross-correlation to estimate the velocity field. In this study, a number of solutions are presented, enabling a more efficient calculation of the velocity field without any significant loss of accuracy. A quick estimation of the predictor displacement field is proposed, based on voxels binning in the first steps of the process. The corrector displacement field is efficiently computed by restricting the search area of the correlation peak. In the initial part of the process, the calculation of a reduced cross-correlation map by using Fast Fourier Transform on blocks is suggested, in order to accelerate the processing by avoiding redundant calculations in case of overlapping interrogations windows. Eventually, direct cross-correlations with a search radius of only 1 pixel in the neighborhood of the estimated peak are employed; the final iterations are consistently faster, since direct correlations can better enjoy the sparsity of the distributions, reducing the number of operations to be performed. Furthermore, three different approaches to reduce the number of redundant calculations for overlapping windows are presented, based on pre-calculations of the contributions to the cross-correlations coefficients along segments, planes or blocks. The algorithms are tested both on synthetic and real images, showing that a potential speed-up of up to 800 times can be obtained, depending on the complexity of the flow field to be analyzed. The challenging application on a real swirling jet results in a speed-up of an order of magnitude
Spatial Filtering Improved Tomographic PIV
No full textIn the present work, an alternative procedure for the tomographic reconstruction of distribution of particles, allowing a remarkable improvement of the accuracy with roughly the same computational cost of the well assessed Multiplicative Algebraic Reconstruction Technique (Herman G.T. & Lent A. (1976) Iterative reconstruction algorithms. Computers in Biology and Medicine, 6, 273-294.), is illustrated. The technique relies on recursive smoothing by means of planar filtering windows orthogonal to the depth direction, with Gaussian distribution of the weighting coefficients. The smoothing of the distribution is much more effective in reducing the intensity of the ghost particles than that of the true ones, on account to the more accentuated elongation of the former ones in the depth direction
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