and extent of flow structures which pervade most turbulent flows. Moreover, turbulence is inherently 3D in nature, and a full description requires a measurement of the 3D velocity field and derivative quantities such as the stress tensor and vorticity vector. Current 3D PIV techniquesThese limitations have led to a number of efforts to develop 3D, 3C PIV-based measurement techniques. Advances such as

A novel method is introduced for increasing the accuracy and extending the dynamic range of time-resolved particle image velocimetry (PIV). The approach extends the concept of particle tracking velocimetry by multiple frames to the pattern tracking by cross-correlation analysis as employed in PIV. The working principle is based on tracking the patterned fluid element, within a chosen interrogation window, along its individual trajectory throughout an image sequence. In contrast to image-pair interrogation methods, the fluid trajectory correlation concept deals with variable velocity along curved trajectories and non-zero tangential acceleration during the observed time interval. As a result, the velocity magnitude and its direction are allowed to evolve in a nonlinear fashion along the fluid element trajectory. The continuum deformation (namely spatial derivatives of the velocity vector) is accounted for by adopting local image deformation. The principle offers important reductions of the measurement error based on three main points: by enlarging the temporal measurement interval, the relative error becomes reduced; secondly, the random and peak-locking errors are reduced by the use of least-squares polynomial fits to individual trajectories; finally, the introduction of high-order (nonlinear) fitting functions provides the basis for reducing the truncation error. Lastly, the instantaneous velocity is evaluated as the temporal derivative of the polynomial representation of the fluid parcel position in time. The principal features of this algorithm are compared with a single-pair iterative image deformation method. Synthetic image sequences are considered with steady flow (translation, shear and rotation) illustrating the increase of measurement precision. An experimental data set obtained by time-resolved PIV measurements of a circular jet is used to verify the robustness of the method on image sequences affected by camera noise and three-dimensional motions. In both cases, it is demonstrated that the measurement time interval can be significantly extended without compromising the correlation signal-to-noise ratio and with no increase of the truncation error. The increase of velocity dynamic range scales more than linearly with the number of frames included for the analysis, which supersedes by one order of magnitude the pair correlation by window deformation. The main factors influencing the performance of the method are discussed, namely the number of images composing the sequence and the polynomial order chosen to represent the motion throughout the trajectory.

edge used in the study of Novara and Scarano (Exp Fluids 52:1027-1041; the second is a swirling jet in a water flow. In both cases, the effective elimination of ghost particles is demonstrated in number and intensity within a short temporal transient of 5-10 frames, depending on the seeding density. The increased value of the velocity space-time correlation coefficient demonstrates the increased velocity field accuracy of SMTE compared with MART.

New experimental data have been obtained for H + CZHt, D + CZH2, H + CpD,, and D + CtDP a t room temperature. Two previously described apparatus were used in order to measure the pressure dependence of the reactions. The absolute rate constants are compared to results from other laboratories. The present results and t.hose of Payne and Stief are used to obtain the high-pressure limiting rate constant at room temperature. When the activation energy from the work of Payne and Stief is considered, it is shown that t.he :l factor for H + C,H? is t.oo low by a factor of -20. If a transmission coeficient is introduced which is const.ant for all isotopic variations, the pressure dependence can be explained in terms of the randomly energized radicals. RRKM theory is then invoked to explain the observed st,atistical nonequilibriurn kinetic isotope effects. (7) 825

The reactions of H and D with silane and monomethyl-, dimethyl-, and trimethylsilanes have been studied at room temperature and 3 Torr of He in a discharge-flow apparatus. The method of detection is time-offlight mass spectrometry. Rate constants based on atom depletion in reactant-rich systems or reactant depletion in atom-rich systems have been measured, and stoichiometry factors have been estimated. Also products of the reactions have been identified, and reaction profile experiments have been carried out. Mechanisms are proposed for these eight reactions which are consistent with observations and invoke considerable atomic cracking through chemical activation. The theoretical basis for this result is qualitatively discussed, and the observed isotope effects are also compared to theoretical BEBO estimates.

A tomographic PIV system composed of 12 cameras has been used to study the object reconstruction accuracy over a wide range of values for the concentration of tracer particles. The relations between particle image density, number of cameras, reconstruction quality, and velocity field accuracy are determined experimentally. The effect of additional cameras is quantified by the reconstruction signal-to-noise ratio and normalized intensity variance. Furthermore, the variation of the reconstruction quality factor with seeding density and number of cameras is estimated considering the 12 cameras case as reference, which has so far only been investigated using numerical simulations. The accuracy of velocity measurements is investigated by comparing two simultaneous measurements obtained by independent tomographic systems. The measurement error is estimated by the quadratic difference between the velocity measured by the two systems. The importance of the number of cameras and seeding density is investigated. The results yield an optimum source density of 0.5 for a three-camera system and greater than 1.0 for a six-camera system. Doubling the number of cameras returns a broad range for the optimum source density and significantly lower measurement errors.

No abstract

A novel 3-D, 3-C PIV technique is described, based on volume illumination and a plenoptic camera to measure a velocity field. The technique is based on plenoptic photography, which uses a dense microlens array mounted near a camera sensor to sample the spatial and angular distribution of light entering the camera. This combination of spatial and angular information is termed the light field, and is measured in this work with a plenoptic camera. Computational algorithms are then used to reconstruct a volumetric intensity field after the image is taken. This paper provides an introduction to the concepts of light field photography and describes the algorithms used to render the intensity field. The algorithms are tested using simulated data to evaluate their reconstruction accuracy and their effectiveness with correlation algorithms. Additionally, the construction of a prototype camera based on a 16-megapixel sensor is described. This work demonstrates the ability to make 3-D, 3-C PIV measurements using simulated data of uniform and vortex flow, and provides the basis for further development using experimental data and more advanced reconstruction algorithms.