Bias and precision errors of digital particle image velocimetry

The bias and precision errors of digital particle image velocimetry are quantified. Uniform displacement images are used to evaluate the uncertainty attributed to various sub-pixel peak finding algorithms. Bias errors are found to exist for all algorithms, and the presence of bias error tends to affect the precision error. The ability to “calibrate” out the bias error is explored using a rectangular free jet experiment. The calibration was effective in removing the bias error in the potential core and less effective in the shear layer. The bias error is found to functionally depend on the displacement gradients present in the interrogation region. The study stresses the need for in situ quantification of DPIV uncertainty. Received: 3 November 1998 / Accepted: 26 June 1999     

Simulation of the Pauli-principle effects in the description of light ion scattering and bound states

A simple model which takes care phenomenologically of the effects of the Pauli principle is proposed to calculate, in the framework of the resonating group method, bound and scattering states of nuclear systems comprised of two light nuclei (n, t and α-particles) without performing a complete antisymmetrization of the wave functions. Retaining only the antisymmetrization between the nucleons belonging to a given cluster, the contributions of the terms corresponding to the exchange of two nucleons belonging to two different clusters are simulated by the matrix elements of an effective central, local, l-dependent, energy-independent nucleon-nucleon potential. The lowenergy levels of ⁸Be and ⁷Li as well as the phase shifts for l = 0 to 4 for energies below 10 MeV (c.m.) have been calculated with this effective potential (added to the regular nucleon-nucleon potential). Good agreement between exact and model calculations is achieved.     

Analysis of α-7Li elastic and inelastic scattering at low energy with a simple model of the pauli principle effects

The elastic and inelastic scattering of α on ⁷Li are calculated at different low energies in the framework of the resonating group method in which the ⁷Li is supposed to be comprised of two clusters, α and t. To reduce the time of calculation, the effects of the Pauli principle due to the exchange of two nucleons belonging to two different clusters are approximated with an effective nucleon-nucleon potential which has been determined previously in order to reproduce correctly the scattering and bound states of the nuclear systems comprised of two particles, 2α or α + t. This calculation takes into account the exchange of the incoming a with the a belonging to ⁷Li as well as the coupling with the ⁸Be + t channel. Without any free parameter, the model calculation reproduces correctly the main features of the experimental data when the projectile energy is less than 5 MeV (lab). It appears that the coupling with the ⁸Be + t channel cannot be neglected.     

Mixing in plasma and low density jets

This study was undertaken to examine the mechanisms which produce the large entrainment measured near the exit of thermal plasma torches. A research facility was constructed to examine low density jet behavior under similar dimensionless conditions as those produced by thermal plasma spray torches; the Reynolds number based on jet diameter and average properties was 1000, and the ratio of jet to ambient density was 0.07. This very low density jet produced organized vortex structures which were partially responsible for the rapid entrainment of external air. The formation of these organized structures could be disrupted by introducing turbulence, but the rapid entrainment process was not significantly affected. The structure of the jet produced by a commercial plasma torch was examined and compared to the low density research jet. At low gas flow rates the plasma jet also displayed the formation of coherent vortex structures, the passage frequency of which compared favorably with that measured in the low density research jet. At higher gas flow rates the shear layer of the plasma jet rapidly broke down producing relatively small scale turbulence. Visualizations of the hot plasma core were compared against measurements of the torch voltage fluctuations caused by arc instabilities. At low flow rates the arc voltage fluctuations were quite low and the plume was very steady. At higher flow rates the arc voltage fluctuations increased and produced surging and whipping in the hot potential core. It is believed that this low frequency unsteadiness is partially responsible for the rapid entrainment measured in plasma torches.The authors would like to thank Dr. Ron Spores for the extensive use of his data and Mr. Stuart Malmberg for help with the plasma core visualizations. This work was supported by IBM and ONR graduate fellowships. Additional funding for the plasma jet research was provided by the National Science Foundation Grant No. ECR-87-2145 and for the low density jet research by the American Chemical Society Grant No. PFR25238-G7E.     

Stabilization effects in flow through helically coiled pipes

When a flow through a straight pipe is passed through a coiled section, two stabilizing effects come into play. First, in a certain Reynolds number range, the flow that is turbulent in the straight pipe becomes completely laminar in the coiled section. Second, the stabilization effect of the coil persists to a certain degree even after the flow downstream of the coil has been allowed to develop in a long straight section. In this paper, we report briefly on aspects related to these two effects.     

Unsteadiness and mixing in thermal plasma jets

This study was undertaken to examine the mechanisms which produce the large entrainment measured at the exit of thermal plasma torches. The experiments studied a Metco 7MB plasma torch with a 706 (6.35 mm diameter) anode nozzle and swirled argon gas injection. The vortex structure produced in the shear layer of the plasma jet was visualized using a laser shadowgraph system with a short exposure lime (10^–4 s). A high-speed video system provided information on the structure and unsteadiness of the hot potential core of the plume. Tile shear layer visualizations were compared to previous measurements of acoustical power spectra and indicate coherent vortex structure formation at low gas flowrates. At higher gas flowrates the shear layer rapidly broke down, producing relatively small scale turbulence. The visualizations of the hot potential core were compared to previous measurements of the torch voltage fluctuations caused by arc instabilities. At low flowrates the arc-produced voltage fluctuations were guile low card the phone was very steady. At higher flowrates the voltage fluctuations increased and produced surging and whipping in the hot potential core.