ALBERT

Notes: The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form "pairs" if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the structures at the interface, scale with a mix of inner (wall shear stress and viscosity) and outer variables. Measurement of the streamwise and spanwise velocities at the free-surface were made by particle imaging velocimetry (PIV) and the surface normal velocity near the free-surface estimated by continuity. The results indicate that the upwellings and spiral eddy regions would be expected to dominate scalar transport rates at high Prandtl/Schmidt numbers. The one-dimensional energy spectra of the flow field at the free-surface compare well with direct numerical simulations and show a region with −5/3 slope at low wave numbers. This experimentally confirms a previous result regarding the two-dimensionality of turbulence near the free surface, based on numerical simulations by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)]. © 1998 American Institute of Physics.

Notes: A method for particle image velocimetry (PIV) is presented which improves upon the accuracy, computational efficiency and dynamic range (i.e., the difference between the largest and smallest resolvable particle displacement vectors) of conventional PIV techniques. The technique is applied to free-surface turbulence to resolve energy spectra for motions with a wide dynamic range. The methodology—based on multi-grid image processing algorithms for rigid body motion analysis, estimates the displacement vectors at discrete particle locations. The essence of this technique is to estimate large scale motions from image intensity patterns of low spatial frequencies and small scale motions from intensity patterns of high spatial frequencies. Cross-correlation between a pair of time separated particle images is implemented by the hierarchical computational scheme of Burt ["Fast filter transforms for image processing," Int. J. Comput. Vision 16, 20 (1981)]. Each image is convolved with a series of band-pass filters and subsampled to obtain a set of images progressively decreasing in resolution and size. A coarse estimate of the displacement field obtained from pairs of lower resolution images are used to obtain more accurate estimates at the next (finer) level. Processing starts at the level of lowest resolution and stops at the highest resolution level, which contains the original image pair. Due to subsampling of low resolution images, the match template size can be kept constant for all stages of computation, thus eliminating the dependence of the largest resolvable displacement on the size of match template. In the present work, the search area at each level is kept constant at 3×3 pixels and the match template size at 5×5 pixels for all levels of computation. The algorithm has been implemented using simple thresholding based on the confidence level of an estimated displacement vector, as suggested by Anandan ["A computational framework and an algorithm for measurement of visual motion," Int. J. Comput. Vision, 2, 283, (1987)]. However, the confidence-level-based smoothing technique for rigid body motions (continuous velocity fields) could not be applied to displacement estimates obtained at discrete points i.e., the particle locations. Instead, smoothing was performed over the area covered by each particle. The algorithm has been tested against direct numerical simulations of turbulent flows when the flow field is known and particle images have been generated from these with the addition of noise. Both the accuracy of motion estimation and the computation time are seen to improve as compared to conventional PIV methods. Finally, video images taken of particle motion on the free-surface of a channel flow have been used to determine the capabilities of the technique in an experimental study. The resulting spectra show a quasi-two-dimensional character of the free-surface turbulent flow field, which corresponds well with the direct numerical simulations. © 1998 American Institute of Physics.

Notes: In this letter, we present results of experiments in which nuclear magnetic resonance (NMR) signals were detected using an untuned pickup coil coupled to a high temperature superconductor (HTS) based superconducting quantum interference device (SQUID). We were able to operate the HTS SQUID in the presence of static fields of up to 500 Gauss and radio frequency fields of up to 6 Gauss. The ability of a HTS SQUID to detect NMR signals opens up the possibility of using a sensitive detector for practical applications involving NMR that require a broad detection bandwidth. © 1997 American Institute of Physics.

Notes: Monte Carlo calculations have been performed on two nominally athermal polymer/solvent mixtures to test molecular theories of mixing properties for these systems. We first used the incremental chemical potential concept to derive an equation of state in the spirit of the generalized Flory dimer theory, without resorting to the concept of excluded volume. The resulting generalized Flory dimer-like theory and a related model, statistical associating fluid theory, were tested against simulation results for the excess volume, excess Gibbs free energy and component activity coefficients. Good agreement was obtained between the statistical associating fluid theory and computer simulations for all properties studied. The generalized Flory dimer theory, when applied self-consistently, was also able to provide quantitative predictions for the thermodynamic properties of these mixtures. An important result that emerges from our calculations is that these polymer solutions behave ideally when examined on the basis of a "Flory-like'' reference state augmented by density effects. This asserts that the effects of molecular size disparity on system thermodynamics are properly captured by this approach. By contrast, the incompressible Flory approach fails to capture the dependence of activity coefficients on composition. © 1995 American Institute of Physics.

Notes: The presence of genotype × environment (GE) interactions in plant breeding experiments has led to the development of several stability parameters in the past few decades. The present study investigated the repeatability of these parameters for 16 chickpea (Cicer arietinum L.) genotypes by correlating their estimates obtained from extreme subsets of environments within a year and also over years. Based on the estimates of response and stability parameters within each trial, the ranking of genotypes in the low-yielding subset differed from that in the high-yielding subset. This indicates poor repeatability for response and stability parameters over the extreme environmental subsets. The estimates of mean yield and stability parameters represented by ecovalence, W2i, were consistent over years, whereas those of response parameters (bi, and S2i) showed poor repeatability. Our results suggest that single-year results for yield and stability can be used effectively for selecting cultivars with stable grain yield if tested in a wider range of environments.