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Notes: Planar visualizations of two compressible free shear layers were performed immediately downstream of centered expansions of differing strengths in order to assess the influence of expansion strength on the embedded large-scale structures. The free shear layers studied here were formed through the separation of an approach flow, either a Mach 2.0 stream or a Mach 2.5 stream, from a planar backstep. In addition to side-view and end-view visualizations, spatial correlations (computed from large image ensembles) and laser Doppler velocimetry surveys of the free shear layers were also examined to discern relationships between the structure dynamics and the underlying pre- and postexpansion velocity fields. The instantaneous images clearly illustrate that ellipsoidal, highly coherent structures were present in both shear layers downstream of the expansion corner. The dissimilar expansion strengths did not appear to produce qualitatively different structures in the shear layers; however, as compared to the weaker expansion, the stronger expansion did result in an increase in the growth rate of the large-scale structures, apparently from an augmentation of the 〈u′v′〉∂U/∂y production term in the TKE equation. Furthermore, quantitative measurements of the mean structure geometry, as determined from the spatial correlation fields, revealed that a stronger expansion strength resulted in a larger aspect ratio of the mean structures (i.e., the structures were stretched preferentially in the streamwise and transverse directions as compared to the spanwise direction during the expansion process). Quadrant decompositions of the instantaneous velocity fluctuations within the approach boundary layers and within the free shear layers indicated a definite increase in structure organization across the expansion region, which is in contrast with studies of expanded supersonic boundary layers without separation. The instantaneous image data, spatial correlations, and velocity decompositions uniformly suggest that the separation process itself, and not the expansion strength, is the primary influence on initial eddy structure in the postexpansion free shear layer. © 2001 American Institute of Physics.

Notes: Instantaneous, quantitative, planar images of molecularly mixed-jet fluid fraction were obtained for the purpose of studying the mixing transition in a gaseous axisymmetric jet from ReD=16 200–29 200. By using a simultaneous nitric oxide and acetone planar laser-induced fluorescence technique, the mixing transition was detected from sudden changes in the molecularly mixed-jet fluid volume fraction, the growth rate of the shear layer, the preferred mixed-jet fluid fraction, and the character of axial/radial probability density functions. The mixing transition for all Reynolds numbers in this regime was found to begin after the first vortex pairing near Rx/λ=6 and was completed by the second vortex pairing near Rx/λ=12, where R=(1−r)/(1+r), r is the low- to high-speed freestream velocity ratio, and λ is the natural instability wavelength. The statistical quantities at all Reynolds numbers were found to collapse when scaled with Rx/λ, with the exception of the mixing layer width. The latter collapsed for all Reynolds numbers when scaled by Rx/λ prior to the mixing transition, and by x/D beyond the mixing transition, as expected for turbulent jets for which r(approximate)0. © 2001 American Institute of Physics.

Notes: Double-pulsed Mie scattering studies were performed to characterize the evolution of large-scale structures embedded within a planar supersonic base flow. Images were obtained at several streamwise stations along the shear layers, at reattachment, and in the near-wake regions. From these time-correlated images, the evolution characteristics of the large-scale structures were examined over a range of nondimensional time delays, as defined by local integral length and velocity scales. The double-pulsed images indicated that for short time delays (i.e., less than the representative eddy rollover time), the structures exhibited a simple translation in the streamwise direction. As the time delay was increased, rotation and elongation of the structures were observed in addition to the translation feature. Time delays that appreciably exceeded the local eddy rollover time generally resulted in a dramatic loss of structure identity. No eddy interactions, such as pairing, were observed at any of the imaging locations. Images obtained near reattachment provided evidence of shocklets moving in concert with the local eddies. In the initial portions of the shear layers, the mean convection velocity was measured to be significantly higher than the isentropic estimate, which is consistent with the results of previous convection velocity studies using mixing layers composed of supersonic/subsonic freestream combinations. The eddies decelerate through the recompression and reattachment regions, presumably due to the influence of the adverse pressure gradient. Downstream of reattachment, the large-scale structures accelerate as the wake develops. © 1999 American Institute of Physics.

Notes: Vortex formation and merging are investigated in the near field of a driven axisymmetric jet. Acoustic forcing is used to obtain repeatable vortex pairing events, and simultaneous passive scalar and cold-chemistry planar laser-induced flourescence are used to obtain instantaneous images of molecularly mixed jet fluid fraction. The time-varying scalar dissipation field and area-averaged stirredness of the vortex core region are measured at various stages of vortex interaction. These mixing properties are analyzed in conjunction with the observed vortex dynamics, such as the time-dependent vortex convection velocity. The results indicate that there are several phases of the pairing event with distinct mixing characteristics, including vortex roll-up, interaction, coalescence, and reentrainment. Vortex roll-up is nearly laminar with molecular diffusion between the layers of jet and co-flow fluid. The most dramatic change in the mixing state of the leading vortex, which includes the appearance of a uniformly mixed core region, occurs as the trailing vortex approaches and interferes with co-flow fluid entrainment. Vortex coalescence is marked by gross deformation and stretching of the trailing vortex, and rapid homogenization of the diffusion layers. Finally, re-entrainment of pure fluid after the pairing event results in an elongated, nonrotating structure. These stages of vortex pairing correspond to the temporal evolution of vorticity observed in previous studies. © 1999 American Institute of Physics.

Notes: The spatial evolution of large-scale turbulent structures in the shear layer of an axisymmetric, supersonic separated flow has been investigated. The experimental diagnostic used was planar visualization of condensed ethanol droplets that were suspended in the supersonic free stream. Spatial correlation analyses of large ensembles of images show that the mean side-view structure is highly strained and elliptical in shape and is inclined toward the local free stream direction. It is also shown that the effect of lateral streamline convergence for this axisymmetric case causes a reduction in side-view structure size and eccentricity at the reattachment point as compared to the planar case. End-view structures are wedge shaped, wider on the free-stream side than on the recirculation region or developing wake side. It is concluded that the wedge shape is caused by the axisymmetric confinement of the shear layer as it approaches the wake centerline. The average number of structures present in the end-view plane decreases significantly from 10–14 at recompression to 4–5 in the developing wake region. Evidence of an amalgamation of end-view structures in the images at the reattachment point illustrates one of the mechanisms responsible for this reduction. © 1999 American Institute of Physics.

Notes: Results from a dual-tracer planar laser-induced fluorescence (PLIF) technique for making instantaneous, quantitative measurements of molecularly mixed fluid fraction are presented for an axisymmetric jet in a slow co-flow. The two-camera, two-laser technique uses PLIF of nitric oxide seeded in a nitrogen jet to mark the unmixed jet fluid fraction, while PLIF of acetone seeded into the low velocity air co-flow marks the total co-flow fluid fraction. By combining data from these two simultaneous images, quantitative measurements of molecularly mixed jet fluid fraction can be made on a pixel-by-pixel basis, while simultaneously allowing visualizations of large-structure behavior and regions of subresolution stirring. Instantaneous images of molecularly mixed jet fluid fraction and jet fluid mixing efficiency, probability density functions (PDFs) of mixed jet fluid fraction, and associated statistics are presented for Rejet=1000, 5000, 10,000, 50,000, and 100,000. For fully turbulent conditions (Rejet≥30,000), stirring at subresolution scales is detected primarily on the jet side of the mixing layer. This creates a hybrid PDF behavior (stationary on the jet side of the mixing layer, marching on the co-flow side) that is not shown by passive scalar methods at equivalent image resolution. © 1999 American Institute of Physics.

Notes: Detailed mean velocity and turbulence data have been obtained with a laser Doppler velocimeter for two axisymmetric shear layers downstream of rapid expansions of different strengths. A comparison of the data in the near field (immediately downstream of separation) and far field (shear layer approaching self-similarity) is presented, and the fluid dynamic effects of the rapid expansion are ascertained for each regime. In general, the rapid expansion was found to distort the initial mean velocity and turbulence fields in the shear layer, in a manner similar to that in rapidly expanded, attached supersonic boundary layers; namely, two distinct regions were found in the initial shear layer: an outer region, where the turbulent fluctuations are quenched primarily due to mean compressibility effects (bulk dilatation), and an inner region, where turbulence activity is magnified due to the interaction of organized large-scale structures in the shear layer with low-speed fluid at the inner edge. With increasing strength of the rapid expansion, the effects in both regions become more pronounced, especially in the inner region, where turbulent fluctuations and mass entrainment rates are greatly magnified. Farther downstream, the turbulence activity of the large-scale eddies remains elevated, due to the rapid expansion, even though the relative distribution of the turbulence energy between the Reynolds stress components (structure of the turbulence) is independent of expansion strength. © 1995 American Institute of Physics.

Notes: Audit of 6 years' experience of breast fine needle aspiration (FNA) cytology using the cytospin method; improvement through multidisciplinary clinical auditA breast FNA cytology service for palpable breast lumps was commenced in 1989 using the cytospin method. Over the following 6 years 2314 aspirates were received. The results were audited in detail in 1990, 1991/1992 and 1994. Multidisciplinary clinical audit meetings followed each audit cycle. Practice change was agreed after each audit. Each audit cycle was followed by demonstrable improvement in the complete sensitivity of the technique, being respectively 79%, 88% and 96%. The cytospin method is a viable alternative to the conventional smear method.