Library

Notes: Mixing enhancement results are presented for compressible (convective Mach number 0.63) planar shear layers perturbed by 2D and 3D disturbances located within the supersonic-side splitter tip boundary layer. The disturbances were parametrically varied in shape, spacing, and thickness, and for each geometry time-resolved end-, side-, and plan-view visualizations of mixed fluid were obtained. The mixing layer thickness and growth rate are measured directly from the averaged images. As an indicator of the pressure loss induced by each disturbance geometry, the streamwise static pressure distribution is also recorded. The visualizations reveal that discrete 3D disturbances induce appreciable spanwise convolution, streamwise structure, and thickening of the mixing layer with disturbances as thin as 5% of the boundary layer displacement thickness. The optimal disturbance appears to have an angle of 30° to the streamwise direction and be located at the splitter tip, rather than upstream. Panoramic side-views show that the far-field growth rate increases (45% in one case) for certain discrete 3D disturbances but not 2D disturbances, despite equivalent area blockage. For the most promising geometry, quantitative measurements of the mixing layer thickness, probability of mixed fluid, and mixing efficiency were made using cold chemistry planar laser-induced fluorescence. The perturbed layer shows a slight improvement (7%) in mixing efficiency and a large increase (48%) in layer thickness, indicating that gains in the total amount of mixed fluid occur primarily by layer thickening. © 1998 American Institute of Physics.

Notes: This study investigates the structure and mixing of the two-dimensional turbulent mixing layer when subjected to longitudinal streamwise curvature. The straight layer is now well known to be dominated by the primary Kelvin–Helmholtz (KH) instability as well as the secondary Taylor–Goertler (TG) instability. For equal density fluids, placing the high-speed fluid on the inside of a streamwise bend causes the TG instability to be enhanced (unstable case), while placing the low-speed fluid on the inside of the same bend leads to the suppression of the TG instability (stable case). The location of the mixing transition is correspondingly altered. Our goal is to study the changes to the mixing field and growth rate resulting from the competition between instabilities.Our studies are performed in a newly constructed blow-down water facility capable of high Reynolds numbers and excellent optical access. Maximum flow speeds are 2 and 0.25 m/sec for the high- and low-speed sides, respectively, leading to maximum Reynolds numbers of 80 000 based on velocity difference and the width of the layer. We are able to dye one stream with a fluorescent dye, thus providing several planar views of the flow under laser sheet illumination. These views are superior to conventional approaches as they are free of wall effects and are not spatially integrating. However, our most useful diagnostic of the structure of the flow is the ability to record high-speed images of the end view of the flow that are then reconstructed by computer using the volume rendering technique of Jiménez et al.1 This approach is especially useful as it allows us to compare the structural changes to the flow resulting from the competition between the KH and TG instabilities. Another advantage is the fact that several hundred frames, covering many characteristic times, are incorporated into the rendered image and thus capture considerably more flow physics than do still images. We currently have our rendering techniques fully operational,2 and are presently acquiring high quality high-speed movies of the various flow cases.Our findings to date, based on planar time-averaged and instantaneous views, show the following: (1) a 50% increase in growth rate from the stable to the unstable case resulting from mild curvature; (2) an enhancement of the TG vortices in the unstable case, but without major disruption of the KH instability which remains relatively intact; and (3) the occurrence of the KH instability at angles tilted with respect to the splitter plate tip, in agreement with the predictions of linear stability theory. This final observation has not been reported to date, primarily because sheet techniques have not been used at Reynolds numbers as high as the present study. The presentation will provide detailed views of the changes between the stable, straight, and unstable cases using our volume rendering approach, and will provide statistical measures such as changes to vortex spacing and size, to quantify such changes.

Notes: Direct visual observations of a high Reynolds number jet are presented. The jet consists of the exhaust plume of a TITAN IV rocket motor, which was discharged upward during ground-based testing producing an estimated Reynolds number of about 2×108. An overall view of the first 2000 ft of the resulting plume is observed and discussed. Image processing is used to enhance the plume appearance and reveal significant events associated with the jet evolution. The most striking finding is the progression of organized structures up through the jet, similar to those observed in laboratory flows at Reynolds numbers of 104. Significant differences are also seen between the time-averaged scalar field, which appears more Gaussian, and the instantaneous scalar field, which appears more top hat. It is concluded that the organization is associated with inviscid instability mechanisms that are Reynolds number independent, and that large-scale organization is an integral part of the evolution of such flows, and not a remnant of transitional behavior.

Notes: Data obtained from the two-dimensional numerical simulation of a plane mixing layer have been used to study the feasibility of tagging one side of the flow by a passive scalar and using the instantaneous concentration of the scalar to detect the typical coherent events in the flow. The study has shown that this technique works quite satisfactorily and yields results similar to those obtained by using the instantaneous vorticity as a detection criterion. The contribution from the coherent events to the time-averaged turbulent momentum and scalar transport has been estimated. It is found that this contribution is of the same order as the time-mean transport during most of the dynamical evolution of the coherent structure. However, it may attain very large values for short periods of time in the neighborhood of pairing. The increase is particularly spectacular in the case of the Reynolds shear stress. While the present findings obtained from a two-dimensional simulation seem to support earlier results obtained from actual experiments, it is desirable to conduct additional studies with three-dimensional simulations when they become available.

Notes: The structure and evolution of natural, circularly and axially forced turbulent round jets in the far field are investigated. A three-dimensional insovalue surface, which compactly shows the global evolution of the large-scale vortical structures, is extracted from x-y-t laser-induced fluorescence data. Correlation techniques are used to determine whether these structures are helical or axisymmetric. The present results suggest that both the helical and axisymmetric instability modes are present in the natural jet, while the helical mode is dominant in both types of forced jets. Circular forcing increases the spreading rate of the jet.

Notes: Abstract Some results are presented on the temporal evolution of a large jet in crossflow consisting of the burning plume from an unabated oil well discharge. The volume rendering approach is used whereby several sequential x-y images of the jet are stacked in time, t, to produce an object in x-y-t space which can be used to conveniently examine the jet dynamics. Similar to earlier findings for free jets, the burning jet in crossflow is seen to consist of a series of large-scale organized structures which convect downstream leading to a quasiperiodic flame tip burnout. The wake region however is seen to be much less organized. Most surprising is the constant speed of the burning structures as they progress from the jet to crossflow direction. It therefore appears that an underlying organization exists in the jet in crossflow, in spite of its relatively complex, three-dimensional structure.

Notes: Abstract  This work identifies a new fluorescent tracer, 5(&6) carboxy-dichlorofluorescein, suitable for use with a Nd:YAG laser for quantitative PLIF concentration measurements in aqueous-phase flows. Fluorescence emission calibrations are performed to validate the use of this laser/dye combination, and to reject other combinations examined. It is found that the commonly used sodium fluorescein dye has a non-linear signal response when excited by the Nd:YAG laser, which makes this laser/dye combination unsuitable for quantitative PLIF measurements, although it is acceptable for purely visualization purposes. The new proposed dye shows a linear response of fluorescent signal both with dye concentration and with laser intensity. Fluid-flow measurements taken with the Nd:YAG/carboxy-dichlorofluorescein system are found in agreement to measurements obtained using a more standard laser/dye combination, namely Copper-vapor with sodium fluorescein, for which signal calibrations tests are also presented.

Notes: Abstract A planar Mie scattering technique is described which allows for the direct visualization of fluid mixing in supersonic flows. The mixed fluid is visualized by laser light sheet scattering from small alcohol droplets which condense as a result of the mixing of a vapor laden subsonic stream with a cold supersonic stream. Issues related to the formation, growth and size of the droplets are addressed. The technique reveals details of the turbulent structure which are masked by the spatial integration of schlieren and shadowgraph methods. Comparative visualizations using the vapor screen method to uniformly mark the high-speed fluid are also shown.

Notes: Abstract The far-field large-scale dynamics of a momentum-driven Re = 2 × 108 non-reacting jet and a Re = 3 × 107 jet diffusion flame are presented and compared. The results are derived from computer graphic volume rendering of a set of sequential images of each flow. When compared to conventional display techniques, volume rendering, by allowing many frames of a movie sequence to be presented simultaneously, more clearly shows the detailed flow evolution. For the non-reacting jet we see the passage and growth of large-scale organized structures up through the jet column, the axial velocity decay of the structures, the fluid entrainment patterns, and occasional pairing events. A rendering of a non-sequential set of images shows no discernible organized component. Volume rendering of the reacting jet shows a similar pattern of burning large-scale organized structures which convert over considerable axial distances but without the corresponding velocity decay, similar to observations of laboratory flames. The images presented here are believed to be some of the most direct visual evidence to date for large-scale organized motions in the far-field of high Reynolds number, fully developed jets and jet flames. Since conditional sampling techniques are not used, we believe that the volume renderings seen here are likely to be representative of the natural development of jet flows.

Notes: Abstract An imaging technique capable of time-resolved, three-dimensional visualizations of compressible flows is described and applied to a supersonic mixing layer. The three-dimensional planar imaging system uses a custom high-speed camera to acquire 10 successive planar images through the mixing layer at a rate of 107 frames per second. Mixed fluid in the layer is visualized by Mie scattering of a laser light sheet from condensed alcohol droplets. After collection, the planar images are corrected for distortions and stacked to form data volumes. Comparative visualizations at low and moderate convective Mach numbers (M c = 0.43 and 0.62) are used to examine the effects of compressibility on large-scale structure in mixing layers. The visualizations graphically reveal the shift from two-dimensional spanwise rollers to three-dimensional structure, such as oblique and V-shaped bands, with increasing compressibility. Additionally, direct comparison between the high- and low-speed edges of the mixing layer shows the high-speed interface to be smoother than its lowspeed counterpart.