Lester K. Su

The structure of fine-scale scalar mixing in gas-phase planar turbulent jets

Fine-scale scalar mixing in gas-phase planar turbulent jets is studied using measurements of three-component scalar gradient and scalar energy dissipation rate fields. Simultaneous planar Rayleigh scattering and planar laser-induced fluorescence, applied in parallel planes, yield the three-dimensional scalar field measurements. The spatial resolution is sufficient to permit differentiation in all three spatial directions. The data span a range of outer-scale Reynolds numbers from 3290 to 8330. Direct measurement of the thicknesses of scalar dissipation structures (layers) shows that the thicknesses scale with outer-scale Reynolds number as Re δ -3/4, consistent with Kolmogorov/Batchelor scaling. Average layer thicknesses are described by the relation λ D = 14.5 δ Re δ -3/4Sc -1/2 . There is no evidence here that Taylor scaling (λ D δ Re δ -1/2) plays a significant role in the scalar dissipation process. The present data resolve a range of length scales from the dissipation scales up to nearly the jet full width, and thus can be used in a priori testing of subgrid models for scalar mixing in large-eddy simulations

Emission spectroscopic measurements and analysis of a pulsed plasma jet

An investigation has been conducted of the characteristics of a freely expanding pulsed plasma jet originating from an electrothermal capillary source. The evolution of the plasma jet was investigated using temporally-resolved emission imaging with a gated intensified CCD camera and temporally- and spatially-resolved emission spectroscopy. The emission images reveal a highly underexpanded plasma jet that forms a distinct barrel shock structure, with a highly luminous Mach disk. The jet structure reaches an approximately quasi-steady state about 70 ps after the initiation of the plasma. An analysis of the emission spectra along the jet axis indicates population temperatures in the expanded region, upstream of the Mach disk, to be 2.02 eV (23 400 K) and 1.4 eV (16 200 K), for time delays of 30 /spl mu/s and 70 ps, respectively. At these same time delays, the electron number densities upstream of the Mach disk are of order 10/sup 17/ cm/sup -3/, but increase by an order of magnitude downstream of the Mach disk. Virtually no change is observed in the electron density across the Mach disk at a delay of 120 /spl mu/s, indicating the shock front has largely dissipated.

Quantitative planar imaging of turbulent buoyant jet mixing

Planar Rayleigh scattering provides quantitative mixing measurements in the developing region of axisymmetric turbulent helium jets issuing into air. The measurements focus on the relatively near field, in which the jets are primarily momentum driven. The imaging parameters are specified to ensure high spatial resolution. The mean jet fluid concentration fields attain self-similarity within the measurement region, though the forms of the mole fraction profiles indicate a reduction in turbulent transport at the jet outer boundary, arising from the reduced jet fluid density. Nevertheless, jet-like scaling pertains for the concentration fields. Mass fraction fluctuations on the jet centreline attain the expected asymptotic value of ≈23 % of the centreline mass fraction values. The scalar dissipation rates, however, show an axial decay rate that is slower than theoretical predictions. The two-dimensional extent of the measurements also allows spatial filtering similar to that inherent in large-eddy simulations (LESs). The results confirm that fluctuation levels and scalar dissipation rates determined for the filtered fields are reduced as the effective resolution is reduced, but while the fluctuation profiles for the filtered fields are similar for the different filter sizes, the forms of the scalar dissipation profiles are highly dependent on filter size. These latter results in particular are of a form that will be useful for grid-dependent assessments of LES results.

Measurements of the three-dimensional scalar dissipation rate field in gas-phase planar turbulent jets

Simultaneous, planar Rayleigh scattering and laser induced fluorescence yields 3D scalar and scalar dissipation rate field information in a planar turbulent jet. The conserved scalar used here is the jet fluid concentration, where the jet consists of propane, which serves as the Rayleigh scattering medium, seeded with acetone for fluorescence. The use of different imaging techniques for the two distinct spatial planes leads to higher signal levels than would, e.g., a two-plane Rayleigh scattering technique. Particular care must be taken to ensure pixel-to-pixel correspondence of the imaged planes. An important issue addressed is the degree to which the Rayleigh scattering and the acetone fluorescence individually and simultaneously mark the jet fluid concentration. Calculated values of the propane-air and acetone-air mass diffusivity suggest a negligible differential diffusion effect, borne out by measurements of the Rayleigh scattering and acetone fluorescence signals in a single spatial plane. This indicates that both techniques accurately measure the concentration of jet fluid. (Author)

Experimental Characterization of a Pulsed Plasma Jet

An investigation has been conducted of the characteristics of a freely expanding pulsed plasma jet originating from an electrotherma l capillary source. The evolution of the plasma jet was investigated using temporally-resolved emission imaging with a gated intensified CCD camera and temporally- and spatially-resolved emission spectroscopy. The emission images reveal a highly underexpanded plasma jet that forms a distinct barrel shock structure, with a highly luminous Mach disk. The jet structure reaches an approximately quasi-steady state about 70 us after the initiation of the plasma. An analysis of the emission spectra along the jet axis indicates population temperatures in the expanded region, upstream of the Mach disk, to be 2.02 eV (23 400 K) and 1.4 eV (16 200 K), for time delays of 30 us and 70 |is, respectively. At these same time delays, the electron number densities upstream of the Mach disk are of order 1017 cm~3, but increase by an order of magnitude downstream of the Mach disk. Virtually no change is observed in the electron density across the Mach disk at a delay of 120 us, indicating the shock front has largely dissipated.

Measurements of multiple mole fraction fields in a turbulent jet by simultaneous planar laser-induced fluorescence and planar Rayleigh scattering

This paper presents two-dimensional measurements of all individual mole fractions in a three-species, non-reacting turbulent flow, using planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering. The flow is an axisymmetric jet of acetone and helium in an air coflow. PLIF measures the acetone mole fraction, while Rayleigh scattering measures a linear combination of the acetone and helium mole fractions. The simultaneous implementation of these techniques allows for the calculation of the helium and air mole fraction fields. The results of this diagnostic method are being used for the study of multicomponent molecular transport effects in turbulent fluid mixing.

Experimental investigations of mixing in turbulent jets with buoyancy

This paper presents measurements of molecular mixing in turbulent buoyant jets of helium issuing into air, using planar laser imaging of the jet uid mole fraction elds. The o ws considered are nominally momentum-driven, so buoyancy eects are presumed to be conned to the small scales of the o w. The measurements focus on the developing region of the jet, which is of particular interest to o ws with combustion. The results suggest that buoyancy aects the details of the evolution of the mixing eld even while the mean eld maintains scaling properties consistent with non-buoyant jets. Specically , the mean jet uid mole fraction proles, and the proles of mole fraction uctuations, show a sharper jet/ambient uid interface relative to non-buoyant jets, possibly indicative of reduced entrainment. The mole fraction uctuations along the jet centerline are also weaker than those reported in non-buoyant jets. Statistics of jet uid mole fractions, conditional on spatial location within the jet, provide one view of the spatial evolution of the jet, while the eld nature of the measurements allows us to investigate the eect of spatial ltering, such as that inherent in large-eddy simulations (LES), on the elds of the turbulent uctuations. The results are intended to inform ongoing eorts to model the mixing process in o ws with density dierences, such as combustion systems.

Scale Relations and Spatial Spectra in a Differentially Diffusing Jet

Planar Rayleigh scattering is used to study differential diffusion in a turbulent propanehelium jet. Using a technique proposed by Bilger & Dibble, the differential diffusion variable ξ, a normalized scalar difference quantity, is extracted from the imaging data [1]. Both statistical and structural aspects of differential diffusion in turbulence are studied, including the distribution of the mean and variance of ξ, the ξ power spectra, and the thickness and distribution of differential diffusion dissipation structures. Comparisons are made to existing data from numerical simulations, particularly in the case of the scalar spectra.

Subgrid modeling approaches for scalar transport and mixing in LES of turbulent shear o ws

LES models for subgrid scalar transport are assessed through large eddy simulation of passive scalar mixing in a turbulent round jet. The simulation is performed on a spherical coordinate grid using semi-implicit time advancement. The models used to close the subgrid scalar ux term are varied while the same modeling approach is used for the momentum model in order to isolate the eects of the scalar modeling. The subgrid scalar ux models tested include the dynamic eddy diusivit y model and the dynamic one-parameter mixed model. Model performance is analyzed by examining mean, resolved-scale quantities and uctuations of the resolved scalar concentration eld. Also addressed in this paper are particular challenges associated with dynamic modeling in spherical-coordinate, round jet simulations. These include explicit test ltering of resolved-scale quantities and spatial averaging of dynamic coecien ts.

Imaging of Turbulent Buoyant Jet Mixing

This paper presents quantitative imaging measurements of jet fluid mole fraction fields in turbulent buoyant jets of helium issuing into air. The measurements use planar laser Rayleigh scattering. Signal levels are low, necessitating a novel approach to background subtraction in the signal processing. The jet flows considered are classified as momentum-driven, meaning that buoyancy effects are presumed to be confined to the small scales of the flow. We focus here on the near-nozzle, developing region of the jet, which is of particular interest to flows with combustion. The results suggest that buoyancy affects the details of the evolution of the mixing field even while the mean field maintains scaling properties consistent with non-buoyant jets. Specifically, the mean jet fluid mole fraction profiles show a sharper jet/ambient fluid interface relative to non-buoyant jets. The mole fraction fluctuations within the jet are also weaker than those reported in non-buoyant jets. These results will inform ongoing efforts to model the mixing process in flows with density differences, such as combustion systems.Copyright