Thin Sheet Of Laser Light Research Articles

The measurement of square channel velocity profiles using a microcomputer-based image analysis system

A technique was developed to perform automated velocity measurements from a sequence of particle images. A very thin sheet of laser light allows determination of essentially two-dimensional velocity profiles in very small conduits. A four image sequence was captured by a microcomputer-based frame grabber. After thresholding to eliminate particles not perfectly centered in the sheet of light, the sequence of pseudo-colored images of a given particle is used to determine its velocity. By measuring several two-dimensional velocity profiles across a square channel, the complete three-dimensional velocity profile was assembled. The experimentally measured velocity profile agrees closely with the known theoretical velocity profile for flow in a square channel.

Structural characteristics of lifted turbulent-jet flames

Instantaneous planar images of the CH4 concentration in a lifted turbulent-jet flame were obtained by collecting Raman-scattered light perpendicular to a thin sheet of laser light passing through the axis of the flame. The instantaneous images were averaged to obtain mean and fluctuating statistics and probability distributions of the CH4 concentration. The timeresolved planar images reveal flow and mixing structure information that is lost when timeaveraged point measurement techniques are used. Spatial variations in the time-averaged statistics and the effects of combustion on these quantities are interpreted in terms of structural characteristics of these flows. Based on the instantaneous images, two sources of turbulent fluctuations are identified: 1) large-scale movement of the interface separating unmixed fuel and air; and 2) smaller scale turbulent fluctuations that are superimposed on the largescale jet structure. Combustion heat release is found to have a significant effect on both the turbulence and mixing characteristics of these flows. The primary differences between a lifted flame and a nonreacting jet are a lower centerline decay rate of CH4, higher instantaneous gradients, and higher concentration fluctuations in the lifted flame. It is shown that the lower centerline decay rate of the CH4 concentration is due to the reduced mass entrainment rate of the central-fuel jet in the lifted flame. The higher rms fluctuations of CH4 in the flame are due to the combined effects of: 1) the generally higher CH4 concentrations along the centerline; 2) the steeper instantaneous CH4 gradients along the central jet boundaries; and 3) the larger turbulence scales in the mixing region.

Scalar dissipation measurements in the developing region of a jet

Instantaneous planar images of CH 4 mass fraction in the developing region ( x d ≤ 17 ) of an isothermal jet were used to calculate instantaneous, mean, root mean square (rms), probability density functions (PDFs), and correlations of scalar dissipation and a scalar. The mass fraction images were derived from CH 4 concentrations obtained by simultaneously recording Raman-scattered light with two cameras oriented perpendicular to a thin sheet of laser light passed through the axis of the jet. Using two cameras enhanced the dynamic range by a factor of ten over a single-camera system and allowed concentration measurements down to a mass fraction of 0.003. Axial and radial components of the scalar dissipation were determined and, by assuming isotropy in the radial and azimuthal directions, total dissipation statistics were obtained. The results were used to evaluate modeling assumptions. The radial and axial dissipation components were found to be strongly nonisotropic in the mixing layer at x d ≤ 13 , but isotropic further downstream. At all locations along the centerline the dissipation was isotropic. The scalar and dissipation were found to be highly correlated in the mixing layer but uncorrelated on the jet axis. The PDF of dissipation on the axis, off axis, and at the stoichiometric surface were all found to be lognormally distributed. This property was used to collapse all of the PDFs onto one plot. The parameters that define the single plot, lognormal PDF mean, and standard deviation are reported. Instantaneous dissipation levels measured at the expected flame liftoff height were found to be consistent with flame extinction limits extrapolated from laminar opposed jet data. For the single condition measured, this data supports the hypothesis that excessive dissipation can extinguish flames in the developing region of jets.

Time-Resolved Three-Dimensional Concentration Measurements in a Gas Jet

Turbulence can greatly influence reaction and heat transfer rates in fluids. The topology of the three-dimensional interface between mixing fluids directly determines the location and degree of reaction. The time-resolved measurement of the three-dimensional concentration field in a transitional gas jet is reported. A thin sheet of laser light was swept through the flow volume by a rotating mirror in a time brief enough that motion of the gas was minimal. The light sheet illuminated different parallel planes within the flow, and light scattered from particles seeding the jet was imaged onto a detector. The series of two-dimensional measurements made during one scan of the flow volume constituted a full three-dimensional mapping of structures within the flow. Computer graphics software was used to reconstruct and visualize three-dimensional surfaces of constant concentration and the magnitude of the concentration gradient vector over such surfaces.

On the coherent structure of the axisymmetric mixing layer: a flow-visualization study

In an effort to resolve some controversies regarding the turbulent mixing-layer structure, the near field of a large (18 cm diameter) air jet has been investigated for the jet exit speed of 30 m s−1. The smoke-laden axisymmetric mixing layer has been illuminated by a thin sheet of laser light in an azimuthal plane passing through the jet axis. High-speed visualization films of the mixing layer in the region of its self-preservation (of which a few picture sequences depicting space-time evolutions of the structure of the layer are presented) reveal that most of the time the mixing layer is in a state of disorganization, consisting of relatively smaller scale, random and diffuse turbulent motions; only occasionally are organized distinct large-scale coherent structures formed. The survival distances of the large-scale structures are found to be comparable to their average sizes. The survival time of these structures is about one ‘turnover’ time, each being roughly about five times the local characteristic time scale of the mixing layer. It is seen that tearing is as dominant a mode of large-scale interaction as pairing is; large-scale structures are continually sheared and typically fragmented due to a segment on the high-speed side being torn and swept away from the slower-moving outer portion. Evolution of the large structures occur not primarily through complete pairing as widely believed but quite frequently through ‘fractional pairing’ between segments which have been torn from different upstream large-scale coherent structures or through ‘partial pairing’ when one structure captures only a part of another. The movies show that along with entrainment of non-vortical ambient fluid, radially outward ejection of vortical fluid into the ambient is an important aspect of jet mixing. From aligned displays of ciné film frame sequences, space-time trajectories of identifiable vortical fluid elements have been traced. The convection velocity variation across the shear layer and even the overall structure convection velocity measured from these trajectories agree with those determined from the wave-number-celerity spectra, obtained from double-Fourier transformation of longitudinal velocity space-time correlation measurements with hot-wires.The visualization films do not bear out the two-street vortex ring model recently propounded by Lau. Based on our observations, we propose that tearing, ‘slippage’ and fractional and partial pairings are responsible for the observed radial variation of structure passage frequency, and the causes of the different coherent structures educed by Bruun on the high- and low-speed sides of the mixing layer and for Yule's failure in educing a coherent structure on the low-speed side of the layer.

Measurement of flow velocity distribution by means of double-exposure holographic interferometry

A method for measuring flow velocity distribution in the interior of a transparent fluid is proposed. The method utilizes a thin sheet of laser light, which illuminates fluid containing small scattering particles. A double-exposure hologram is made with the scattered light as a signal wave. In the reconstructed image we can observe interference fringes which correspond to the distribution of particle displacements between the two exposures and consequently to the distribution of flow velocity. Simple experiments were performed that confirmed the usefulness of the method. A discussion of the relation of this method to laser Doppler velocimetry shows that the two methods utilize different aspects of the same phenomenon.