Spectroscopic study of the properties of a supersonic plasma stream

A plasma diagnosis has been carried out by spectroscopic methods. The behavior of temperature and concentration of charged particles was measured along the stream as a function of polarity for a stream with a shock wave and a stream with periodic structure. An explanation of the observed phenomena is given. The physical processes occuring in a supersonic plasma stream are quite distinctive [1], Spectroscopic investigations are necessary since they provide more detailed information on the physical state of the plasma. It is also of interest to study wave processes taking place in the stream from the point of view of the additional plasma heating thus obtained.We take this opportunity to express our sincere thanks to M. A. El'yashevich for discussing the results.     

Simultaneous DPTV/PLIF measurements of a turbulent jet

Simultaneous measurements of instantaneous velocity and concentration fields were performed using digital particle tracking velocimetry and planar laser- induced fluorescence for a turbulent jet at a Reynolds number of 3000. The measurements of mean velocity, turbulent stresses, mean concentration, concentration variance, and turbulent flux of tracer all collapse onto self-similar profiles in the far field of the jet. The measurements showed excellent agreement with previous point velocity and concentration measurements. It is concluded that the system is an effective means of measuring the velocity and concentration distributions and turbulent characteristics. Received: 7 July 1999/Accepted: 9 February 2000     

Criteria for quantitative PLIF experiments using high-power lasers

In order to interpret concentration measurements using planar laser-induced fluorescence (PLIF), it is necessary to understand the systematic and statistical errors that accompany those measurements. High-power lasers provide both increased signal-to-noise ratios (desirable) and intensity-induced deviations from linearity (undesirable). This paper reviews and argues for modification of current criteria for use of fluorophores for quantitative PLIF imaging, and makes use of a modified new protocol to qualify the fluorophore Rhodamine-WT for use in quantitative PLIF imaging. The key element is the decision to operate the imaging experiment under optically thin conditions.     

Electro-mechanical properties and electrostriction response of a rubbery polymer for EAP applications

The mechanical response of a widely used electro-active polymer (EAP) material, and the sensitivity of this response to imposed strain rate, are measured by performing tension and compression tests at constant strain rate as well as relaxation experiments. Simple actuators are constructed and used to determine the sensitivity of the material’s relative dielectric permittivity to excitation frequency and imposed equibiaxial strain. Actuators are subjected to both a monotonic, slowly increasing electrical excitation and to a more rapid oscillating electric field; the differences in the resulting actuation strains reveal the importance of the material’s viscosity in its performance as an actuator.     

The effect of photobleaching on PLIF

Organic dyes commonly used for planar laser-induced fluorescence (PLIF) are susceptible to photobleaching under laser excitation. Previous work has shown that photobleaching can induce significant errors in laser-induced fluorescence measurements made with single-point techniques. This paper presents an analytical model and experimental results that quantify the extent of photobleaching in the more common planar configuration of the technique. Experimental results for two common dyes, fluorescein and Rhodamine 6G, agree well with analytical predictions for typical PLIF parameters. Results indicate that even dyes such as fluorescein that have large photobleaching constants can be used with little error in many PLIF experiments.     

Combined PIV/PLIF measurements of a steady density current front

A novel method for combined particle image velocimetry and laser induced fluorescence is described and results from an experiment in a stratified flow are presented. A standard two-dimensional, one camera particle image velocimetry configuration is used, acquiring images of the seeding particles and the dye marking the current simultaneously and separating the two fields digitally. The implementation of the postprocessing method, its capabilities and the necessary conditions for its use are discussed in detail. The proposed method is applied to an arrested density current front. The front is made stationary by opposing a uniform velocity profile, obtained from the combination of a moving floor and the recirculation of fresh water in the channel. To improve the quality of the images, the current is made optically homogeneous by matching the refractivity index throughout the domain. Instantaneous and time averaged fields are obtained for both velocity and density. Simultaneous measurements of these fields provide insight in the mixing processes at the front of the density current. In particular persistent billow generation, similar to that found in shear layers and associated with Kelvin–Helmholtz instabilities, is observed. Support for this work was partially provided by the US Office of Naval Research through the Coastal Geosciences Program and by Exxon Mobil Exploration Company. The authors would like to thank Dr. Wing Lai of TSI, Inc, for loaning part of the PIV equipment used in this study.     

Passive scalar measurements in a planar mixing layer by PLIF of acetone

 Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re _δ up to 10⁴ using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer. This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer. The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re _δ  ≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal mixing before the transition point. Received: 5 October 1999 / Accepted: 9 February 2001     

PLIF imaging of mean temperature and pressure in a supersonic bluff wake

 Planar laser-induced fluorescence (PLIF) of seeded nitric oxide was used to obtain mean 2-D temperature and pressure fields in the near-wake region of a thick flat plate in a Mach 3 flow. A two-line ratio technique was used to obtain the temperature field, while an image obtained at the limit of low quenching rate was used to infer the pressure field. An analysis shows that these time-average measurements can suffer from significant weighted averaging bias errors in regions where there are large temperature fluctuations; however, these bias errors can be minimized by judicious selection of the absorption lines used. The resulting temperature field reveals the warm upstream boundary layer, the temperature jump across the recompression shocks and the expected minimum and maximum temperatures in the expansion and recirculation regions, respectively. The pressure measurements indicate a uniform low pressure in the base region, a rapid increase near reattachment, followed by a gradual approach to the free stream value farther downstream. Received: 25 July 1996 / Accepted: 11 September 1997     

Scalar interactions between parallel jets measured using a two-channel PLIF technique

A two-channel PLIF technique that simultaneously quantifies two scalar fields is presented. The technique consists of two independently operated single-color PLIF systems that synchronously image a common region. Two dyes (fluorescein sodium salt and oxazine 725) are excited by two lasers (argon-ion and krypton-ion), and the resulting fluorescence is imaged by a pair of cameras. The two-channel system is used to study mixing between two parallel jets, each transporting a different scalar species. Time-averaged and instantaneous mixing statistics are calculated and used to show the effects of turbulent structure on scalar mixing. In particular, the existence of positive spatial correlations between the two scalar fields is demonstrated in off-axis mixing regions.     

PLIF thermometry in shock tunnel flows using a Raman-shifted tunable excimer laser

Planar laser-induced fluorescence is performed in a free-piston shock tunnel by using a Raman-shifted tunable excimer laser to excite nitric oxide molecules in the flow. Two different flowfields are examined to test the difficulties associated with applying the technique to shock tunnels: the bluff body flow produced by a 25 mm diameter cylinder; and the oblique shock and expansion fan produced by a 35° half-angle wedge. For the cylinder, the maximum flow enthalpy was limited to 4.1 MJ kg due to high flow luminosity which is produced by metallic contaminants in the flow. A reflective filter is used to reduce the influence of flow luminosity making these measurements feasible. Freestream temperature measurements are in excellent agreement with those predicted from numerical flow calculations. Large uncertainties were observed for the high-temperature post-shock results. Several higher enthalpy shots (14 MJ kg) were also performed with the wedge and showed an insignificant amount of contaminant emission. Received 5 June 1996 / Accepted 8 February 1998     

A calibration scheme for quantitative concentration measurements using simultaneous PIV and PLIF

A new approach for calibration of planar laser-induced fluorescence (PLIF) measurements is presented. The calibration scheme is based on the fact that there is a constant concentration flux through each cross-section of a fluorescent plume in a given flow field and makes use of simultaneous measurements of particle image velocimetry (PIV) and PLIF. The following are the advantages of the current technique: (1) it is experimentally less demanding and (2) it does not require in situ calibration for generating the calibration curves. The technique can be implemented in many experimental setups (both in water and gaseous flows) provided the geometry of the time-averaged scalar field is known. Using the calibration scheme, an analysis is carried out on the measurements of concentration fields in grid turbulence to validate the proposed technique. To demonstrate the feasibility of the scheme, the distributed second-order moments (μ ₂), and concentration and velocity correlations ( á u^￠c^￠ ñ \left\langle {u^{\prime}c^{\prime}} \right\rangle and á v^￠c^￠ ñ \left\langle {v^{\prime}c^{\prime}} \right\rangle ) are computed. Good agreement is found with previous studies. In addition, a quantitative appraisal of a simple closure approximation of the moment-based transport equation is also presented using simultaneous PIV and PLIF.     

Covering properties on H-spaces and applications

Several theorems on closed(resp.open)covering properties of H-spaces areobtained which improve and generalize the corresponding results of Sperner,Klee,Alexandroff-Pasynkoff,Berge,Ghouila-Houri,Danzer-Gr(?)nbaum-Klee,Ky Fan,Shih-Tan,Horvath and Lassonde.As application an almost fixed point theorem for lowersemi-continuous map in l.c.-spaces and a generalization of Tychonoffs fixed pointtheorem are proved in l.c.-spaces which improve those results of Ky Fan and Horvath.     

A PLIF and PIV study of liquid mixing enhanced by a lateral synthetic jet pair

In this paper, enhancement of mixing between two water streams of the same flow rate in a planar channel by means of a lateral synthetic jet pair is studied at a net flow Reynolds number of 83 using PLIF and PIV. The synthetic jet pair is operated 180° out-of-phase at a range of actuation frequencies and displacements, with the latter being characterized by the dimensionless stroke length. The extent of mixing is evaluated using PLIF data at a location further downstream in the mixing channel. It is found that at a fixed actuation frequency a higher dimensionless stroke length produces a better mixing, and as the actuation frequency increases a lower dimensionless stroke length is required to achieve a given mixing degree. At a sufficiently high frequency or dimensionless stroke length, a nearly homogenous mixing with a mixing degree greater than 0.9 can be obtained. A functional relationship between actuation frequency and dimensionless stroke length is also obtained by best fitting the experimental data, which can be used for selecting the synthetic jet operating conditions to ensure a good mixing. Furthermore, both PLIF and PIV results show that each synthetic jet actuation cycle produce two opposing vortex pairs, which play an important role in prompting mixing between the two fluid streams. The excellent mixing obtained at a high frequency or a high dimensionless stroke length is found to be largely caused by a strong interaction between these opposing vortex pairs.     

Development of a strain multiplier for fatigue-sensor applications

The successive imposition of strain cycles on the fatigue sensor causes a progressive increase in its unstrained resistance which is thus a measure of fatigue experience. Consideration of the sensor as an equivalent device to the counting accelerometer, having the advantages of negligible size and mass, implies its application to a wide range of structural locations which, in most instances, will receive cyclic strain intensities of only moderate amplitude. For such applications, the low sensitivity of the sensor requires the employment of a strain multiplier. The development of a multiplier having the required characteristics is described, and its construction and application detailed. Typical results for the sensor and multiplier combination under static and fluctuating strains are given.     

Phase-Locked Temperature Measurements by Two-Line OH PLIF Thermometry of a Self-Excited Combustion Instability in a Gas Turbine Model Combustor

The phenomenon of periodic combustion instabilities has been investigated in a gas turbine model combustor by application of two-line planar laser induced fluorescence (PLIF) of OH for the determination of temperature distributions. The measurement technique has been evaluated using laser Raman scattering for comparison. The results showed that even with a lower accuracy compared to single-point techniques like CARS or Raman, valuable information concerning the stabilization mechanism can be drawn from the phase-locked mean temperature. The fact that the 2D technique is less time consuming compared to single-point techniques makes it attractive for phase-resolved measurements. The investigation showed that the two-line OH-PLIF thermometry technique can very well contribute to the understanding of combustion instabilities phenomena and assist the validation and the improvement of CFD models.