This paper presents the application of pressure-sensitive paint (PSP) measurement technology to a large-scale commercial turbine-engine test stand. In this work, the test article is the engine-inlet bell mouth. A sol-gel-based PSP is applied to the inlet and illuminated using the blue (460-nm) output of eleven LED arrays. PSP data are acquired using a scientific-grade CCD camera. The application of PSP measurements in the engine-test-stand environment requires test instrumentation to be fixed within a housing located upstream of the test article. Challenges associated with performing PSP in this hostile environment are discussed, with focus on the strategies implemented to recover surface-pressure distributions on the engine-inlet bell mouth

Velocity measurements using the Planar Doppler Velocimetry (PDV) technique were made in a small jet facility and in a large wind tunnel to study the feasibility of the technique including sources of error and means to minimize them. PDV is a non-intrusive technique which allows the measurement of one or more velocity components everywhere in a plane within a flow field. Its potential to yield instantaneous, simultaneous velocities throughout a plane and at high spatial resolution in a single measurement makes it a promising tool for turbulence studies. PDV measurements are made by detecting the Doppler shift produced when laser light is scattered from moving particles in a flow field. The current PDV system utilizes a pulsed, injection-seeded, frequency-doubled Nd:YAG laser capable of producing narrow linewidth light (-140 MHz) that can be tuned to frequencies associated with the absorption lines of iodine. An iodine filter may therefore be used to discriminate the frequency of the laser light. A system utilizing two scientific-grade CCD cameras was used to record images produced by the scattered laser light. An unusual feature of the experimental arrangement was a frequency monitoring system which tracked the laser set point frequency on an image to image basis, providing resolution to better than 4 MHz. In a Mach 1.36 free jet, mean flow velocities in the core (averaged from three streamwise stations) were measured by PDV to within 2.4% of the value obtained by LDV (to within 6.4 m/s). For the high speed core of the jet, instantaneous velocity fluctuations greater than 3.4% (about 9 m/s) were measurable by PDV, but lesser fluctuations were masked by PDV system noise. In the wind tunnel (3.05 x 2.13 m test section), uniform flow runs at 68 and 96 m/s indicated bias and random errors of less than 2 and 4 m/sec, respectively. Mean measurements of the flow over a delta wing at a 23° angle-of-attack were similar to the results of a CFD calculation by Rizzetta (1996). The dominant source of random error in the velocity measurements arose from the phenomenon of laser speckle and the dominant source of bias error came from the characterization of the iodine filter profiles.

We report the first confirmed fluorescence lifetime measurement for a diazirine. We have obtained time-correlated single-photon counting fluorescence decays for adamantyldiazirine in a variety of solvents, over a wide range of temperatures (77-320 K) and across the diazirine absorption band (330-371 nm). The fluorescence Lifetime of the primary decay component is on the order of 240 ps at ambient temperature and increases at lower temperatures. Arrhenius treatment of the fluorescence lifetime data indicates that the rate-limiting barrier for activated processes in the diazirine excited state is between 2.7 and 2.9 kcal/mol. Adamantyldiazirine's fluorescence lifetime appears to be unaffected by deuteration of the solvent, solvent polarity, or excitation energy, We also report and discuss the steady-state absorption and fluorescence emission spectra of adamantyldiazirine in a variety of solvents, as well as the infrared spectrum (KBr). We interpret the spectra with the help of ab initio (RHF/6-31G* and CIS/6-31G*), density functional (B3LYP/6-31G*), and semiempirical (PM3) calculations. The fluorescence quantum yield of adamantyldiazirine at ambient temperature was calculated to be similar to 0.0012. Analysis of our data in the light of previous research leads us to conclude that little or no intermolecular chemistry is attributable to photoexcited adamantyldiazirine in solution at ambient temperature. Rather, fluorescence competes with one or more intramolecular photochemical processes.

Three techniques integrated into a single instrumented platform allow simultaneous measurement in real time of particle-size growth rate, surface mass-deposition rate, and concentration of dissolved oxygen in thermally stressed jet fuel. Particle growth rate is studied using Photon Correlation Spectroscopy (PCS); surface mass-deposition rate is measured with a Quartz Crystal Microbalance (QCM); and dissolved-oxygen concentration is monitored by Pyrene Fluorescence Quenching (PFQ). The three techniques have been used simultaneously to study the behavior of a jet fuel undergoing thermal stressing and to evaluate the performance of an anti-oxidant, a dispersant, and a metal deactivator in another jet fuel.

An optical method for the quantitation of dissolved molecular oxygen in aviation fuels has been developed to aid the study of thermally induced fuel oxidation. The technique is based on the propensity of dissolved molecular oxygen to quench probe molecule fluorescence excited with a pulsed nitrogen laser. Linear calibration curves based on Stern-Volmer kinetics are generated through measurement of the time-resolved fluorescence signal produced by pyrene doped into aviation fuel at parts per million (ppm) levels. The advantages of nondestructive in-situ monitoring, reduced measurement time, and enhanced capabilities (including spatially resolved and rapidly time-varying measurements) make this optical technique a very attractive complement to current chromatographic and electrochemical methods. Application to flowing-fuel simulators demonstrates the technique

We describe a multipass dye laser amplifier pumped by a cavity-dumped argon-ion laser for the amplification of ultrashort pulses at megahertz repetition rates. We present the operation and performance characterization of the system for the amplification of picosecond pulses from a synchronously pumped, cavity-dumped dye laser. We find that the multipass configuration of the amplifier provides conversion efficiencies of approximately 25% of the available energy provided by the pump laser. These results are consistent with appropriate models for the gain of the amplifier.

In picosecond time-resolved Raman spectroscopy, the depolarization ratio is expected to change as the time delay is varied if an anisotropy distribution of the excited state is introduced by using linearly polarized pumping light and the time resolution of the measuring system is shorter than the reorientation lifetime. The time dependence of the depolarization ratios for two intense Raman bands of trans-stilbene in the first singlet excited (S1) state was measured to demonstrate this effect. The results are explained well by a simple model that assumes the alignment of the S1 distribution and its subsequent randomization.

We usually assume randomly orientated molecules when discussing depolarization ratios in Raman spectroscopy. This assumption does not hold, however, in picosecond time-resolved Raman spectroscopy. Because samples are, in most cases, excited with linearly polarized laser light and the time resolution of the measurement is shorter than the reorientation lifetime of the sample, the probability distribution of the excited molecules is largest along the axis of the polarization of the laser light and smallest for the perpendicular direction. The depolarization ratios at t=0 should be different from the “normal” values because of this anisotropy and should change as the time delay increases. We have measured the time dependence of the depolarization ratio of trans-stilbene in the lowest excited singlet (S1) state and observed that the depolarization ratio changes depending on the time delay, which can be explained well by our simple, but realistic model.

The infrared and spontaneous Raman spectra of the cyanine dye 3,3-diethyloxadicarbocyanine iodide (DODCI) and its six analogues, 3,3'-diethylthiacarbocyanine iodide (DTCI), 3,3'-diethyl-9-methylthiacarbocyanine iodide (MDTCI), 3,3'-diethylthiadicarbocyanine iodide (DTDCI), 3,3'-diethylselenacarbocyanine iodide (DSCI), 3,3'-diethyloxacarbocyanine iodide (DOCI), and 3,3'-diethyloxatricarbocyanine iodide (DOTCI), have been measured. A Raman spectrometer consisting of a Ti:sapphire laser and a CCD detector was found to be advantageous in avoiding the strong fluorescence of these cyanine dyes. The spectra have been compared, and several of the vibrational bands have been assigned. The locations of the CC and CN stretching vibrations suggest considerably weakened bond alternations for these molecules in solution.

We obtain the picosecond transient Raman spectra of S1 trans-stilbene in acetonitrile and n-hexane. We observe mode-specific, solvent-dependent variations in the vibrational spectra. In acetonitrile, the vibrational bands in S1 trans-stilbene associated with the phenyl portion of the molecule shift to higher energy and increase in bandwidth, relative to n-hexane, owing to the increased coupling of the excited state to the more polar solvent. In both solvents, the vibrational motions associated with the olefin portion of the molecule change peak position and bandwidth with delay, while the peak position and bandwidth of the phenyl modes in each solvent remain constant with delay. The change in peak position and bandwidth of the olefin modes depends on the excitation frequency used to excite the molecule. We attribute these changes, in part, to vibrational relaxation via resonance energy exchange involving low-frequency vibrations. Additional factors, likely attributable to conformational changes, appear to give rise to a portion of the effects we observe.

We demonstrate broadband picosecond transient absorption spectroscopy at MHz repetition rates using stimulated Raman scattering from a single-mode polarization-preserving optical fiber. We use the output of the fiber to probe the absorption of the excited state of the cyanine dye DODCI.