Search

A single-image nitric-oxide molecular tagging velocimetry (NO-MTV) is reported and employed in a real air driven hypersonic shock tunnel and provided velocities of 3240 ± 170(5.2%) and 3030 ± 160(5.3%) m s − 1 , insignificantly different from previous results of 3037 ± 98(3.2%) obtained by an ordinary multi-image MTV technique. The proposed methodology relies on an one-dimensional analytical description of the spatial intensity profile registered by a single MTV image.

Superconducting radio-frequency (SRF) cavities cooled by superfluid 4He (He II) are building blocks of many modern particle accelerators due to their high quality factor. However, Joule heating from sub-millimeter surface defects on cavities can lead to cavity quenching, which limits the maximum acceleration gradient of the accelerators. Developing a non-contacting detection technology to accurately locate these surface hot spots is the key to improve the performance of SRF cavities and hence the accelerators. In a recent proof-of-concept experiment (Phys. Rev. Applied, 11, 044003 (2019)), we demonstrated that a molecular tagging velocimetry (MTV) technique based on the tracking of a He 2 * molecular tracer line created nearby a surface hot spot in He II can be utilized to locate the hot spot. In order to make this technique practically useful, here we describe our further development of a stereoscopic MTV setup for tracking the tracer line’s motion in three-dimensional (3D) space. We simulate a quench spot by applying a transient voltage pulse to a small heater mounted on a substrate plate. Images of the drifted tracer line, taken with two cameras from orthogonal directions, are used to reconstruct the line profile in 3D space. A new algorithm for analyzing the 3D line profile is developed, which incorporates the finite size effect of the heater. We show that the center location of the heater can be reproduced on the substrate surface with an uncertainty of only a few hundred microns, thereby proving the practicability of this method.

Molecular tagging velocimetry (MTV) is an optic experimental technique widely employed for measuring the velocity field in fluid flows. The measuring principle is based on the tracking of molecules able to emit light in response to a laser excitation. By seeding the flow with this tracer, local velocity measurements can be carried out by following the displacement of the emitting molecules. While this technique has already been successfully applied in liquid and gas flows, the application to rarefied and confined gas flows is still a challenge due to the high molecular diffusion and the low emitted light from the tracer at low pressures. The interest in applying MTV in rarefied conditions derives from the absence of local experimental data that can allow a better understanding on the mechanisms of interaction between the gas molecules and the wall surface. Theoretical and numerical analysis predicted a non-zero velocity at the wall, which has been, up to now, only indirectly measured by means of global quantities, such as the mass flow rate in microchannel flows. In this work, an experimental analysis of the intensity and lifetime of the photoluminescence of the molecular tracers employed, i.e., acetone and diacetyl, is presented. This study revealed the importance of the triplet-triplet annihilation phenomenon in determining the observed phosphorescence decay. Moreover, this analysis allowed to estimate the best working conditions for applying MTV to rarefied gas flows. Thus, MTV has been applied to gas-tracer mixtures at low pressures in a millimetric rectangular channel producing the first gas flow visualizations in the slip flow regime and the first preliminary measurements of the slip velocity at the wall.; Le marquage moléculaire est une technique expérimentale permettant d’effectuer de manière peu intrusive de la vélocimétrie au sein des écoulements. La mesure est basée sur le suivi de molécules capable d’émettre de la lumière suite à une excitation par une source laser. La mesure locale de vitesse est alors déduite de la visualisation du déplacement des molécules traceuses. Bien que la vélocimétrie par marquage moléculaire (MTV) ait déjà été utilisée avec succès pour des écoulements liquides ou gazeux, son application à des écoulements gazeux raréfiés internes reste encore un défi en raison de la diffusion moléculaire élevée et de la faiblesse de l’émission lumineuse à basse pression. Néanmoins, pouvoir appliquer la MTV en condition de gaz raréfié est particulièrement intéressant du fait du manque de données expérimentales locales nécessaires à une meilleure compréhension des mécanismes d’interaction moléculaire entre le gaz et une surface solide. Les analyses théoriques et numériques mettent en évidence une vitesse non nulle à paroi, qui, jusqu’à maintenant, a pu être quantifiée expérimentalement uniquement de manière indirecte, par des mesures de quantités globales telles que le débit massique à travers des microcanaux. Dans ce travail, une étude expérimentale a été menée sur l’intensité et le temps de vie de la photoluminescence des traceurs moléculaires utilisés, à savoir l’acétone et le diacétyle vapeurs. Cette étude a démontré l’importance du processus d’annihilation triplet-triplet sur l’émission phosphorescente expérimentalement observée. En outre, cette analyse a permis d’estimer les conditions expérimentales optimales pour l’application de la MTV aux gaz raréfiés. Ainsi, la MTV a été appliquée à des écoulements de mélanges gaz-traceur à basses pressions dans un canal millimétrique de section rectangulaire, fournissant ainsi les premiers résultats de vélocimétrie en régime d’écoulement légèrement raréfié.

Molecular tagging velocimetry (MTV) is an optic experimental technique widely employed for measuring the velocity field in fluid flows. The measuring principle is based on the tracking of molecules able to emit light in response to a laser excitation. By seeding the flow with this tracer, local velocity measurements can be carried out by following the displacement of the emitting molecules. While this technique has already been successfully applied in liquid and gas flows, the application to rarefied and confined gas flows is still a challenge due to the high molecular diffusion and the low emitted light from the tracer at low pressures. The interest in applying MTV in rarefied conditions derives from the absence of local experimental data that can allow a better understanding on the mechanisms of interaction between the gas molecules and the wall surface. In this work, an experimental analysis of the intensity and lifetime of the photoluminescence of the molecular tracers employed, i.e., acetone and diacetyl, is presented. This analysis allowed to estimate the best working conditions in order to be able to apply MTV to rarefied gas flows. Thus, MTV has been applied to gas-tracer mixtures at low pressures in a millimetric rectangular channel producing the first preliminary results in the slip flow regime.; Le marquage moléculaire est une technique expérimentale permettant d’effectuer de manière peu intrusive de la vélocimétrie au sein des écoulements. La mesure est basée sur le suivi de molécules capable d’émettre de la lumière suite à une excitation par une source laser. La mesure locale de vitesse est alors déduite de la visualisation du déplacement des molécules traceuses. Bien que la vélocimétrie par marquage moléculaire (MTV) ait déjà été utilisée avec succès pour des écoulements liquides ou gazeux, son application à des écoulements gazeux raréfiés internes reste encore un défi en raison de la diffusion moléculaire élevée et de la faiblesse de l’émission lumineuse à basse pression. Néanmoins, pouvoir appliquer la MTV en condition de gaz raréfié est intéressant du fait du manque de données expérimentales locales nécessaires à une meilleure compréhension des mécanismes d’interaction moléculaire entre le gaz et une surface solide. Dans ce travail, une étude expérimentale a été menée sur l’intensité et le temps de vie de la photoluminescence des traceurs moléculaires utilisés, à savoir l’acétone et le diacétyle vapeurs. Cette analyse a permis d’estimer les conditions expérimentales optimales pour l’application de la MTV aux gaz raréfiés. Ainsi, la MTV a été appliquée à des écoulements de mélanges gaz-traceur à basses pressions dans un canal millimétrique de section rectangulaire, fournissant ainsi les premiers résultats de vélocimétrie en régime d’écoulement légèrement raréfié.

Streamwise velocity profiles and their wall-normal derivatives were used to investigate the properties of turbulent channel flow in the low polymer drag reduction (DR) regime, \(DR = 6.5\)–\(26\%\). Streamwise velocity data were obtained over a friction Reynolds number ranging from 650 to 1800 using the single velocity component version of molecular tagging velocimetry (1c-MTV). This adaptation of the MTV technique has the ability to accurately capture instantaneous profiles at very high spatial resolution, and thus generate well-resolved derivative information as well. Owing to this ability, the present study is able to build upon and extend the recent numerical simulation analysis of White et al. [6] that examined the mean dynamical structure of polymer drag reduced channel flow at friction Reynolds numbers up to 1000.

In this paper, a new type molecular tagging velocimetry based on NH fluorescence was developed and validated for the velocity measurements of a high-enthalpy rarefied gas wind tunnel where the maximum flow velocity exceeds 7 km/s at 0.2 Pa. The feasibility of this technique using the short-lived NH fluorescence was demonstrated in the hypersonic rarefied gas flow with yielding velocity profiles at multiple downstream locations from the nozzle exit. The total uncertainty of the measured flow velocities was estimated to be less than 6% when the flow velocity above 2000 m/s. For the new tagging technique, only a single laser and a single time-gated camera are required for velocity measurement, due to the existence of NH radicals in the arc-discharged N2 mixed with small an amount of H2. Therefore, the NH-MTV not only shows great promise for tagging in high-enthalpy rarefied gas of nitrogen or air flows without seeding but also possesses high practicability and facility for velocity measurement.

A novel velocimetry method is proposed for point velocity measurement, which is based on tracking a laser-induced plasma in a flow. The plasma’s behaviour is first analysed spatially, temporally and spectrally in quiescent air. The dependence of this technique on the delay time between subsequent plasma images and the processing methods are described. It is found that, for optimized operation of the technique in a turbulent air jet (exit diameter 10.0 mm from a 480 mm long pipe; with an averaged velocity of 50 m/s at the jet exit resulting in Reynolds number of 34,000) with 100 µs time delay between plasma images, the systematic and random components of the velocity uncertainty are − 0.51 m/s and ± 3.6 m/s along the laser beam direction, and 1.25 m/s and ± 0.86 m/s along other directions perpendicular to the laser beam. These uncertainties are mainly caused by the asymmetric laser energy deposition during the formation of plasma, and the associated spatial resolution (in this realisation of the instrument) of 5 mm. The mean velocity measurements in the turbulent air jet flow are consistent with the reported flow behaviour in the literature for mean velocity: the turbulent intensity of axial velocity fluctuations is comparable to those in the literature but difference arises due to the limited spatial resolution. This velocimetry method is an alternative to traditional tracer-based velocimetry methods, because it does not require ‘seeding’ of particles or other substances in the flow. It also has the ability to measure local gas mixture composition, using laser-induced breakdown spectroscopy approach, simultaneously with flow velocity, but this aspect is not explored in the current study. Concept of Laser-Induced Plasma Image Velocimetry (LIPIV) applied to a turbulent jet flow. The LIPIV technique measures the flow velocity vector by using the temporal displacement of the laser-induced plasma in a flowing fluid. For this reason, two sequential images of the plasma with time delay Δt are used.

International audience; Rarefied gas flows have a central role in microfluidic devices for many applications in various scientific fields. Local thermodynamic non-equilibrium at the wall-gas interface produces macroscopic effects, one of which is a velocity slip between the gas flow and the solid surface. Local experimental data able to shed light on this physical phenomenon are very limited in the literature. The molecular tagging velocimetry (MTV) could be a suitable technique for measuring velocity fields in gas micro flows. However, the implementation of this technique in the case of confined and rarefied gas flows is a difficult task: the reduced number of molecules in the system, which induces high diffusion, and the low concentration of the molecular tracer both drastically reduce the intensity and the duration of the exploitable signal for carrying out the velocity measures. This work demonstrates that the application of the 1D-MTV by direct phosphorescence to gas flows in the slip flow regime and in a rectangular long channel is, actually, possible. New experimental data on phosphorescence emission of acetone and diacetyl vapors at low pressures are presented. An analysis of the optimal excitation wavelength is carried out to maximize the intensity and the lifetime of the tracer emission. The experimental results demonstrate that a little concentration of about 5-10 % of acetone vapor excited at 310 nm or of diacetyl vapor excited at 410 nm in a helium mixture at pressures on the order of 1 kPa provides an intense and durable luminescent signal. In a 1-mm deep channel, a gas flow characterized by these thermodynamic conditions is in the slip flow regime. Moreover, numerical experiments based on DSMC simulations are carried out to demonstrate that an accurate measurement of the velocity profile in a laminar pressure-driven flow is possible for the rarefied conditions of interest.

Femtosecond laser electronic excitation tagging (FLEET) is a molecular tagging velocimetry technique that can be applied in combustion flow fields, although detailed studies of its application in combustion are still needed. We report the applicability of FLEET in premixed CH4–air flames. We found that FLEET can be applied in all of the combustion areas (e.g., the unburned region, the burned region and the reaction zone). The FLEET signal in the unburned region is significantly higher than that in the burned region. This technique is suitable for both lean and rich CH4–air combustion flow fields and its performance in lean flames is better than that in rich flames.

The interaction of vortex rings with thin wire mesh screens is investigated using laser-induced fluorescence (LIF) and molecular tagging velocimetry (MTV). The existence of vortex shedding from individual wires of the porous screens, suggested by prior works, is shown and compared to flow visualization results. A range of interaction Reynolds numbers and screen porosities are studied to determine the conditions affecting the interaction. Transmitted vortex (TV) ring formation is shown to be a function of vortex shedding and the shedding Reynolds number, but not a function of porosity. Screen porosity is shown to affect the TV convective speed but did not impact the formation behaviors. Three major flow regimes existed for the interaction: TV formation with no vortex shedding, TV formation with visible vortex shedding, and no downstream formation with strong shed vortices.

Numerical simulations of the gas-seeding strategies required for planar laser-induced fluorescence in a Mach 10 (approximately Mach 8.2 postshock) airflow were performed. The work was performed to understand and quantify the adverse effects associated with gas seeding and to assess various types of seed gas that could potentially be used in future experiments. In prior experiments, NO and NO2 were injected through a slot near the leading edge of a flat-plate wedge model used in NASA Langley Research Center’s 31 in. Mach 10 air tunnel facility. In this paper, nitric oxide, krypton, and iodine gases were simulated at various injection rates. Simulations showing the deflection of the velocity boundary layer for each of the cases are presented. Streamwise distributions of velocity and concentration boundary-layer thicknesses, as well as vertical distributions of velocity, temperature, and mass distributions, are presented for each of the cases. A comparison between simulated streamwise velocity profiles and e...

Simulations of gas seeding into a hypersonic boundary-layer flow were performed using OpenFOAM® to investigate and quantify errors associated with quantitative planar laser-induced fluorescence thermometry and velocimetry techniques. A modified version of the compressible rhoCentralFoam solver was used to simulate multicomponent chemically reactive flows. Simulations replicated conditions used in NASA Langley Research Center’s 31 in. Mach 10 facility with a wedge model oriented at various angles of attack with respect to the freestream flow in the test section. Adverse chemistry effects from the reaction of nitric oxide with molecular oxygen were investigated at various facility running conditions. Specifically, the effect of heat release on velocity and temperature profiles that would be obtained using the nonintrusive laser measurement techniques was assessed. The effect of any potential adverse chemistry reactions was found to be negligible.

The molecular tagging velocimetry (MTV) is a well-suited technique for velocity field measurement in gas flows. Typically, a line is tagged by a laser beam within the gas flow seeded with light emitting acetone molecules. Positions of the luminescent molecules are then observed at successive times and the velocity field is deduced from the analysis of the tagged line displacement and deformation. However, the displacement evolution is expected to be affected by molecular diffusion, when the gas is rarefied. Therefore, there is no direct and simple relationship between the velocity field and the measured displacement of the initial tagged line. This paper addresses the study of tracer molecules diffusion through a background gas flowing in a channel delimited by planar walls. Tracer and background species are supposed to be governed by a system of coupled Boltzmann equations, numerically solved by the direct simulation Monte Carlo (DSMC) method. Simulations confirm that the diffusion of tracer species becomes significant as the degree of rarefaction of the gas flow increases. It is shown that a simple advection–diffusion equation provides an accurate description of tracer molecules behavior, in spite of the non-equilibrium state of the background gas. A simple reconstruction algorithm based on the advection–diffusion equation has been developed to obtain the velocity profile from the displacement field. This reconstruction algorithm has been numerically tested on DSMC generated data. Results help estimating an upper bound on the flow rarefaction degree, above which MTV measurements might become problematic.

High spatial resolution measurements of local flow velocities and temperature distributions are crucial for understanding turbulent, high-speed, and reacting flows. In this paper, we demonstrate that femtosecond laser electronic excitation tagging, a recently developed molecular tagging velocimetry method, can also be used to simultaneously measure temperature. A 150 femtosecond pulse populates excited molecular nitrogen states, driving significant emission from the nitrogen second positive and first negative systems. Spectroscopy of this emission allows temperature to be determined from the distribution of rotational molecular energy. Temperature profiles are measured with better than 10% uncertainty and 500 μm spatial resolution between 300 and 650 K in heated jets. This method is expected to be valid up to at least 2000 K. Femtosecond laser electronic excitation tagging temperature and velocity measurements involve separate nitrogen transitions and can be conducted simultaneously without interference.

Particle image velocimetry (PIV) has become a powerful tool for flow velocity measurements in wind tunnel testing. However, it is generally difficult to apply the PIV technique to supersonic flows because of unreliable particle traceability. In the present study, the PIV and MTV (molecular tagging velocimetry) techniques are applied to transonic and supersonic flows, in which a normal shock wave appears, to evaluate particle traceability. Based on this work, it is found that the PIV data largely deviate from the MTV data behind a normal shock wave for both flows. The drag coefficient is also estimated for a particle from the velocity data measured by the two techniques. Its value is then compared to the drag coefficient value calculated from an empirical formula for particle Mach numbers ranging between 0.1 and 0.9. Based on the results, it is found that the experimental data can be reproduced reasonably well by the formula for particle Reynolds numbers higher than ～1. However, the data associated with particle Reynolds numbers lower than ～1 deviate largely from the formula., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1620002000

This review article reports the recent progress in the development of a new group of molecule-based flow diagnostic techniques, which include molecular tagging velocimetry (MTV) and molecular tagging thermometry (MTT), for both qualitative flow visualization of thermally induced flow structures and quantitative whole-field measurements of flow velocity and temperature distributions. The MTV and MTT techniques can also be easily combined to result in a so-called molecular tagging velocimetry and thermometry (MTV&T) technique, which is capble of achieving simultaneous measurements of flow velocity and temperature distribution in fluid flows. Instead of using tiny particles, the molecular tagging techniques (MTV, MTT, and MTV&T) use phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, as the tracers for the flow velocity and temperature measurements. The unique attraction and implementation of the molecular tagging techniques are demonstrated by three application examples, which include: (1) to quantify the unsteady heat transfer process from a heated cylinder to the surrounding fluid flow in order to examine the thermal effects on the wake instabilities behind the heated cylinder operating in mixed and forced heat convection regimes, (2) to reveal the time evolution of unsteady heat transfer and phase changing process inside micro-sized, icing water droplets in order to elucidate the underlying physics pertinent to aircraft icing phenomena, and (3) to achieve simultaneous droplet size, velocity and temperature measurements of “in-flight” droplets to characterize the dynamic and thermodynamic behaviors of flying droplets in spray flows.

The effects of light-driven enhancement of Evans Blue dye complexes with blood plasma proteins were observed for the first time, both in vitro and in vivo. The possible background of the effect concerns the photochemical cis-trans isomerization of the azo dye molecules. The effect was induced in the solution with a red laser with a wavelength of 638 nm, which corresponds to the peak of the dye absorption. The lifetime of the enhanced fluorescence is approximately 1 second and enables its use as an optically tagged molecular flow tracer for blood flow velocity measurements. Utilizing the effect, we performed for the first time the intravital molecular tagging velocimetry of the blood velocity in blood vessels in a living animal. The results of the measurements of the blood flow velocities in the cerebral veins of a group of healthy mice are presented.

Proceedings 18th International Symposium on Flow Visualization

The measurement of velocity and scalar quantities is critical to increase the knowledge base to understand the behavior of fluid flows. The study of rapidly evolving fluid flows, particularly those involving complex phenomena such as chemical reactions, abrupt gradients, turbulence and nonequilibrium conditions, requires diagnostic tools capable of providing as much information as possible, ideally quantifying two or more variables simultaneously with high time resolution. Accurate modeling of fluctuations in a flow requires of experimental information about the coupling between different quantities, and the development of experimental methods that provide an insight into the coupling of the fluid dynamics and processes such as energy transfer and temperature fluctuations, specifically methods that allow simultaneous determination of velocity and scalar fields, is highly desirable. This chapter describes the development and application of nitric oxide LIF-based methods to provide simultaneous 2-D measurements of the mean and instantaneous fluctuations in two-component velocity and temperature in high-speed gaseous flows in a nonintrusive manner. Specifically, instantaneous and simultaneous two-component velocity and temperature measurements in gaseous flow fields are possible by combining two PLIF-based methods: two-component molecular tagging velocimetry (MTV) and two-line thermometry, employing nitric oxide (NO) as a molecular tracer. Although a series of recent experiments in cold gas expansions are described, these measurements are potentially applicable to combustion and other reactive or high-enthalpy flow fields of wide interest.

Since 2000 there has been a revolution in diagnostics of high-speed air flows. The foundations for this revolution were laid over the past few decades, but with the development of new short pulse and pulse burst laser technologies, higher laser powers and higher pulse energies, new high-speed cameras, better laser control and improved detection and laser delivery methodologies, many very effective new capabilities have been developed. Newly developed methods for molecular tagging velocimetry provide high fidelity visualization of transport properties and may be extended to simultaneous temperature measurements. Rapid field imaging with frequency tunable pulse burst lasers shows instantaneous flow structure and complex boundary and mixing interactions. Extending these pulse burst concepts to swept volumetric imaging is very promising for full volumetric data collection. Fast wavelength modulation spectroscopy follows real-time flow variation, and three-dimensional particle imaging extends particle imaging velocimetry to volumetric data acquisition.

A molecular tagging velocity (MTV) technique is developed to non-intrusively measure velocity in an integral effect test (IET) facility simulating a high temperature helium-cooled nuclear reactor in accident scenarios. In these scenarios, the velocities are expected to be low, on the order of 1 m/s or less, which forces special requirements on the MTV tracer selection. Nitrous oxide (N2O) is identified as a suitable seed gas to generate NO tracers capable of probing the flow over a large range of pressure, temperature, and flow velocity. The performance of N2O-MTV is assessed in the laboratory at temperature and pressure ranging from 295 to 781 K and 1 to 3 atm. MTV signal improves with a temperature increase, but decreases with a pressure increase. Velocity precision down to 0.004 m/s is achieved with a probe time of 40 ms at ambient pressure and temperature. Measurement precision is limited by tracer diffusion, and absorption of the tag laser beam by the seed gas. Processing by cross-correlation of single shot images with high signal-to-noise ratio reference images improves the precision by about 10% compared to traditional single shot image correlations. The instrument is then deployed to the IET facility. Challenges associated with heat, vibrations, safety, beam delivery, and imaging are addressed in order to successfully operate this sensitive instrument in-situ. Data are presented for an isothermal depressurized conduction cool-down. Velocity profiles from MTV reveal a complex flow transient driven by buoyancy, diffusion, and instability taking place over short ( 30 min) time-scales at sub-meter per second speed. The precision of the in-situ results is estimated at 0.027, 0.0095, and 0.006 m/s for a probe time of 5, 15, and 35 ms, respectively.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used to study the flowfield around a symmetric, transonic airfoil in the NASA Langley 0.3-m TCT facility. A nominal Mach number of 0.85 was investigated with a total pressure of 125 kPa and total temperature of 280 K. Two-components of velocity were measured along vertical profiles at different locations above, below, and aft of the airfoil at angles of attack of 0°, 3.5°, and 7°. Velocity profiles within the wake showed sufficient accuracy, precision, and sensitivity to resolve both the mean and fluctuating velocities and general flow physics such as shear layer growth. Evidence of flow separation is found at high angles of attack. Velocity measurements were assessed for their accuracy, precision, dynamic range, spatial resolution, and overall measurement uncertainty as they relate to the present experiments. Measurement precisions as low as 1 m/s were observed, while the velocity dynamic range was found to be nearly a factor of 500. The spatial resolution of between 1 mm and 5 mm was found to be primarily limited by the FLEET spot size and advection of the flow. Overall measurement uncertainties ranged from 3 to 4 percent.

Physical interpretations and especially analytical considerations benefit from the ability to accurately estimate derivatives of experimentally measured statistical profiles. Toward this aim, experiments were conducted to investigate the efficacy of single-component molecular tagging velocimetry (1c-MTV) to measure mean velocity profiles that can be differentiated multiple times. Critical effects here pertain to finite measurement uncertainty in the presence of high spatial resolution. Measurements acquired in fully developed turbulent channel flow over a friction Reynolds number range from 390 to 1800 are used to investigate these issues. Each measured profile contains about 880 equally spaced data points that span from near the edge of the viscous sublayer to the channel centreline. As a result of the high spatial resolution, even very small levels of uncertainty in the data adversely affect the capacity to produce smooth velocity derivative profiles. It is demonstrated that the present 1c-MTV measurements can be differentiated twice, with the resulting profile remaining smooth and accurate. The experimental mean velocity profiles and their wall-normal derivatives up to second order are shown to convincingly agree with existing DNS data, including the apparent variations with Reynolds number.

In this paper, a new type molecular tagging velocimetry based on NH fluorescence was developed and validated for the velocity measurements of a high-enthalpy rarefied gas wind tunnel where the maximum flow velocity exceeds 7 km/s at 0.2 Pa. The feasibility of this technique using the short-lived NH fluorescence was demonstrated in the hypersonic rarefied gas flow with yielding velocity profiles at multiple downstream locations from the nozzle exit. The total uncertainty of the measured flow velocities was estimated to be less than 6% when the flow velocity above 2000 m/s. For the new tagging technique, only a single laser and a single time-gated camera are required for velocity measurement, due to the existence of NH radicals in the arc-discharged N2 mixed with small an amount of H2. Therefore, the NH-MTV not only shows great promise for tagging in high-enthalpy rarefied gas of nitrogen or air flows without seeding but also possesses high practicability and facility for velocity measurement.

A new micro molecular tagging velocimetry (μMTV) setup has been developed to analyze velocity fields in confined internal gas flows. MTV is a little-intrusive velocimetry technique. It relies on the properties of molecular tracers which can experience relatively long lifetime luminescence once excited by a laser beam with an appropriate wavelength. The technique has been validated for acetone seeded flows of argon inside a 1 mm depth rectangular minichannel, with a multilayer design offering two optical accesses. Velocity profiles have been obtained using a specific data reduction process, with a resolution in the order of 15 μm. The experimental data are compared to theoretical velocity profiles of compressible pressure-driven flows. A good agreement is observed, except close to the walls, where the accuracy would still need to be improved. Following these first results obtained at atmospheric pressure, the influence of pressure on the luminescence intensity of acetone molecules is analyzed. The obtained data lead to a discussion of MTV applicability to rarefied flows and its possible use for a direct measurement of velocity slip at the channel walls.

Numerical predictions of the Mars Science Laboratory reaction control system jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide planar laser-induced fluorescence data. The steady Reynolds-averaged Navier–Stokes equations using the Baldwin–Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental fluorescence data used for comparison consist of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the fluorescence signal equation, computational flow images were produced and directly compared to the qualitative fluorescence data. Post processing of the experimental and simulation data enabled the jet trajectory to be extracted for a more quantitative comparison. The two-dimensional velocity fields were reconstructed through interpolation of a series of single-component velocity profiles. Each...

Planar laser-induced fluorescence of naturally occurring nitric oxide has been used to provide insight into baseline flow conditions of the Hypersonic Materials Environmental Test System 400 kW arc-heated wind tunnel at NASA Langley Research Center via radial and axial velocity measurements and instantaneous flow-visualization images. This represents both the first flow-tagging velocity measurements and the first application of nitric-oxide planar laser-induced fluorescence flow visualization in an arcjet facility. Results are presented at selected facility run conditions, including some in a simulated Earth atmosphere (75% nitrogen, 20% oxygen, 5% argon) and others in a simulated Martian atmosphere (71% carbon dioxide, 24% nitrogen, 5% argon) for specific bulk enthalpies ranging from 6.5 to 18.4 MJ/kg. Flow-visualization images reveal the presence of large-scale unsteady flow structures and indicate nitric-oxide fluorescence signal over more than 70% of the core flow for specific bulk enthalpies below a...

Nitric-oxide planar laser-induced fluorescence was used to visualize the flow in the wake of a Mars Science Laboratory entry capsule with activated reaction control system jets in NASA Langley Research Center’s 31 in. Mach 10 air tunnel facility. Images were processed using the virtual diagnostics interface method, which brings out the three-dimensional nature of the flow visualization data while showing the relative location of the data with respect to the model. Comparison of wind-on and wind-off results illustrates the effect that the hypersonic crossflow has on the trajectory and structure of individual reaction control system jets. The visualization and comparison of both single and multiple activated jets indicate low levels of jet–jet interaction. Quantitative streamwise velocity was also obtained via molecular tagging velocimetry.

A variation on molecular tagging velocimetry (MTV) [1] designated iodine tagging velocimetry (ITV) is demonstrated. Molecular iodine is tagged by two-photon absorption using an Argon Fluoride (ArF) excimer laser. A single camera measures fluid displacement using atomic iodine emission at 206 nm. Two examples ofMTVfor cold-flowmeasurements areN2OMTV [2] and Femtosecond Laser Electronic Excitation Tagging [3]. These, like most MTV methods, are designed for atmospheric pressure applications. Neither can be implemented at the low pressures (0.1- 1 Torr) in typical hypersonic wakes. Of all the single-laser/singlecamera MTV approaches, only Nitric-Oxide Planar Laser Induced Fluorescence-based MTV [4] has been successfully demonstrated in a Mach 10 wake. Oxygen quenching limits transit times to 500 ns and accuracy to typically 30%. The present note describes the photophysics of the ITV method. Off-body velocimetry along a line is demonstrated in the aerothermodynamically important and experimentally challenging region of a hypersonic low-pressure near-wake in a Mach 10 air wind tunnel. Transit times up to 10 μs are demonstrated with conservative errors of 10%.

We demonstrate a surface-emitting laser, based on III-V semiconductor technology with an integrated metasurface, generating vortex-like coherent state in the Laguerre-Gauss basis.24 We use a first order phase perturbation to introduce a weak orbital anisotropy, based on a dielectric metasurface and non-linear laser dynamics, allowing selecting vortex handedness. Moreover, similarly to linear Doppler Shift, light carrying orbital angular momentum L, scattered by a rotating object at angular velocity, experiences a rotational Doppler shift L. We show that this fundamental light matter interaction can be detected exploiting self-mixing in a vortex laser under Doppler-shifted optical feedback, with quantum noise-limited light detection.25 This will allow us to combine a velocity sensor with optical tweezers for micro-manipulation applications, with high performances, simplicity and compactness. Such high performance laser opens the path to widespread new photonic applications.

Selective Two-Photon Absorptive Resonance Femtosecond-Laser Electronic-Excitation Tagging (STARFLEET), a non-seeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and non-reactive flows. STARFLEET is pumped via a two-photon resonance in N2 using 202.25-nm 100-fs light. STARFLEET greatly reduces the per-pulse energy required (30 μJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and non-reactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N2 and in a premixed flame show good agreement with theoretical velocities and further demonstrate the significantly less-intrusive nature of STARFLEET.

Hydroxyl tagging velocimetry (HTV) is a molecular tagging technique that relies on the photo-dissociation of water vapor into OH radicals and their subsequent tracking using laser induced fluorescence. Velocities are then obtained from time-of-flight calculations. At ambient temperature in air, the OH species lifetime is relatively short (

We present a demonstration of the simultaneous measurement of spatially resolved three-component velocity and temperature in gaseous flow fields using a variant of the vibrationally excited nitric oxide monitoring (VENOM) technique, based on planar laser induced fluorescence and molecular tagging velocimetry methods. Three-component velocity determinations were derived from two-dimensional molecular tagging velocity measurements employing sequential fluorescence image pairs obtained simultaneously by two cameras in stereoscopic configuration. Probing two different rotational states of nitric oxide (Xsup2/sup∏, υsup''/sup=1), produced via fluorescence and collisional quenching from initial excitation to the A Σsup+/sup2 state, for the sequential velocimetry images allows simultaneous determination of the temperature field. Experimental measurements of velocity and temperature across an oblique shock result in mean values within 21 m/s for the three components of velocity and 20 K for planar temperature when compared to oblique shock calculations.

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU (General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuti, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence.

A new seeded velocity measurement technique, N2O molecular tagging velocimetry (MTV), is developed to measure velocity in wind tunnels by photochemically creating an NO tag line. Nitrous oxide “laughing gas” is seeded into the air flow. A 193 nm ArF excimer laser dissociates the N2O to O(1D) that subsequently reacts with N2O to form NO. O2 fluorescence induced by the ArF laser “writes” the original position of the NO line. After a time delay, the shifted NO line is “read” by a 226-nm laser sheet and the velocity is determined by time-of-flight. At standard atmospheric conditions with 4% N2O in air, ∼1000 ppm of NO is photochemically created in an air jet based on experiment and simulation. Chemical kinetic simulations predict 800–1200 ppm of NO for 190–750 K at 1 atm and 850–1000 ppm of NO for 0.25–1 atm at 190 K. Decreasing the gas pressure (or increasing the temperature) increases the NO ppm level. The presence of humid air has no significant effect on NO formation. The very short NO formation time (

Nitric-oxide planar laser-induced fluorescence was used to perform velocity measurements in hypersonic flows by generatingmultiple tagged lines thatfluoresce as they convect downstream.Determination of axial velocitywasmade by application of a cross-correlation analysis of the horizontal shift of individual tagged lines. A single interline, progressive scan-intensified charge-coupled device camera was used to obtain two sequential images of the nitricoxide molecules that had been tagged by the laser. The charge-coupled device allowed for submicrosecond acquisition of both images, resulting in submicrosecond temporal resolution as well as submillimeter spatial resolution (0.5 mm horizontal, 0.7 mm vertical). Quantification of systematic errors, the contribution of gating/ exposure duration errors, and the influence of collision rate on temporal uncertainty were made. This velocity measurement technique has been demonstrated for two hypersonic flow experiments: 1) a reaction control system jet on an Orion crew exploration vehicle wind-tunnel model and 2) a 10 deg half-angle wedge with a 2-mm-tall 4-mmwide cylindrical boundary-layer trip. Mean-velocity uncertainties below 30 m=s (2.7% of the measured average velocity) and single-shot uncertainties below 100 m=s (9.7%of themeasured average velocity) have been obtained in regions with optimal signal intensities using this technique.

Understanding turbulence kinetic energy (TKE) budget in gas–liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. In this study, a molecular tagging velocimetry based on photobleaching reaction was applied to turbulent bubbly flows with sub-millimeter bubbles in a vertical square duct to examine the applicability of the k–e models to the bubbly flows. Effects of bubbles on TKE budget are discussed and a priori tests of the standard and low Reynolds number k–e models are carried out to examine the applicability of these models to the bubbly flows. The conclusions obtained are as follows: (1) The photobleaching molecular tagging velocimetry is of use for validating turbulence models. (2) The bubbles increase the liquid velocity gradient in the near wall region, and therefore, enhance the production and dissipation rates of TKE. (3) The k–e models can reasonably evaluate the production rate of TKE in the bubbly flows. (4) The modulations of diffusion due to the bubbles have different characteristics from the diffusion enhancement due to shear-induced turbulence. Hence, the k–e models fail in evaluating the diffusion rate in the near wall region in the bubbly flows. (5) The k–e models represent the trends of the production, dissipation, and diffusion rates of e in the bubbly flow, although more accurate experimental data are required for quantitative validation of the e equation.

In 2D molecular tagging velocimetry (MTV), tags are written into a fluid flow with a laser grid and imaged at discrete times. These images are analyzed to calculate Lagrangian displacement vectors, often by direct cross correlation. The cross correlation method is inherited from particle imaging velocimetry, where the correlated images contain a random pattern of particles. A template matching method is presented here which takes advantage of the known geometry of laser written tag grids in MTV to achieve better accuracy. Grid intersections are explicitly located in each image by correlation with a template with several linear and rotational degrees of freedom. The template is a continuous mathematical function, so the correlation may be optimized at arbitrary sub-pixel resolution. The template is smooth at the spatial scale of the image noise, so random error is substantially suppressed. Under typical experimental conditions at low imaging resolution, displacement uncertainty is reduced by a factor of 5 compared to the direct cross correlation method. Due to the rotational degrees of freedom, displacement uncertainty is insensitive to highly deformed grids, thus permitting longer delay times and increasing the relative accuracy and dynamic range of the measurement. In addition, measured rotational displacements yield velocity gradients which improve the fidelity of interpolated velocity maps.

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU(General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuchi, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry is characterized for the first time at high-pressure, low-temperature conditions. FLEET signal intensity and signal lifetime data are examined for their thermodynamic dependences; temperatures range from 89 K to 275 K while pressures are varied from 85 kPa to 400 kPa. The FLEET signal intensity is found to scale linearly with the flow density. An inverse density dependence is observed in the FLEET signal lifetime data, with little independent sensitivity to the other thermodynamic conditions apparent. FLEET velocimetry is demonstrated in the NASA Langley 0.3-m Transonic Cryogenic Tunnel. Velocity measurements are made over the entire operational envelope: Mach numbers from 0.2 to 0.75, total (stagnation) temperatures from 100 K to 280 K, and total pressures from 100 kPa to 400 kPa. The velocity measurement accuracy is assessed over this domain of conditions. Measurement errors below 1.15 percent are typical, with slightly decreasing accuracy as temperatures are decreased. Assessment of the measurement precision finds a zero-velocity precision of 0.4 m/s. The precision is observed to have a weak temperature dependence as well, likely a result of the shorter lifetimes experienced at higher densities. The velocity dynamic range is found to have a nominal value of 650. Finally the spatial resolution of the measurements is found to be a dominated by the physical size of the FLEET signal and advective motion. The transverse spatial resolution is found to be 1 mm, while the streamwise spatial resolution is dependent on velocity with a minimum of 2 mm and a maximum of 3.3 mm.

In this work, the effect of experimental parameters on the error levels associated with simultaneous measurement of displacement and temperature using Molecular Tagging Velocimetry/Thermometry (MTV/T) was quantified via simulated images. Images were simulated using Gaussian profile laser lines. Noise was added to the images using a uniform random distribution and a Gaussian random distribution to simulate electronic noise and shot noise, respectively. The results showed that the error levels in the displacement and temperature measurements were inversely related for most experimental parameters including the laser line thickness, fluid temperature and image delay times. It is concluded that the dynamic range of the technique depends on the flow speeds and temperatures and must be determined for each experiment individually. Error levels, for 95% confidence, were found to be better than 0.3°C for temperature and 0.2 pixels for displacement for typical real-world experimental parameters.

The method of laser Doppler visualization and measurement of 2D velocity vector field in flows with minimization of influence of a multi–particle scattering is discussed. The investigated section in the flow is illuminated with a laser sheet. Our approach is based on reception of a pair wise difference of normalized frequency–demodulated images of a laser sheet in light beams scattered in various directions. The 2D velocities field is formed from linear combinations of images of the laser sheet in frequency–demodulated light scattered in directions orthogonal to it.

We report a laser diagnostic technique which relies on planar laser-induced fluorescence of vibrationally excited nitric oxide (NO v=1 ) molecules produced from the 355 nm photodissociation of seeded NO 2 for molecular tagging velocimetry applications. The technique was applied toward an axisymmetric highly underexpanded jet flowfield to yield single-component (streamwise) velocity maps. Detection of the photodissociated NO v=1 molecules would be valuable in flow environments where molecular tagging velocimetry would be highly desirable, but where there are also significant background concentrations of NO. The technique would also be valuable in high-quenching and/or low-velocity flow conditions due to the long-lived nature of the photodissociated NO molecules. Single-shot streamwise velocity uncertainties were about 5% and could be lowered by increasing signal to noise. In addition, the vibrational relaxation of NO was explored in support of a U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative project and it was found that the vibrational decay of NO was heavily dependent on collisional vibrational relaxation with oxygen atom formed through NO 2 photodissociation.

The formation and evolutionary characteristics of the blade tip vortices generated by a hovering rotor were studied using dual-plane stereoscopic particle image velocimetry. The dual-plane stereoscopic particle image velocimetry technique permitted noninvasive measurement of the three components of the velocity field and the nine components of the velocity gradient tensor, a capability not possible with classical particle image velocimetry. The dual-plane stereoscopic particle image velocimetry method is based on coincident flow measurements made over two differentially spaced laser sheet planes. A polarization-based approach was used in which the two laser sheets were given orthogonal polarizations, with filters and beam-splitting optical cubes placed so that the cameras imaged Mie scattered light from only one or other of the laser sheets. The digital processing of the images used a deformation grid correlation algorithm that was optimized for the high velocity gradient and small-scale turbulent flows found inside the blade tip vortices. High-resolution flow imaging of the vortex sheet in the wake behind the rotor blade, combined with detailed turbulence measurements, revealed the presence of several turbulent flow features during the vortex roll-up process that appear to play an important role in its final evolution. The dual-plane stereoscopic particle image velocimetry measurements also included the fluctuating terms involved in the Reynolds-averaged stress transport equations. The results confirm that an isotropic assumption of turbulence is invalid inside blade tip vortices and that stress should not be represented as a linear function of strain. The dual-plane stereoscopic particle image velocimetry measurements were also compared with velocity and turbulence measurements made using a laser Doppler velocimeter system, with good correlation.