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A novel configuration of focused laser differential interferometry (FLDI) was developed, simulated, and tested. The new configuration is referred to as cylindrical FLDI or CFLDI. Using cylindrical ...

In experimental fluid dynamics, it would be invaluable to have solids with a refractive index similar to that of water, hence simplifying the use of existing facilities to study flows around complex geometries. Here we report a polymer that is refractive-index-matched with water at room temperature and can be cast in thick specimens. We successfully demonstrated its use with three common laser-based diagnostics: laser-induced fluorescence, particle image velocimetry, and molecular tagging velocimetry.

Molecular tagging velocimetry (MTV) is a nonintrusive optical diagnostic with typically three variants in liquids distinguished by the dye that they rely on: phosphorescent, photochromic, or caged. The latter scheme, initially called photo-activated nonintrusive tracking of molecular motion (PHANTOMM), required both a UV and a dedicated blue laser. Here, we demonstrate caged-dye-based MTV with pulsed UV and green lasers for the first time, simplifying its deployment in most experimental fluid dynamics laboratories.

Femtosecond laser electronic excitation tagging (FLEET) is a nonintrusive optical diagnostic technique that has been demonstrated to provide direct measurement of velocity and other transport prope...

Selective two-photon absorptive resonance femtosecond laser electronic excitation tagging (STARFLEET) velocimetry is demonstrated for the first time in a NASA Langley Research Center wind tunnel wi...

The High Temperature Test Facility (HTTF) is a quarter-scale integral-effect facility designed to study pressurized and depressurized conduction cooldowns (PCC and DCC, respectively) in high temperature, gas-cooled reactors (HTGR). This study focuses on DCC and aims to characterize the gas exchange between the reactor core and the reactor cavity to assess the risk for air ingress and subsequent potential for natural circulation core cooldowns. High-resolution velocity profile measurements are performed at the coolant pipe break location over a long time-scale using molecular tagging velocimetry. This non-intrusive, laser-based technique is applied for the first time in the field of nuclear thermal hydraulics, providing new insights for understanding the underlying phenomena at play during a DCC event. Such data are also valuable for validation of numerical simulations such as Reactor Excursion and Leak Analysis Program (RELAP). Results are reported here for various gas mixtures of helium (the coolant) and nitrogen (surrogate for air) to study the effect of density ratio. The short transient (30 s) lock-exchange phase is well captured with helium flow velocity between 0.8 and 2 m/s. The flow is shown to persist over much longer time-scales (hours) at a velocity of about 0.2 m/s, a likely consequence of gas mixing in the hot leg and in the lower plenum of the reactor. Flow visualization at the exit of the hot leg shows shear instabilities, which supports the hypothesis of significant mixing.

Various physical principles can be utilized for molecular tagging velocimetry (MTV). In this article, the capabilities and limitations of photoisomerization in MTV are studied. The molecular rotor 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) exists in two isomers. Although the E isomer, which is present in the absence of light, yields a fluorescent behavior, the Z isomer is a photoproduct with no detectable luminescence. This regenerative behavior is utilized for MTV. The two time constants define the experimental limitations of the method for tagging and recovery. It is shown that, whereas the tagging time constant is in the range of milliseconds to seconds, the recovery time is in the range of minutes to hours, enabling measurements of slow flow phenomena. A flow in a rectangular channel with a Reynolds number of 9×10−3 and a Péclet number of 1009 is investigated. Furthermore, the addition of methyl-β-cyclodextrin (methyl-β-CD) to the solution increases the fluorescent quantum yield and the isomeric rate constants, supporting the hypothesis of a complex formation with CCVJ. It is shown that photoisomeric MTV is suitable for analyzing slow flow regimes in the liquid phase. The addition of methyl-β-CD increases the dynamic range of the results. In comparison with existing MTV principles, photoisomerization has the advantage of an intrinsic regenerative behavior. The cost-efficient and simple setup make it an alternative in biological and environmental flow studies., Physical Review Fluids, 6 (4), ISSN:2469-990X

Shock-wave–turbulence interactions are important problems with ubiquitous applications in high-speed flight and propulsion. The complex physical processes during the interaction are not fully understood, where contemporary high-fidelity numerical simulations have brought into question classical linear interaction analyses (LIA). The differences are most pronounced at high Mach number (2). The objective of this study was to experimentally examine the role of a normal shock wave on the modification of velocity and temperature fluctuations to provide an empirical basis to help close the emerging knowledge gap between classical and contemporary theories. The experiments were performed in a pulsed wind tunnel facility at Mach 4.4. The free-stream disturbances provided the test bed for the study. A Mach-stem normal shock was generated through the interaction of two mirrored oblique shock waves. Molecular tagging velocimetry and two-line planar laser induced fluorescence thermometry were conducted upstream and downstream of the normal shock wave and the fluctuating intensities were compared. The measured axial velocity fluctuation amplification factor was nominally 1.1–1.2 over the Reynolds number range tested. The measured values were more consistent with LIA than contemporary theory. The temperature fluctuation amplification factor was found to vary between 3.0 and 4.5, where the lowest Reynolds number condition saw the highest free-stream disturbances and largest amplification. The free-stream fluctuations were primarily in the entropic mode, which is believed to lead to the significantly higher amplification of the entropic mode reported in these measurements.

The present study focuses on the collapse length of compressible rectangular microjets whose Mach numbers range between 0.3 and 1.5. The microjets are created from a convergent rectangular nozzle whose height is 500 μm at its exit. The techniques of planar laser-induced fluorescence (PLIF) and molecular tagging velocimetry (MTV) are used to visualize the microjets. The PLIF images reveal that each microjet spreads abruptly at a certain location. It is confirmed from the instantaneous MTV images that this location corresponds to the location where the jet starts to collapse. The jet collapse length, which is defined as a distance between the nozzle exit and jet collapse location, is estimated from the PLIF image. The plot of the collapse length versus the jet Reynolds number reveals that the collapse length is inversely proportional to the Reynolds number for subsonic and ideally expanded microjets. On the other hand, the collapse length for underexpanded microjets is almost uniform when the jet Reynolds number is higher than a certain value (~ 103) although the length is inversely proportional to the Reynolds number for the lower Reynolds numbers. To clarify the reason why such peculiarities appear in the underexpanded microjets, the numerical flow simulations are carried out. The results reveal that the collapse length remains constant as long as a jet screech occurs. Consequently, the collapse length of the screeching jet is related to the feedback length in the jet. The critical Reynolds numbers at which laminar-turbulent transition occurs are estimated from the collapse lengths of the microjets without screeching and plotted against the convective Mach number. It is found that the critical Reynolds number increases with the convective Mach number.

A new scheme is described and characterized to generate very fine structured illumination patterns that can be used to significantly improve the spatial resolution of molecular tagging velocimetry (MTV) and other techniques that rely on structured illumination. MTV is particularly suited to measure spatial gradients of velocity as well as second-order derivatives, but has traditionally suffered from a limited resolution. MTV has gained a broad adoption in high-speed gas dynamics and has also been applied to a lesser extent in aqueous flows. The present work makes use of the optical Talbot effect, a near-field diffraction phenomenon, to generate an array of quasi-parallel beamlets with very flexible beam spacing and diameter. Both pitch and width have been demonstrated down to the order of 10 $${\upmu \mathrm {m}}$$, which is about ten times smaller than that performed in previous MTV studies. Structured illumination with Talbot effect has potential for even smaller sizes. Several parameters are extensively characterized, including beamlet size, spacing, and persistence. Optimization of the optical components is also discussed, including lasers and lenses. Simple analytical models enable to assist in the design of experiments with Talbot effect. The technique is demonstrated with 1D multi-line and 2D grids with simple MTV tests in water. The selected caged dye is used for the first time in MTV and conveniently enables the use of green PIV lasers to read the tagged lines.

International audience; Direct measurements of the slip velocity in rarefied gas flows produced by local thermodynamic non-equilibrium at the wall represent crucial information for the validation of existing theoretical and numerical models. In this work, molecular tagging velocimetry (MTV) by direct phosphorescence is applied to argon and helium flows at low pressures in a 1-mm deep channel. MTV has provided accurate measurements of the molecular displacement of the gas at average pressures of the order of 1 kPa. To the best of our knowledge, this work reports the very first flow visualizations of a gas in a confined domain and in the slip flow regime, with Knudsen numbers up to 0.014. MTV is cross-validated with mass flowrate measurements by the constant volume technique. The two diagnostic methods are applied simultaneously, and the measurements in terms of average velocity at the test section are in good agreement. Moreover, preliminary results of the slip velocity at the wall are computed from the MTV data by means of a reconstruction method.

Femtosecond laser electronic excitation tagging (FLEET) is a new non-seed molecular tagging velocity measurement method. Fluorescent light excited by femtosecond laser is used as tagging filament, and will disturb local temperature which may affect the flow field measurement precision. In this paper, the Rayleigh scattered of nanosecond laser is used to measure the ambient air temperature changing. The feasibility of this measurement method is proved. The key data processing of low signal to noise ratio (SNR) images including the filament segmentation, noise filtering, skeleton extraction of filament position is tested. The preliminary test on the heating process between 1us and 800us is demonstrated. The limitations of this measurement method due to the gas media condition changing are discussed briefly.

Helium-4 in the superfluid phase (He II) is a two-fluid system that exhibits fascinating quantum hydrodynamics with important scientific and engineering applications. However, the lack of high-precision flow measurement tools in He II has impeded the progress in understanding and utilizing its hydrodynamics. In recent years, there have been extensive efforts in developing quantitative flow visualization techniques applicable to He II. In particular, a powerful molecular tagging velocimetry (MTV) technique, based on tracking thin lines of He$^*_2$ excimer molecules created via femtosecond laser-field ionization in helium, has been developed in our lab. This technique allows unambiguous measurement of the normal-fluid velocity field in the two-fluid system. Nevertheless, there are two limitations of this technique: 1) only the velocity component perpendicular to the tracer line can be measured; and 2) there is an inherent error in determining the perpendicular velocity. In this paper, we discuss how these issues can be resolved by advancing the MTV technique. We also discuss two novel schemes for tagging and producing He$^*_2$ tracers. The first method allows the creation of a tagged He$^*_2$ tracer line without the use of an expensive femtosecond laser. The second method enables full-space velocity field measurement through tracking small clouds of He$^*_2$ molecules created via neutron-$^3$He absorption reactions in He II., 15 pages, 6 figures

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 ($DR=6.5\,\%$to$26\,\%$), as realized via polymer injection at the channel surface. 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 (${\gtrsim}850$data points per wall-normal profile), 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 Whiteet al. (J. Fluid Mech., vol. 834, 2018, pp. 409–433) that examined the mean dynamical structure of polymer drag-reduced channel flow at friction Reynolds numbers up to$1000$. Consistently, the present mean velocity profiles indicate that the extent of the logarithmic region diminishes with increasing polymer concentration, while statistically significant increases in the logarithmic profile slope begin to occur for drag reductions less than$15\,\%$. Profiles of the r.m.s. streamwise velocity indicate that the maximum moves farther from the wall and increases in magnitude with reductions in drag. Similarly, with increasing drag reduction, the profile of the combined Reynolds and polymer shear stress exhibits a decrease in its maximum value that also moves farther from the wall. Correlations are presented that estimate the location and value of the maximum r.m.s. streamwise velocity and combined Reynolds and polymer shear stress. Over the range of$DR$investigated, these effects consistently exhibit approximately linear trends as a function of$DR$. The present measurements allow reconstruction of the mean momentum balance (MMB) for channel flow, which provides further insights regarding the physics described in the study by Whiteet al. In particular, the present findings support a physical scenario in which the self-similar properties on the inertial domain identified from the leading-order structure of the MMB begin to detectably and continuously vary for drag reductions less than$10\,\%$.

Molecular tagging velocimetry (MTV) is a non intrusive velocimetry technique based on laser spectroscopy. It is particularly adequate in challenging gas flow conditions encountered in thermal hydraulics where particle-based methods such as particle image (or tracking) velocimetry do not perform well. The main principles for designing and operating this diagnostic are presented as well as a set of gases that have been identified as potential seeds. Two gases (H2O and N2O) have been characterized extensively for thermodynamic conditions ranging from standard temperature and pressure to environments encountered in integral effect test (IET) facilities for high temperature gas reactors (HTGR). A flexible, modular, and trans- portable laser system has been designed and demonstrated with H2O and N2O seed gases. The laser system enabled to determine the optimum excitation wavelength, tracer concentration, and timing parameters. Velocity precision and thermodynamic domain of applicability are discussed for both tracers. The spectroscopic nature of the diagnostics enables to perform first principle uncertainty analysis which makes it attractive for validating numerical models. MTV is demonstrated for two flows. First, in blow down tests with H2O seed, the unique laser system enables one of the largest dynamic ranges reported to date for velocimetry: 5,000:1 (74 dB). N2O-MTV is then deployed in-situ in an IET, the high-temperature test facility at Oregon State University during a depressurized condition cooldown (DCC) event. Data enable to gain insights into flow instabilities present during DCC. Thus, MTV shows a strong potential to gain fundamental understanding of gas flows in nuclear thermal hydraulics and to provide validation data for numerical solvers.

Nanoparticle tracking velocimetry (NPTV), a novel particle tracking velocimetry method using nano-sized tracer particles, was investigated as a hybrid tool for the determination of the flow velocity in a microscale area. In contrast to the common optical microscopic velocimetry techniques with submicron/micron sized tracer particles, the NPTV method is based on the observation of the motion of bright spots attributed to the light scattering from individual nano-sized tracer particles. We observed light scattering from individual nano-sized tracers induced by high-intensity laser light, which are difficult to observe by direct observation of tracers using optical microscope in common flow velocimetry systems. Although the PTV/PIV method using fluorescent nano-tracer such as quantum dots make it possible to use nano-tracer in the flow velocity assessment, our light scattering observable optical system can be applied to nano-tracers of any kinds of materials. Because the determination of the flow velocity in the NPTV method is affected by the random Brownian motion of the nano-sized tracers, we demonstrated that reliable flow velocity values could be successfully obtained by ensemble averaging the displacement of each particle with an accurate correction of the effect of the Brownian motion of the nano-sized tracers. The NPTV method developed in this study can be utilized for various liquid samples owing to non-limited variety of nano-sized tracer materials. In addition, because the velocimetry by nano-sized tracers enables the determination of the flow velocity in a narrower area than that studied with submicron/micron tracers, this novel method could play an important role in the flow velocity measurement in the design of nanoscale/microscale reaction/mixing channel processes.

Objectives: To provide comprehensive literature survey of laminar separation bubble over a low Reynolds number airfoil with reference to the conventional and non-conventional experimental technique using wind tunnel. Methods/ Statistical Analysis: For more convenient understanding, the various wind tunnel experimental techniques and advanced research progress in this field is presenting in table format. The table shows characteristics of laminar separation bubble over airfoil at different Reynolds number for various conditions. The table critiques the proof-based entire data about the Laminar Separation Bubble (LSB) traits (height and period), go with the flow traits differentiated, transition and reattachment locations and go with the flow visualization over airfoil were experimentally discovered at various angles of attack and Reynolds range, respectively. Findings: The current evidence supports the view that laminar separation bubble characteristics, behaviour and its effect over an airfoil. Most studies in this field have been done experimentally using Wind Tunnel with various techniques i.e., Surface Oil Flow Technique, Particle Image Velocimetry (PIV), Infrared Thermography (IT), Force Measurement and Hot-Wire Experiments, Smoke-Wire Experiment, Multi-line Molecular Tagging Velocimetry, Volumetric Three-Component Velocimetry (V3V), Time-resolved surface pressure sensor arrays, ESP (Electronically Scanned Pressure), Embedded Laser Doppler Velocimetry (ELDV), Fast Fourier Transform (FFT). Application/ Improvements: This paper describes the various techniques and characteristics of LSB which will be beneficial to design the low Reynolds number airfoil in order to minimize the drag and increases the aerodynamic efficiency for industrial applications. Keywords: Flow Visualization Techniques, Low Reynolds Number, Laminar Separation Bubble Characteristics, Wind Tunnel Experiments

A novel, to the best of our knowledge, optical arrangement is evaluated for performing single-shot femtosecond laser electronic excitation tagging in a 16-point grid (Grid-FLEET) with single-ended optical access. The optical arrangement includes a diffractive optical element beam splitter to produce a grid of laser beams in a simplified, flexible, and efficient manner for tracer-free multi-component molecular tagging velocimetry in a two-dimensional field. Analysis of the optical element with respect to beam forming is described, and Grid-FLEET measurements are evaluated relative to the precision of previously described single-point FLEET measurements using Lagrangian tracking for flow in a laminar jet and around a sharp corner. Utilizing a conventional 1-kHz laser source coupled to a high-speed intensified camera, it is also feasible to achieve measurement rates of 100 kHz or higher by mapping the Lagrangian grid to one or more Eulerian measurement points. The data further indicate that enhancement of the instantaneous vector fields and spatial velocity gradients can be analyzed to enhance the understanding of multi-dimensional flow physics in applications in which the use of tracers may be difficult and where multi-directional optical access may be limited.

While photobleaching can be detrimental in applications focusing on fluorescence as it lowers the signal strength, it can advantageously provide non-fluorescent tracers in a fluorescent flow and hence be implemented in tracking techniques such as molecular tagging velocimetry (MTV). The photobleaching of rhodamine 6G under a single UV pulse is described with a simple three-level model of fluorescence. It offers a convenient estimate of the order of magnitude of irradiance required to observe significant photobleaching for a uniform beam. In an effort to improve the application to MTV, analytical formulas enable to determine the photobleached signal strength from a Gaussian UV laser beam overlapping with a green laser sheet and imaged from the side, as well as the apparent width of the imaged photobleached line.

Molecular tagging velocimetry is a little-intrusive technique based on the properties of specific molecules able to emit luminescence once properly excited. Several variants of this technique have been successfully developed for analyzing external gas flows or internal gas flows in large systems. There is, however, very few experimental data on molecular tagging velocimetry for gas flows in mini or microsystems, and these data are strongly affected by the molecular diffusion of the tracer molecules. In the present paper, it is demonstrated that the velocity field in gas microflows cannot be directly deduced from the measured displacement field without taking into account Taylor dispersion effects. For that purpose, the benchmark case of a Poiseuille gas flow through a rectangular channel 960 μm in depth is experimentally investigated by micro molecular tagging velocimetry using acetone vapor as a molecular tracer. An appropriate reconstruction method based on the advection diffusion equation is used to process the data and to correctly extract the velocity profiles. The comparison of measured velocities with flowrate data and with theoretical velocity profiles shows a good agreement, whereas when the reconstruction method is not implemented, the extracted velocity field exhibits qualitative and quantitative anomalies, such as a non-physical slip at the walls. The robustness of the reconstruction method is demonstrated on flows with light and heavy molecular species, namely helium and argon, at atmospheric as well as at lower pressure conditions, for which diffusion effects are stronger. The obtained results represent an encouraging step for future analysis of rarefied gas microflows.

Biacetyl phosphorescence has been the commonly used molecular tagging velocimetry (MTV) technique to investigate in-cylinder flow evolution and cycle-to-cycle variations in an optical engine. As the phosphorescence of biacetyl tracer deteriorates in the presence of oxygen, nitrogen was adopted as the working medium in the past. Recently, nitrous oxide MTV technique was employed to measure the velocity profile of an air jet. The authors here plan to investigate the potential application of this technique for engine flow studies. A possible experimental setup for this task indicated different permutations of image signal-to-noise ratio (SNR) and laser line width. In the current work, a numerical analysis is performed to study the effect of these two factors on displacement error in MTV image processing. Also, several image filtering techniques were evaluated and the performance of selected filters was analyzed in terms of enhancing the image quality and minimizing displacement errors. The flow displacement error without image preprocessing was observed to be inversely proportional to SNR and directly proportional to laser line width. The mean filter resulted in the smallest errors for line widths smaller than 9 pixels. The effect of filter size on subpixel accuracy showed that error levels increased as the filter size increased.

To gain insight into the roll wave instability in particulate flow systems, the spatiotemporal velocities of particle and liquid phases in a clear-fluid layer are simultaneously measured by means of a photobleaching molecular tagging velocimetry (PB-MTV). A grid pattern tag is generated by intermittent irradiation of two intense laser beams, which facilitates measurement of a wall vicinity flow induced by the Boycott effect. To cope with a plurality of tagline intersections, which largely deform in shear flows, a novel spatial correlation technique is developed with a template-matching method. A priori study using artificial images of tags in a temporally oscillating flow is performed to examine the effects of the signal to noise (S/N) ratio and the degree of unsteadiness on the accuracy of the velocimetry. An experimental validation of the present PB-MTV is also conducted on the statistical values of the streamwise velocity through comparison with a laser-induced fluorescence and particle tracking velocimetry (LIF-PTV). A series of experiments are carried out using a suspension, which consists of tap water and spherical hollow light particles, varying the particle diameter and the particle volume fraction. The liquid velocity fluctuation tends to increase with decrease in the inter-particle distance compared to the thickness of the clear-fluid layer. We conclude that the roll wave instability occurs only under the fluid-like condition, where the suspension behaves as a continuum, and is affected by the sharpness of flow stratification between the clear-fluid and particle-rich phases.

A dual-dye-laser Laser-induced Fluorescence (LIF) system is established for flow field investigations in JF-10, a H 2 /O 2 -detonation-driven shock tunnel. The shock tunnel generates 400 mm-wide hypersonic flow with total enthalpy of 16.1 MJ/kg and static pressure at only105Pa, where LIF measurement is extremely challenging. In this LIF system, the laser sheet of a conventional planar LIF is rotated 90 degrees along its propagation direction, so that the fluorescence signal collected by the camera concentrates on a sharp line. The S/N ratio is increased, thus single-shot measurements are achievable. The rotational temperature of the flow measured by two-line thermometry (TLT) with simultaneous excitation of two $\gamma$ (0,0) transitions of NO at distinct rotational levels. Compared with single-dye-laser set-up, the current system is less affected by the fluctuation of the flow field from each facility operation, thus capable of reflect the dramatic temperature change across the shock layer. With precise control of the proper of the laser, and through evaluation of the error of the data, the temperature measurement and numerical simulation in the flow is consistent. Meanwhile, the velocity of the flow is measured by molecular tagging velocimetry (MTV) with a long-life fluorescence excitation, and the result agrees with the expected value. The UV LIF technique is proven useful in flow diagnostics in high-enthalpy regime.

We present the development of molecular tagging velocimetry (MTV) capability for the NASA Zero Boil-Off Tank (ZBOT) experiment. The challenges of chemical solubility posed by the fluorocarbon working fluid and small range of velocities in the experiment are met using MTV based on the photobleaching of coumarin-153. The feasibility of measurements is demonstrated by one-component velocimetry (1c-MTV) in a setup with geometric characteristics that mimic the actual ZBOT test section.

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.

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen ( N 2 ) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves “writing” via photo-dissociating nitrogen molecules and “tracking” by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.