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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

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.

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.

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.

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

Proceedings 18th International Symposium on Flow Visualization

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.

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.

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.

We propose a point measurement technique for simultaneous gas temperature and velocity measurement based on thermographic phosphor particles dispersed in the fluid. The flow velocity is determined from the frequency of light scattered by BaMgAl10O17:Eu2+ phosphor particles traversing the fringes like in conventional laser Doppler velocimetry. Flow temperatures are derived using a two-color ratio method applied to the phosphorescence from the same particles. This combined diagnostic technique is demonstrated with a temperature precision of 4%-10% in a heated air jet during steady operation for flow temperatures up to 624 K. The technique provides correlated vector-scalar data at high spatial and temporal resolution.

We exploit the coupled emission-states of a single-chip semiconductor InAs/GaAs quantum-dot laser emitting simultaneously on ground-state (λ(GS) = 1245 nm) and excited-state (λ(ES) = 1175 nm) to demonstrate coupled-two-state self-mixing velocimetry for a moving diffuse reflector. A 13 Hz-narrow Doppler beat frequency signal at 317 Hz is obtained for a reflector velocity of 3 mm/s, which exemplifies a 66-fold improvement in width as compared to single-wavelength self-mixing velocimetry. Simulation results reveal the physical origin of this signal, the coupling of excited-state and ground-state photons via the carriers, which is unique for quantum-dot lasers and reproduce the experimental results with excellent agreement.

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu. Laser-induced fluorescence and phosphorescence properties of gaseous acetone in argon are measured and analyzed in a pressure ranging from 10(5) to 10(2) Pa, with the aim of analyzing by molecular tagging velocimetry gas microflows in rarefied regimes which requires operation at low pressure. Acetone is excited at a wavelength of 266 nm and immediately emits short lifetime fluorescence rapidly followed by long lifetime phosphorescence. At atmospheric pressure, the early phosphorescence intensity is more than 600 times lower than the fluorescence one. The phosphorescence signal is rapidly decreasing with time, closely following a power law. Both fluorescence and phosphorescence signals are decreasing with pressure. The systematic analysis of fluorescence and phosphorescence of acetone molecules shows that although the signal is dramatically reduced at low pressure, the on-chip integration technique and the optimization of the acquisition parameters provide an exploitable signal for molecular tagging velocimetry in rarefied microflows, in a Knudsen number range corresponding to the early slip flow regime.

International audience; In order to develop velocimetry methods for confined geometries, we propose to combine image registration and volumetric reconstruction from a monocular video of the draining of a Hele-Shaw cell filled with water. The cell’s thickness is small compared to the other two dimensions (e.g. 1x400 x 800 mm3). We use a technique known as molecular tagging which consists in marking by photobleaching a pattern in the fluid and then tracking its deformations. The evolution of the pattern is filmed with a camera whose principal axis coincides with the cell’s gap. The velocity of the fluid along this direction is not constant. Consequently, tracking the pattern cannot be achieved with classical methods because what is observed is the integral of the marked molecules over the entire cell’s gap. The proposed approach is built on top of direct image registration that we extend to specifically model the volumetric image formation. It allows us to accurately measure the motion and the velocity profiles for the entire volume (including the cell’s gap) which is something usually hard to achieve. The results we obtained are consistent with the theoretical hydrodynamic behaviour for this flow which is known as the Poiseuille flow.

Experiments in Fluids, 52 (4), ISSN:0723-4864, ISSN:1432-1114

International audience; The goal of the present work is to implement Molecular Tagging Velocimetry (MTV) for the analysis of internal gas flows in mini-channels. A MTV experimental setup has been designed. Tagging and detecting steps are respectively insured by a UV laser and a CCD camera coupled to intensified relay optics. A specific channel with 1 x 5 mm 2 rectangular cross section has been designed and equipped with integrated temperature sensors along its 20 cm length. It has been manufactured in PEEK (PolyEtherEther-Ketone) and Suprasil® optical windows have been integrated for the tagging access. Image processing allows extraction of velocity profiles for a pressure driven steady flow of argon through this channel. These profiles are compared to the theoretical profiles of laminar flows and the accuracy of the method is discussed. The MTV potential for the analysis of internal gaseous flows is commented on, with a discussion on perspectives for velocity measurement in rarefied flows and direct access to slip velocity at the walls.

The Molecular Tagging Velocimetry (MTV) technique has been widely used for analyzing velocity fields in liquid mini- and microflows. Concerning gaseous flows, only few works describe the implementation of MTV at millimetric scale, and these studies are limited to the analysis of external flows, such as jet flows. The goal of the present work is to develop this technique for the analysis of internal gas flows in minichannels. It is a first step toward the visualization of velocity profiles in rarefied conditions, and direct measurement of velocity slip at the walls. A specific experimental setup has been designed. Its features are detailed. Velocity profiles are obtained in a pressure driven steady flow of argon through a long rectangular minichannel of 1.2 × 5 mm2 cross-section and 15 cm length using acetone molecules as tracer. Experiments are carried out at atmospheric pressure, in a laminar continuum flow regime. The accuracy of the method is discussed by comparison between experimental and theoretical velocity profiles. The potential of the MTV technique for analyzing mini or micro gaseous internal flows is commented on. Perspectives of the work for discussing the validity of boundary conditions in the slip flow regime are presented.

We describe a scheme of molecular tagging velocimetry in air in which nitric oxide (NO) molecules are created out of O2 and N2 molecules in the focus of a strong laser beam. The NO molecules are visualized a while later by laser-induced fluorescence. The precision of the molecular tagging velocimetry of gas flows is affected by the gradual blurring of the written patterns through molecular diffusion. In the case of turbulent flows, molecular diffusion poses a fundamental limit on the resolution of the smallest scales in the flow. We study the diffusion of written patterns in detail for our tagging scheme which, at short (micros) delay times is slightly anomalous due to local heating by absorption of laser radiation. We show that our experiments agree with a simple convection-diffusion model that allows us to estimate the temperature rise upon writing. Molecular tagging can be a highly nonlinear process, which affects the art of writing. We find that our tagging scheme is (only) quadratic in the intensity of the writing laser.

Molecular tagging velocimetry using air photolysis and recombination tracking (APART) was applied to the wake flow of a 30° wedge in a small Mach 3 wind tunnel. Average velocities could be determined with a standard deviation down to 0.4% (free stream) and 7% (expansion fan behind wedge). Instantaneous velocities (measurement time 7 µs) could be recorded with a standard deviation down to 2%, limited by the available laser power. The measured velocity profile behind the wedge is compared to a simulation.

In this paper we discuss the possibilities of Molecular Tagging Velocimetry (MTV) method called APART for fundamental turbulence research. We have investigated relevant characteristics of the method such as signal intensities, lifetime of the tagged molecules, spatial resolution and velocity resolution to determine whether the method is suitable for measuring turbulence over a large range of scales. Tagging is done at the outflow of a strongly turbulent free jet. It is shown that 2 mJ/pulse is minimally needed to create LIF in the 4 mm2 'read' area. Tagging lines down to 50 μm cross section can now be reached. Using a Gaussian fit algorithm velocities with an accuracy of 0.8 ms-1 can be calculated.

In molecular tagging velocimetry, patterns of tagged molecules are written in a flow using lasers. They can be lines, crosses or grids. When time proceeds, these patterns are deformed and displaced by the flow. The patterns are followed in time, and from their deformation and displacement, the velocity field can be deduced. In strongly turbulent flow, written lines may be severely deformed or even ruptured, and then, it becomes a challenge to find them in images. We discuss the technique of active contours to trace deformed lines and crosses in turbulence. These patterns can be used to measure the statistical properties of the flow. We illustrate our technique using a variant of molecular tagging in a turbulent flow of air in which NO molecules are formed from N-2 and O-2 using UV laser beams. A short while later, these patterns are visualized by inducing fluorescence of the NO molecules.