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A new methodology to analyze two-component molecular tagging velocimetry image pairs is presented. Velocity measurements with high spatial resolution are achieved by determining grid displacements at the intersections as well as along the grid lines using a multivariate adaptive regression splines parameterization along the segments connecting adjacent grid intersections. The methodology can detect the orientation of the grid, contains redundant steps for increased reliability, and handles cases where parts of the grid are missing, indicating potential for automation. Initial demonstration of the algorithm’s performance was illustrated using synthetic data sets derived from Computational Fluid Dynamics simulations and compared to Hough-transform and cross-correlation methodologies. Besides providing comparable results in terms of precision and accuracy to previously reported methodologies, the analysis of images by the proposed methodology results in significantly increased spatial resolution of the flow displacement determinations along the grid lines with comparable precision and accuracy. This methodology’s ability to handle different grid orientations without modifications was assessed using synthetic datasets with grids formed by sets of parallel lines at 90, 45, and 30 degrees from the vertical axis. Comparable results in terms of precision and accuracy were obtained across grid orientations, with all uncertainties below 0.1 pixel for images with signal-to-noise levels exceeding 5, and within 0.5 pixel for the noisiest image sets.

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.

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.

We report improvements to the molecular tagging velocimetry and thermometry (MTV&T) technique for the simultaneous measurement of velocity and temperature fields in fluid flows. A phosphorescent molecule, which can be turned into a long lifetime tracer upon excitation by photons of appropriate wavelength, is used as a tracer for both velocity and temperature measurements. A pulsed laser is used to 'tag' the regions of interest, and those tagged regions are imaged at two successive times within the lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules provides the estimate of the fluid velocity vector. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the two images. In relation to the original molecular tagging thermometry work of Thompson and Maynes (2001 J. Fluid Eng. 123 293–302), the improvements reported here are the use of lifetime imaging as a ratiometric method to enhance the robustness and accuracy of temperature measurements and the extension of the technique to simultaneous whole-field planar mapping of velocity and temperature fields. Compared with other simultaneous velocity and temperature measurement techniques such as combined PIV-LIF (Sakakibara et al 1997 Int. J. Heat Mass Transfer 40 3163–76, Grissino et al 1999 Proc. 3rd Int. Workshop on Particale Image Velocimetry (Santa Barbara, CA, USA, 16–18 September 1999)) and the DPIV/T technique (Park et al 2001 Exp. Fluids 30 327–38), this method accomplishes the same objectives but with a completely molecular-based approach. Because of its molecular nature, issues such as tracking of the flow by the seed particles and the thermal response of the thermal tracer particles are eliminated. In addition, the use of a single molecular tracer and a dual-frame CCD camera provides for a much reduced burden on the instrumentation and experimental set-up. The implementation and application of the new technique are demonstrated by conducting simultaneous velocity and temperature measurements in the wake region of a heated circular cylinder at a Richardson number of 0.36, a value large enough for the buoyancy effects to potentially influence the flow.

We report a new molecular tagging velocimetry method for use in pure nitrogen gas with a small impurity of water vapor on the order of 0.1%. This two laser method can produce a 25 mm long tag line of NH (imidogen) radicals at a standoff of 1 m which is imaged by laser-induced fluorescence after a delay of a few microseconds. The signal-to-noise ratio of the tag image can exceed 50 at standard temperature and pressure, allowing excellent spatial resolution. This new method called imidogen tagging velocimetry is useful for measuring spatially resolved velocity in flow facilities which run pure nitrogen rather than air, contain or can be seeded with trace water vapor, and in which other seeded or intrusive velocimetry methods are impractical.

Molecular tagging velocimetry (MTV) involves intensive data reduction that extracts flow velocity information from the Lagrangian tracking of phosphorescing fluid material. A computationally efficient algorithm for the data reduction is thus of practical interest for processing large MTV data sets. We were motivated by this consideration into developing a simplified version of the spatial correlation technique, the decoupled spatial correlation technique, in an effort to seek a balance between accuracy and efficiency. By Taylor series analysis it is shown that, if the Lagrangian displacement vector can be roughly pre-determined, the two components in the displacement vectors that have to be solved simultaneously using the spatial correlation technique can now be determined independently in two orthogonal directions. This decoupling results in about an order of magnitude decrease in the CPU time required. An accuracy estimate based on artificial images that follow the motion of a line vortex indicates that the technique can determine displacements to within 0.08 pixel. This technique was also used to process MTV images acquired in a cross stream plane of the transverse jet. This flow is characterized by a large scale counter-rotating vortex pair (CVP). The velocity fields obtained clearly show the existence of this CVP, which provides further verification of this technique.

Two complementary unseeded molecular flow tagging techniques for gas-flow velocity field measurement at low and high temperature are demonstrated. Ozone tagging velocimetry (OTV) is applicable to low-temperature air flows whereas hydroxyl tagging velocimetry (HTV) is amenable to use in high-temperature reacting flows containing water vapour. In OTV, a grid of ozone lines is created by photodissociation of O_2 by a narrowband 193 nm ArF excimer laser. After a fixed time delay, the ozone grid is imaged with a narrowband KrF laser sheet that photodissociates the ozone and produces vibrationally excited O_2 that is subsequently made to fluoresce by the same KrF laser light sheet via the O_2 transition B^3Σ_u^-(v'=0,2) ← X^3Σ_g^-(v"=6,7). In HTV, a molecular grid of hydroxyl (OH) radicals is written into a flame by single-photon photodissociation of vibrationally excited H_2O by a 193 nm ArF excimer laser. After displacement, the OH tag line position is revealed through fluorescence caused by OH A^2Σ^+_-X^2Π (3←0) excitation using a 248 nm tunable KrF excimer laser. OTV and HTV use the same lasers and can simultaneously measure velocities in low and high temperature regions. Instantaneous flow-tagging grids are measured in air flows and a flame. The velocity field is extracted from OTV images in an air jet using the image correlation velocimetry (ICV) method.

A new technique for the simultaneous measurement of velocity and concentration fields is described. We describe here applications in liquid-phase flows, but the methodology can be extended to gas-phase flows with appropriate tracers. In this single-laser, two-tracer approach, molecular tagging velocimetry (MTV) based on the use of a phosphorescent compound is combined with laser induced fluorescence (LIF) using fluorescein as a tracer. Results show that one can design experiments with minimal cross-talk between the LIF and MTV signals. Applications of the simultaneous MTV-LIF technique are demonstrated by performing simultaneous flow visualization and vorticity measurements in a low Reynolds number forced wake and simultaneous velocity-concentration measurements in a turbulent mixing layer. Preliminary data on the mean and RMS fluctuation of velocity and concentration are presented, together with the correlation between velocity and concentration fluctuations.

A technique is proposed to correct the bias error associated with one-component molecular tagging velocimetry (1c-MTV). 1c-MTV is typically used to capture the velocity component normal to a single or multiple lines of tagged fluid molecules with very high spatial resolution. However, the measurements are affected by an inherent error, which arises from the presence of a velocity component parallel to the lines. In the present study, a Taylor-series-based approach is used to derive a general mathematical expression for this error. The derived expression, which is validated using simulated MTV measurements in laminar channel flow with suction/blowing, is used as the basis for introducing a correction method involving the acquisition of MTV images at multiple time delays. To examine the effectiveness of this ‘multi-time-delay’ approach for correcting 1c-MTV data, an experiment is conducted whereby a known bias error is deliberately imposed on measurements in laminar channel flow. The corrected measurements agree with the true velocity profile to better than 2%, thus validating the correction method.

Recent advances in ultra-high repetition rate (100?kHz and above) laser diagnostics for fluid dynamic measurements are reviewed. The development of the pulse burst laser system, which enabled several of these advances, is described. The pulse burst laser system produces high repetition rate output by slicing the output of a low power continuous wave laser and passing the resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systems have been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generated at between 50 and 1000?kHz. Combined with the capabilities of several types of commercially available high-speed cameras, these systems have been used to make a wide variety of high repetition rate and 3D flow measurements. Several examples of various high repetition rate laser diagnostics are described, including flow visualization, filtered Rayleigh scattering, planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence, molecular tagging velocimetry and 3D flow visualization.

Recent progress made in the development of novel molecule-based flow diagnostic techniques, including molecular tagging velocimetry (MTV) and lifetime-based molecular tagging thermometry (MTT), to achieve simultaneous measurements of multiple important flow variables for micro-flows and micro-scale heat transfer studies is reported in this study. The focus of the work described here is the particular class of molecular tagging tracers that relies on phosphorescence. Instead of using tiny particles, especially designed phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, are used as tracers for both flow velocity and temperature measurements. A pulsed laser is used to 'tag' the tracer molecules in the regions of interest, and the tagged molecules are imaged at two successive times within the photoluminescence lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules provides the estimate of the fluid velocity. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the acquired two phosphorescence images. The implementation and application of the molecular tagging approach for micro-scale thermal flow studies are demonstrated by two examples. The first example is to conduct simultaneous flow velocity and temperature measurements inside a microchannel to quantify the transient behavior of electroosmotic flow (EOF) to elucidate underlying physics associated with the effects of Joule heating on electrokinematically driven flows. The second example is to examine the time evolution of the unsteady heat transfer and phase changing process inside micro-sized, icing water droplets, which is pertinent to the ice formation and accretion processes as water droplets impinge onto cold wind turbine blades.