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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.

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$10\,\%$.

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

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.

A computer mouse has been modified for use as a low-cost laser Doppler interferometer and used to measure the two-component fluid velocity of a flowing soap film. The mouse sensor contains two vertical cavity surface emitting lasers, photodiodes, and signal processing hardware integrated into a single package, approximately 1 cm2 in size, and interfaces to a host computer via a standard USB port. Using the principle of self-mixing interferometry, whereby laser light re-enters the laser cavity after being scattered from a moving target, the Doppler shift and velocity of scatterers dispersed in the flow are measured. Observations of the boundary layer in a turbulent soap film channel flow demonstrate the capabilities of the sensor.

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.

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.

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.

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

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.

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...

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.

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.

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.

This presentation covers an innovative approach to probing equilibrium and non-equilibrium (non-Maxwell Boltzmann velocity distribution) gas flow via coherent detection of elastically and inelastically scattered laser light.

The two dimensional molecular tagging velocimetry (2D-MTV) has been used to measure velocity fields of the flow in a micro mixer. Instead of commonly used micro particles an optical tagging of the flow has been performed by using a caged dye. The pattern generation is done by imaging a mask for the first time. This allows to generate nearly any imaginable pattern. The flow induces a deformation of the optically written pattern that can be tracked by laser induced fluorescence. The series of raw images acquired in this way were analyzed quantitatively with a novel optical flow based technique. The reference measurements have been carried out allowing to draw conclusions about the accuracy of this procedure. A comparison to the standard technique of μPIV has also been conducted. Apart from measuring flow velocities in microfluidic mixing processes, the spatial distribution of concentration fields for different species has also been measured. To this end, a new technique has been developed that allows spatial measurements from Planar Spontaneous Raman Scattering (PSRS). The Raman stray light of the relevant species has been spectrally selected by a narrow bandpass filter and thus detected unaffectedly by the Raman stray light of other species. The successful operation of this measurement procedure in micro flows will be demonstrated exemplary for a mixing process of water and ethanol.

A correction method based on the Stokes drag law was developed for flow velocimetry using tracer particles. The present method can be used to calculate the corrected velocity from the measured velocity and its spatial gradient without the need to solve the whole flowfield. Velocity measurement was conducted for sonic transverse injection into Mach 1.8 flow using particle image velocimetry. The correction method was applied to the velocity field obtained from the particle image velocimetry data. The corrected data showed the positions and shapes of the shock waves in the measurement region more clearly. The corrected peak velocity downstream of the shock waves became closer to the theoretical values. The jet streamlines calculated from the corrected and original particle image velocimetry data were compared. The streamlines from the corrected data went lower than those from the original data.

A new non-intrusive surface pressure and shear stress diagnostics technique is introduced in the present work. The technique is enabled by the unique ability of one-component molecular tagging velocimetry (1c-MTV) to provide measurements at very high spatial resolution (at every pixel along a tagged line of molecules) in the imme- diate proximity of surfaces. The utility of the new method to provide accurate measurements of the wall pressure and shear stress is demonstrated in an experiment involving a cylinder in cross flow at a Reynolds number of 6000. These measurements agree very well with experimental and com- putational data published in literature. The new technique is particularly advantageous in situations where embed- ding sensor arrays in the wall is impractical or cost/time prohibitive.

The effects of solid particles on the flow structure in the near field region of a coaxial water jet are investigated non-intrusively using molecular tagging velocimetry. Glass beads of 240 μm and specific gravity SG of 2.46 are used at three volume loadings of γv=0.03, 0.06, and 0.09% in the central water jet with a Reynolds number of 4.1×104. Measurements are acquired for four annular to central jet velocity ratios in the range 0.11≤ Uo/Ui≤1.15 at downstream distances up to six inner jet diameters and the results are analyzed for the effects of solid particles on the characteristics of flow. It is found that the addition of particles does not affect the mean fluid velocity profile in this region. The results also indicate a small and moderate enhancement of axial turbulent velocity and radial gradients of velocity fluctuations, respectively, due to the presence of particles.

Laser measurement techniques are becoming more commonly applied to many areas of thermofluids and heat transfer. An area of their application, which presents highly challenging requirements, is in the measurement of aerospace flows. In this case the experimentalist may encounter high mass flowrates, high-speed flows and in some cases high temperatures, and the measurements may also have to be taken with restricted optical access. Such types of flow have already been measured using established laser techniques such as particle image velocimetry, laser Doppler anemometry, laser-induced fluorescence and liquid crystal thermography. Other more recent techniques which have seen application include Doppler global velocimetry and molecular tagging velocimetry for velocity measurement and thermographic phosphor thermometry for surface temperature measurement. This paper reviews the most recent advances in laser techniques and their application to aerospace environments.

We demonstrate a new variation of molecular-tagging velocimetry for hypersonic e ows based on laser-induced e uorescence. A thin line of nitric-oxide molecules is excited with a laser beam and then, after a time delay, a e uorescence image of the displaced line is acquired. One component of velocity is determined from the time of e ight.Thismethodisappliedtomeasurethevelocityproe leinaMach8.5laminar,hypersonicboundarylayerinthe Australian National University’ s T2 free-piston shock tunnel. The single-shot velocity measurement uncertainty in the freestream was found to be 3.5%, based on 90% cone dence. The method is also demonstrated in the separated e ow region forward of a blunt e n attached to a e at plate in a Mach 7.4 e ow produced by the Australian National University’ s T3 free-piston shock tunnel. The measurement uncertainty in the blunt e n experiment is approximately 30%, owing mainly to low e uorescence intensities, which could be improved signie cantly in future experiments. This velocimetry method is applicableto very high-speed e ows that have low collisional quenching of the e uorescing species. It is particularly convenient in facilities where planar laser-induced e uorescence is already being performed.

We present results of a combined experimental computational study of free jet flow produced by a 1 mm (height)×5 mm (span) nominally Mach 2 supersonic jet. Two-dimensional maps of ux, the component of velocity parallel to the principal flow axis, are obtained experimentally, by acetone molecular tagging velocimetry (MTV), and computationally, by the direct simulation Monte Carlo (DSMC) method, at a stagnation pressure and temperature of 10 torr and 300 K, respectively. In all cases, direct comparison of the experimental data and the predictions from DSMC showed excellent agreement, with only minor deviations which, in most cases, can be ascribed to either the inherent uncertainty in the MTV or small uncertainties in the measured wall pressures.

Fluid flow through microtubes is of interest to many industries and there exists a need for detailed measurements of the velocity field. Velocity profile data are critical for momentum, mass, and heat transport analysis, and thus the design of devices utilizing microgeometries. This paper outlines a measurement technique that has led to time-resolved measurements of velocity profiles in microtubes (less than 1,000 μm). The research program was experimental in nature and consisted of an extension of molecular tagging velocimetry to the microscale. Average velocity and rms profile data in the fully developed region, in addition to mass flow rate and pressure drop data, are presented for numerous Reynolds numbers ranging from 600 to 5,000 in a tube of diameter 705 μm.

Supersonic microjets issuing from a convergent rectangular nozzle whose height is 500 µm at its exit are visualized using the acetone-based PLIF (planar laser-induced fluorescence) and MTV (molecular tagging velocimetry) techniques. The jet Reynolds numbers are ranged between 200 and 7000 and the fully-expanded jet Mach-number is set to 1.5. The PLIF visualization results reveal that the jets spread abruptly at a certain location. Instantaneous MTV images are captured in the vicinity of the location where a jet spreads abruptly and the process of the jet collapse is successfully visualized. The distance between the nozzle exit and the jet-collapse location is estimated from each PLIF image. The distance is found to be constant for the Reynolds numbers higher than ~700 and to be inversely proportional to the Reynolds number for the numbers lower than ~700. Moreover, the distance changes abruptly at the Reynolds number of ~700.

Rotational Doppler effect can be used to measure the rotation of particles that are similar in scale to the wavelength of light used, even when the particle in question is subject to a noisy Brownian environment.

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.

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.

‡‡ Streamwise velocity profile behavior in a hypersonic laminar boundary layer in the presence of an isolated roughness element is presented for an edge Mach number of 8.2. Two different roughness element types are considered: a 2-mm tall, 4-mm diameter cylinder, and a 2-mm radius hemisphere. Measurements of the streamwise velocity behavior using nitric oxide (NO) planar laser-induced fluorescence (PLIF) molecular tagging velocimetry (MTV) have been performed on a 20-degree wedge model. The top surface of this model acts as a flat-plate and is oriented at 5 degrees with respect to the freestream flow. Computations using direct numerical simulation (DNS) of these flows have been performed and are compared to the measured velocity profiles. Particular attention is given to the characteristics of velocity profiles immediately upstream and downstream of the roughness elements. In these regions, the streamwise flow can experience strong deceleration or acceleration. An analysis in which experimentally measured MTV profile displacements are compared with DNS particle displacements is performed to determine if the assumption of constant velocity over the duration of the MTV measurement is valid. This assumption is typically made when reporting MTV-measured velocity profiles, and may result in significant errors when comparing MTV measurements to computations in regions with strong deceleration or acceleration. The DNS computations with the cylindrical roughness element presented in this paper were performed with and without air injection from a rectangular slot upstream of the cylinder. This was done to determine the extent to which gas seeding in the MTV measurements perturbs the boundary layer flowfield.

Two-state-laser self-mixing is demonstrated. An additional Doppler-signal originating from the two-state-interaction yields the potential of high-velocity detection. Simulations verify the results and identify the relevant mechanism for the Doppler signal-generation.

We report a precise velocity measurement of a fast atomic lithium beam using a calibrated solid etalon. This technique is important to our program of precision atomic lifetime measurements which includes the 2p states of neutral 7Li. For our lifetime measurements, we use a dye laser crossed with a fast lithium beam to selectively excite the 2 p 2 P 1 2 , 3 2 states from the 2 s 2 S 1 2 ground state, and photon-counting detectors record the decay in flight of the fluorescence. With the laser aligned antiparallel to the atomic beam direction, we use a calibrated solid etalon to measure the Doppler shift of the 2 s 2 S 1 2 → 2 p 2 P 3 2 resonance to be 1742.67(53) GHz. The resulting velocity converts our position scale to a time scale with a precision of 0.03%.

Measurements and computations of the separation and reattachment locations are reported for the steady flow over a SD7003 airfoil at different angles of attack and chord Reynolds number in the range 2 × 104–4 × 104. The experiments are based on multi-line molecular tagging velocimetry, and the computations employ an implicit large eddy simulation approach. Comparisons of experimental results with current computations and previous experiments point to challenges involved in the experimental determination of the separation bubble characteristics. The results also underline the importance of the facility-dependent freestream turbulence level on the experimental data. The collective effect of experimental uncertainty and facility-dependent issues, examined systematically herein, appear to clarify the discrepancy among the various experimental and computational results. The findings also suggest that accurate characterization of the separation bubble over airfoils at low Reynolds number is more difficult than generally recognized and presents a challenge in comparing results between different experiments, and between experiments and computations. Moreover, this complicates the validation of computational data against experiments within this flow regime.

Measurements of mean streamwise velocity, fluctuating streamwise velocity, and instantaneous streamwise velocity profiles in a hypersonic boundary layer were obtained over a 10-degree half-angle wedge model. A laser-induced fluorescence-based molecular tagging velocimetry technique was used to make the measurements. The nominal edge Mach number was 4.2. Velocity profiles were measured both in an untripped boundary layer and in the wake of a 4-mm diameter cylindrical tripping element centered 75.4 mm downstream of the sharp leading edge. Three different trip heights were investigated: k = 0.53 mm, k = 1.0 mm and k = 2.0 mm. The laminar boundary layer thickness at the position of the measurements was approximately 1 mm, though the exact thickness was dependent on Reynolds number and wall temperature. All of the measurements were made starting from a streamwise location approximately 18 mm downstream of the tripping element. This measurement region continued approximately 30 mm in the streamwise direction. Additionally, measurements were made at several spanwise locations. An analysis of flow features show how the magnitude, spatial location, and spatial growth of streamwise velocity instabilities are affected by parameters such as the ratio of trip height to boundary layer thickness and roughness Reynolds number. The fluctuating component of streamwise velocity measured along the centerline of the model increased from approximately 75 m/s with no trip to +/-225 m/s with a 0.53-mm trip, and to +/-240 m/s with a 1-mm trip, while holding the freestream Reynolds number constant. These measurements were performed in the 31-inch Mach 10 Air Tunnel at the NASA Langley Research Center.

Measurements of mean and instantaneous streamwise velocity profiles in a hypersonic boundary layer with variable rates of mass injection (blowing) of nitrogen dioxide (NO2) were obtained over a 10-degree half-angle wedge model. The NO2 was seeded into the flow from a slot located 29.4 mm downstream of the sharp leading edge. The top surface of the wedge was oriented at a 20 degree angle in the Mach 10 flow, yielding an edge Mach number of approximately 4.2. The streamwise velocity profiles and streamwise fluctuating velocity component profiles were obtained using a three-laser NO2->NO photolysis molecular tagging velocimetry method. Observed trends in the mean streamwise velocity profiles and profiles of the fluctuating component of streamwise velocity as functions of the blowing rate are described. An effort is made to distinguish between the effect of blowing rate and wall temperature on the measured profiles. An analysis of the mean velocity profiles for a constant blowing rate is presented to determine the uncertainty in the measurement for different probe laser delay settings. Measurements of streamwise velocity were made to within approximately 120 gm of the model surface. The streamwise spatial resolution in this experiment ranged from 0.6 mm to 2.6 mm. An improvement in the spatial precision of the measurement technique has been made, with spatial uncertainties reduced by about a factor of 2 compared to previous measurements. For the quiescent flow calibration measurements presented, uncertainties as low as 2 m/s are obtained at 95% confidence for long delay times (25 gs). For the velocity measurements obtained with the wind tunnel operating, average single-shot uncertainties of less than 44 m/s are obtained at 95% confidence with a probe laser delay setting of 1 gs. The measurements were performed in the 31-inch Mach 10 Air Tunnel at the NASA Langley Research Center.

Numerical predictions of the Mars Science Laboratory (MSL) reaction control system (RCS) jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide (NO) planar laser-induced fluorescence (PLIF) data. The steady Reynolds Averaged Navier Stokes (RANS) equations using the Baldwin-Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental PLIF data used for comparison consists of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the PLIF signal equation, computational flow images (CFI) were produced and directly compared to the qualitative PLIF 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 distribution of single-component velocity was obtained using molecular tagging velocimetry (MTV). After validating the numerical model, the numerical solution was further examined to gain insight into hypersonic jet-in-crossflow interaction. Future NO-PLIF experiments are proposed based on this analysis.

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

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 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. Although the accuracy of measured velocity and velocity gradients were examined in the previous study, the accuracy of the TKE budget has not been examined yet due to lack of information on the TKE budget. In this study, we measure TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number condition as DNS data provided by Iwamoto, and compare the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. The photobleaching is an irreversible photodegradation of a fluorescent dye induced by intense illumination. Uranin (fluoresceine sodium salt) is used for the fluorescent dye which is photobleached by an intense laser beam (wave length λ = 488.0 nm) to form a molecular tag. Velocity and velocity gradients are evaluated from translation and deformation of the molecular tag between two consecutive images with a time interval δt, respectively. The size of the tag is about 40 to 100 micrometers, and smaller than the Kolmogorov scale in the turbulent flow. The cross-section and length of the duct are 50×50 mm and 1.6 m, respectively. The measured result indicates that the TKE budget evaluated by PB-MTV agrees well with the DNS data. The PB-MTV is also applied to turbulent bubbly flows in the square duct.

Nitric-oxide planar laser-induced fluorescence (NO PLIF) was used to visualize the flow in the wake of a Mars Science Lab (MSL) entry capsule with activated reaction control system (RCS) jets in NASA Langley Research Center s 31-Inch Mach 10 Air Tunnel facility. Images were processed using the Virtual Diagnostics Interface (ViDI) 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 RCS jets. The visualization and comparison of both single and multiple activated RCS jets indicate low levels of jet-jet interaction. Quantitative streamwise velocity was also obtained via NO PLIF molecular tagging velocimetry (MTV).

Laser Doppler velocimetry is used to measure very accurately the velocity and position of a microparticle propelled and guided by laser light in liquid-filled photonic crystal fiber. Periodic variations in particle velocity are observed that correlate closely with modal beating between the two lowest order guided fiber modes.

Measurements and computations of the separation and reattachment locations are reported for the SD7003 airfoil with the chord Reynolds number in the range 2 – 4 ×10. Measurements are based on multi-line Molecular Tagging Velocimetry. Comparisons of experimental results with current computations and previous experiments point to challenges involved in the experimental determination of separation bubble characteristics. Results also bring into focus the facility dependent issues, specifically freestream turbulence, that impacts the experimental data. Preliminary results indicate a downstream/upstream movement of separation/reattachment, respectively, with added freestream turbulence. This observation may explain some of the discrepancies among the various experimental and computational results.

Planar laser-induced fluorescence (PLIF) of naturally occurring nitric oxide (NO) has been used to obtain instantaneous flow visualization images, and to make both radial and axial velocity measurements in the HYMETS (Hypersonic Materials Environmental Test System) 400 kW arc-heated wind tunnel at NASA Langley Research Center. This represents the first application of NO PLIF flow visualization in HYMETS. 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 MJ/kg 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 about 11 MJ/kg, but over less than 10% of the core flow for specific bulk enthalpies above about 16 MJ/kg. Axial velocimetry was performed using molecular tagging velocimetry (MTV). Axial velocities of about 3 km/s were measured along the centerline. Radial velocimetry was performed by scanning the wavelength of the narrowband laser and analyzing the resulting Doppler shift. Radial velocities of +/- 0.5 km/s were measured.

NO2 Molecular Tagging Velocimetry (MTV) at repetition rates as high as 500 kHz is demonstrated in a laboratory scale Mach 5 wind tunnel. The method tags a flow by means of the photo-dissociation process NO2 + hϑ → NO + O, with subsequent interrogation of the NO produced using ordinary Planar Laser Induced Fluorescence (PLIF) imaging. In this work a pulse burst Nd:YAG laser, operating a repetition rate as high as 500 kHz, is used to both generate the required 355 nm photo-dissociation beam and as a pump source for a home built Optical Parametric Oscillator which generates the 226 nm output required for NO PLIF imaging. NO2 MTV images were obtained both upstream and downstream of the bow shock created by cylindrical model in a 1% NO2 seeded Mach 5 N2 flow with free stream static pressure and temperature of approximately 0.7 Torr and 50 K, respectively. Least squares curve fitting of the grayscale spatial distribution in the interrogated images enabled extraction of velocity images with statistical uncertainty of approximately 11 m/sec, or 1.6% of the average free stream flow speed of 719m/sec.

We report the development of a new method of instantaneously measuring three-dimensional velocity profiles and structure in air and oxygen. No seeding of particles, molecules, or atomic species is required. The method combines Raman excitation and laser-induced electronic fluorescence to generate a time-gated image of the moving oxygen molecules.