Ross A. Burns

Femtosecond Laser Tagging Characterization of a Sweeping Jet Actuator Operating in the Compressible Regime

A sweeping jet (SWJ) actuator operating over a range of nozzle pressure ratios (NPRs) was characterized with femtosecond laser electronic excitation tagging (FLEET), single hot-wire anemometry (HWA) and high-speed/phase-averaged schlieren. FLEET velocimetry was successfully demonstrated in a highly unsteady, oscillatory flow containing subsonic through supersonic velocities. Qualitative comparisons between FLEET and HWA (which measured mass flux since the flow was compressible) showed relatively good agreement in the external flow profiles. The spreading rate was found to vary from 0.5 to 1.2 depending on the pressure ratio. The precision of FLEET velocity measurements in the external flow field was poorer (is approximately equal to 25 m/s) than reported in a previous study due to the use of relatively low laser fluences, impacting the velocity fluctuation measurements. FLEET enabled velocity measurements inside the device and showed that choking likely occurred for NPR 2.0, and no internal shockwaves were present. Qualitative oxygen concentration measurements using FLEET were explored in an effort to gauge the jets mixing with the ambient. The jet was shown to mix well within roughly four throat diameters and mix fully within roughly eight throat diameters. Schlieren provided visualization of the internal and external flow fields and showed that the qualitative structure of the internal flow does not vary with pressure ratio and the sweeping mechanism observed for incompressible NPRs also probably holds for compressible NPRs.

Experimental and Computational Studies of Mixing in Supersonic Flow

A preliminary combined experimental and computational investigation is conducted on the mixing characteristics of a strut-based hypermixer in a Mach 3 freestream. The hypermixing flow-field is generated by an injection pylon with expansive wedges to enhance the streamwise vorticity. Two different scalar visualization techniques are used to examine the underlying mixing processes. Planar laser scattering of condensed carbon dioxide gas is used to visualize the freestream flow, whereas two-photon planar laser-induced fluorescence (PLIF) of krypton gas is used to mark the injected jet fluid. The experimental results are compared directly to a large-eddy simulation (LES) of the same flow-field. The results obtained are complimentary because the experimental data can aid in the validation of LES models, and the simulations provide information about the thermodynamic property variations that affect the interpretation of the krypton PLIF signal.

Simultaneous krypton PLIF, LII and PIV measurements in a sooting jet flame

Experiments to measure the concentration of a conserved passive scalar (krypton), with the ultimate aim to determine the mixture fraction, by means of planar laser induced fluorescence (PLIF) were performed in the soot inception region of a turbulent non-premixed ethylene-nitrogen-krypton-air jet flame with a jet-exit Reynolds of number 8300. These measurements were conducted simultaneously to experiments to measure the distribution of soot within the flame (laser induced incandescence) and three components of velocity in a plane (stereoscopic particle image velocimetry). Krypton has an absorption peak at 214.7 nm that leads to the emission of fluorescence at 760 nm and it is this fluorescence that is subsequently detected. Whilst the technique has previously been used in non-sooting flames this study shows that it is also viable in sooting flames, and can be implemented to investigate the significance of mixture fraction in the formation and transport of soot. It is observed that concentrations of up to 8% krypton by mole fraction can be used to seed the flame without self-quenching becoming an issue at the soot inception region of the flame. Extinction of the incident 214.7 nm radiation, either through scattering by the PIV tracer particles or soot, or absorption by polycyclic aromatic hydrocarbons (PAH) present in the flame or absorption by the krypton itself plays an important role in determining the krypton mole fraction. Soot particles directly obstructing the incident 214.7 nm radiation are also noted to strongly attenuate the PLIF signal. The results confirm that soot structures in a jet flame of this Reynolds number are intermittent and “spotty” and that they tend to form in regions of low Reynolds stresses. A strong correlation is found in the region of peak mean soot volume fraction between the LII and krypton PLIF signals.

Unseeded Velocity Measurements Around a Transonic Airfoil Using Femtosecond Laser Tagging

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.

FLEET Velocimetry Measurements on a Transonic Airfoil

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 deg, 3.5 deg, and 7deg. Measurements were assessed for their accuracy, precision, dynamic range, spatial resolution, and overall measurement uncertainty in the context of the applied flowfield. Measurement precisions as low as 1 m/s were observed, while overall uncertainties ranged from 4 to 5 percent. 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.

Improved Large-Eddy Simulation Validation Methodology: Application to Supersonic Inlet/Isolator Flow

A process was demonstrated that enables more meaningful comparisons of experimental and large-eddy simulation results of the same flowfield. This approach was motivated by recent studies, which show the importance of modeling the measurement device or process when making comparisons of this nature. In the current case, a large-eddy simulation of a Mach 5 inlet/isolator flow is the subject of the validation process, a flow that was previously validated using particle image velocimetry measurements. In the present study, the particle image velocimetry measurement process is emulated by adding particles with inertia to the large-eddy simulation and using synthetic particle image velocimetry to mimic the process by which velocity data are extracted. Analysis of the particle response reveals modifications to the underlying flow consistent with the original particle image velocimetry measurements, including a drastic decrease in rms velocities in the vicinity of shockwaves and weaker velocity gradients througho...

Scalar and Velocity Measurements in a Mach 3 Hypermixing Flowfield

Mach 3 hypermixer flowfields are studied by using scalar and velocity imaging techniques. Two strut-based hypermixer configurations are used that feature parallel gas injection through a castellated base with (i) expansive wedges flanking the base, and (ii) a combination of compressive and expansive wedges near the base. Two-component particle image velocimetry (PIV) is used to obtain the velocity fields, planar laser scattering (PLS) of condensed carbon dioxide gas is used to visualize the freestream flow structures, and planar laser-induced fluorescence (PLIF) of acetone vapor is used to tag the injected jet fluid. In the cases with no base injection, the two wakes show notable differences, with the compressive mixer entraining large quantities of freestream fluid within the wake in contrast to the expansive mixer. With the addition of base injection, the expansive mixer is found to induce notable transverse spread of the injected jet fluid though spanwise spread is limited. The compressive mixer enhances the spread of the injected fluid in both the spanwise and transverse directions. The velocity measurements show that rapid transverse spread of the jet in the compressive hypermixer is the result of a large transverse velocity component induced by the strong streamwise vortices. These vortices are so strong as to create a bifurcation of the jet fluid along the midplane of the hypermixer.

Development of Hydroxyl Tagging Velocimetry for Low Velocity Flows

Hydroxyl tagging velocimetry (HTV) is a molecular tagging technique that relies on the photo-dissociation of water vapor into OH radicals and their subsequent tracking using laser induced fluorescence. Velocities are then obtained from time-of-flight calculations. At ambient temperature in air, the OH species lifetime is relatively short (<50 s), making it suited for high speed flows. Lifetime and radicals formation increases with temperature, which allows HTV to also probe low-velocity, high-temperature flows or reacting flows such as flames. The present work aims at extending the domain of applicability of HTV, particularly towards low-speed (<10 m/s) and moderate (<500 K) temperature flows. Results are compared to particle image velocimetry (PIV) measurements recorded in identical conditions. Single shot and averaged velocity profiles are obtained in an air jet at room temperature. By modestly raising the temperature (100-200 degC) the OH production increases, resulting in an improvement of the signal-to-noise ratio (SNR). Use of nitrogen - a non-reactive gas with minimal collisional quenching - extends the OH species lifetime (to over 500 s), which allows probing of slower flows or, alternately, increases the measurement precision at the expense of spatial resolution. Instantaneous velocity profiles are resolved in a 100degC nitrogen jet (maximum jet-center velocity of 6.5 m/s) with an uncertainty down to 0.10 m/s (1.5%) at 68% confidence level. MTV measurements are compared with particle image velocimetry and show agreement within 2%.

Planar Imaging Investigation of Supersonic Hypermixer Configurations

An experimental investigation of the mixing characteristics of several Mach 3 hypermixer flowfields is conducted. Through a combination of laser-based diagnostics including planar laser scattering (PLS) of CO2, planar laser Mie scattering (PLMS) of seeded TiO2 particles, and PIV, four different hypermixer configurations are studied for determination of their wake structure and dispersion characteristics. Two of the mixers tested, one featuring a combination of compressive and expansive ramps and another featuring purely expansive ramps, exhibit far greater dispersion characteristics than the other geometries tested. The mixing enhancement afforded by the compressive mixer was largely due to the strength of the streamwise vortices shed in addition to a broader base region that allowed the injected fluid to expand rapidly after injection. The principle dispersion direction for this mixer was transverse to the flow. The expansive configuration exhibited a modest enhancement to transverse dispersion, and was the only mixer capable of sustained spanwise dispersion in the far-field of the mixer while maintaining the potential advantage of minimal total pressure loss.