Doctor of Philosophy, The Ohio State University, 2021, Mechanical Engineering

Kinetics of the excited metastable state of molecular nitrogen, N2(A3Σu+), and the low-temperature hydroperoxyl radical, HO2, have been studied using newly developed laser diagnostics, Cavity Ring Down Spectroscopy (CRDS) and Tunable Diode Laser Spectroscopy (TDLAS). Measurements of these species have been made in three different environments, where they have been generated by a repetitive, ns pulse, double dielectric barrier discharge plasmas, (i) high-pressure, low-temperature plasma in nitrogen, N2-O2, N2-H2, and N2-NO mixtures; (ii) supersonic flows of nitrogen in a nonequilibrium flow, blowdown wind tunnel (Mach 4-5), excited by an electric discharge in the plenum; and (iii) H2-O2, CH4-O2, and C2H4-O2 mixtures diluted in argon in a heated plasma flow reactor. Experimental results obtained in the first environment (low-temperature plasma cell) include absolute time-resolved populations of N2 in the excited A3Σu+ electronic state, measured by both CRDS and TDLAS. CRDS measurements of N2(A3Σu+,v=0-2) populations are made in the discharge afterglow at pressures of 22 and 39 Torr. The data reduction procedure takes into account the linewidth of the pulsed laser source, which is comparable with the absorption linewidth and results in a non-single exponential ring down decay. Peak N2(A3Σu+,v=0-2) populations after a 10-pulse ns discharge burst are in the range of 1012 - 1013 cm-3. In the afterglow, these populations exhibit a relatively slow decay with the characteristic time of approximately 500 μs, most likely due to the quenching by N atoms. TDLAS data have been taken at a higher pressure of 132 Torr. Absolute time-resolved N2(A3Σu+,v=0,1) number densities are measured during ns pulse discharge bursts up to 5 pulses long and in the afterglow. The results indicate that N2(A3Σu+) is generated after every discharge pulse on a 20-50 μs time scale, much longer compared to the discharge pulse duration of ~100 ns, and subsequently decays between the pulses. The decay (open full item for complete abstract)

Committee: Igor Adamovich (Advisor); Jefferey Sutton (Committee Member); Datta Gaitonde (Committee Member); Mo Samimy (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2020, Aero/Astro Engineering

Non-self-sustained hybrid plasmas are formed by the overlap of two separate voltage waveforms with significantly different reduced electric field values (E/N), one of them below the ionization threshold, to produce excited species and radicals selectively. In this work, a stable, capacitively coupled ns pulse – RF waveform hybrid discharge is operated in nitrogen and mixtures of nitrogen with other molecular gases at 50 – 100 Torr pressure, using a single pair of electrodes mounted externally to the reactor cell. The purpose of the ns pulse discharge is to generate ionization and electronic excitation of the mixture components, while the below-breakdown RF voltage couples additional energy to the vibrational modes of the mixture components. Based on the broadband plasma emission imaging, the plasma volume appears to be enhanced by the RF waveform, compared to ns pulse discharge, due to the drift oscillations of electrons induced by the RF waveform. Coherent Anti-Stokes Raman Spectroscopy (CARS) measurements in the hybrid discharge operated in nitrogen show that the RF waveform significantly enhances the vibrational excitation of N2 in the ground electronic state, populating vibrational levels up to at least v=3, and increasing the vibrational temperature of N2 from TV = 1210 ± 110 K in the ns pulse train plasma to TV = 1810 ± 170 K in the ns-RF hybrid discharge. The translational- rotational temperature at these conditions remains low, TR = 315 ± 15 K. To evaluate the potential of this plasma to operate in other gas mixtures, 1% of H2 is added to nitrogen. CARS measurements reveal a moderate N2 vibrational relaxation by hydrogen, reducing the vibrational temperature in the hybrid plasma to TV = 1700 ± 150 K and increasing in the translational-rotational temperature to TR = 396 ± 10 K. Time-resolved measurements of the number density of the first electronically excited state of nitrogen, N2(A3Σ), obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS) in n (open full item for complete abstract)

Committee: Igor Adamovich (Advisor); Jeffrey Sutton (Committee Member) Subjects: Chemistry; Energy; Engineering; Environmental Engineering

Master of Science, The Ohio State University, 2017, Mechanical Engineering

Flow separation leading to stall imposes considerable performance penalties on lifting surfaces. Limitations in flight envelope and loss of control are among the chief reasons for the interest in the aeronautical research community for better understanding of this phenomenon. Modern flow control techniques explored in this work can potentially alleviate the performance penalties due to flow separation. Experiments were designed to investigate excitation of flow over an airfoil with leading edge separation at a post-stall angle of attack with nanosecond pulse dielectric barrier discharge actuators. The subject airfoil is designed with a small radius of curvature that potentially challenges the task of flow control as more centrifugal acceleration around leading is required to successfully reattach the flow. The Reynolds number based on the chord was fixed at 5·105, corresponding to a freestream flow of approximately 37 m/s. An angle of attack of 19° was used and a single plasma actuator was mounted near the leading edge of the airfoil. Fully separated flow on the suction side extended well beyond the airfoil with naturally shed vortices generated at a Strouhal number of 0.60. Excitation at very low to moderate (~1) Strouhal numbers at the leading edge generated organized coherent structures in the shear layer over the separated region with a shedding Strouhal number corresponding to that of the excitation, synchronizing the vortex shedding from leading and trailing edges. Excitation around the shedding Strouhal number promoted vortex merging while excitation at higher Strouhal numbers resulted in smaller, weaker structures that quickly developed and disintegrate over the airfoil. The primary mechanism of control is the excitation of instabilities associated with the vortices shed from leading edge. The excitation generates coherent large-scale structures that entrain high-momentum fluid into the separation region to reduce the separation and/or accelerate the flow ov (open full item for complete abstract)

Committee: Mo Samimy (Advisor); James Gregory (Committee Member); Igor Adamovich (Committee Member) Subjects: Aerospace Engineering; Experiments; Fluid Dynamics

Doctor of Philosophy, The Ohio State University, 2015, Mechanical Engineering

This dissertation presents the results of development of a picosecond four wave mixing technique and its use for electric field measurements in nanosecond pulse discharges. This technique is similar to coherent anti-Stokes Raman spectroscopy and is well suited for electric field measurements in high pressure plasmas with high spatial and temporal resolution. The results show that the signal intensity scales proportionally to the square of the electric field, the signal is emitted as a coherent beam, and is polarized parallel to the electric field vector, making possible electric field vector component measurements. The signal is generated when a collinear pair of pump and Stokes beams, which are generated in a stimulated Raman shifting cell (SRS), generate coherent excitation of molecules into a higher energy level, hydrogen for the present work. The coherent excitation mixes with a dipole moment induced by an external electric field. The mixing of these three ``waves'' allows the molecules to radiate at their Raman frequency, producing a fourth, signal, wave which is proportional to the square of the electric field. The time resolution of this technique is limited by the coherence decay time of the molecules, which is a few hundred picoseconds.

Committee: Igor Adamovich (Advisor) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

In recent years, plasma assisted ignition and flame-holding in high speed flows has attracted considerable attention due to potential applications for turbojet engines and afterburners operating at high altitudes, as well as scramjet engines. Conventional methods of igniting a flow in the combustor using a spark or an arc discharge are known to be ineffective at low pressures and high flow velocities, since the ignition kernel is limited by a small volume of the spark or arc filament. Single photon LIF spectroscopy is used to study hydroxyl radical formation and loss kinetics in low temperature hydrogen-air repetitively pulsed nanosecond plasmas. Nanosecond pulsed plasmas are created in a rectangular cross section quartz channel / plasma flow reactor. Flow rates of hydrogen-air mixtures are controlled by mass flow controllers at a total pressure of 40-100 torr, initial temperature T0=300-500 K and a flow velocity of approximately u=0.1-0.8 m/sec. Two rectangular copper plate electrodes, rounded at the corners to reduce the electric field non-uniformity, are attached to the outside of the quartz channel. Repetitively pulsed plasmas are generated using a Chemical Physics Technologies (CPT) power supply which produces ~25 nanosecond pulses with ~20 kV peak voltage. Absolute hydroxyl radical mole fraction is determined as both a function of time after application of a single 25 nsec pulse, and 60 microseconds after the final pulse of a variable length “burst” of pulses. Relative LIF signal levels are put on an absolute mole fraction scale by means of calibration with a standard near-adiabatic Hencken flat flame burner at atmospheric pressure. By obtaining OH LIF data in both the plasma and the flame, and correcting for differences in the collisional quenching and Vibrational Energy Transfer (VET) rates, absolute OH mole fraction can be determined. For a single discharge pulse at 27 °C and 100 °C, the absolute OH temporal profile is found to rise rapidly during the initi (open full item for complete abstract)

Committee: Walter R. Lempert PhD (Advisor); Igor V. Adamovich PhD (Committee Member); Joseph W. Rich PhD (Committee Member); Jeffrey A. Sutton PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2007, Mechanical Engineering

The present research examines the relationship between the large-scale structure dynamics of a jet and the far-field sound. This was achieved by exploring the flowfield and the far field of an axisymmetric Mach 0.9 jet with a Reynolds number of approximately 0.76 million. The jet is controlled by eight plasma actuators, which operate over a large frequency range and have independent phase control allowing excitation of azimuthal modes (m) 0, 1, 2, and 3. The jet's far field is probed with a microphone array positioned at 30 degrees with respect to the downstream jet axis. The array is used to estimate the origin of peak sound events in space, and find the sound pressure level (SPL) and overall sound pressure level (OASPL). The lower forcing Strouhal numbers (StDF's) increase the OASPL and move noise sources upstream while higher StDF's decrease the OASPL and have noise source distributions similar to the baseline jet. The flowfield was investigated using particle image velocimetry (PIV). A Reynolds decomposition of the PIV data emphasized the importance of the streamwise velocity fluctuations for the symmetric azimuthal modes (m = 0 and 2) and the cross-stream velocity fluctuations for the asymmetric azimuthal modes (m = 1 and 3). A proper orthogonal decomposition of the PIV data was performed to extract information about how forcing affects the large-scale flow features and conditionally average the PIV data. When forcing at StD's other than the preferred mode, the conditional-averaged images show large-scale flow features that grow, saturate, and decay closer to the nozzle exit. When exciting a symmetric azimuthal mode, m = 0, near the preferred StDF, the streamwise phase-averaged velocity grows quickly and saturates over a relatively long spatial range. When exciting an asymmetric azimuthal mode, m = 1, near the preferred StDF, the cross-stream phase-averaged velocity grows slowly, saturates, and then decays relatively quickly. The noise source distribution occur (open full item for complete abstract)

Committee: Mo Samimy (Advisor) Subjects: Engineering, Mechanical

Doctor of Philosophy, The Ohio State University, 2005, Mechanical Engineering

The role of compressibility on the convective velocity of large-scale structures in axisymmetric jets is studied using a home-built pulse burst laser system and newly developed high-repetition rate experimental diagnostics. A pulse burst laser system was designed and constructed with the ability to produce a burst of short duration (10 nsec), high energy (order of 10 -100 mJ/pulse) pulses over a ~150 microsecond period with inter-pulse timing as low as 1 microsecond (1 MHz). The application of the pulse burst laser for flow measurements was investigated through the development of MHz rate flow visualization and MHz rate planar Doppler velocimetry (PDV). MHz rate PDV is a spectroscopic technique that produces 28 time-correlated realizations of the velocity over a plane with a maximum repetition rate of up to 1 MHz and accuracies on the order of 5%. Space-time correlations were used to track structures within the flow field and determine their convective velocity. Data produced using flow visualization images agrees with previous research and indicates a strong departure of the convective velocity from theory. Data produced using velocity data, however, shows starkly different trends and does not produce the same measurements of convective velocity. This difference in measurement is attributed to a misinterpretation of the use of space-time correlation for tracking structures. The presence of a distinct boundary between the mixing layer and the jet core as well as the mixing layer and ambient air in the flow visualization data and some of the velocity data leads to a bias in the measurement. The space-time correlation is found to preferentially follow these boundaries, thus leading to faster and/or slower measurements of convective velocity. For the Mach 2.0 jet, velocity data was obtained with seed particles marking the jet core and the mixing layer, but not the ambient air. This lack of velocity measurements on the low-speed side of the jet's mixing layer biased the (open full item for complete abstract)

Committee: Mo Samimy (Advisor) Subjects:

Doctor of Philosophy, Case Western Reserve University, 2008, Mechanical Engineering

The scope of this dissertation is to develop and apply a non-intrusive molecular Rayleigh scattering diagnostic that is capable of providing time-resolved simultaneous measurements of gas temperature, velocity, and density in unseeded turbulent flows at sampling rates up to 32 kHz. Molecular Rayleigh scattering is elastic light scattering from molecules; the spectrum of Rayleigh scattered light contains information about the gas temperature and velocity of the flow. Additionally, the scattered signal is directly proportional to the molecular number density. These characteristics are utilized in the development of the measurement technique. This dissertation results in the following: 1. Development of a point-based Rayleigh scattering measurement system that provides time-resolved simultaneous measurement of temperature, velocity, and density at sampling rates up to 32 kHz. 2. Numerical modeling of the light scattering and detection process to evaluate uncertainty levels and capabilities of the measurement technique. 3. Validation of the developed measurement system in benchmark flow experiments in which velocity and temperature fluctuations were decoupled and independently forced at various amplitudes and frequencies. 4. Demonstration of simultaneous measurement of all three quantities in an electrically-heated free jet facility at NASA Glenn Research Center. 5. Comparison of Rayleigh scattering measurements in all experiment phases with thermal anemometry measurements. The experimental measurements are presented in terms of first-order time-series results that are measured directly by the technique, and second-order statistics, such as power spectral density and rms fluctuations, which are calculated from the direct time-resolved quasi-instantaneous measurements. Temperature fluctuation results are compared with constant current anemometry measurements and velocity fluctuation results are compared with constant temperature anemometry measurements. Experiments were (open full item for complete abstract)

Committee: Chih-Jen Sung (Advisor) Subjects: Engineering, Mechanical