Sean O’Byrne

Electric field measurements in a dielectric barrier nanosecond pulse discharge with sub-nanosecond time resolution

The paper presents the results of time-resolved electric field measurements in a nanosecond discharge between two plane electrodes covered by dielectric plates, using picosecond four-wave mixing diagnostics. For absolute calibration, the IR signal was measured in hydrogen at a pressure of 440 Torr, for electrostatic electric field ranging from 0 to 8 kV cm−1. The calibration curve (i.e. the square root of IR signal intensity versus electric field) was shown to be linear. By measuring the intensities of the pump, Stokes, and IR signal beam for each laser shot during the time sweep across the high-voltage pulse, temporal evolution of the electric field in the nanosecond pulse discharge was determined with sub-nanosecond time resolution. The results are compared to kinetic modeling predictions, showing good agreement, including non-zero electric field offset before the main high voltage pulse, breakdown moment, and reduction of electric field in the plasma after breakdown. The difference between the experimental results and model predictions is likely due to non-1D structure of the discharge. Comparison with the kinetic modeling predictions shows that electric field in the nanosecond pulse discharge is controlled primarily by electron impact excitation and charge accumulation on the dielectric surfaces.

Baseline correction for stray light in log-ratio diode laser absorption measurements.

Log-ratio detection is a convenient technique for making temperature and concentration measurements using sensors based on tunable diode laser absorption spectroscopy. In many practical sensing applications, it is difficult to avoid stray light falling on the signal photodiode of the sensor. This stray light acts as noncommon-mode interference and introduces a systematic error in absorption measurements, which is not removed by baseline subtraction. This paper analyzes the factors that determine this systematic error and also presents a calibration method that can correct for it. This correction method is verified using a simple experiment.

Molecular dynamics study on the structural and dynamic properties of xanthan gum in a dilute solution under the effect of temperature

Xanthan gum (XG) is considered one of the most industrially important polysaccharides, with applications ranging from food products such as ice creams and salad dressings to pharmaceuticals and oil well drilling fluids. The wide application of XG is due to its favourable rheological properties and its capability to resist degradation under a high shear or high temperature environment. It is generally accepted that both inter- and intramolecular interactions, including hydrogen bonding (HB), are responsible for its unique properties. To date, there is still a lack of comprehensive examination on the HB mechanism in polysaccharides. Therefore, the study proposed here was conducted using molecular dynamics (MD) simulations that are able to provide insights with an unparalleled temporal and spatial resolution. Since XG is used over a broad range of temperatures, the implications of thermal effect on the structure and molecular interactions of XG in an aqueous solution are discussed in this paper. MD simulations were run at an isobaric-isothermal condition with 1 atm target pressure and five temperatures ranging between 283K and 353K. From the simulation results, an increasingly extended conformation of XG is observed as the temperature rises, and this finding matches qualitatively with the results published in the literature. The radius of gyration, radial pair distribution functions and intramolecular HB of XG were also discussed. The outcomes of the present study may serve as a stepping stone for the future studies on polysaccharides using MD simulations.Xanthan gum (XG) is considered one of the most industrially important polysaccharides, with applications ranging from food products such as ice creams and salad dressings to pharmaceuticals and oil well drilling fluids. The wide application of XG is due to its favourable rheological properties and its capability to resist degradation under a high shear or high temperature environment. It is generally accepted that both inter- and intramolecular interactions, including hydrogen bonding (HB), are responsible for its unique properties. To date, there is still a lack of comprehensive examination on the HB mechanism in polysaccharides. Therefore, the study proposed here was conducted using molecular dynamics (MD) simulations that are able to provide insights with an unparalleled temporal and spatial resolution. Since XG is used over a broad range of temperatures, the implications of thermal effect on the structure and molecular interactions of XG in an aqueous solution are discussed in this paper. MD simulatio...

Multiple receivers in a high-resolution near-infrared heterodyne spectrometer.

The paper describes a modification of a high-resolution heterodyne NIR spectrometer described by Rodin, et al. in Opt. Express22, 13825 (2014), wherein the noise amplitude of the heterodyne signal squared was proportional to the power of the incoherent radiation source (the sun). The addition of a second receiver collecting radiation from the sun into a second single-mode fiber created up to a factor of two increase in detected source power spectrum. The ability to add uncorrelated heterodyne IF signals as Gaussian noise (variance) provides the means by which multiple heterodyne receivers of signal from an incoherent source can increase the detected source power by the same multiple, thus avoiding the limit imposed by the antenna theorem for a single receiver (A. E. Siegman, Appl. Opt.5, 1588 (1966)).

Comparison of density-sensitive and fluorescence visualization in low-density separated flow

This paper compares two methods used to visualize low-density hypersonic separated flow fields: resonantly enhanced shearing interferometry (RESI) and planar laser-induced fluorescence (PLIF) of nitric oxide. The two methods are both used on a leading-edge separated flow configuration, and the values of the recompression neck position and thickness are compared between the two methods. It is seen that the lithium seeding in the RESI technique has a discernible effect on the flow behavior, changing the motion of the forebody shock wave during the experiment compared with unseeded flow, and the seeding also appears to prevent the recompression neck from moving further downstream during the tunnel run compared with the unseeded flow. The seeded lithium itself is also found to only enhance the density gradients in the flow during the first 550 µs of flow time. The PLIF images are consistent with the RESI results for both neck position and thickness to within the measurement uncertainty, indicating that the RE...

DSMC computations of hypersonic flow separation and re-attachment in the transition to continuum regime

The flow over a ‘tick’ shaped configuration is performed using two Direct Simulation Monte Carlo codes: the DS2V code of Bird and the code from Sandia National Laboratory, called SPARTA. The configuration creates a flow field, where the flow is expanded initially but then is affected by the adverse pressure gradient induced by a compression surface. The flow field is challenging in the sense that the full flow domain is comprised of localized areas spanning continuum and transitional regimes. The present work focuses on the capability of SPARTA to model such flow conditions and also towards a comparative evaluation with results from DS2V. An extensive grid adaptation study is performed using both the codes on a model with a sharp leading edge and the converged results are then compared. The computational predictions are evaluated in terms of surface parameters such as heat flux, shear stress, pressure and velocity slip. SPARTA consistently predicts higher values for these surface properties. The skin fric...

Measurement of gas viscosity using photonic crystal fiber

A new measurement technique for gas viscosity coefficient is designed and demonstrated using the technique of tunable diode laser absorption spectroscopy (TDLAS). Gas flow is driven by a pressure gradient between two gas cells, through a photonic crystal fiber (PCF) surrounded by a furnace for temperature adjustment. PCF with 20-micron diameter affords physical space for gas-light interaction and provides a basis for gas viscosity measurement by determining the time for flow to exit a capillary tube under the influence of a pressure gradient. Infrared radiation from a diode laser is coupled into the fiber to be guided through the gas, and the light attenuation due to absorption from the molecular absorbing species is measured by a photo detector placed at the exit of the fiber. A numerical model from Sharipov and Graur describing local number density distribution in a unsteady state is applied for the determination of gas viscosity, based on the number density of gas measured by the absorption of the lase...

Hypersonic High-Enthalpy Flow in a Leading-Edge Separation

Flow separation in/over a hypersonic space vehicle is an important phenomenon which occurs due to flow interaction with various geometric elements of the vehicle. This however can lead to adverse pressure gradient and localised intense heating resulting in detrimental consequences for the successful performance of the vehicle.

A direct simulation Monte Carlo study of hypersonic leading-edge separation with rarefaction effects

Hypersonic laminar flow separation preceded by a strong expansion can occur in practical situations such as the base flow of re-entry vehicles and flow over deflected control surfaces. A leading-edge separation configuration provides a case for studying fundamental aspects of such flow separation and reattachment in the absence of a pre-existing boundary layer. In hypersonic low-density flows, the onset of separation is complicated by the presence of rarefaction and thermal non-equilibrium. The reattachment process is also characterized by high compressibility. A computational study of the characteristics of flow separation and reattachment over such a configuration has been carried out using the direct simulation Monte Carlo code SPARTA. The salient features of separation are explained from a fluid dynamic perspective, and distinct characteristics in surface parameters are identified and discussed in detail. The physical mechanisms behind the formation as well as co-existence of primary and secondary vortices are described. The region close to the leading-edge is analyzed in detail, and the prevailing non-equilibrium aspects are presented. Various theoretical concepts, developed based on continuum flow separation, are applied to the present configuration to explore its applicability in the presence of slip, and the resulting relative variations are highlighted. The temporal evolution of flow structures at various wall temperatures is studied, and its strong dependence on the wall-to-stagnation temperature ratio is elucidated.Hypersonic laminar flow separation preceded by a strong expansion can occur in practical situations such as the base flow of re-entry vehicles and flow over deflected control surfaces. A leading-edge separation configuration provides a case for studying fundamental aspects of such flow separation and reattachment in the absence of a pre-existing boundary layer. In hypersonic low-density flows, the onset of separation is complicated by the presence of rarefaction and thermal non-equilibrium. The reattachment process is also characterized by high compressibility. A computational study of the characteristics of flow separation and reattachment over such a configuration has been carried out using the direct simulation Monte Carlo code SPARTA. The salient features of separation are explained from a fluid dynamic perspective, and distinct characteristics in surface parameters are identified and discussed in detail. The physical mechanisms behind the formation as well as co-existence of primary and secondary vor...

Characterization of a High Temporal Resolution TDLAS System for Measurements in a Shock Tube Facility

Transient heating and pressurization of a gas by shock waves can be useful for a variety of purposes, particularly for configurations involving shock wave focussing. Unless the geometry is particularly simple, the time history of temperature can be difficult to predict accurately. Hence, a non-intrusive measurement technique with high temporal resolution is required to record the time history of the very rapidly changing temperature of a shock-heated flow. One promising measurement techniques for these high-speed flows is Tunable Diode Laser Absorption Spectroscopy (TDLAS)