R.B. Miles

• Professor (Full) at Texas A&M University

Laser Thomson scattering, which provides thermal properties of electrons, has been extensively utilized in plasma diagnostics. However, being a non-resonant linear light scattering technique, it often encounters several challenges in weakly-ionized plasma diagnostics due to spectral overlap with different scatterings induced from other existing spe...

Chemical kinetic schemes have been developed for hypersonic flows with ablative carbon and carbonaceous components; however, experimental data for the validation of these schemes are limited. Therefore, in this study, we use a carbon-ablation chemical kinetics model to identify changes in the refractive index field near a hypersonic vehicle as well...

This article presents the first demonstration of the tunable 5th harmonic of a Nd:YAG pulse-burst laser used for single-shot velocity and acceleration measurements via Nitric-oxide ionization induced Flow Tagging and Imaging. This demonstration was performed in a highly transient hypersonic flowfield created by injecting a supersonic jet perpendicu...

This article presents the first demonstration of the tunable 5th harmonic of a Nd:YAG pulse-burst laser used for single-shot velocity and acceleration measurements via Nitric-oxide ionization induced Flow Tagging and Imaging. This demonstration was performed in a highly transient hypersonic flowfield created by injecting a supersonic jet perpendicu...

Laser Thomson scattering, which provide thermal properties of electrons, has been extensively utilized in plasma diagnostics. However, being a non-resonant linear light scattering technique, it often encounters several challenges in weakly-ionzed plasma diagnostics due to spectral overlap with different scatterings induced from other existing speci...

Laser Thomson scattering, which provide thermal properties of electrons, has been extensively utilized in plasma diagnostics. However, being a non-resonant linear light scattering technique, it often encounters several challenges in weakly-ionzed plasma diagnostics due to spectral overlap with different scatterings induced from other existing speci...

The paper presents a detailed analysis of the change in the refractive index field during the decay of laser plasma in nitrogen at atmospheric pressure. The mathematical model combines a three-temperature nonequilibrium plasma model with detailed kinetics and a semi-classical refractive index model based on species polarizabilities. A detailed char...

Spontaneous Raman scattering, also known as common old ordinary Raman scattering (COORS), is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-alti...

In our recent work, we successfully demonstrated single-shot common old ordinary Raman scattering (COORS) for localized one-dimensional (1D) hypersonic flow diagnostic, measuring gas density and temperature distributions across a bow shock in front of a blunt wedge model. Herein, we describe several details of the single-shot 1D COORS technique, in...

New research has been conducted on a canonical hypersonic inlet test article. Two experimental campaigns involving two different diagnostic techniques were carried out to obtain data at different locations on the test article. The first experimental campaign focused on improving existing infrared (IR) thermography results from prior work. By changi...

This Letter describes the first, to the best of our knowledge, demonstration of a velocity measurement by nitric oxide ionization induced flow tagging and imaging (NiiFTI) of a high-enthalpy hypersonic flow utilizing naturally formed nitric oxide. The measurements were conducted in the hypervelocity expansion tunnel (HXT) at Texas A&M University in...

This article presents the first demonstration of burst-mode nitric oxide planar laser-induced fluorescence measurement of a reacting environment via Nd:YAG 5th harmonic excitation. A tunable pulse-burst laser generated 1 ms bursts of resonant 213 nm radiation at 200 kHz, 500 kHz, and 1 MHz repetition rates with 16 mJ, 3.5 mJ, and 0.75 mJ average pu...

This article presents the first demonstration of burst-mode nitric oxide planar laser-induced fluorescence measurement of a reacting environment via Nd:YAG 5th harmonic excitation. A tunable pulse-burst laser generated 1 ms bursts of resonant 213 nm radiation at 200 kHz, 500 kHz, and 1 MHz repetition rates with 16 mJ, 3.5 mJ, and 0.75 mJ average pu...

In this paper we study defocusing and non-paraxial effects associated with nanosecond pulsed laser beam propagation through femtosecond laser generated plasmas. Simulations were performed using a non-reduced wave propagation equation to highlight the role of spatial beam shaping of the nanosecond pulse in improving the laser propagation characteris...

Common old ordinary Raman scattering (COORS) is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross-section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-altitude hypersonic flights. Utilizing a recently...

Common old ordinary Raman scattering (COORS) is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross-section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-altitude hypersonic flights. Utilizing a recently...

In this paper, we evaluate the thermal response of graphite samples to high-power continuous wave laser heating. Experimental work is performed at the Aerospace Laboratory for Lasers, Electromagnetics, and Optics (ALLEMO) at Texas A&M University. Experimental results are compared to the results of an in-house finite-volume heat transfer model. The...

The ubiquitous presence of sodium in hypersonic and combustion environments makes it a candidate for planar laser induced fluorescence (PLIF) imaging. However use of the strong 589.0 and 589.6 nm transitions encounters difficulties with background laser scattering and optical thickness effects. In high enthalpy flows and combusting environments, th...

In this paper, the authors describe an experimental demonstration of Nitric-oxide ionization induced Flow Tagging and Imaging (NiiFTI) in near-surface velocimetry applications. The flowfield of interest including boundary layers was produced by an Ogive hypersonic model in a Mach 5.7 air flow seeded with 1 % nitric oxide. The diagnostic setup consi...

This letter describes the first demonstration of a velocity measurement by Nitric-oxide ionization induced Flow Tagging and Imaging (NiiFTI) of a high-enthalpy hypersonic flow utilizing naturally formed nitric oxide. The measurements were conducted in the Hypervelocity Expansion Tunnel at Texas A&M University in Mach 8.5 and Mach 10 flows near an o...

This letter describes the first demonstration of a velocity measurement by Nitric-oxide ionization induced Flow Tagging and Imaging (NiiFTI) of a high-enthalpy hypersonic flow utilizing naturally formed nitric oxide. The measurements were conducted in the Hypervelocity Expansion Tunnel at Texas A&M University in Mach 8.5 and Mach 10 flows near an o...

In this article we explore (a) optical and (b) thermochemical properties of reactive hypersonic flows, with an emphasis on oxygen dynamics and time resolved measurement capabilities. First, we discuss experimental and numerical analysis of reactive hypersonic flows of an argon–oxygen mixture over a blunt wedge geometry. The experimental campaign fo...

This article presents an experimental demonstration of a spectroscopic method based on the dispersion of the scattering spectrum from laser-illuminated liquid water collected through a rubidium atomic vapor prism cell. Resonant absorption at 780 nm suppresses Mie/Rayleigh scattering and the steep gradients in refractive index near the 780 nm absorp...

In this work, the potential for hypersonic leading edge cooling by electron emission is demonstrated. To overcome space charge limitations the experiments are carried out in an argon discharge at 1 Torr. Cooling is observed with time-resolved measurements of the electron emission current and surface temperature, taking advantage of well controlled...

This work reports the measurement of two-dimensional electron properties over a nanosecond scale integration time across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the $$\sim 100$$ ∼ 100 $$\upmu$$ μ m diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-d...

A self-consistent, multi-dimensional three-temperature plasma model for laser-induced non-equilibrium plasma is integrated with a semi-classical model for refractive index calculations to explore refractive index modifications in femtosecond laser-induced filaments. The investigation focuses on single filaments in nitrogen at atmospheric pressure,...

In this work, the authors discussed an experimental investigation of different design solutions and pumping arrangements for efficient ultraviolet pumping of an Optical Parametric Oscillator by a Pulse-Burst Laser. Cavity length, pump beam profile, and pump pulse duration were considered and their effects on the resulting conversion efficiency were...

In this work, we investigated the concept of a bore sight FLEET geometry, a single-ended configuration where filament imaging is performed along the axis of the excitation laser. Multiple methods to achieve bore sight FLEET are discussed. The bore sight approach was employed to characterize the intensity dependence of FLEET on density and lifetime....

This Letter describes, to the best of our knowledge, a new approach to flow tagging, nitric oxide (NO) Ionization Induced Flow Tagging and Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard...

This letter describes a new approach to flow tagging, Nitric-oxide Ionization Induced Flow Tagged Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard deviation of as low as 3.4 m/s and mean v...

Femtosecond laser electronic excitation tagging (FLEET) velocimetry is applied in a hypersonic boundary layer behind an array of turbulence-inducing trips. One-dimensional mean velocity and root-mean-square (RMS) of velocity fluctuation profiles are extracted from FLEET emissions oriented across a test article and through a boundary layer in two te...

This letter describes a new approach to flow tagging, Nitric-oxide Ionization Induced Flow Tagged Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard deviation of as low as 3.4 m/s and mean v...

This letter describes a new approach to flow tagging, Nitric-oxide Ionization Induced Flow Tagged Imaging (NiiFTI), and presents the first experimental demonstration for single-shot velocimetry in a near Mach 6 hypersonic flow at 250 kHz. The mean velocity of 860 m/s was measured with a single-shot standard deviation of as low as 3.4 m/s and mean v...

This work reports the measurement of two-dimensional electron properties across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the ~100 um diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-dimensional high-resolution laser Thomson scattering with a spatial...

In this paper we consider changes in the refractive index at the high-speed flow over the wedge, using a combination of the high-fidelity Direct Molecular Simulation, the semi-classical model of the polarizability and interferometry measurements. The behavior of the Gladstone-Dale constant of air at various Mach numbers and vibrational non-equilibr...

In this article, the authors present an experimental investigation of the thermochemical relaxation regions downstream of symmetric Mach reflections generated in Mach 8.5 flows with stagnation enthalpies from 7 to 10 MJ/kg. The experiment was performed in the Hypervelocity Expansion Tunnel at the National Aerothermochemistry and Hypersonics Laborat...

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as a spectral/angular filter, have often been used as a point measurement technique with spatial resolution as low as a few hundred µm defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with sev...

This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sen...

A kinetic model of a non-equilibrium water vapor plasma created by a nanosecond pulsed discharge at elevated temperatures and at low pressure is presented. The model includes equations for electron, vibrational and gas temperatures combined with equations for neutral, charged, electronically and vibrationally excited species. Simulation results are...

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as a spectral/angular filter, have often been used as a point measurement technique with spatial resolution as low as a few hundred um defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with sev...

View Video Presentation: https://doi.org/10.2514/6.2022-3877.vid The effect of plasma sheath on thermionic current density is usually analyzed assuming that the sheath is isothermal and collisionless. In certain applications, however, the gas pressure can be high enough to yield a collisional sheath. In this study, the deviation of plasma sheath ch...

View Video Presentation: https://doi.org/10.2514/6.2022-3580.vid Laboratory experiments on electron transpiration cooling (ETC) are conducted using a rapidly modulated laser as a controllable means of surface heating. The modulated laser source can apply a time-varying surface heat influx in a well-determined manner, correspondingly controlling the...

View Video Presentation: https://doi.org/10.2514/6.2022-3440.vid Nonequilibrium effects are important to accurately predict optical distortions in high-speed flows. In this paper, the refractive index of a pure nitrogen flow was simulated at different cross-sections over a wedge at Mach number M=20. A semi-classical model, based on characteristic a...

View Video Presentation: https://doi.org/10.2514/6.2022-3365.vid In this work, we demonstrate a new high throughput, high resolution spectroscopic technique based on dispersion in cesium atomic vapor and use it to image the Rayleigh-Brillouin spectrum from air. This is accomplished by utilizing a prism shaped cesium vapor cell and taking advantage...

View Video Presentation: https://doi.org/10.2514/6.2022-3477.vid Femtosecond laser electronic excitation tagging was applied in a transitioning boundary layer behind an array of turbulence inducing trips. The Texas A&M University Actively Controlled Expansion tunnel in which FLEET was performed operated at Mach 5.7 at a Reynolds number of 5.2 × 10^...

View Video Presentation: https://doi.org/10.2514/6.2022-3495.vid In this study, we examine the effect of finite collection solid angle on the integrated spectral lineshape of Brillouin scattering of water in a forward scattering geometry. This geometry leads to spectral line shifts of the Brillouin sidebands and provides guidance for similar spectr...

A model is developed for a collisionless plasma sheath assuming isentropic electrons, in contrast to the standard isothermal electron assumption. The conservation of entropy leads to a modified Boltzmann relation and a modified Bohm criterion. The predicted floating sheath potential is in excellent agreement with experimental data. Takamura's model...

The concept of electron transpiration cooling (ETC) uses thermionic emission of electrons from a low work function material to cool surfaces in hypersonic flight. A theoretical estimate of the emission current is given by the Richardson-Dushman equation. In hypersonic flights, the emission current can deviate from this estimate as the ambient air is...

View Video Presentation: https://doi.org/10.2514/6.2022-0985.vid Spatial and temporal variations of air density due to atmospheric turbulence lead to wavefront distortions and irradiance fluctuations in free-space laser propagation. Many factors, including the specific structure of the refractive index fluctuations, determine the amplitude and spec...

A rapid surface temperature drop is observed on a negatively biased, thermally controlled, thermionic emitter at the initiation of an "ignited mode" plasma in argon and attributed to electron transpiration cooling (ETC). A transient current surge accompanies the observed rapid temperature drop, indicating that the ignited mode eliminates space char...

View Video Presentation: https://doi.org/10.2514/6.2022-0067.vid Unsteady flow phenomena induced by shock interactions in hypersonic propulsion flowpaths can significantly impact the performance and robustness of flight vehicles but remain imperfectly characterized and present significant modeling and prediction challenges. While several canonical...

View Video Presentation: https://doi.org/10.2514/6.2022-0981.vid In hypersonic flights, the intense ionization of the surrounding air leads to the formation of a weakly ionized plasma nearby the vehicle surface. If the electron oscillation frequency in the plasma is higher than the communication signal frequency, the plasma blocks the electromagnet...

View Video Presentation: https://doi.org/10.2514/6.2022-0986.vid We consider a combination of the analytical solution for the turbulent pulsations of temperature, shell model of turbulence and ray tracing algorithm to simulate the refractive index spectrum behavior. The approach give us a possibility to capture changes of the refractive index struc...

View Video Presentation: https://doi.org/10.2514/6.2022-1747.vid In this study, we calculate the delayed output of light passing an atomic vapor cell where the input signal is relatively broad. We use a deconvolution/convolution method to decompose a signal into Gaussian portions. We calculate the outputs of these portions and add them together to...

View Video Presentation: https://doi.org/10.2514/6.2022-1525.vid The development of laser diagnostic methods based on Optical Parametric Oscillators (OPOs) has enabled high speed non-intrusive measurements of velocity, temperature and species composition in unsteady hypersonic flows at repetition rates exceeding dye laser performance. Here, we repo...

View Video Presentation: https://doi.org/10.2514/6.2022-0982.vid Air flow around a hypersonic vehicle is subjected to shocks, which can raise the temperature of the surrounding air and create a weakly ionized plasma near the surface. Electron transpiration cooling (ETC) is proposed as a cooling mechanism for hypersonic vehicles to counter the heat...

Modern “non-intrusive” optical methods are providing revolutionary capabilities for diagnostics of hypersonic flow fields. They generate accurate information on the performance of ground test facilities and provide local time accurate measurements of near-wall and off-body flow fields surrounding hypersonic test articles. They can follow the true m...

Coherent microwave scattering from laser-induced plasmas, including weakly ionized plasma, laser sparks, multiphoton ionization, and resonance enhanced multi-photon ionizations (radar REMPI) has achieved much successes in plasma, reactive and nonreactive flow diagnostics. Under illumination of microwaves (radar), electrons inside the laser-induced...

View Video Presentation: https://doi.org/10.2514/6.2021-3112.vid The dispersion effects caused by refractive index gradients of atomic vapors such as mercury, rubidium, cesium, potassium and sodium can be used for dispersion of Rayleigh scattering. A multi-prism atomic vapor cell based technique has multiple surface interactions between the glass a...

View Video Presentation: https://doi.org/10.2514/6.2021-3119.vid We consider the concept of electron transpiration cooling for leading edges of hypersonic vehicles, which is based on the thermionic emission of electrons from the surface, covered by a liquid cesium layer to lower the work function. The electron emission results in a net heat flux fr...

View Video Presentation: https://doi.org/10.2514/6.2021-3113.vid In this study, we examine the effect of finite solid angle collection on the integrated lineshape of Thomson and Rayleigh-Brillouin scattering. This angular averaging process can be especially important when capturing signals with a fast lens or scattering in the near-forward directio...

The energy flux of a nanosecond pulsed cold atmospheric pressure (CAP) plasma jet in contact with a substrate surface was measured to improve the understanding of the correlation between energy flux, flow dynamics and applied electrical power. The flow pattern properties of the CAP jet were imaged using Rayleigh scattering showing a transition from...

Submitted for the TSS21 Meeting of The American Physical Society: Analysis of the effect of the flux functions, boundary conditions, and Bohm criterion on the stability of plasma sheaths in the presence of the thermionic emission.

The long-term hydrodynamics of laser generated plasma regions are dictated by the spatial distribution of the deposited laser energy. Using an integrated chemical-optical solver to track energy deposition from single and dual nanosecond laser pulses, we show that self-defocusing results in a biased growth of the plasma region towards the focusing l...

We considered plasma decay in pure water vapor at high temperatures. The conductivity decay in a pure water vapor was significantly slower compared with the plasma decay in the mixture of water vapor with argon leading the 50 % conductivity decrease at the microsecond time scale. The effect of the depopulation of the tail of the electron energy dis...

We report the first application of Femtosecond Laser Electronic Excitation Tagging (FLEET) and Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy to measurement of velocity and, respectively temperature in Mach 18 nitrogen flow at the AEDC Hypervelocity Wind Tunnel 9. We have refined our measurement system used in previous testing campaigns...

In order to evaluate the effects of both gas composition and multiple pulses on nanosecond repetitively pulsed (NRP) discharges, spatially and temporally resolved measurements of plasma parameters in mixtures of argon and water vapor have been performed using laser Thomson scattering (LTS) and laser Rayleigh scattering (LRS). Bursts of 6-ns, 14-kV...

The present paper is concerned with the computational fluid dynamic model of a cesium-wetted tungsten surface and associated thermionic emission under hypersonic flight conditions. A set of Navier-Stokes equations coupled with the finite-rate cesium-air plasma model and Ohm's law is used to model a two-dimensional viscous, thermally conductive, and...

We considered two shell models of turbulence to simulate turbulent cascade behavior for buoyancy driven turbulence. The statistical properties of turbulence do not change with the buoyancy force but the actual effect of the temperature gradient is seen directly in density pulsations and total entropy dynamics in a wave space. It was shown that the...

Femtosecond laser electronic excitation tagging velocimetry is experimentally characterized in pure nitrogen at pressures (0.228–101 kPa) and temperatures (72–298 K) relevant to high-speed ground testing. Evaluated metrics of performance include signal intensity/lifetime and velocity accuracy/precision. A fast-framing camera lens-coupled to a bandp...