Refractive index measurements are critical for characterizing the properties of hypersonic flows, but moderate- to high-pressure experiments require alternative methods to traditional interferometric fringe counting. In this work, we introduce a novel, to the best of our knowledge, multi-wavelength phase-correlation interferometric technique to estimate the refractive index changes across nearly discrete shock wave boundaries and also simultaneously capture optical dispersion and vibrational relaxation times. By comparing the interference pattern of three or more wavelengths against each other, the refractive index can be accurately determined. To demonstrate this technique, laser diodes in two wavelength combinations are tested producing refractive index resolutions on the order of 2.65 × 10-7. Results in air across a range of initial pressure conditions (P1 = 2.66 to 5.33 kPa) and incident wave speeds (Mach 2 to 5) show density changes that agree with theoretical estimates within 2%. Single-shot dispersion and vibrational relaxation measurements with this method also illustrate good agreement with other techniques.

Understanding the thermophysical properties and fracture mechanics of energetic materials is critical for ensuring safety and reliability during manufacturing, transport, and operation. In this work, we utilized digital image correlation (DIC) at high magnification to study the thermal expansion of energetic materials and simulants. The experimental setup and DIC algorithm was validated by measuring thermal expansion of common materials over a range of temperatures. Coefficients of thermal expansion (CTEs) obtained from DIC strain calculations for each material agreed well with available literature values. The validated high magnification experimental setup was then used to measure the CTE of TKP pellets, pentaerythritol crystals (PE), and pentaerythritol tetranitrate crystals (PETN). Additionally, crack formation in single crystals of sucrose and PETN were also imaged with the experimental setup and analyzed with DIC. Overall, this work demonstrates how this robust diagnostic can be used to study thermophysical properties and fracture mechanics in energetic materials. Not only will this contribute to a better understanding of the operation of energetic materials, but also to a better understanding of the effects of aging on the structure and performance.

One of the barriers preventing the adoption of civil supersonic transportation (CST) is the environmental impacts of a prospective CST fleet. Current research utilizing a low-emissions Lean Premixed Prevaporized (LPP) concept combustor shows promising results for potential use in a larger CST engine. However, the validation of combustor stability and emissions metrics are still needed in order to properly model the combustor. In this work, our research centers on the design and characterization of an LPP concept combustor using exhaust sampling and laser diagnostic techniques. This paper discusses the design of the premixer, combustion chamber, water-cooled exhaust, and emissions probe. Experimental results of planar laser-induced fluorescence, OH* chemiluminescence, and relay-imaging digital in-line holography are also presented and discussed. By studying the combustion dynamics for this system, we hope to improve modeling of the LPP combustion process for future supersonic transportation applications.

Detonation engines can produce thrust with few or no moving parts, making these engines compact, lightweight, and capable of high maximum efficiencies. While gas-phase detonations and detonation engines have been studied extensively, there are comparatively fewer studies of liquid-fueled detonations. The complex interactions between liquid breakup, bow shocks, and vaporization in detonation regimes are currently not well understood and more data is needed to accurately model these processes. In this work, we utilize digital inline holography techniques to observe microscale liquid droplet interactions with detonation waves. The design of a detonation tube facility is first discussed followed by a description of the digital inline holography diagnostic. Preliminary experimental pressure trace data from hydrogen-air and hydrogen-oxygen detonations are then illustrated up to Mach 4.14. Next, high temporal resolution holograms of detonation wave interactions with liquid droplets are described. By obtaining high magnification three-dimensional holograms of the droplet breakup phenomena, it is possible to simultaneously image detonation waves and droplets as well as determine breakup, vaporization, and combustion timescales. This information can be used to understand the conditions necessary to promote detonation propagation for liquid-fueled systems. This, in turn, can provide validation data for models and can help improve designs for future liquid-fueled detonation engines.

Laser-induced incandescence (LII) has proven to be a useful diagnostic for studying non-volatile particulate matter (nvPM) in combustion environments. The decay of time-resolved LII (TiRe-LII) signals can be used to estimate soot primary particle sizes. While many models for time-resolved LII exist, there are currently no validated models for the combustion of Jet A fuels at high pressures. In this work, we compare the results from a TiRe-LII model with that of an in-situ extractive soot sampling technique at 1, 2.4, and 3.8 bar pressures in a laminar, prevaporized, premixed Jet A/air combustor. A single camera, single laser shot technique is used to gather planar time-series images with an ultra-high-speed camera. At the same time, soot particles are extracted from the pressurized combustor using a custom designed thermophoretic soot sampler system, and examined through a transmission electron microscope (TEM) to determine its physical dimensions. The TiRe-LII data is validated with the help of the particle size distributions obtained by processing the obtained TEM images. Thus, the model developed from this work can be used to provide estimates of soot particle sizes for complex combustor systems.

Understanding the optical properties of air is essential for the validation and characterization of plasmas and hypersonic flows. Beyond 6000 K, the dissociation of nitrogen and oxygen molecules, along with other reactions, alters the equilibrium composition of air, causing a temperature and pressure dependence in the Gladstone–Dale coefficient. Due to measurement complexities, there is currently very little experimental data to validate model predictions under these conditions. In this work, a unique quadrature fringe imaging interferometer technique is applied to high temperature and pressure measurements of air in the Sandia free-piston high enthalpy shock tube. The diagnostic method combines a narrowband and broadband source to capture large, nearly-discrete changes in the index of refraction by calibrating to interference pattern changes. For the experiments, the reflected shock front is used to generate temperatures between 6000 and 7800 K at pressures up to 300 psi (20 bars). Results behind the shock front exhibit complex flow bifurcation and tail shock feature before equilibrium conditions are reached. Measurements in these flows show close agreement with theoretical predictions of the nonconstant Gladstone–Dale coefficient at high temperatures and high pressures, providing new validation data for chemical equilibrium gas models.

Three-dimensional (3D) printing concepts that combine electrically conductive and electrically insulating materials opens up new opportunities for the design and manufacturing of electromagnetic actuators. While significant research has been conducted to 3D print antennas and planar circuits using silver nanoparticle inks, little focus has been given towards high power (>1 W) actuator applications. In this work, we design a novel 3D printed, centimeter-scale, multi-layer electromagnetic actuator consisting of syringe deposited silver nanoparticle ink on layers of copper-particle-filled polylactic acid (PLA) polymer filament. The Cu-PLA material is not only electrically insulating at moderately high temperatures but is also higher density and more thermally conductive than traditional polymer filaments. These features enable higher operating temperatures, higher burst forces, and longer sustained output. To demonstrate this concept, we first outline the design, material selection, and 3D printing process for a 16-layer, single trace electromagnetic coil. Then, models for the thermal characteristics, force distribution, and mechanical response are developed and compared with experimental results. Measurements show that the electromagnetic coil can produce up to 46 mN of force over 4 mm of stroke with 6.3 W of input power, and can operate indefinitely with 4.2 W of input power at 140 ◦ C without external cooling. Several applications are demonstrated including a small compliant joint gripper and a speaker. Finally, a fully-integrated, multi-material, single-print actuator and gripper combination is demonstrated to illustrate how this work can be used to create fully-operational single-print mechatronic and robotic systems.

Several simultaneous optical and probe-based diagnostics were used to characterize the behavior of a novel lean premixed prevaporized (LPP) combustor with relevance to future commercial supersonic transport (CST) applications. The burner was tested at several fuel/air ratios while subjected to externally applied forcing. The use of fuel planar laser induced fluorescence (PLIF) revealed the stiffness of the fuel injection system to periodic oscillations within the combustor. High-speed OH* chemiluminescence and stereo particle image velocimetry were used to analyze the flow-flame coupling across different cases. Overall, the combustor exhibited no bifurcations as the fuel/air ratio and forcing frequency and amplitude were varied.

Direct injection studies of liquid jets in supersonic crossflows are critical for understanding combustion in scramjet engines. Exploring these fluid dynamic interactions is not only an important step towards characterizing fundamental liquid breakup properties but also key for improving engine design. Understanding instabilities and primary breakup modes, for example, can contribute to improved steady power output, reduced NOx emission, and increased efficiency. Previous studies in the literature have examined how supersonic crossflows affect gaseous and liquid jet breakup characteristics using backlit imaging or schlieren techniques. In this work, we aim to study jet instabilities and droplet breakup characteristics using digital in-line holography. Experiments are conducted with a heated Mach 1.71 crossflow and a transitional regime liquid jet (slenderness ratio L/D of 19) with a diameter of 0.5 mm. High-speed and high-resolution digital in-line holography and schlieren methods are utilized to spatially resolve the jet breakup characteristics near the injection point. By analyzing the time-resolved data of the column instabilities, validation data can be obtained to help improve mathematical models, optimize injector geometry, and refine scramjet engine designs.

Emissions and optical diagnostics were used to characterize a novel lean premixed pre-vaporized (LPP) combustor of relevance to future commercial supersonic transport (CST) applications. The burner consists of four annular flames with cylindrical bluff bodies, stabilized by a swirling pilot flame. Experiments were performed at pressures at and above nominal cruise, with various air inlet temperatures and fuel/air ratios. NOx, CO and unburnt hydrocarbons (UHC) emissions were measured using a traversable water-cooled probe. OH* chemiluminescence, fuel droplet Mie scattering, fuel vapor planar laser induced fluorescence and 2D laser induced incandescence were used to describe the heat release distribution, any residual fuel spray, fuel/air mixing and nonvolatile particulate matter, respectively. The optical diagnostics demonstrate flame structures corresponding to partially premixed bluff-body stabilized flames, albeit with significant spatial and temporal variations. The measured emissions demonstrate an encouraging potential of LPP combustion for CST.

Multi-material three-dimensional (3D) printing methods that combine conductive and non-conductive materials create new opportunities for the design of electrical circuits as well as electromagnetic actuators. While silver nanoparticle inks have previously been used in 3D printing to enable a wide range of dynamic electromechanical systems, most designs have focused on single-layer and micro-scale devices. In this work, we develop a fully 3D printed, centimeter-scale, 12-layer electric solenoid using conductive silver nanoparticle ink deposited via syringe extrusion and non-conductive polymers manufactured via fused deposition modeling. The design and fabrication techniques for the solenoid are first described. Then, a model for the magnetic force distribution in the solenoid is developed and compared with experimental results. Finally, the mechanical response is analyzed using stochastic system identification techniques. Results show that the solenoid can produce up to 15 mN of force with 6 W of input power. A mechanical bandwidth up to 36 Hz was also observed. After characterization, a 3D printed speaker application is demonstrated using this multi-layer additively manufactured solenoid.

The measurement of high temperature gas properties is key for characterizing high speed flows. Nearly discrete changes in density across shock waves, in particular, are difficult to resolve through traditional fringe-counting interferometric methods. Existing techniques for estimating large fringe jumps are either resolution limited or require specialized window configurations. In this Letter, we describe a unique hybrid interferometric technique that combines narrowband fringes for high resolution and broadband fringes as an absolute reference to measure changes in refractive index with a resolution of up to 7 × 10−8 across nearly discrete index changes of up to 1.5 × 10−4. By capturing fringes with an ultrahigh-speed camera, the refractive index changes across discrete shock fronts can be estimated inside a shock tube with high accuracy and time resolution. First, a novel hybrid calibration method for tracking finite fringes is discussed. Next, this technique is used to measure the post-initial-shock refractive indices for Mach 2.7 to 4.2 flows (pressures from 90.4 to 228.4 kPa). Results are then compared with theoretical values showing agreement within 2%.

Titanium particles are commonly used in igniters and pyrotechnics to improve combustion performance. Understanding particle kinetics and dynamics in these systems is vital to improving their functionality and preventing accident scenarios. Despite their importance, most prior published studies focus on isolated titanium particle combustion experiments. In this work, we aim to study in-situ titanium particle combustion statistics in Ti/KClO4 pyrotechnic igniters. To achieve this, we use simultaneous electric field holography, which captures undistorted three-dimensional holograms of the particle field during combustion, and split-image two-color pyrometry, which estimates projected titanium particle surface temperatures. Here, we discuss the measurement theory, present preliminary experimental results, and describe measurement statistics in order to show the feasibility of these diagnostics for studying in-situ titanium particle combustion in extreme pyrotechnic environments.

Accurately measuring aero-optical properties of non-equilibrium gases is critical for characterizing compressible flow dynamics and plasmas. At thermochemical non-equilibrium conditions, excited molecules begin to dissociate, causing optical distortion and non-constant Gladstone-Dale behavior. These regions typically occur behind a strong shock at high temperatures and pressures. Currently, no experimental data exists in the literature due to the small number of facilities capable of reaching such conditions and a lack of diagnostic techniques that can measure index of refraction across large, nearly-discrete gradients. In this work, a quadrature fringe imaging interferometer is applied at the Sandia free-piston high temperature shock tube for high temperature and pressure Gladstone-Dale measurements. This diagnostic resolves high-gradient density changes using a narrowband analog quadrature and broadband reference fringes. Initial simulations for target conditions show large deviations from constant Gladstone-Dale coefficient models and good matches with high temperature and pressure Gladstone-Dale models above 5000 K. Experimental results at 7653 K and 7.87 bar indicate that the index of refraction approaches high temperature and pressure theory, but significant flow bifurcation effects are noted in reflected shock.

Measuring aero-optical properties is key for both characterizing optical aberrations and understanding complex physical processes in compressible flows. However, existing interferometric measurement techniques are unable to validate theoretical estimates of the Gladstone-Dale constant above 6000 K at moderate pressures, particularly in shock-tube environments. In this work, we conduct simulations of the temperature-dependent Gladstone-Dale constant to illustrate regions with nonlinear dependency and where experimental validation data are needed. Then, we develop a quadrature fringe imaging interferometer for measuring index of refraction. By utilizing two quadrature signals for high precision and white light interference for absolute index measurements, this technique is able to track fringe movement across discontinuous shock wave boundaries. Resolution and uncertainty levels are then characterized from calibration measurements. Finally we show techniques for estimating index of refraction from measured images for applications in shock tube environments.

Detonation of explosive devices produces extremely hazardous fragments and hot, luminous fireballs. Prior experimental investigations of these post-detonation environments have primarily considered devices containing hundreds of grams of explosives. While relevant to many applications, such large- scale testing also significantly restricts experimental diagnostics and provides limited data for model validation. As an alternative, the current work proposes experiments and simulations of the fragmentation and fireballs from commercial detonators with less than a gram of high explosive. As demonstrated here, reduced experimental hazards and increased optical access significantly expand the viability of advanced imaging and laser diagnostics. Notable developments include the first known validation of MHz-rate optical fragment tracking and the first ever Coherent Anti-Stokes Raman Scattering (CARS) measures of post-detonation fireball temperatures. While certainly not replacing the need for full-scale verification testing, this work demonstrates new opportunities to accelerate developments of diagnostics and predictive models of post-detonation environments.

In extreme supersonic, hypersonic, or explosive environments, the presence of gas-phase shocks cause coherent imaging distortions and inhibit object tracking. In this work, we aim to remove these distortions by measuring the relative phase of the light using digital phase-sensitive holography techniques including a two-step phase-shifting technique and a single-shot polarization phase-shifting technique. Once the phase of the shock-wave is acquired, the distortion is numerically canceled from the image and the un-distorted object image is recovered. This work discusses the theory, provides simulation results, and presents preliminary experimental data showing how this concept can be applied to remove shock-wave phase distortions created by supersonic air jets.

Aluminum particle combustion is a critical component in solid propellant operation. Understanding these processes is essential for improving specific impulse and other performance metrics. Prior studies of aluminum particle combustion in the literature have focused on spatial and temperature statistics for a single propellant strand size, which is typically significantly smaller than the full grain size used in aerospace and defense applications. In this work, we aim to determine the effect of increasing propellant strand size on several key properties of aluminum particle combustion at atmospheric pressure. To accomplish this, we use simultaneous high speed holography and imaging pyrometry to obtain temporally resolved spatial and temperature information. Here, we discuss how agglomerate size, velocity, and temperature statistics vary as a function of propellant strand size from 6 mm up to 19 mm in diameter. By understanding how the statistics scale as a function of strand size, we can determine how to extrapolate lab-scale experimental data to full-scale propellant burns.

Solid rocket motors operate at high temperatures that can potentially damage engineering materials used for motor nozzles or thrust vanes. In this work, surface temperature measurements were collected on solid rocket motor components of varying materials exposed to the combustion of ammonium perchlorate-based solid propellants. Spatially-resolved temperatures were measured using a novel two-color pyrometer built from an acousto-optic tunable filter (AOTF) coupled to a near IR camera. Temperatures were then verified using a single-point visible spectrometers. Experiments conducted in low pressure conditions inside a high altitude chamber at Sandia National Laboratories illustrate the temporal and spatial evolution of temperature gradients across the nozzle and vanes.