X-ray stereo digital image correlation (DIC) measurements were performed at 10 kHz on the internal surface of a jointed structure in a shock tube at a shock Mach number of 1.42 and compared with optical stereo DIC measurements on the outer, visible surface of the structure. The shock tube environment introduces temperature and density gradients in the gas through which the structure was imaged, resulting in spatial and temporal index of refraction variations. These variations cause bias errors in optical DIC measurements due to beam-steering but have minimal influence on x-ray DIC measurements. These results demonstrate the utility of time-resolved x-ray DIC measurements in complicated environments where optical measurements suffer severe errors and/or are precluded by lack of optical access.
High-enthalpy hypersonic flight represents an application space of significant concern within the current national-security landscape. The hypersonic environment is characterized by high-speed compressible fluid mechanics and complex reacting flow physics, which may present both thermal and chemical nonequilibrium effects. We report on the results of a three-year LDRD effort, funded by the Engineering Sciences Research Foundation (ESRF) investment area, which has been focused on the development and deployment of new high-speed thermochemical diagnostics capabilities for measurements in the high-enthalpy hypersonic environment posed by Sandia's free-piston shock tunnel. The project has additionally sponsored model development efforts, which have added thermal nonequilibrium modeling capabilities to Sandia codes for subsequent design of many of our shock-tunnel experiments. We have cultivated high-speed, chemically specific, laser-diagnostic approaches that are uniquely co-located with Sandia's high-enthalpy hypersonic test facilities. These tools include picosecond and nanosecond coherent anti-Stokes Raman scattering at 100-kHz rates for time-resolved thermometry, including thermal nonequilibrium conditions, and 100-kHz planar laser-induced fluorescence of nitric oxide for chemically specific imaging and velocimetry. Key results from this LDRD project have been documented in a number of journal submissions and conference proceedings, which are cited here. The body of this report is, therefore, concise and summarizes the key results of the project. The reader is directed toward these reference materials and appendices for more detailed discussions of the project results and findings.
A new reflected shock tunnel has been commissioned at Sandia capable of generating hypersonic environments at realistic flight enthalpies. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25 cm core flow with 4.6 MJ/kg total enthalpy is achieved over an approximately 1 millisecond test time. Analysis shows that increasing piston mass should extend test time by a factor of 2-3.
Efforts at Sandia National Laboratories have focused on fundamental experiments to understand the dispersal of dense particle distributions in high-speed compressible flow. The experiments are conducted in shock tube facilities where the flow conditions and the initial conditions of the particle distributions are well controlled and well characterized. An additional advantage of the shock tube is that it is more readily able to accommodate advanced measurement diagnostics in comparison to explosive field tests.
A high-speed temperature diagnostic based on spontaneous Raman scattering (SRS) was demonstrated using a pulse-burst laser. The technique was first benchmarked in near-adiabatic H2-air flames at a data-acquisition rate of 5 kHz using an integrated pulse energy of 1.0 J per realization. Both the measurement precision and accuracy in the flame were within 3% of adiabatic predictions. This technique was then evaluated in a challenging free-piston shock tube environment operated at a shock Mach number of 3.5. SRS thermometry resolved the temperature in post-incident and post-reflected shock flows at a repetition rate of 3 kHz and clearly showed cooling associated with driver expansion waves. Collectively, this Letter represents a major advancement for SRS in impulsive facilities, which had previously been limited to steady state regions or single-shot acquisition.
Here we present results from experiments within Sandia National Labs’ multiphase shock tube on the shock-induced dispersal of dense particle curtains. This study builds on previous work by examining the effect of particle density on the dynamics of a shock-particle interaction in a dense volume fraction regime. We present results gathered from high-speed schlieren images used to track the propagation of the upstream and downstream fronts of the particle curtain. The effect of particle density on the curtain spread rate was examined by comparing curtains comprised of soda lime, stainless steel, and tungsten particles at two distinct volume fractions ϕp = 9% and ϕp ≈ 20%, and various incident shock strengths. Time scales of the spreading process were non-dimensionalized using two scaling methods from literature; one defined by the pressure ratio across a reflected shock and the other related to the incompressible drag through a grid. Both scaling methods successfully collapsed the spreading rate of curtains with different particle densities, while only the drag based scaling could account for variation in volume fraction. In addition, a new scaling based on a simple force balance that uses the pressure ratio across the curtain was found to achieve the tightest collapse of all methods tested.
A high-speed thermometry diagnostic based on spontaneous Raman scattering (SRS) was demonstrated using a pulse-burst laser at a 3-kHz data acquisition rate, with a pulse duration of 200 ns and wavelength of 532 nm. The technique was evaluated in a challenging free-piston shock tube environment operated at conditions up to 1653 K and 112 bar following an incident shock Mach number of 3.5 and a reflected shock Mach number of 2.2. The SRS thermometry resolved the temperature in post-incident and post-reflected shock flows and clearly showed cooling associated with driver expansion waves. A detailed spectral physics model inferred temperatures within 1% of the predicted post-shock temperatures, when SNR was greater than 2.0. This was a significant advancement of spontaneous Raman vibrational thermometry.
Measurements of bifurcated reflected shocks over a wide range of incident shock Mach numbers, 2.9 < Ms < 9.4, are carried out in Sandia’s high temperature shock tube. The size of the non-uniform flow region associated with the bifurcation is measured using high speed schlieren imaging. Measurements of the bifurcation height are compared to historical data from the literature. A correlation for the bifurcation height from Petersen et al. [1] is examined and found to over estimate the bifurcation height for Ms > 6. An improved correlation is introduced that can predict the bifurcation height over the range 2.15 < Ms < 9.4. The time required for the non-uniform flow region to pass over a stationary sensor is also examined. A non-dimensional time related to the induced velocity behind the shock and the distance to the endwall is introduced. This non-dimensional time collapses the data and yields a new correlation that predicts the temporal duration of the bifurcation.
Digital Image Correlation (DIC) is a well-established, non-contact diagnostic technique used to measure shape, displacement and strain of a solid specimen subjected to loading or deformation. However, measurements using standard DIC can have significant errors or be completely infeasible in challenging experiments, such as explosive, combustion, or fluid-structure interaction applications, where beam-steering due to index of refraction variation biases measurements or where the sample is engulfed in flames or soot. To address these challenges, we propose using X-ray imaging instead of visible light imaging for stereo-DIC, since refraction of X-rays is negligible in many situations, and X-rays can penetrate occluding material. Two methods of creating an appropriate pattern for X-ray DIC are presented, both based on adding a dense material in a random speckle pattern on top of a less-dense specimen. A standard dot-calibration target is adapted for X-ray imaging, allowing the common bundle-adjustment calibration process in commercial stereo-DIC software to be used. High-quality X-ray images with sufficient signal-to-noise ratios for DIC are obtained for aluminum specimens with thickness up to 22.2 mm, with a speckle pattern thickness of only 80 μm of tantalum. The accuracy and precision of X-ray DIC measurements are verified through simultaneous optical and X-ray stereo-DIC measurements during rigid in-plane and out-of-plane translations, where errors in the X-ray DIC displacements were approximately 2–10 μm for applied displacements up to 20 mm. Finally, a vast reduction in measurement error—5–20 times reduction of displacement error and 2–3 times reduction of strain error—is demonstrated, by comparing X-ray and optical DIC when a hot plate induced a heterogeneous index of refraction field in the air between the specimen and the imaging systems. Collectively, these results show the feasibility of using X-ray-based stereo-DIC for non-contact measurements in exacting experimental conditions, where optical DIC cannot be used.
Many liquid metals form surface oxides, which can affect atomization processes during thermal spray coating and metal powder formation. In this work, we experimentally investigate the behaviors and morphologies of a liquid metal under a shockwave-induced cross-flow. Specifically, we use Galinstan, a non-toxic room temperature liquid metal that forms thin elastic oxide layers. By utilizing backlit imaging and digital in-line holography (DIH) of liquid columns inside a shock tube, we are able to compare the behavior of Galinstan with water. Morphological differences and drag properties are investigated as a function of Weber number in the bag, multimode, and sheet thinning regimes. We show that surface oxides appear to drive liquid metal Galinstan to break up earlier in non-dimensional time and cause the formation of more non-spherical breakup shapes and droplets. This investigation of surface oxide behaviors helps to further the understanding of liquid metal breakup.
X-ray stereo digital image correlation (DIC) measurements were performed at 10 kHz on a jointed-structure in a shock tube at a shock Mach number of 1.42. The X-ray results were compared to optical DIC using visible light. In the X-ray measurements, an internal surface with a tantalum-epoxy DIC pattern was imaged, whereas the optical DIC imaged an external surface. The environment within the shock tube caused temperature and density gradients in the gas through which the structure was imaged, therefore leading to spatial and temporal index of refraction variations. These variations caused beam-steering effects that resulted in bias error in optical DIC measurements. X-rays were used to mitigate the effects of beam-steering caused by the shock tube environment. Beam displacements measured using X-ray DIC followed similar trends (slopes, oscillations amplitudes and frequencies) as optical DIC data while ignoring beam-steering effects. Power spectral densities of both measurements showed peaks at the natural frequencies of the structure. X-ray DIC also has the advantage of being able to image internal structural responses, whereas optical DIC is only capable of measurements on the outer surface of objects.
A high-speed Raman thermometry diagnostic was evaluated in lean H2-air flames at a data acquisition rate of 5 kHz. Bursts of nanosecond pulses were generated at a 10 kHz burst rate with energy of E ≈ 13 J/burst at λ = 532 nm. The pulses had a duration of ≈ 200 ns and were used to interrogate a stabilized flat flame burner. Spectra were collected using an electron multiplying charge-coupled device (EMCCD) detector. Raman spectra were integrated over the full burst to map adiabatic flame temperature versus equivalence ratio. The measured spectra resolved vibrational band features to infer temperature. A detailed spectral fitting model was used in the burst-integrated and burst-mode spectra. Two pulses were used for each burst-mode measurement resulting in a 5 kHz rate up to flame temperatures of ≈ 2100 K. The measurement precision in burst mode was 23 K and 62 K at flame temperatures of 1160 K and 2080 K, respectively. The measurement accuracy was benchmarked against the spectrally fitted full-burst spectra, chemical equilibrium calculations and previous coherent anti-Stokes Raman scattering (CARS) measurements. In summary, the measurement precision and accuracy were within 3% of the measured and adiabatic equilibrium temperatures, respectively.
A new capability has been added to study shock-particle interactions in the Sandia High-Temperature Shock Tube (HST). The apparatus to do so featured a high-speed pneumatic actuator with high-pressure engineered seals. Like previous studies in a lower-strength facility, the particle curtain was comprised of 100-micron glass spheres at an initial volume fraction of approximately 20%. A shock-particle interaction was investigated using 210 kHz Schlieren imaging where the incident shock Mach number was 3.3. The initially uniform curtain was distorted by recoil in the HST. Nevertheless, the interaction dynamics were observed to be qualitatively similar to those in previous studies. Future efforts will work to decouple the recoil from the curtain formation and push the interaction towards stronger shocks.
Here, experiments were performed within Sandia National Labs’ Multiphase Shock Tube to measure and quantify the shock-induced dispersal of a shock/dense particle curtain interaction. Following interaction with a planar travelling shock wave, schlieren imaging at 75 kHz was used to track the upstream and downstream edges of the curtain. Data were obtained for two particle diameter ranges ($d_{p}=106{-}125$,$300{-}355~\unicode[STIX]{x03BC}\text{m}$) across Mach numbers ranging from 1.24 to 2.02. Using these data, along with data compiled from the literature, the dispersion of a dense curtain was studied for multiple Mach numbers (1.2–2.6), particle sizes ($100{-}1000~\unicode[STIX]{x03BC}\text{m}$) and volume fractions (9–32 %). Data were non-dimensionalized according to two different scaling methods found within the literature, with time scales defined based on either particle propagation time or pressure ratio across a reflected shock. The data refelct that spreading of the particle curtain is a function of the volume fraction, with the effectiveness of each time scale based on the proximity of a given curtain’s volume fraction to the dilute mixture regime. It is observed that volume fraction corrections applied to a traditional particle propagation time scale result in the best collapse of the data between the two time scales tested here. In addition, a constant-thickness regime has been identified, which has not been noted within previous literature.