Propagation Code Research Articles

Investigation of the compression–shear fracture propagation for rocks accounting for confining pressure and crack surface friction

A series of fracture experiments were carried out to study the coupling effects of confining pressure and crack surface friction on the propagation of compression-shear fracture in a self-developed triaxial compartment. Meanwhile, a fracture propagation code (FPC) that can effectively simulate the initiation and propagation of closed cracks was developed for the corresponding numerical research. The results show that the crack of the centrally cracked Brazilian disc (CCBD) specimen is still mainly of tension-shear fracture in a certain range of crack inclination angle under a high confining pressure coefficient. Compared with confining pressure, the influence of crack surface friction on fracture propagation is more prominent after crack closure. In addition, the fracture mode for closed crack is determined by confining pressure, crack surface friction, relative crack length and crack inclination angle.

Implementation of a sonic boom propagation algorithm with the augmented Burgers equation

For the purpose of sonic boom carpet determination for an optimized routing of supersonic aircraft around landmasses, a sonic boom propagation code was implemented at the Institute of Air Transportation Systems. The implementation is based on the well-known augmented Burgers equation for stratified atmospheres and extends the sole geometric ray tracing that has been used at the institute to determine the lateral cutoff distance of sonic booms so far. For the quantification of sonic boom signatures on ground, algorithms for the computation of noise metrices, namely, the perceived level of noise Stevens Mk VII (PL) and the sound exposure level (SEL) with several frequency weightings are also implemented. This work describes the implementation of the sonic boom propagation algorithm and the methods used for loudness calculation briefly. For the evaluation of the implemented code, pre-defined cases from the Third Sonic Boom Prediction Workshop are simulated, and the computed results are compared to the published results of that workshop. The similarities and differences are discussed in detail, such that the general implementation of the propagation code can be verified, whereas potential inaccuracies, especially for the off-track determination, can be detected and quantified.

Characterization of High Harmonic Frequencies in Reactor Noise Experiments Within the CORTEX Project

We present a novel technique of neutron noise detection and experimental data interpretation developed during the EU H2020 project CORTEX aiming to improve the capabilities for identification and localization of neutron noise sources. The experimental data analysis is performed in the frequency domain by extracting the spectral power density and the phase angle using a novel spectral variance reduction technique based on per cycle-based bootstrapping with replacement. This technique allows for variance reduction of measured spectral power and phase angle not only at base frequency, but at higher harmonic frequency contributions as well. This allows for a more representative treatment of experimental data and validation of codes for neutron noise propagation, some of which have been developed within the project. The detector response is not necessarily linearly dependent on the oscillator movement, and the study of nonlinear terms provides additional information which can improve the accuracy of neutron noise source identification and localization. Moreover, these terms can be used in a Taylor series to identify a more complex dependence. The process is simplified in the sense that these contributions are linearized in the frequency domain as higher harmonic frequency contributions and can be easily identified and extracted due to spectral peaks prominence provided by the bootstrapping with replacement method. Combined with the spectral power and phase, we present a preliminary investigation of usability of higher-order terms for noise source identification and localization. In this article, we outline the CORTEX project, the experiments, and the measurement analysis methodology based on the bootstrapping with replacement along with the initial developments on the study of nonlinear terms in CROCUS reactor, using several different noise source configurations at a set frequency.

Accelerating IceCube’s Photon Propagation Code with CUDA

The IceCube Neutrino Observatory is a cubic kilometer neutrino detector located at the geographic South Pole designed to detect high-energy astrophysical neutrinos. To thoroughly understand the detected neutrinos and their properties, the detector response to signal and background has to be modeled using Monte Carlo techniques. An integral part of these studies are the optical properties of the ice the observatory is built into. The simulated propagation of individual photons from particles produced by neutrino interactions in the ice can be greatly accelerated using graphics processing units (GPUs). In this paper, we (a collaboration between NVIDIA and IceCube) reduced the propagation time per photon by a factor of up to 3 on the same GPU. We achieved this by porting the OpenCL parts of the program to CUDA and optimizing the performance. This involved careful analysis and multiple changes to the algorithm. We also ported the code to NVIDIA OptiX to handle the collision detection. The hand-tuned CUDA algorithm turned out to be faster than OptiX. It exploits detector geometry and only a small fraction of photons ever travel close to one of the detectors.

Systematic modeling of electrostatic radiation shields for deep space flight

Astronauts are exposed to a unique radiation environment during space missions. This environment is dominated by high charge and energy (HZE) ions with sources that include solar energetic particles (SEPs), galactic cosmic rays (GCRs), and trapped electrons and protons around planetary bodies with substantial magnetic fields. Traditional shielding strategies aim to reduce astronaut radiation exposure by increasing vehicle mass, but this is ineffective for the extremely penetrating GCRs. Use of electromagnetic fields has been investigated previously, but these studies have been limited to single point designs. The NASA Active Shielding project has instead developed a workflow that includes a fast and accurate particle propagation code (Active Shielding Particle Pusher), which can be used to rapidly evaluate the shielding efficacy of many different permutations of electromagnetic shielding designs. While some validation of the code has been previously done, additional validation is needed to increase user confidence in simulation results involving more complicated electrostatic conductor configurations. In the present study, we evaluated shielding properties of spherical electrode arrays and planar grid structures against 2 MeV or 6 MeV H+ and 22.5 MeV Fe6+ ions using ASPP, SIMION, and COMSOL, and compared these results to laboratory-based measurements. Many numerical experiments were conducted to reproduce experimental ion beam tests that measure the consequential particle reduction rate in a safe zone compared to the initial number of ions. The experimental visualizations were also compared with the present numerical results and found to be in close agreement with each other for all considered voltage combinations applied to electrode structures. This comparison allows us to use numerical models to predict the behavior of complex shielding structures at much larger scales not feasible in a laboratory environment.

Wire, hybrid, and laser-cut X-pinches as Talbot–Lau backlighters for electron density diagnostics

Talbot–Lau x-ray deflectometry (TXD) enables refraction-based imaging for high-energy-density physics experiments, and thus, it has been studied and developed with the goal of diagnosing plasmas relevant to inertial confinement and magnetic liner inertial fusion. X-pinches, known for reliably generating fast (∼1 ns), small (∼1 µm) x-ray sources, were driven on the compact current driver generator for ablation structure and implosion studies (∼200 kA, 150 ns) as a potential backlighter source for TXD. Considering that different X-pinch configurations have characteristic advantages and drawbacks as x-ray generating loads, three distinct copper X-pinch configurations were studied: the wire X-pinch, the hybrid X-pinch, and the laser-cut X-pinch. The Cu K-shell emission from each configuration was characterized and analyzed regarding the specific backlighter requirements for an 8 keV TXD system: spatial and temporal resolution, number of sources, time of emission, spectrum, and reproducibility. Recommendations for future experimental improvements and applications are presented. The electron density of static objects was retrieved from Moiré images obtained through TXD. This allowed to calculate the mass density of static samples within 4% of the expected value for laser-cut X-pinches, which were found to be the optimal X-pinch configuration for TXD due to their high reproducibility, small source size (⩽5 µm), short duration (∼1 ns), and up to 106 W peak power near 8 keV photon energy. Plasma loads were imaged through TXD for the first-time using laser-cut X-pinch backlighting. Experimental images were compared with simulations from the x-ray wave-front propagation code, demonstrating that TXD can be a powerful x-ray refraction-based diagnostic for dense Z-pinch loads. Future plans for Talbot–Lau interferometry diagnostics in the pulsed-power environment are described.

Gold-nanoparticle-enriched breast tissue in breast cancer treatment using the INTRABEAM® system: a Monte Carlo study.

Using a 50-kV INTRABEAM® system after breast-conserving surgery, breast skin injury and long treatment time remain the challenging problems when large-size spherical applicators are used. This study has aimed to address these problems using gold (Au) nanoparticles (NPs). For this, surface and isotropic doses were measured using a Gafchromic EBT3 film and a water phantom. The particle propagation code EGSnrc/Epp was used to score the corresponding doses using a geometry similar to that used in the measurements. The simulation was validated using a gamma index of 2%/2mm acceptance criterion in the gamma analysis. After validation Au-NP-enriched breast tissue was simulated to quantify any breast skin dose reduction and shortening of treatment time. It turned out that the gamma value deduced for validation of the simulation was in an acceptable range (i.e., less than one). For 20mg-Au/g-breast tissue, the calculated Dose Enhancement Ratio (DER) of the breast skin was 0.412 and 0.414 using applicators with diameters of 1.5cm and 5cm, respectively. The corresponding treatment times were shortened by 72.22% and 72.30% at 20mg-Au/g-breast tissue concentration, respectively. It is concluded that Au-NP-enriched breast tissue shows significant advantages, such as reducing the radiation dose received by the breast skin as well as shortening the treatment time. Additionally, the DERs were not significantly dependent on the size of the applicators.

Fire-spotting generated fires. Part II: The role of flame geometry and slope

This is the second part of a series of two papers concerning fire-spotting generated fires. While, in the first part, we focus on the impact of macro-scale factors on the growth of the burning area by considering the atmospheric stability conditions, in the present study we focus on the impact of meso-scale factors by considering the effects of the flame geometry and terrain slope. First, we discuss the phenomenological power law that relates flame length and fireline intensity by reporting literature data, analysing a formula originally proposed by Albini, and deriving an alternative formula based on the energy conservation principle. Subsequently, we extend the physical fire-spotting parametrisation RandomFrontadopted in the first part by including flame geometry and slope. Numerical examples show that fire-spotting is affected by flame geometry and, therefore, cannot be neglected in simplified fire-spread models used in operational software codes for wildfire propagation. Meanwhile, we observe that terrain slope enhances the spread of a fire at a higher rate than the augmentation of fire-spotting generated fires, such that a rapid merging occurs among independent fires.

Progress toward an acoustic point to point ray tracing method in a stratified atmosphere with wind

NASA is investigating acoustic propagation capabilities to accommodate a point to point propagation prediction scheme defining a ray between a specific source–observer pair in an inhomogeneous stratified atmosphere with wind. A 4D space-time ray method based on the null geodesic equations yields two main benefits over classical 3D Euclidian reduced ray equations: (1) it poses a valid two point boundary value problem for the rays connecting two locations; and (2) the solution method is valid throughout the whole domain including at ray turning points without the need for piecewise definitions. The resulting two point boundary value problem and the constant horizontal slowness vectors, which are a result of the stratified medium assumption, allow the desired (t, x, y, z) ray trace. The presentation highlights the equations and inputs required to solve the 4D ray trace. The solution to this two point boundary value problem will create the groundwork for efficient future NASA propagation codes.

Acoustic propagation in heterogeneous environments and its implications on the active space of rock ptarmigan

Acoustic communication networks among birds are widely studied in homogeneous environment like tropical forest or open field. There is no study in heterogeneous environment to our knowledge. However, the propagation of an acoustic signal is strongly influenced by topography, meteorological and ground effects, especially at long distance. For bird species living in high mountains, these effects can have a prominent impact on how their vocalization spreads. Here we try to redefine the active space notion, using the rock ptarmigan as a model. We develop a sound propagation code dedicated to bioacoustic studies based on the parabolic equation method and taking into account the above mentioned effects. Propagation measurements were carried out at a mountain site to provide information on typical conditions encountered and to test the validity of our model in this context. Using this model, we describe how the mountainous conditions affect active spaces of communication compared to typical homogeneous situations. We further test how singing during display flights can be a good strategy for a transmitter to expand its active space in such a context. A fine modeling of the propagation of acoustic signals is likely to provide important cues for understanding communication networks of mountainous species.

A mixed Fourier transform for impedance boundary conditions of acoustic waves at grazing incidence

Acoustic propagation in the atmosphere near the Earth’s surface at grazing incidence is influenced by ground impedance. Consequently, numerical simulations must use an impedance ground boundary condition in order to accurately predict ground effects associated with grazing incidence. A mixed Fourier transform method for impedance boundary conditions has been developed for parabolic approaches by the electromagnetics community. Here, the mixed Fourier transform method is adapted and validated for acoustic wave propagation. The one-way solution of the Helmholtz equation is computed with a modified angular spectrum approach to account for a locally reacting surface using a mixed Fourier transform in the vertical direction. Numerical simulations are presented that focus on sonic boom propagation near the ground in the shadow zone. The simulations demonstrate the utility of the mixed Fourier transform method to account for ground impedance effects that are ignored by ray theory-based propagation codes. Consequently, the method extends sonic boom prediction to the lateral cutoff and shadow zone regions. [This research is supported by the Commercial Supersonic Technology Project of the National Aeronautics and Space Administration under Grant No. 80NSSC19K1685.]

STORM: A semi-analytical orbit propagator for assessing the compliance with Mars Planetary Protection Requirements

STORM is a general semi-analytical propagation code developed at Airbus Defence and Space and originally designed to perform long-term Mars orbit simulations, for mission analysis purposes. Since the 1960’s, humans have sent 18 spacecraft to study the Red Planet, 8 of which are active as of February 2021. While space-debris surveillance and mitigation have been gathering momentum in the past decades in the case of Earth, the issue of artificial objects left in orbit around Mars is another cause for concern in view of the Committee of Space Research’s Planetary Protection Policy. In that perspective, STORM is introduced as a potential tool for assessing the compliance of spacecraft with orbital lifetime requirements. The programme makes it possible to study the evolution of orbits over several decades by propagating a set of mean equinoctial orbital elements, with a comprehensive set of gravitational and non-gravitational perturbations. Particular emphasis is laid on atmospheric entry cases as the programme is interfaced with the European Mars Climate Database, which is the current best knowledge of the state of the Martian atmosphere. This work is focused on the semi-analytical propagator’s implementation and its actual use for predicting entry dates. A sensitivity analysis is conducted to ascertain the influence of atmosphere modelling and initial conditions, providing insight into how to account for uncertainties in a probabilistic way.

Propagation Analysis Using sBOOM for the Third AIAA Sonic Boom Prediction Workshop

The Third Sonic Boom Prediction Workshop was held on January 4–5, 2020, in conjunction with the AIAA SciTech conference. The workshop had 23 participants who represented 6 different countries and 18 unique organizations that included government, industry, and academia. The workshop was attended by over 50 individuals. The motivation for the workshop stems from the goal of obtaining supersonic commercial overland flight. To replace the current prohibition with a certification standard, an international effort is required to quantify the accuracy and reliability of prediction methods. The workshops also identify deficiencies in existing methods where further research should be focused. The workshop consisted of two days: the first day focused on near-field computational fluid dynamics and the second day on propagation techniques. For the atmospheric propagation portion of the workshop, two required cases and one optional case were prescribed for the participants to exercise their propagation implementations. This paper presents and discusses the results that were obtained using the sonic boom propagation code sBOOM. Results show that sonic boom carpets could be much different under arbitrary atmospheric conditions as opposed to standard atmospheric conditions. Additionally, several unique features when using sBOOM are presented.

Magnetic diffusion and interaction effects on ultrahigh energy cosmic rays: Protons and nuclei

The flux of ultrahigh energy cosmic rays reaching the Earth is affected by the interactions with the cosmic radiation backgrounds as well as with the magnetic fields that are present along their trajectories. We combine the SimProp cosmic ray propagation code with a routine that allows to account for the average effects of a turbulent magnetic field on the direction of propagation of the particles. We compute in this way the modification of the spectrum which is due to the magnetic horizon effect, both for primary nuclei as well as for the secondary nuclei resulting from the photo-disintegration of the primary ones. We also provide analytic parameterizations of the attenuation effects, as a function of the magnetic field parameters and of the density of cosmic ray sources, which make it possible to obtain the expected spectra in the presence of the magnetic fields from the spectra that would be obtained in the absence of magnetic fields. The discrete nature of the distribution of sources with finite density also affects the spectrum of cosmic rays at the highest energies where the flux is suppressed due to the interactions with the radiation backgrounds, and parameterizations of these effects are obtained.

Inverse ray tracing on icosahedral tetrahedron grids for non-linear laser plasma interaction coupled to 3D radiation hydrodynamics

A novel approach to efficiently model 3-D laser plasma interactions at fluid scales is presented. This method, implemented in the IFRIIT propagation code developed at CELIA, relies on inverse ray tracing to compute laser fields at arbitrary locations in a plasma. This enables to describe the fields at high order in space compared to standard forward ray tracing approaches. In addition, inverse ray tracing enables the use of etalon integral methods to reconstruct caustic fields and greatly speeds up calculations of cross-beam energy transfer by decoupling the ray amplitude and ray phase calculations. A comparison of the inverse and forward methods for 3-D calculations of fields in presence or not of cross-beam energy transfer illustrates the significant advantages of the inverse method. Conversely, while the inverse method is well suited to most spherical plasma profiles, it currently cannot treat concave profiles or target holders. The coupling of IFRIIT with the 3-D ASTER radiative hydrodynamics code developed at the Laboratory for Laser Energetics is then presented. ASTER and IFRIIT resolve their respective equations on separate grids which communicate through interpolation. As such, IFRIIT uses a dedicated laser grid adapted to the computations at play, which also allows to use different parallelization methods for both codes: block decomposition for the hydrodynamics versus domain duplication for the laser. Applications to direct-drive implosions for inertial confinement fusion are presented, for which a geodesic icosahedron grid is implemented in IFRIIT. The performances of the ASTER/IFRIIT coupling are demonstrated by conducting simulations of cryogenic implosions performed on the OMEGA laser system, in presence of various sources of 3-D effects; laser port geometry, cross-beam energy transfer, beam imbalance and target mis-alignment. Comparison with neutron data, measured through bang-time, for a cryogenic implosion experiment shows an excellent agreement for the laser-plasma coupling.

Impedance models for single and two degree of freedom linings and correlation with grazing flow duct testing

Presented here is the development of a predictive model for impedance of single-degree-of-freedom (SDOF) and two-degree-of-freedom (2DOF) acoustic linings that is suitable for the design stage of suppression of inlet noise for turbo-fan engines. It is required that over a probable range of lining physical parameters and operating conditions the impedance spectrum is predicted with accuracy sufficient to support a lining design process and assessment of achievable attenuation. The starting point is a published impedance model for SDOF linings that primarily focuses on the transfer impedance of conventional and micro-perforate face sheets with grazing flow. This is expanded here to include 2DOF linings, introducing new issues related to transfer impedance of the inserted septum. Problems addressed are related to the septum insertion process that can change thickness, hole diameter and open area ratio of the uninstalled septum, and introduce blockage. Required empiricism is discussed and models for face sheet and septum-in-core transfer impedance are derived, applicable to a specific range of sheet thickness, hole diameter, and open area ratio. Manufacturing processes considered are mechanical drilling in the case of the carbon fiber laminate face sheet that is conventional perforate, and laser drilling in the case of the epoxy film micro-perforate septum material. Benchmarking is carried out by comparison of acoustic field predictions, using the proposed lining model in an FEM propagation code, with measured data from a grazing flow duct facility. Test samples include SDOF and 2DOF linings, including cases with three segments, each with distinct physical properties. Example results of comparisons are shown to highlight the fidelity of the impedance model over a frequency range compatible with the grazing flow duct geometry.

Three-Dimensional, Time-Dependent Analysis of High- and Low-Q Free-Electron Laser Oscillators

Free-electron lasers (FELs) have been designed to operate over virtually the entire electromagnetic spectrum, from microwaves through to X-rays, and in a variety of configurations, including amplifiers and oscillators. Oscillators can operate in both the low and high gain regime and are typically used to improve the spatial and temporal coherence of the light generated. We will discuss various FEL oscillators, ranging from systems with high-quality resonators combined with low-gain undulators, to systems with a low-quality resonator combined with a high-gain undulator line. The FEL gain code MINERVA and wavefront propagation code OPC are used to model the FEL interaction within the undulator and the propagation in the remainder of the oscillator, respectively. We will not only include experimental data for the various systems for comparison when available, but also present, for selected cases, how the two codes can be used to study the effect of mirror aberrations and thermal mirror deformation on FEL performance.

Inferring the Evolution of a Large Earthquake From Its Acoustic Impacts on the Ionosphere

AbstractWe investigate the possibility to constrain the evolution of the 2016 M7.8 Kaikoura earthquake evolution based on Global Positioning System signal‐derived ionospheric total electron content (TEC) perturbations, that represent plasma responses to infrasonic acoustic waves (IAWs) generated by surface motion. This earthquake exhibited unusual complexity and some first‐order aspects of its evolution remain unclear; for example, how and when the Papatea fault (PF) and the corresponding large surface deformation occurred. For various earthquake models, a seismic wave propagation code is used to simulate time‐dependent surface deformations, which then excite IAWs in a 3D compressible nonlinear atmospheric model, coupled with a 2D nonlinear multispecies ionospheric plasma dynamic model. Our preferred finite‐fault model reproduces the amplitudes, shapes, and time epochs of appearance of detected TEC perturbations well. Additionally, the incorporation of the PF, ruptured during the earthquake, results in the closest agreement between simulated and observed near‐zenith vertical TEC perturbations, whereas its absence shows significant discrepancy. This supports the hypothesis that the PF was ruptured during the Kaikoura earthquake. Furthermore, the IAWs and resulting ionospheric plasma disturbances contain additional information on the PF rupture progression, including the timing of initiation and propagation direction, indicating new opportunities to further constrain the PF rupture with low elevation angle “slant” TEC data. The results confirm the ability for TEC measurements to constrain evolutions of large crustal earthquakes to provide new insight beyond traditional seismic and geodetic data sets.

Scattering of low-frequency acoustic waves from a moving source by the sea surface.

This paper examines the scattering of a monochromatic acoustic wave by sea-surface gravity waves in the 1-200 Hz frequency range. The source is moving in a straight line at a constant speed, and the acoustic waves are traveling upward in a refractive channel. Considering the scales of the problem, the small perturbation method coupled with the normal-mode theory and an asymptotic analysis are used to derive the first-order scattered pressure field p1. This method, established by Labianca and Harper [J. Acoust. Soc. Am. 61(2), 378-389 (1977)], allows p1 to be expressed with normal-mode functions, which are computed numerically using the in-house modal propagation code MOCTESUMA for any sound-speed profile. The pressure field is calculated in a deep-water configuration with a moving source inside a summer thermocline. First, the spatial distribution of p1 is found to follow the diffraction grating formula. Particular attention is drawn to the border between the propagative and evanescent regimes in which singularities in the theory lead to computational difficulties. Subsequently, the power spectral density of the pressure field is computed and the Doppler sidebands, asymmetrically shifted from the carrier frequency, are examined.

Probability distributions of sonic boom noise metrics in isotropic turbulent fields

Sonic boom waveform distortion caused by isotropic turbulence is examined with numerical simulations of acoustic propagation through randomly generated 3D turbulent fields. A recently developed planar, one way acoustic propagation code is used in this study to simulate the propagation of sonic boom waveforms. Computations of the signal account for diffraction, signal scattering by turbulence, nonlinear distortion, and absorption. Two types of waveforms are considered, one type is an N-wave and the other is a shaped boom. The intensity of the turbulent field is systematically varied for each set of simulations. For each condition, simulations through hundreds of randomly generated fields are conducted to obtain the statistical means and variances of PL, ISBAP, and select weighted SEL metrics. The computational results provide insight on the dependence of the probability distribution of each metric on the turbulence intensity and propagation distance. The data is further examined to compute probability density functions for each metric as a function of the turbulence intensity at different propagation distances. These functions provide insight on the effects of turbulence on the loudness levels of sonic booms. Future work and applicability of the results to atmospheric boundary layer turbulence is considered. [NASA Fellowship Activity Grant No. 80NSSC19K1685.]