Acoustic effects of sprays

Since the early 1960's, it has been known that realistic combustion models for liquid fuel rocket engines should contain at least a rudimentary treatment of atomization and spray physics. This is of particular importance in transient operations. It has long been recognized that spray characteristics and droplet vaporization physics play a fundamental role in determining the stability behavior of liquid fuel rocket motors. This paper gives an overview of work in progress on design of a numerical algorithm for practical studies of combustion instabilities in liquid rocket motors. For flexibility, the algorithm is composed of semi-independent solution modules, accounting for different physical processes. Current findings are report and future work is indicated. The main emphasis of this research is the development of an efficient treatment to interactions between acoustic fields and liquid fuel/oxidizer sprays

Development of a CFD Code for Analysis of Fluid Dynamic Forces in Seals

The aim is to develop a 3-D computational fluid dynamics (CFD) code for the analysis of fluid flow in cylindrical seals and evaluation of the dynamic forces on the seals. This code is expected to serve as a scientific tool for detailed flow analysis as well as a check for the accuracy of the 2D industrial codes. The features necessary in the CFD code are outlined. The initial focus was to develop or modify and implement new techniques and physical models. These include collocated grid formulation, rotating coordinate frames and moving grid formulation. Other advanced numerical techniques include higher order spatial and temporal differencing and an efficient linear equation solver. These techniques were implemented in a 2D flow solver for initial testing. Several benchmark test cases were computed using the 2D code, and the results of these were compared to analytical solutions or experimental data to check the accuracy. Tests presented here include planar wedge flow, flow due to an enclosed rotor, and flow in a 2D seal with a whirling rotor. Comparisons between numerical and experimental results for an annular seal and a 7-cavity labyrinth seal are also included

Multiscale Modelling of Blast-Induced TBI Mechanobiology - From Body to Neuron to Molecule

Blast induced Traumatic Brain Injury (bTBI) has become a signature wound of the recent military operations and is becoming a significant factor of recent civilian blast explosion events. In spite of significant clinical and preclinical research on TBI, current understanding of injury mechanisms is limited and little is known about the short and long-term outcomes. Mathematical models of bTBI may provide capabilities to study brain injury mechanisms, perhaps accelerating the development of neuroprotective strategies and aiding in the development of improved personal protective equipment. The paper presents a novel multiscale simulation framework that couples the body/brain scale biomechanics with micro-scale mechanobiology to study the effects of “primary” micro-damage to neuro-axonal structures with the “secondary” injury and repair mechanisms. Our results show that oligodendrocyte myelinating processes distribute strains among neighbor axons and cause their off-axis deformations. Similar effects have been observed at the finer scale for the Tau-Microtubule interaction. The paper also discusses the need for coupled modeling of primary injury biomechanics, secondary injury mechanobiology and model based assessment of injury severity scores. A new integrated computational and experimental approach is described coupling micro-scale injury criteria for the primary micro-mechanical damage to brain tissue/cells as well as to investigate various secondary injury mechanisms.

Different anatomic components, material models and corresponding material parameters used in the finite element model.

<p>In this table, ρ is the material density,E is the young's modulus, ν is the poisson ratio, C₀ is the speed of sound, S is the linear Hugoniot slope coefficient, Γ₀ is the Gruneisen gamma at the reference state, η is the shear viscosity, α is the Ogden material constant, C₀₁ and C₁₀ are the Mooney-Rivlin material constants, K is the bulk modulus, μ is the shear modulus.</p

Do blast induced skull flexures result in axonal deformation?

<div><p>Subject-specific computer models (male and female) of the human head were used to investigate the possible axonal deformation resulting from the primary phase blast-induced skull flexures. The corresponding axonal tractography was explicitly incorporated into these finite element models using a recently developed technique based on the embedded finite element method. These models were subjected to extensive verification against experimental studies which examined their pressure and displacement response under a wide range of loading conditions. Once verified, a parametric study was developed to investigate the axonal deformation for a wide range of loading overpressures and directions as well as varying cerebrospinal fluid (CSF) material models. This study focuses on early times during a blast event, just as the shock transverses the skull (< 5 milliseconds). Corresponding boundary conditions were applied to eliminate the rotation effects and the resulting axonal deformation. A total of 138 simulations were developed– 128 simulations for studying the different loading scenarios and 10 simulations for studying the effects of CSF material model variance–leading to a total of 10,702 simulation core hours. Extreme strains and strain rates along each of the fiber tracts in each of these scenarios were documented and presented here. The results suggest that the blast-induced skull flexures result in strain rates as high as 150–378 s^-1. These high-strain rates of the axonal fiber tracts, caused by flexural displacement of the skull, could lead to a rate dependent micro-structural axonal damage, as pointed by other researchers.</p></div

Table showing the maximum axonal strains and strain rates using different CSF material descriptions.

<p>A frontal blast loading simulation of the model head model was developed with the different CSF material descriptions and used it to tabulate the above results.</p

CORA ratings for the different brain-skull relative displacement validation plots (impact loading–parietal).

<p>Models were subjected to impact loading conditions, same as that of the experimental study by Hardy et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190881#pone.0190881.ref035" target="_blank">35</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190881#pone.0190881.ref036" target="_blank">36</a>].</p

Flexural bending displacements experienced by the skull and the resulting strain rates experienced by the axonal fiber tracts for an overpressure loading of 600 kPa in anterior-posterior direction.

<p>A maximum axonal strain rate of 80 s^-1 was observed in this scenario. This figure also emphasizes the fact that embedded element based head model allows for a high-resolution visualization of the model. (i) Flexural displacements of the skull at (a) t = 1 ms. (b) t = 2 ms. (c) t = 3 ms. (d) t = 4 ms. (ii) Strain rates experienced by the axonal fiber tracts (e) t = 1 ms. (f) t = 2 ms. (g) t = 3ms. (h) t = 4ms. Here, the red color cross-sectional view of the skull represents the original skull shape while the blue color cross-sectional view represents the displaced skull’s cross-sectional view.</p

Different loading conditions were used to determine the effect of variation in loading (direction and magnitude) on the axonal response.

<p>(a) ConWep blast loading curves. The different loading magnitudes simulated here include 1500 kPa, 1200 kPa, 900 kPa, 600 kPa, 300 kPa, 200 kPa, 100 kPa, 50 kPa. These blast loads are simulated using the ConWep tool in ABAQUS. (b) Blast loading conditions in comparison to Bowen’s lung threshold curve. This plot shows that all the loading conditions opted here fall below the threshold—indicating that the injury will not result in the death of the subject. (c) Arrangement of detonation points around the head form. This arrangement allows us to study the effect of variation in loading direction on the resulting axonal response. (d) Table shows the different ConWep parameters (Overpressure, ConWep charge, Detonation Distance and Positive Phase Duration) for the corresponding loading values used in this paper.</p

CORA ratings for the different intracranial pressure validation plots.

<p>Models were subjected to impact loading conditions same as that of the experimental study by Nahum et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190881#pone.0190881.ref034" target="_blank">34</a>].</p