Initial Density Difference Research Articles

A new perspective on Lüders band formation in medium-Mn steels based on Lüders-strain-rate and dislocation evolution

Propagating Lüders bands with pronounced strain localization are often observed in medium-Mn transformation-induced plasticity (TRIP) steels, which adversely limits the application of such alloys in engineering structures. The present work aims to understand this phenomenon from a dislocation-based perspective, with a focus on the specific role of dislocation multiplication. The material studied is a typical dual-phase medium-Mn steel with a ferrite matrix and a second phase of retained austenite. A warm-rolling process was used as a pre-straining approach to adjust the microstructure of the material, such that materials with two different microstructural states can be obtained: as-received material and pre-strained material. They show distinct differences in initial dislocation density. Stress-controlled rather than conventional strain-controlled tensile testing was adopted, which allowed the Lüders band to develop in an unconstrained manner. In this way, the intrinsic Lüders-strain-rate can be obtained. The results show that the Lüders band-related mechanical parameters, including Lüders strain, Lüders-strain-rate and yield stress drop, all depend on the initial microstructure state and the dislocation evolution process during Lüders band formation. In particular, Lüders-strain-rate is a critical microstructure-sensitive factor. It is closely related to the dislocation multiplication in the formation of Lüders band, which can be characterized and quantified using scanning transmission electron microscope and synchrotron X-ray diffraction. Finally, a quantitative relationship between Lüders-strain-rate and dislocation multiplication rate is established, which allows providing a better understanding of the exceptional Lüders band phenomenon in medium-Mn steels.

Wood Degradation by Fomitiporia mediterranea M. Fischer: Exploring Fungal Adaptation Using Metabolomic Networking

Fomitiporia mediterranea M. Fischer (Fmed) is a white-rot wood-decaying fungus associated with one of the most important and challenging diseases in vineyards: Esca. To relieve microbial degradation, woody plants, including Vitis vinifera, use structural and chemical weapons. Lignin is the most recalcitrant of the wood cell wall structural compounds and contributes to wood durability. Extractives are constitutive or de novo synthesized specialized metabolites that are not covalently bound to wood cell walls and are often associated with antimicrobial properties. Fmed is able to mineralize lignin and detoxify toxic wood extractives, thanks to enzymes such as laccases and peroxidases. Grapevine wood's chemical composition could be involved in Fmed's adaptation to its substrate. This study aimed at deciphering if Fmed uses specific mechanisms to degrade grapevine wood structure and extractives. Three different wood species, grapevine, beech, and oak. were exposed to fungal degradation by two Fmed strains. The well-studied white-rot fungus Trametes versicolor (Tver) was used as a comparison model. A simultaneous degradation pattern was shown for Fmed in the three degraded wood species. Wood mass loss after 7 months for the two fungal species was the highest with low-density oak wood. For the latter wood species, radical differences in initial wood density were observed. No differences between grapevine or beech wood degradation rates were observed after degradation by Fmed or by Tver. Contrary to the Tver secretome, one manganese peroxidase isoform (MnP2l, jgi protein ID 145801) was the most abundant in the Fmed secretome on grapevine wood only. Non-targeted metabolomic analysis was conducted on wood and mycelium samples, using metabolomic networking and public databases (GNPS, MS-DIAL) for metabolite annotations. Chemical differences between non-degraded and degraded woods, and between mycelia grown on different wood species, are discussed. This study highlights Fmed physiological, proteomic and metabolomic traits during wood degradation and thus contributes to a better understanding of its wood degradation mechanisms.

RASER MRI: Magnetic resonance images formed spontaneously exploiting cooperative nonlinear interaction.

The spatial resolution of magnetic resonance imaging (MRI) is limited by the width of Lorentzian point spread functions associated with the transverse relaxation rate 1/T2*. Here, we show a different contrast mechanism in MRI by establishing RASER (radio-frequency amplification by stimulated emission of radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying radio-frequency pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. Small local differences in initial population inversion density can create stronger image contrast than conventional MRI. This different contrast mechanism is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Contrast mechanism and artifacts are explored experimentally and predicted by simulations on the basis of a proposed RASER MRI theory.

Winners and losers in dryland reforestation: species survival, growth, and recruitment along a 33‐year planting chronosequence

Reforestation in the Lower Rio Grande Valley (LRGV) of Texas began in the 1960s and to date over 6,475 ha of land has been reforested. However, there has been minimal assessment to understand differential species success, compositional trends, and the aboveground carbon sequestration potential of these developing forests. We coupled quantitative planting information of >50 native woody tree and shrub species with surveys of 5,223 stems of 4,606 individuals in a chronosequence of restored forests ranging in age from 1 to 33 years to estimate species‐specific mortality rates, biomass accumulation and recruitment, as well as compositional trends in the herbaceous understory. We show that 7–15 years are required for mortality rates of the transplanted cohort to stabilize to background levels observed in other dry forests. A small number of species, mostly N‐fixing trees with a deep rooting habit, persisted on the landscape beyond 15 years. Even so, aboveground biomass (corrected for differences in initial planting density) accumulated at an average rate of 1.41 Mg ha−1 yr−1 compared to 0.35 Mg ha−1 yr−1 for a fallow old‐field. Species biomass growth rates increased with decreasing mortality, as did the abundance of recruits, suggesting a degree of reproduction by initial planted cohorts. However, a suite of highly competitive exotic grasses increases in density over a 25‐year period, which we link to suppressed seedling recruitment. This poses a serious challenge to the long‐term sustainability of planted forests in the LRGV. We highlight potential avenues of research and modification to restoration methodologies.

Three‐dimensional submerged wall jets and their transition to density flows: Morphodynamics and implications for the depositional record

AbstractJets that expand from an orifice into an ambient water body represent a basic flow model for depositional environments related to expanding flows. Momentum‐dominated jets evolve into gravity‐dominated density flows. To understand this transition and its sedimentological relevance, three‐dimensional tank experiments with submerged wall jets were conducted, systematically varying parameters such as the initial density difference, bed slope, grain size and sediment supply. Bedform successions could be subdivided into those related to the jet and those related to the density flow. Jet deposits included early‐stage bedforms, scours and mouth bars. Early‐stage bedforms are asymmetrical dunes that spread concentrically from the orifice. Sediment entrainment by eddies from the expanding jets led to the formation of scours and mouth bars. Flows with lesser initial density difference produced more elongate scours. Conversely, scours became deeper for denser incoming flows. Coarser‐grained sediment caused the formation of higher and steeper mouth bars and vice versa. The transition from momentum‐dominated jets to gravity‐dominated density flows occurred approximately at the mouth‐bar crest. Hydraulic jumps were absent in the expanding jet regions and at the transitions to density flows. Instead, complex flow patterns and circulations were inferred from the velocity measurements within the scour and at the mouth‐bar crests. Bedform trains related to the density flow were controlled by the grain size and sediment supply. Coarse‐grained sediment and high supply rates caused strong mouth‐bar aggradation and flow splitting, leading to the formation of bedform trains laterally adjacent to the mouth bar. Fine‐grained sediment and low supply rates caused the formation of bedform trains downflow of the mouth bar. The symmetrical bedforms deposited by the density flows always displayed an in‐phase relationship with the flow, indicating that they were antidunes. The experimental jet deposits resemble successions known from subaqueous ice‐contact fans and deep water channel‐lobe transition zones.

Hydrodynamics of weakly and strongly stratified two-layer lock-release gravity currents

A series of lock-release experiments are carried out to investigate the hydrodynamics of weakly and strongly stratified two-layer gravity currents (GC) on a flat bottom. A particle image velocimetry (PIV) system combined with an optical method is used to obtain the mixing process, front velocity, and velocity profiles of the two-layer GCs, focusing on the effect of the initial height ratio and lock aspect ratio. For a weakly stratified two-layer GC, the two layers mix thoroughly earlier with a higher initial height ratio and lock aspect ratio. A theoretical model is improved to predict the time that is necessary for the thorough mixing during the propagation of weakly stratified two-layer GCs. For a strongly stratified two-layer GC, a higher initial height ratio leads to a larger velocity of the upper layer because of the larger initial density difference, which results in a shorter time for the upper layer to outrun the lower layer. After the upper layer outruns the lower layer, the current has a constant-velocity phase and then a deceleration phase. The present investigation indicates that the maximum velocity in a vertical velocity profile of a strongly stratified two-layer GC is located at the position near to, or higher than the upper limit for single-layer GCs due to the smaller drag resistance at the upper boundary.

Quantitative determination of rapid biomass formation on pyro-electrified polymer sheets

The ability of a bacterial strain to form a biofilm is strictly related to its pathogenicity. Bacterial adherence and early biofilm formation are influenced by chemical, physical and biological factors that determine their pathogenic properties. We recently presented in literature the ability of pyro-electrified polymer sheets to promote rapid biofilm formation, based on what we called biofilm electrostatic test (BET) carriers. Here we performed a step forward by presenting a comprehensive characterization of the BET methodology through a quantitative evaluation of the biomass on the BET-carrier in the very early stages of incubation. Two bacterial suspensions of Escherichia coli were added to the surface of the BET-carrier, with one order of magnitude difference in initial optical density. The biofilms were stained at different incubation times, while the crystal violet assay and the live/dead reaction kit were used for evaluating the biomass and the viability, respectively. The BET-carrier systematically promoted a faster biofilm formation even in case of very diluted bacterial concentration. The results suggest that the BET-carrier could be used for evaluating rapidly the ability of bacteria to form biofilms and thus their inclination to pathogenicity, thanks to the challenging acceleration in biofilm formation.

Characterization of intruder particle motion in a bladed mixer

Granular mixing is important to many industries, but the mixing process is still poorly understood. In this study, motion of a single intruder particle introduced in a shallow bed of native particles was investigated. A parametric investigation was carried out to quantify the effects of initial intruder particle location, blade speed, and density difference between similarly sized native and intruder particles on the motion of the intruder particle during the mixing process. Intruder particles were color-tagged and videos were obtained from the top free surface of the shallow bed throughout the mixing duration. Additional experiments were performed to investigate the effect of density ratio between the intruder and native particles on the mean radial location (MRL) of the intruder particle. In the shallow bed system, all particles were found to be consistently under the direct influence of the blade and moved in discrete tracks unless randomly excited by the blade or neighboring particles. Intruder particles that were denser than the native particles were found to rotate closer to the hub with a smaller mean radial location, whereas less dense intruders were found to be further away from the mixer center on average. Initial location was important only for the first few rotations, and the blade speed had a negligible impact on the intruder particle flow behavior.

Influence of thermal boundary condition on gas drainage from a semi-sealed narrow channel

Draining a viscous gas from a semi-sealed narrow conduit is a pore-scale problem of fundamental interest to primary fluid recovery from a petroleum reservoir as well as other applications. Such a drainage flow is entirely driven by the volumetric expansion of the gas and its mass flow rate is determined by the time-rate of decrease of the gas density within the conduit. It was found previously that when thermal effect is completely neglected, the drainage rate differs significantly from that based on the lubrication theory. The present work parametrically explores the influence of thermal boundary condition on the non-isothermal drainage flow from a narrow channel. It is found that as the wall transitions from adiabatic to isothermal condition, the excess density changes from a plane wave solution to a non-plane wave solution; and the drainage rate increases due to thermal damping on the wall, as a strong damping of the acoustic wave accelerates the process towards the final equilibrium. It is shown that when the exit is also cooled and the wall is non-adiabatic, the total recovered fluid mass exceeds the amount based on the isothermal theory determined by the initial and final density difference alone. The isothermal wall condition with the wall maintained at the initial gas temperature produces the most amount of fluid in the shortest time.

Dose‐dependent density change of lens epithelial cells after in vivo UVR exposure

AbstractIntroductionOur research group focuses on the mechanism of ultraviolet radiation (UVR) induced lens damage. Posterior capsule opacification (PCO) is the most common complication for cataract surgery. PCO results from the growth and abnormal proliferation of lens epithelial cells (LECs) on the posterior capsule. Lens epithelium is the first target for toxic factors such as ultraviolet radiation. UVR‐300 nm (UVR‐B) is especially an important risk factor for cataract development. Our previous findings show that exposure to UVR‐B can damage the lens epithelium, affect the expression of proteins, induce apoptosis and lead to cataract formation. Therefore, it is possible to prevent PCO by sufficient dose exposure of ultraviolet radiation to residual lens epithelial cells after cataract surgery.PurposeTo determine the effect of different doses of UVR on the distribution of lens epithelial cells (LECs).MethodsAltogether, 40 Sprague Dawley rats were unilaterally exposed in vivo to 1, 3, 6 and 8 kJ/m2 UVR‐300 nm for 15 min. One week after UVR exposure, exposed and contralateral non‐exposed lenses were removed. One midsagittal section from each lens was stained with DAPI for fluorescence microscopy in order to determine the spatial distribution of LECs.ResultsThe LECs density difference between exposed and contralateral non‐exposed eyes at exposure dose of 1, 3, 6 and 8 kJ/m2 was estimated as a CI (95%), −0.1 ± 2.1, 0.1 ± 3.0, −4.1 ± 1.4 and −2.9 ± 2.7 Cell μm−1 10−2, respectively. Data was fit to a linear model considering the initial density difference at 0. The inclination coefficient was estimated as CI (95%), −0.4 ± 0.1 Cell μm kJ−1 1010.ConclusionUVR‐300 nm induces significant LECs density decrease at exposure dose ≥6 kJ/m2. With fitting to a linear model the evolution of dose‐cell density dependence can be determined.

Rollover Phenomenon in Liquefied Natural Gas Storage Tank

Liquefied natural gas (LNG) rollover refers to the sudden mixing of stratified LNG layers, which can cause the generation of significant amounts of boil-off gas which create safety issues significantly in LNG storage tanks. Therefore, understanding of the phenomenon is very important for prevention purpose. Heat leak is a major challenge as it causes slow LNG boil-off leading to LNG stratification and LNG rollover phenomenon which has negative impact on people, environment, company asset and reputation. In this work, rollover phenomenon evolution is modeled and simulated using ANSYS FLUENT 16.0 software to study and analyze the effect of different ambient temperatures on the rollover phenomenon in LNG storage tanks and to study the stress distribution due to rollover and its effects on inner wall of the tank and determine the rollover time. This study examined the effect of different ambient temperatures ranged from 283 to 323 K on the LNG rollover inside a 5000 m3 LNG storage tank. Results show that higher temperature will cause higher heat transfer leaks to the LNG tank. The heat transfer through the tank walls is higher than the heat leaks to the tank through tank base. The pressure inside the LNG storage tank is strongly affected by the surrounding temperature. Higher temperature will cause higher pressure to build up in the tank especially along the area between the roof of the tank and the side walls. The simulations results show that significant initial density differences (∆ρ) between the stratified layers lead to late rollover events but last for longer time. At ∆ρ = 30, 15 and 7 kg/m3, the rollover will last for 3.1, 2.5 and 2.2 h, respectively.

On the total entrained flow rate of artificial downwelling

Artificial downwelling potentially counteracts severe eutrophication and hypoxia in coastal regions. The entrainment that commonly occurs in the downwelling-induced plume can develop around a compensatory downward flow at a scale of kilometers, which favors below-pycnocline ventilation and thus mitigates hypoxia in bottom water. However, little discussion has been made on the entrainment of artificial downwelling. The paper focus on the total entrained (TE) flow rate that represents the magnitude of entrainment transport. Injection of a negatively buoyant jet from a round pipe into a stagnant and homogeneous ambient medium is numerically studied using a verified standard k- ε turbulence model, in which different initial pipe flow speeds, density differences and pipe radii are involved. Previous theories and experiments confirm the numerical model. An empirical formula is fitted to the simulation data to calculate the TE flow rate, which permit quantitative analysis of the capacity of ventilation for artificial downwelling. The efficiency index of ventilation (EIV) is also defined and applied to study a typical downwelling device, i.e., a tidal pump, to suggest optimal engineering parameters.

Structural representation of additively manufactured 316L austenitic stainless steel

Three 316L stainless steel materials are studied and reported upon; wrought, as-built additively manufactured (AM), and heat-treated AM material. The AM material was produced from the laser engineered net shaping (LENS) process. Macroscopic uniaxial compression stress-strain curves were obtained for all three materials. The curves were similar for the wrought and heat-treated AM materials but the as-built AM material demonstrated approximately 1.7 times greater flow stress at any given level of strain than the other two materials. Electron-backscatter diffraction analysis of the materials also showed that the microstructures of the three materials differed; with complex grain morphology for the as-built AM material. The mean grain size of each of the three materials also differed. The initial dislocation density was also measured with neutron diffraction and line-profile analysis for both wrought and as-built AM materials with the density in the AM material approximately 2.5 times greater. A single crystal model was proposed to represent the essential features of the three FCC materials accounting for dislocation interactions and representation of grain size via a simple Hall-Petch type term. The strength of this term is evaluated through independent experimental results on traditionally manufactured materials. The model was applied to each of the three materials by simulation of the uniaxial compression experiments by direct numerical simulation of electron-backscatter diffraction images. This allowed for representation of the size of each grain in the simulations. The results suggest that the difference in initial dislocation density of the three materials is the primary factor causing the difference in stress-strain response. Although the differences in grain size contribute to a higher stress for the as-built AM material, the effect is small. Other factors such as internal stress and grain morphology could play a role in mechanical behavior difference and these two factors are also discussed.

The influence of the chemical composition representation according to the number of species during mixing in high-pressure turbulent flows

Mixing of several species in high-pressure (high-$p$) turbulent flows is investigated to understand the influence of the number of species on the flow characteristics. Direct numerical simulations are conducted in the temporal mixing layer configuration at approximately the same value of the momentum ratio for all realizations. The simulations are performed with mixtures of two, three, five and seven species to address various compositions at fixed number of species, at three values of initial vorticity-thickness-based Reynolds number, $Re_{0}$, and two values of the free-stream pressure, $p_{0}$, which is supercritical for each species except water. The major species are C7H16, O2 and N2, and the minor species are CO, CO2, H2 and H2O. The extensive database thus obtained allows the study of the influence not only of $Re_{0}$ and $p_{0}$, but also of the initial density ratio and of the initial density difference between streams, $\unicode[STIX]{x0394}\unicode[STIX]{x1D70C}$. The results show that the layer growth is practically insensitive to all of the above parameters; however, global vortical aspects increase with $Re_{0},p_{0}$ and the number of species; nevertheless, at the same $Re_{0},p_{0}$ and density ratio, vorticity aspects are not influenced by the number of species. Species mixing produces strong density gradients which increase with $p_{0}$ and otherwise scale with $\unicode[STIX]{x0394}\unicode[STIX]{x1D70C}$ but, when scaled by $\unicode[STIX]{x0394}\unicode[STIX]{x1D70C}$, are not affected by the number of species. Generalized Korteweg-type equations are developed for a multi-species mixture, and a priori estimates based on the largest density gradient show that the Korteweg stresses, which account for the influence of the density gradient, have negligible contribution in the momentum equation. The species-specific effective Schmidt number, $Sc_{\unicode[STIX]{x1D6FC},\mathit{eff}}$, is computed and it is found that negative values occur for all minor species – particularly for H2 – thus indicating uphill diffusion, while the major species experience only regular diffusion. The probability density function (p.d.f.) of $Sc_{\unicode[STIX]{x1D6FC},\mathit{eff}}$ shows strong variation with $p_{0}$ but weak dependence on the number of species; however, the p.d.f. substantially varies with the identity of the species. In contrast, the p.d.f. of the effective Prandtl number indicates dependence on both $p_{0}$ and the number of species. Similar to $Sc_{\unicode[STIX]{x1D6FC},\mathit{eff}}$, the species-specific effective Lewis-number p.d.f. depends on the species, and for all species the mean is smaller than unity, thus invalidating one of the most popular assumptions in combustion modelling. Simplifying the mixture composition by reducing the number of minor species does not affect the crucial species–temperature relationship of the major species that, for accuracy, must be retained in combustion simulations, but this relationship is affected for the minor species and in regions of uphill diffusion, indicating that the reduction is nonlinear in nature.

Control of Rayleigh-Taylor instability onset time and convective velocity by differential diffusion effects.

Fingering instabilities of a miscible interface between two fluids in a gravitational field can develop due to adverse density gradients as in the well-known Rayleigh-Taylor (RT) and double-diffusive (DD) instabilities. In the absence of differential diffusion, the mixing rate and the onset time of the RT instability developing when a denser solution of a given solute A overlies a less dense solution of a solute B are respectively proportional and inversely proportional to the initial density difference Δρ_{0} between the two superposed layers. We show here both experimentally and theoretically for porous media flows that when the mechanisms of RT and DD instabilities are combined, the properties of the convective growth of the fingers are controlled by the dynamic density jump Δρ_{m} of the nonmonotonic density profile induced by the differential diffusion effects. In particular, the onset time and mixing rate can be controlled by varying the ratio of the diffusion coefficients of the solutes.

The role of fluid turbulence on contact electrification of suspended particles

A probabilistic version of a well-known phenomenological model for contact electrification is used to examine the effect of fluid turbulence on charge development for suspended particles as a function of the particle Stokes number. The distribution of particle collisions and particle charge appear to approach asymptotic states for high values of the Kolmogorov-scale Stokes numbers, exhibiting approximately normal distributions. The influence on particle contact electrification of differences in initial charge carrier density and in particle size are examined.

Simulating on rollover phenomenon in LNG storage tanks and determination of the rollover threshold

Rollover is a potential risk to the safety of LNG storage tanks during the LNG storage process, so study of its prevention method is very important. In this paper, rollover phenomenon in a liquefied natural gas (LNG) storage tank is modeled physically and mathematically. Its evolution is simulated using FLUENT™ software from the breakdown of stratification to the occurrence of rollover. Results show that the evolution consists of three phases: the initial phase where rollover occurs near the side wall of the storage tank; the turbulent phase where rollover transfers to the center of the tank; and the final phase where new layers evolve. Based on these phases, rollovers in 160,000, 30,000, and 5000 m3 LNG storage tanks are simulated at varying initial density differences, and a rollover coefficient is defined to describe rollover intensity. The simulations show that the rollover coefficient initially increases within a small scope and then increases rapidly with the increment of initial density difference. This turning point is chosen to be the rollover threshold, which is regarded as the critical density difference in this study. The critical density differences obtained from the simulation results of the 160,000, 30,000, and 5000 m3 LNG storage tanks are 3, 5, and 7 kg/m3, respectively, which can be used as their rollover criteria to ensure the safety of LNG storage tanks.

Linking resource selection and mortality modeling for population estimation of mountain lions in Montana

To be most effective, the scale of wildlife management practices should match the range of a particular species’ movements. For this reason, combined with our inability to rigorously or regularly census mountain lion populations, several authors have suggested that mountain lions be managed in a source-sink or metapopulation framework. We used a combination of resource selection functions, mortality estimation, and dispersal modeling to estimate cougar population levels in Montana statewide and potential population level effects of planned harvest levels. Between 1980 and 2012, 236 independent mountain lions were collared and monitored for research in Montana. From these data we used 18,695 GPS locations collected during winter from 85 animals to develop a resource selection function (RSF), and 11,726 VHF and GPS locations from 142 animals along with the locations of 6343 mountain lions harvested from 1988–2011 to validate the RSF model. Our RSF model validated well in all portions of the State, although it appeared to perform better in Montana Fish, Wildlife and Parks (MFWP) Regions 1, 2, 4 and 6, than in Regions 3, 5, and 7. Our mean RSF based population estimate for the total population (kittens, juveniles, and adults) of mountain lions in Montana in 2005 was 3926, with almost 25% of the entire population in MFWP Region 1. Estimates based on a high and low reference population estimates produce a possible range of 2784 to 5156 mountain lions statewide. Based on a range of possible survival rates we estimated the mountain lion population in Montana to be stable to slightly increasing between 2005 and 2010 with lambda ranging from 0.999 (SD=0.05) to 1.02 (SD=0.03). We believe these population growth rates to be a conservative estimate of true population growth. Our model suggests that proposed changes to female harvest quotas for 2013–2015 will result in an annual statewide population decline of 3% and shows that, due to reduced dispersal, changes to harvest in one management unit may affect population growth in neighboring units where smaller or even no changes were made. Uncertainty regarding dispersal levels and initial population density may have a significant effect on predictions at a management unit scale (i.e. 2000km2), while at a regional scale (i.e. 50,000km2) large differences in initial population density result in relatively small changes in population growth rate, and uncertainty about dispersal may not be as influential. Doubling the presumed initial density from a low estimation of 2.19 total animals per 100km2 resulted in a difference in annual population growth rate of only 2.6% statewide when compared to high density of 4.04 total animals per 100km2 (low initial population estimate λ=0.99, while high initial population estimate λ=1.03). We suggest modeling tools such as this may be useful in harvest planning at a regional and statewide level.

Designing ultrasound fields to control the morphology of engineered microvessel networks

Spatial patterning of endothelial cells using ultrasound standing wave fields (USWF) can promote extensive microvessel network formation throughout the volume of three-dimensional collagen hydrogels. The goal of this study was to identify acoustic exposure parameters that generate specific initial spatial patterns of cells, in order to control resultant microvessel morphology. Endothelial cells were suspended in soluble collagen and exposed to a 1-MHz continuous wave USWF for 15 min during collagen gel polymerization. Samples were exposed to peak USWF pressures of 0, 0.1, 0.2, or 0.3 MPa. Samples were either imaged immediately post-exposure using high frequency ultrasound or cultured for ten days. Analysis of B-mode ultrasound images confirmed differences in initial cell band spacing and cell band density between the four exposure pressures tested. After ten days in culture, USWF-induced cell patterning resulted in three distinct microvessel morphologies. Specifically, 0.1 MPa exposure resulted in capillary-like networks, 0.2 MPa exposure resulted in non-branching vessel structures, and 0.3 MPa exposure resulted in hierarchical branching microvessel networks. Spatial characteristics of initial cell bands were then correlated with resulting microvessel network morphology. Results of these investigations allow for the capability to predictively, reproducibly, and rapidly form microvessel networks of known morphology within 3D engineered hydrogels.

Effective buoyancy ratio: a new parameter for characterizing thermo-chemical mixing in the Earth's mantle

Abstract. Numerical modeling has been carried out in a 2-D cylindrical shell domain to quantify the evolution of a primordial dense layer around the core–mantle boundary. Effective buoyancy ratio, Beff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. Beff decreases with time due to (1) warming of the compositionally dense layer, (2) cooling of the overlying mantle, (3) eroding of the dense layer through thermal convection in the overlying mantle and (4) diluting of the dense layer through inner convection. When Beff reaches the instability point, Beff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (Beff = 0) over a short time period. A parabolic relationship was revealed between the initial density difference of the layers and the mixing time. Morphology of large low-shear-velocity provinces and results from seismic tomography and normal mode data suggest a value of Beff ≥ 1 for the mantle.