The Double Asteroid Redirection Test (DART) is a NASA-sponsored mission that will be the first direct test of the kinetic impactor technique for planetary defense. The DART spacecraft will impact into Didymos-B, the moon of the binary system 65803 Didymos and the resulting period change will be measured from Earth. Impact simulations will be used to predict the crater size and momentum enhancement expected from the DART impact. Because the specific material properties (strength, porosity, internal structure) of the Didymos-B target are unknown, a wide variety of numerical simulations must be performed to better understand possible impact outcomes. This simulation campaign will involve a large parameter space being simulated using multiple different shock physics hydrocodes. In order to understand better the behaviors and properties of numerical simulation codes applicable to the DART impact, a benchmarking and validation program using different numerical codes to solve a set of standard problems was designed and implemented. The problems were designed to test the effects of material strength, porosity, damage models, and target geometry on the ejecta following an impact and thus the momentum transfer efficiency. Several important results were identified from comparing simulations across codes, including the effects of model resolution and porosity and strength model choice: 1) Momentum transfer predictions almost uniformly exhibit a larger variation than predictions of crater size; 2) The choice of strength model, and the values used for material strength, are significantly more important in the prediction of crater size and momentum enhancement than variation between codes; 3) Predictions for crater size and momentum enhancement tend to be similar (within 15-20%) when similar strength models are used in different codes. These results will be used to better design a modeling plan for the DART mission as well as to better understand the potential results that may be expected due to unknown target properties. The DART impact simulation team will determine a specific desired material parameter set appropriate for the Didymos system that will be standardized (to the extent possible) across the different codes when making predictions for the DART mission. Some variation in predictions will still be expected, but that variation can be bracketed by the results shown in this study. IntroductionThe Double Asteroid Redirection Test (DART) is a NASA-sponsored mission, currently in Phase C development (as of January, 2019). DART is the first direct test of kinetic impactor technology for planetary defense, and involves the impact of a spacecraft into the moon of a binary system in order to monitor momentum transfer to the target. High-fidelity impact simulations are one tool used to better predict the results of this impact. Because the target of the DART impact is a binary system that has not previously been visited by spacecraft, many of the target properties are unknown and the potential modeling parameter space is large....
In fast-transcribing prokaryotic genes, such as an rrn gene in Escherichia coli, many RNA polymerases (RNAPs) transcribe the DNA simultaneously. Active elongation of RNAPs is often interrupted by pauses, which has been observed to cause RNAP traffic jams; yet some studies indicate that elongation seems to be faster in the presence of multiple RNAPs than elongation by a single RNAP. We propose that an interaction between RNAPs via the torque produced by RNAP motion on helically twisted DNA can explain this apparent paradox. We have incorporated the torque mechanism into a stochastic model and simulated transcription both with and without torque. Simulation results illustrate that the torque causes shorter pause durations and fewer collisions between polymerases. Our results suggest that the torsional interaction of RNAPs is an important mechanism in maintaining fast transcription times, and that transcription should be viewed as a cooperative group effort by multiple polymerases.
Through the computation of the most-unstable modes, we perform a systematic analysis of the linear Rayleigh–Taylor instability at a spherical interface separating two different homogeneous regions of incompressible viscous fluids under the action of a radially directed acceleration over the entire parameter space. Using the growth rate as the dependent variable, the parameter space is spanned by the spherical harmonic degree n and three dimensionless variables: the Atwood number A, the viscosity ratio s, and the dimensionless variable B=(aRρ22/μ22)1/3R, where aR, ρ2, and μ2 are the local radial acceleration at the interface and the density and viscosity of the denser overlying fluid, respectively. To understand the effect of the various parameters on the instability behavior and to identify similarities and differences between the planar and spherical configurations, we compare the most-unstable growth rates αP* (planar) and αS* (spherical) under homologous driving conditions. For all A, when s ≪ 1, the planar configuration is more unstable than the spherical (αP*>αS*) within the interval 0 < B < ∞. However, as s increases to O(1), there is a region for small values of B where αS*>αP*, whereas for larger values of B, αP*>αS* once again. When s ∼ 2, the maximum of αS* for the n = 1 mode is greater than αS* for any other mode (n ≥ 2). For s∼O(10), αS*>αP* for all A within 0 < B < ∞. We find that the instability behavior between the planar and spherical systems departs from each other for s ≳ 2 and diverges considerably for s ≫ 1. In the limit when s → ∞, the planar configuration reduces to the trivial solution αP*≡0 for all B and A, whereas αS* has a non-zero limiting value for the n = 1 mode but vanishes for all the other modes (n ≥ 2). We derive an equation for αS* in this limit and obtain closed form solutions for the maximum of αS* and the value of B at which this occurs. Finally, we compare the most-unstable growth rates between the exact dispersion relation and three different approximations to highlight their strengths and weaknesses.
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