Habitat Bennu: Design Concepts for Spinning Habitats Constructed From Rubble Pile Near-Earth Asteroids

The authors explore the possibility that near-earth, rubble pile asteroids might be used as habitats for human settlement by increasing their rotation to produce spin gravity. Using previously published scaling by Maindl et al. and studies of asteroid populations, it is shown that there is no class of hollowed body that would survive the spin-up process on its own without additional reinforcement. Large solid-rock asteroids (diameter D > 10 km) would not have the tensile strength to withstand the required rotation rates and would fracture and break apart. Smaller asteroids, being ‘rubble piles’, have little tensile strength and would quickly disperse. The possibility of containing the asteroid mass using higher-strength materials like carbon nanofiber is instead considered. It is found that a moderate tensile strength container can maintain the integrity of a large spinning cylinder composed of dispersed asteroid regolith. The research extends the range of possible asteroid habitat candidates, since it may become feasible to construct habitats from the more numerous smaller bodies, including NEAs (Near Earth Asteroids). The required tensile strength of the container material scales with habitat radius and thickness and is ∼ 200 MPa for a starting asteroid body of radius 300 m that is spun up to provide 0.3 g⊕ while increasing its radius to 3 km and maintaining a rubble and regolith shield thickness of 2 m to protect against cosmic rays. Ambient solar power can be harvested to aid in spin-up and material processing.

The Observation and Analysis of Stellar Photospheres

This textbook describes the equipment, observational techniques, and analysis used in the investigation of stellar photospheres. Now in its fourth edition, the text has been thoroughly updated and revised to be more accessible to students. New figures have been added to illustrate key concepts, while diagrams have been redrawn and refreshed throughout. The book starts by developing the tools of analysis, and then demonstrates how they can be applied. Topics covered include radiation transfer, models of stellar photospheres, spectroscopic equipment, how to observe stellar spectra, and techniques for measuring stellar temperatures, radii, surface gravities, chemical composition, velocity fields, and rotation rates. Up-to-date results for real stars are included. Written for starting graduate students or advanced undergraduates, this textbook also includes a wealth of reference material useful to researchers. eBook formats include color imagery while print formats are greyscale only; a wide selection of the color images are available online.

New single vortex generation method for flame/vortex interaction using flow behind a rotating cylinder

This paper investigates a new experimental method to generate a single two-dimensional translated vortex for flame/vortex interaction studies. A rotating cylinder is immersed in a uniform flow and, its rotating speed is impulsively reduced. This sudden action triggers the generation of a single vortex when both the initial and the final rotation speeds are in the range of a steady-state regime. Flow visualization allows confirming the applicability of this method, while a complementary two-dimensional numerical simulation is conducted to understand the vortex formation process. A vorticity layer is detached from the cylinder, initiating a feeding process and gradual growth of a single leading vortex. The feeding process is saturated at a specific distance from the cylinder and, vortex separation from the vorticity layer is observed. At the final stage of the formation process, the generated vortex is advected away and, a steady-state regime is again established behind the cylinder. The vortex characteristics appear to be related to the normalized reduction in the rotation rate ∆α, defined as the initial and final rotation rates difference normalized by the initial rotation rate. Several combinations of initial and final rotation rates corresponding to different normalized reductions are investigated experimentally and numerically. The results allow understanding the effect of this parameter; a higher normalized reduction generates a stronger, more rapidly growing vortex. However, its trajectory is related to the wake deviation corresponding to the final rotation rate.

Convolutional Neural Networks for Classification of Drones Using Radars

The ability to classify drones using radar signals is a problem of great interest. In this paper, we apply convolutional neural networks (CNNs) to the Short-Time Fourier Transform (STFT) spectrograms of the simulated radar signals reflected from the drones. The drones vary in many ways that impact the STFT spectrograms, including blade length and blade rotation rates. Some of these physical parameters are captured in the Martin and Mulgrew model which was used to produce the datasets. We examine the data under X-band and W-band radar simulation scenarios and show that a CNN approach leads to an F1 score of 0.816±0.011 when trained on data with a signal-to-noise ratio (SNR) of 10 dB. The neural network which was trained on data from an X-band radar with 2 kHz pulse repetition frequency was shown to perform better than the CNN trained on the aforementioned W-band radar. It remained robust to the drone blade pitch and its performance varied directly in a linear fashion with the SNR.

3D structure of single and multiple vortices in a flow under rotation

In this paper, we analyze the 3D structure of vortices developed in a rotating cylinder nonhomogeneously heated from below, when the rotation rate is increased. The analysis has been done by using nonlinear simulations. For a fixed Rayleigh number, the rotation rate is the bifurcation parameter. At low rotation rates, one single vortex is developed. When the rotation on the system is increased, another coexistent vortex appears at mid-levels in the cell. If the rotation is high enough, multiple-vortex structures with three or four vortices are developed at different heights. For larger rotation, complex multiple vortices appear with a chaotic behavior. A force balance analysis permits to study the role of the forces being determinant.

Exploring the Stellar Rotation of Early-type Stars in the LAMOST Medium-resolution Survey. II. Statistics

Angular momentum is a key property regulating star formation and evolution. However, the physics driving the distribution of the stellar rotation rates of early-type main-sequence stars is as yet poorly understood. Using our catalog of 40,034 early-type stars with homogeneous v sin i parameters, we review the statistical properties of their stellar rotation rates. We discuss the importance of possible contaminants, including binaries and chemically peculiar stars. Upon correction for projection effects and rectification of the error distribution, we derive the distributions of our sample’s equatorial rotation velocities, which show a clear dependence on stellar mass. Stars with masses less than 2.5 M ⊙ exhibit a unimodal distribution, with the peak velocity ratio increasing as stellar mass increases. A bimodal rotation distribution, composed of two branches of slowly and rapidly rotating stars, emerges for more massive stars (M > 2.5 M ⊙). For stars more massive than 3.0 M ⊙, the gap between the bifurcated branches becomes prominent. For the first time, we find that metal-poor ([M/H] < −0.2 dex) stars only exhibit a single branch of slow rotators, while metal-rich ([M/H] > 0.2 dex) stars clearly show two branches. The difference could be attributed to unexpectedly high spin-down rates and/or in part strong magnetic fields in the metal-poor subsample.

Turbulent jet through porous obstructions under Coriolis effect: an experimental investigation

The present study has the main purpose to experimentally investigate a turbulent momentum jet issued in a basin affected by rotation and in presence of porous obstructions. The experiments were carried out at the Coriolis Platform at LEGI Grenoble (FR). A large and unique set of velocity data was obtained by means of a Particle Image Velocimetry measurement technique while varying the rotation rate of the tank and the density of the canopy. The main differences in jet behavior in various flow configurations were assessed in terms of mean flow, turbulent kinetic energy and jet spreading. The jet trajectory was also detected. The results prove that obstructions with increasing density and increased rotation rates induce a more rapid abatement of both jet velocity and turbulent kinetic energy. The jet trajectories can be scaled by a characteristic length, which is found to be a function of the jet initial momentum, the rotation rate, and the drag exerted by the obstacles. An empirical expression for the latter is also proposed and validated. Graphic abstract

Fast Rotating Neutron Stars: Oscillations and Instabilities

In this review article, we present the main results from our most recent research concerning the oscillations of fast rotating neutron stars. We derive a set of time evolution equations for the investigation of non-axisymmetric oscillations of rapidly rotating compact objects in full general relativity, taking into account the contribution of a dynamic spacetime. Using our code, which features high accuracy at comparably low computational expense, we are able to extract the frequencies of non-axisymmetric modes of compact objects with rotation rates up to the Kepler limit. We propose various universal relations combining bulk properties of isolated neutron stars as well as of binary systems before and after merger; these relations are independent of the true equation of state and may serve as a valuable tool for gravitational wave asteroseismology. We also present an introductory example using a Bayesian analysis.