Unsteady aerodynamics and flow control for flapping wing flyers

The creation of micro air vehicles (MAVs) of the same general sizes and weight as natural fliers has spawned renewed interest in flapping wing flight. With a wingspan of approximately 15 cm and a flight speed of a few meters per second, MAVs experience the same low Reynolds number (10⁴–10⁵) flight conditions as their biological counterparts. In this flow regime, rigid fixed wings drop dramatically in aerodynamic performance while flexible flapping wings gain efficacy and are the preferred propulsion method for small natural fliers. Researchers have long realized that steady-state aerodynamics does not properly capture the physical phenomena or forces present in flapping flight at this scale. Hence, unsteady flow mechanisms must dominate this regime. Furthermore, due to the low flight speeds, any disturbance such as gusts or wind will dramatically change the aerodynamic conditions around the MAV. In response, a suitable feedback control system and actuation technology must be developed so that the wing can maintain its aerodynamic efficiency in this extremely dynamic situation; one where the unsteady separated flow field and wing structure are tightly coupled and interact nonlinearly. For instance, birds and bats control their flexible wings with muscle tissue to successfully deal with rapid changes in the flow environment. Drawing from their example, perhaps MAVs can use lightweight actuators in conjunction with adaptive feedback control to shape the wing and achieve active flow control. This article first reviews the scaling laws and unsteady flow regime constraining both biological and man-made fliers. Then a summary of vortex dominated unsteady aerodynamics follows. Next, aeroelastic coupling and its effect on lift and thrust are discussed. Afterwards, flow control strategies found in nature and devised by man to deal with separated flows are examined. Recent work is also presented in using microelectromechanical systems (MEMS) actuators and angular speed variation to achieve active flow control for MAVs. Finally, an explanation for aerodynamic gains seen in flexible versus rigid membrane wings, derived from an unsteady three-dimensional computational fluid dynamics model with an integrated distributed control algorithm, is presented.     

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfvén wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 R _E. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 R _E). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.     

Analysis of Theories of Fully Ionized Space Plasma

The physical sense of the main ideas, presently used in plasma physics, is discussed. An attempt is made to clarify the concepts, used in plasma physical calculations. The concept of `Coulomb collisions' with the implicitly introduced rapid stochastization plays the main negative role in the physics of fully ionized plasma. Statistical methods, which are adequate for the neutral gas and for the partially ionized plasma, are not applicable for the completely ionized case. It is the cause of large errors in evaluating real plasma parameters. A new concept is considered: a fully ionized space plasma should be treated as a dynamical system with a low level of chaos. Further progress in space physics requires a serious renewal of plasma theory.     

Einstein observations of high luminosity X-ray binaries

Recent observational advances in the study of high luminosity x-ray binaries have permitted investigation of the interaction of the outgoing x-radiation with the accreting matter surrounding the compact object. In two sources, 4U1822-37 and 4U2129+47, extended EINSTEIN coverage has led to the detection of partial x-ray eclipses, which indicate that the x-ray emitting regions must be extended in size. These have been interpreted as evidence for a large Compton-thick corona produced by evaporation of cool material off the surface of an accretion disk. In three other sources, 4U1915-05, 4U1624-49, and Cygnus X-2, evidence has been found for short x-ray absorption dips which are likely to be associated with obscuration by cool dense matter at the outer edge of the disk. In 4U1915-05, these dips are strictly periodic and determine the binary period for the system. In Cygnus X-2, the dips appear to be quasiperiodic, while in 4U1624-49, insufficient coverage has prevented clarification of the temporal properties of the absorption.For the brightest cosmic x-ray source, Scorpius X-1, the EINSTEIN objective grating spectrometer has provided high resolution spectra (λ/Δλ ～50) in the wavelength range 40-10 Å. The spectra reveal absorption features due to intervening helium, nitrogen, and oxygen. The implied nitrogen and oxygen abundances are anomalous and suggest that the absorbing material is intrinsic accreting matter which has been transferred from the surface of an evolved companion. Constraints on the inclination of the system then imply that this cool dense material must be well out of the orbital plane of the binary.     

The potential for lithoautotrophic life on Mars: application to shallow interfacial water environments

We developed a numerical model to assess the lithoautotrophic habitability of Mars based on metabolic energy, nutrients, water availability, and temperature. Available metabolic energy and nutrient sources were based on a laboratory-produced Mars-analog inorganic chemistry. For this specific reference chemistry, the most efficient lithoautotrophic microorganisms would use Fe(2+) as a primary metabolic electron donor and NO(3)(-) or gaseous O(2) as a terminal electron acceptor. In a closed model system, biomass production was limited by the electron donor Fe(2+) and metabolically required P, and typically amounted to approximately 800 pg of dry biomass/ml ( approximately 8,500 cells/ml). Continued growth requires propagation of microbes to new fecund environments, delivery of fresh pore fluid, or continued reaction with the host material. Within the shallow cryosphere--where oxygen can be accessed by microbes and microbes can be accessed by exploration-lithoautotrophs can function within as little as three monolayers of interfacial water formed either by adsorption from the atmosphere or in regions of ice stability where temperatures are within some tens of degrees of the ice melting point. For the selected reference host material (shergottite analog) and associated inorganic fluid chemistry, complete local reaction of the host material potentially yields a time-integrated biomass of approximately 0.1 mg of dry biomass/g of host material ( approximately 10(9) cells/g). Biomass could also be sustained where solutes can be delivered by advection (cryosuction) or diffusion in interfacial water; however, both of these processes are relatively inefficient. Lithoautotrophs in near-surface thin films of water, therefore, would optimize their metabolism by deriving energy and nutrients locally. Although the selected chemistry and associated model output indicate that lithoautotrophic microbial biomass could accrue within shallow interfacial water on Mars, it is likely that these organisms would spend long periods in maintenance or survival modes, with instantaneous biomass comparable to or less than that observed in extreme environments on Earth.     

Methane production by methanogens following an aerobic washing procedure: simplifying methods for manipulation

The recent discovery of methane in the martian atmosphere is arguably one of the most important discoveries in the field of astrobiology. One possible source of this methane could be a microorganism analogous to those on Earth in the domain Archaea known as methanogens. Methanogens are described as obligately anaerobic, and methods developed to work with methanogens typically include anaerobic media and buffers, gassing manifolds, and possibly anaerobic chambers. To determine if the time, effort, and supplies required to maintain anaerobic conditions are necessary to maintain viability, we compared anaerobically washed cells with cells that were washed in the presence of atmospheric oxygen. Anaerobic tubes were opened, and cultures were poured into plastic centrifuge tubes, centrifuged, and suspended in fresh buffer, all in the presence of atmospheric oxygen. Washed cells from both aerobic and anaerobic procedures were inoculated into methanogenic growth media under anaerobic conditions and incubated at temperatures conducive to growth for each methanogenic strain tested. Methane production was measured at time intervals using a gas chromatograph. In three strains, significant differences were not seen between aerobically and anaerobically washed cells. In one strain, there was significantly less methane production observed following aerobic washing at some time points; however, substantial methane production occurred following both procedures. Thus, it appears that aerobic manipulations for relatively short periods of time with at least a few species of methanogens may not lead to loss of viability. With the discovery of methane in the martian atmosphere, it is likely that there will be an increase in astrobiology-related methanogen research. The research reported here should simplify the methodology.     

Oxidant enhancement in martian dust devils and storms: storm electric fields and electron dissociative attachment

Laboratory studies, numerical simulations, and desert field tests indicate that aeolian dust transport can generate atmospheric electricity via contact electrification or "triboelectricity." In convective structures such as dust devils and dust storms, grain stratification leads to macroscopic charge separations and gives rise to an overall electric dipole moment in the aeolian feature, similar in nature to the dipolar electric field generated in terrestrial thunderstorms. Previous numerical simulations indicate that these storm electric fields on Mars can approach the ambient breakdown field strength of approximately 25 kV/m. In terrestrial dust phenomena, potentials ranging from approximately 20 to 160 kV/m have been directly measured. The large electrostatic fields predicted in martian dust devils and storms can energize electrons in the low pressure martian atmosphere to values exceeding the electron dissociative attachment energy of both CO2 and H2O, which results in the formation of the new chemical products CO/O- and OH/H-, respectively. Using a collisional plasma physics model, we present calculations of the CO/O- and OH/H- reaction and production rates. We demonstrate that these rates vary geometrically with the ambient electric field, with substantial production of dissociative products when fields approach the breakdown value of approximately 25 kV/m. The dissociation of H2O into OH/H- provides a key ingredient for the generation of oxidants; thus electrically charged dust may significantly impact the habitability of Mars.     

The Juno Radiation Monitoring (RM) Investigation

The Radiation Monitoring Investigation of the Juno Mission will actively retrieve and analyze the noise signatures from penetrating radiation in the images of Juno’s star cameras and science instruments at Jupiter. The investigation’s objective is to profile Jupiter’s \(>10\mbox{-MeV}\) electron environment in regions of the Jovian magnetosphere which today are still largely unexplored. This paper discusses the primary instruments on Juno which contribute to the investigation’s data suite, the measurements of camera noise from penetrating particles, spectral sensitivities and measurement ranges of the instruments, calibrations performed prior to Juno’s first science orbit, and how the measurements may be used to infer the external relativistic electron environment.     

Cometary Dust

This review presents our understanding of cometary dust at the end of 2017. For decades, insight about the dust ejected by nuclei of comets had stemmed from remote observations from Earth or Earth’s orbit, and from flybys, including the samples of dust returned to Earth for laboratory studies by the Stardust return capsule. The long-duration Rosetta mission has recently provided a huge and unique amount of data, obtained using numerous instruments, including innovative dust instruments, over a wide range of distances from the Sun and from the nucleus. The diverse approaches available to study dust in comets, together with the related theoretical and experimental studies, provide evidence of the composition and physical properties of dust particles, e.g., the presence of a large fraction of carbon in macromolecules, and of aggregates on a wide range of scales. The results have opened vivid discussions on the variety of dust-release processes and on the diversity of dust properties in comets, as well as on the formation of cometary dust, and on its presence in the near-Earth interplanetary medium. These discussions stress the significance of future explorations as a way to decipher the formation and evolution of our Solar System.     

Origin of Molecular Oxygen in Comets: Current Knowledge and Perspectives

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard the Rosetta spacecraft has measured molecular oxygen (O₂) in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) in surprisingly high abundances. These measurements mark the first unequivocal detection of O₂ in a cometary environment. The large relative abundance of O₂ in 67P/C-G despite its high reactivity and low interstellar abundance poses a puzzle for its origin in comet 67P/C-G, and potentially other comets. Since its detection, there have been a number of hypotheses put forward to explain the production and origin of O₂ in the comet. These hypotheses cover a wide range of possibilities from various in situ production mechanisms to protosolar nebula and primordial origins. Here, we review the O₂ formation mechanisms from the literature, and provide a comprehensive summary of the current state of knowledge of the sources and origin of cometary O₂.