Aircraft control system

An aircraft control system is described which is particularly suited to rotary wing aircraft. Longitudinal acceleration and course rate commands are derived from a manual control stick to control translational velocity of the aircraft along a flight path. In the collective channel the manual controls provide vertical velocity commands. In the yaw channel the manual controls provide sideslip or heading rate commands at high or low airspeeds, respectively. The control system permits pilots to fly along prescribed flight paths in a precise manner with relatively low work load

Fixed-base simulation evaluation of various low-visibility landing systems for helicopters

Fixed base simulation evaluation of one fully automatic and six manual low visibility landing systems for helicopter

Selective trapping of DNA using glass microcapillaries

We show experimentally that a cheap glass microcapillary can accumulate {\lambda}-phage DNA at its tip and deliver the DNA into the capillary using a combination of electro-osmotic flow, pressure-driven flow, and electrophoresis. We develop an efficient simulation model for this phenomenon based on the electrokinetic equations and the finite-element method. Using our model, we explore the large parameter space of the trapping mechanism by varying the salt concentration, the capillary surface charge, the applied voltage, the pressure difference, and the mobility of the analyte molecules. Our simulation results show that this system can be tuned to capture a wide range of analyte molecules, such as DNA or proteins, based on their electrophoretic mobility. Our method for separation and pre-concentration of analytes has implications for the development of low-cost lab-on-a-chip devices.Comment: 9 pages, 4 figure

A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean

The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasi-conservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (εNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 εNd units along the flow path of Circumpolar Deep Water; εNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards εNd≈−11.5 offshore northern Madagascar, whereas εNd≈−7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment εNd, an isotope mass balance suggests a deep water residence time for Nd of ≤400 yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records εNd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset εNd values and reduced glacial–interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic εNd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition

Reactivity of neodymium carriers in deep sea sediments: Implications for boundary exchange and paleoceanography

The dissolved neodymium (Nd) isotopic distribution in the deep oceans is determined by continental weathering inputs, water mass advection, and boundary exchange between particulate and dissolved fractions. Reconstructions of past Nd isotopic variability may therefore provide evidence on temporal changes in continental weathering inputs and/or ocean circulation patterns over a range of timescales. However, such an approach is limited by uncertainty in the mechanisms and importance of the boundary exchange process, and the challenge in reliably recovering past seawater Nd isotopic composition (εNd) from deep sea sediments. This study addresses these questions by investigating the processes involved in particulate–solution interactions and their impact on Nd isotopes. A better understanding of boundary exchange also has wider implications for the oceanic cycling and budgets of other particle-reactive elements. Sequential acid-reductive leaching experiments at pH ∼2–5 on deep sea sediments from the western Indian Ocean enable us to investigate natural boundary exchange processes over a timescale appropriate to laboratory experiments. We provide evidence that both the dissolution of solid phases and exchange processes influence the εNd of leachates, which suggests that both processes may contribute to boundary exchange. We use major element and rare earth element (REE) data to investigate the pools of Nd that are accessed and demonstrate that sediment leachate εNd values cannot always be explained by admixture between an authigenic component and the bulk detrital component. For example, in core WIND 24B, acid-reductive leaching generates εNd values between −11 and −6 as a function of solution/solid ratios and leaching times, whereas the authigenic components have εNd ≈ −11 and the bulk detrital component has εNd ≈ −15. We infer that leaching in the Mascarene Basin accesses authigenic components and a minor radiogenic volcanic component that is more reactive than Madagascan-derived clays. The preferential mobilisation of such a minor component demonstrates that the Nd released by boundary exchange could often have a significantly different εNd composition than the bulk detrital sediment. These experiments further demonstrate certain limitations on the use of acid-reductive leaching to extract the εNd composition of the authigenic fraction of bulk deep sea sediments. For example, the detrital component may contain a reactive fraction which is also acid-extractible, while the incongruent nature of this dissolution suggests that it is often inappropriate to use the bulk detrital sediment elemental chemistry and/or εNd composition when assessing possible detrital contamination of leachates. Based on the highly systematic controls observed, and evidence from REE patterns on the phases extracted, we suggest two approaches that lead to the most reliable extraction of the authigenic εNd component and good agreement with foraminiferal-based approaches; either (i) leaching of sediments without a prior decarbonation step, or (ii) the use of short leaching times and low solution/solid ratios throughout

Strong and weak constraint variational assimilations for reduced order fluid flow modeling

International audienceIn this work we propose and evaluate two variational data assimilation techniques for the estimation of low order surrogate experimental dynamical models for fluid flows. Both methods are built from optimal control recipes and rely on proper orthogonal decomposition and a Galerkin projection of the Navier Stokes equation. The techniques proposed di er in the control variables they involve. The first one introduces a weak dynamical model defined only up to an additional uncertainty time-dependent function whereas the second one, handles a strong dynamical constraint in which the dynamical system's coe cients constitute the control variables. Both choices correspond to di erent approximations of the relation between the reduced basis on which is expressed the motion field and the basis components that have been neglected in the reduced order model construction. The techniques have been assessed on numerical data and for real experimental conditions with noisy Image Velocimetry data

On hydrodynamic shear turbulence in Keplerian disks: via transient growth to bypass transition

This paper deals with the problem of hydrodynamic shear turbulence in non-magnetized Keplerian disks. We wish to draw attention to a route to hydrodynamic turbulence which seems to be little known by the astrophysical community, but which has been intensively discussed among fluid dynamicists during the past decade. In this so-called `bypass' concept for the onset of turbulence, perturbations undergo a transient growth, and they may reach an amplitude that is sufficiently large to allow positive feedback through nonlinear interactions. This transient growth is linear in nature, and thus it differs in principle from the well-known nonlinear instability. We describe the type of perturbations that according to this process are the most likely to lead to turbulence, namely non-axisymmetric vortex mode perturbations in the two dimensional limit. We show that the apparently inhibiting action of the Coriolis force on the dynamics of such vortical perturbations is substantially diminished due to the pressure perturbations, contrary to current opinion. We stress the similarity of the turbulent processes in Keplerian disks and in Cartesian flows and conclude that the prevalent skepticism of the astrophysical community on the occurrence of hydrodynamic shear turbulence in such disks is not founded.Comment: 8 pages, 1 figure, accepted in A &

First Nd isotope record of Mediterranean–Atlantic water exchange through the Moroccan Rifian Corridor during the Messinian Salinity Crisis

We present the first neodymium isotope reconstruction of Mediterranean–Atlantic water exchange through the Moroccan (‘Rifian’) Corridor 8–5 Ma. This covers the late Miocene Messinian Salinity Crisis (MSC); a period when progressive tectonic restriction of the Mediterranean–Atlantic seaways resulted in extreme, basin-wide Mediterranean salinity fluctuations. The Rifian Corridor was one of these seaways and until now, relatively poor age constraints existed for the timing of Corridor closure, due to the impact of uplift and erosion on the sedimentary record. The bottom water Nd isotope record from the continuous Bou Regreg Valley succession in northwest Morocco allows us to explore corridor connectivity with the Atlantic. Data from the interior and Mediterranean edge of the Rifian Corridor (respectively, the Taza–Guercif and Melilla basins, northern Morocco) provide new information on corridor shallowing and the provenance of water flowing through the seaway. As a result, we can constrain the age of Rifian Corridor closure to 6.64–6.44 Ma. We also find no evidence of the siphoning of Atlantic waters through the seaway (7.20–6.58 Ma). Our results cannot exclude the possibility that at times during the Messinian Salinity Crisis, Mediterranean Outflow Water reached the Atlantic

Planktonic events may cause polymictic-dimictic regime shifts in temperate lakes

Water transparency affects the thermal structure of lakes, and within certain lake depth ranges, it can determine whether a lake mixes regularly (polymictic regime) or stratifies continuously (dimictic regime) from spring through summer. Phytoplankton biomass can influence transparency but the effect of its seasonal pattern on stratification is unknown. Therefore we analysed long term field data from two lakes of similar depth, transparency and climate but one polymictic and one dimictic, and simulated a conceptual lake with a hydrodynamic model. Transparency in the study lakes was typically low during spring and summer blooms and high in between during the clear water phase (CWP), caused when zooplankton graze the spring bloom. The effect of variability of transparency on thermal structure was stronger at intermediate transparency and stronger during a critical window in spring when the rate of lake warming is highest. Whereas the spring bloom strengthened stratification in spring, the CWP weakened it in summer. The presence or absence of the CWP influenced stratification duration and under some conditions determined the mixing regime. Therefore seasonal plankton dynamics, including biotic interactions that suppress the CWP, can influence lake temperatures, stratification duration, and potentially also the mixing regime

Coherent deglacial changes in western Atlantic Ocean circulation

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the 231Pa/230Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1