Integration of first-principles methods and crystallographic database searches for new ferroelectrics: Strategies and explorations

In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb$_{1/2}$Mn$_{1/2}$)O$_3$ as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite $M$Sb$_2$O$_4$; and (3) ferroelectric semiconductors with formula $M_2$P$_2$(S,Se)$_6$. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties.Comment: 13 pages, 5 figures, 4 table

Prediction and Simulator Verification of Roll/Lateral Adverse Aeroservoelastic Rotorcraft–Pilot Couplings

The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a technique to predict the adverse aeroservoelastic rotorcraft–pilot couplings, specifically between a rotorcraft’s roll motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive lead–lag mode that strongly interacts with the roll modeat a frequency within the biodynamic band of the pilots. These predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse couplings experienced while using only rigid-body models in the flight simulator verified that the observed phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the observed experimental results indicates that the techniques developed in this paper can be used to highlight the proneness of new or existing designs to pilot-assisted oscillation

Overview of the experimental tests in prototype

Experimental tests in prototype are necessary to understand the dynamic behaviour of the machine during different operating points. Hydraulic phenomena as well as its effect on the structure need to be studied in o rder to avoid instabilities during operation and to extend the life - time of the different components. For this purpose, a complete experimental study of a large Francis turbine prototype has been performed installing several sensors along the machine. Pres sure sensors were installed in the penstock, spiral case, runner and draft tube, strain gauges were installed in the runner, vibration sensors were used in the stationary parts and different electrical and operational parameters were also measured. All the se signals were acquired simultaneously for different operating points of the turbine.Postprint (published version

S5 0836+710: An FRII jet disrupted by the growth of a helical instability?

The remarkable stability of extragalactic jets is surprising, given the reasonable possibility of the growth of instabilities. In addition, much work in the literature has invoked this possibility in order to explain observed jet structures and obtain information from these structures. For example, it was recently shown that the observed helical structures in the jet in S5 0836+710 could be associated with helical pressure waves generated by Kelvin-Helmholtz instability. Our aim is to resolve the arc-second structure of the jet in the quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found by present observing arrays, as this lack implies a loss of jet collimation before interaction with the intergalactic medium. In this work, we use an observation performed in 2008 using EVN and MERLIN. The combined data reduction has provided a complete image of the object at arc-second scales. The lack of a hot-spot in the arc-second radio structure is taken as evidence that the jet losses its collimation between the VLBI region and the region of interaction with the ambient medium. This result, together with the previous identification of the helical structures in the jet with helical pressure waves that grow in amplitude with distance, allow us to conclude that the jet is probably disrupted by the growth of Kelvin-Helmholtz instability. This observational evidence confirms that the physical parameters of jets can be extracted using the assumption that instability is present in jets and can be the reason for many observed structures. Interestingly, the observed jet is classified as a FRII object in terms of its luminosity, but its large-scale morphology does not correspond to this classification. The implications of this fact are discussed.Comment: Accepted for publication in Astronomy & Astrophysic

Adaptive Detection of Instabilities: An Experimental Feasibility Study

We present an example of the practical implementation of a protocol for experimental bifurcation detection based on on-line identification and feedback control ideas. The idea is to couple the experiment with an on-line computer-assisted identification/feedback protocol so that the closed-loop system will converge to the open-loop bifurcation points. We demonstrate the applicability of this instability detection method by real-time, computer-assisted detection of period doubling bifurcations of an electronic circuit; the circuit implements an analog realization of the Roessler system. The method succeeds in locating the bifurcation points even in the presence of modest experimental uncertainties, noise and limited resolution. The results presented here include bifurcation detection experiments that rely on measurements of a single state variable and delay-based phase space reconstruction, as well as an example of tracing entire segments of a codimension-1 bifurcation boundary in two parameter space.Comment: 29 pages, Latex 2.09, 10 figures in encapsulated postscript format (eps), need psfig macro to include them. Submitted to Physica