The Universal Scaling Exponents of Anisotropy in Turbulence and their Measurement

The scaling properties of correlation functions of non-scalar fields (constructed from velocity derivatives) in isotropic hydrodynamic turbulence are characterized by a set of universal exponents. It is explained that these exponents also characterize the rate of decay of the effects of anisotropic forcing in developed turbulence. This set has never been measured in either numerical or laboratory experiments. These exponents are important for the general theory of turbulence, but also for modeling anisotropic flows. We propose in this letter how to measure these exponents using existing data bases of direct numerical simulations and by designing new laboratory experiments.Comment: 10 pages, latex, no figures, online (html) version available at http://lvov.weizmann.ac.il/EXP/EXP.htm

Service Life Extension of the Propulsion System of Long-Term Manned Orbital Stations

One of the critical non-replaceable systems of a long-term manned orbital station is the propulsion system. Since the propulsion system operates beginning with the launch of station elements into orbit, its service life determines the service life of the station overall. Weighing almost a million pounds, the International Space Station (ISS) is about four times as large as the Russian space station Mir and about five times as large as the U.S. Skylab. Constructed over a span of more than a decade with the help of over 100 space flights, elements and modules of the ISS provide more research space than any spacecraft ever built. Originally envisaged for a service life of fifteen years, this Earth orbiting laboratory has been in orbit since 1998. Some elements that have been launched later in the assembly sequence were not yet built when the first elements were placed in orbit. Hence, some of the early modules that were launched at the inception of the program were already nearing the end of their design life when the ISS was finally ready and operational. To maximize the return on global investments on ISS, it is essential for the valuable research on ISS to continue as long as the station can be sustained safely in orbit. This paper describes the work performed to extend the service life of the ISS propulsion system. A system comprises of many components with varying failure rates. Reliability of a system is the probability that it will perform its intended function under encountered operating conditions, for a specified period of time. As we are interested in finding out how reliable a system would be in the future, reliability expressed as a function of time provides valuable insight. In a hypothetical bathtub shaped failure rate curve, the failure rate, defined as the number of failures per unit time that a currently healthy component will suffer in a given future time interval, decreases during infant-mortality period, stays nearly constant during the service life and increases at the end when the design service life ends and wear-out phase begins. However, the component failure rates do not remain constant over the entire cycle life. The failure rate depends on various factors such as design complexity, current age of the component, operating conditions, severity of environmental stress factors, etc. Development, qualification and acceptance test processes provide rigorous screening of components to weed out imperfections that might otherwise cause infant mortality failures. If sufficient samples are tested to failure, the failure time versus failure quantity can be analyzed statistically to develop a failure probability distribution function (PDF), a statistical model of the probability of failure versus time. Driven by cost and schedule constraints however, spacecraft components are generally not tested in large numbers. Uncertainties in failure rate and remaining life estimates increase when fewer units are tested. To account for this, spacecraft operators prefer to limit useful operations to a period shorter than the maximum demonstrated service life of the weakest component. Running each component to its failure to determine the maximum possible service life of a system can become overly expensive and impractical. Spacecraft operators therefore, specify the required service life and an acceptable factor of safety (FOS). The designers use these requirements to limit the life test duration. Midway through the design life, when benefits justify additional investments, supplementary life test may be performed to demonstrate the capability to safely extend the service life of the system. An innovative approach is required to evaluate the entire system, without having to go through an elaborate test program of propulsion system elements. Evaluating every component through a brute force test program would be a cost prohibitive and time consuming endeavor. ISS propulsion system components were designed and built decades ago. There are no representative ground test articles for some of the components. A 'test everything' approach would require manufacturing new test articles. The paper outlines some of the techniques used for selective testing, by way of cherry picking candidate components based on failure mode effects analysis, system level impacts, hazard analysis, etc. The type of testing required for extending the service life depends on the design and criticality of the component, failure modes and failure mechanisms, life cycle margin provided by the original certification, operational and environmental stresses encountered, etc. When specific failure mechanism being considered and the underlying relationship of that mode to the stresses provided in the test can be correlated by supporting analysis, time and effort required for conducting life extension testing can be significantly reduced. Exposure to corrosive propellants over long periods of time, for instance, lead to specific failure mechanisms in several components used in the propulsion system. Using Arrhenius model, which is tied to chemically dependent failure mechanisms such as corrosion or chemical reactions, it is possible to subject carefully selected test articles to accelerated life test. Arrhenius model reflects the proportional relationship between time to failure of a component and the exponential of the inverse of absolute temperature acting on the component. The acceleration factor is used to perform tests at higher stresses that allow direct correlation between the times to failure at a high test temperature to the temperatures to be expected in actual use. As long as the temperatures are such that new failure mechanisms are not introduced, this becomes a very useful method for testing to failure a relatively small sample of items for a much shorter amount of time. In this article, based on the example of the propulsion system of the first ISS module Zarya, theoretical approaches and practical activities of extending the service life of the propulsion system are reviewed with the goal of determining the maximum duration of its safe operation

Corona with Streamers in Atmospheric Pressure Air in a Highly Inhomogeneous Electric Field

The paper presents research data on positive and negative coronas in atmospheric pressure air in a highly inhomogeneous electric field. The data show that irrespective of the polarity of pointed electrodes placed in a high electric field (200 kV/cm), this type of discharge develops via ball streamers even if the gap voltage rises slowly (0.2 kV/ms). The start voltage of first positive streamers, compared to negative ones, is higher and the amplitude and the frequency of their current pulses are much lower: about two times and more than two orders of magnitude, respectively. The higher frequency of current pulses from negative streamers provides higher average currents and larger luminous areas of negative coronas compared to positive ones. Positive and negative cylindrical streamers from a pointed to a plane electrode are detected and successive discharge transitions at both polarities are identified

Identification of conditions for nanostructured burnishing and subsurface shear instability

Numerical as well as physical modeling of nanostructuring burnishing has been carried out to find out the process parameter limiting levels, which serve both to provide appropriate surface quality and positive deformation-induced structural modification of the subsurface layers as well as to avoid shear instability in the subsurface layers of burnished metal. The effects of load, burnishing speed, tool tip material, tool pass number and tribological transfer on the burnished surface roughness have been elucidated by the example of quenched and tempered steel 20X (EN 20Cr4). It was shown that overloading results in quasi-viscous flow of the subsurface material, deterioration of the surface and ruining the positive effect of nanostructuring burnishing

Ediacara biota flourished in oligotrophic and bacterially dominated marine environments across Baltica.

Middle-to-late Ediacaran (575-541 Ma) marine sedimentary rocks record the first appearance of macroscopic, multicellular body fossils, yet little is known about the environments and food sources that sustained this enigmatic fauna. Here, we perform a lipid biomarker and stable isotope (δ15Ntotal and δ13CTOC) investigation of exceptionally immature late Ediacaran strata (<560 Ma) from multiple locations across Baltica. Our results show that the biomarker assemblages encompass an exceptionally wide range of hopane/sterane ratios (1.6-119), which is a broad measure of bacterial/eukaryotic source organism inputs. These include some unusually high hopane/sterane ratios (22-119), particularly during the peak in diversity and abundance of the Ediacara biota. A high contribution of bacteria to the overall low productivity may have bolstered a microbial loop, locally sustaining dissolved organic matter as an important organic nutrient. These oligotrophic, shallow-marine conditions extended over hundreds of kilometers across Baltica and persisted for more than 10 million years

Таємниця одного адміністратора

Провінційний чиновник, особливо дореформених часів, – і досі рідкісний гість на сторінках вітчизняних наукових монографій, дисертаційних досліджень, ба, навіть, статей у спеціалізованих періодичних виданнях. Поясненням цього феномену є, мабуть, сформована ще класичною російською літературою і діяча-ми доби Великих Реформ слава більшості миколаївських чиновників чи то як «сірості», посередності (стор. 73), чи то як хабарників і крадіїв [1]. З іншого боку, тривалій відразі до студій з адміністративної історії прислужилася логіка т.зв. «національного наративу», за якою місцевий представник імперської адміністра-ції видавався своєрідним «чужинцем» – колоніалістом чи колабораціоністом. Втім, останні десять років досі так само національно орієнтовані англо- і російськомовна історіографії демонструють тенденцію до «імперського пово-роту» («imperial turn»), зокрема, актуалізуючи і переосмислюючи проблематику адміністрування Російської імперії [2]. Вкупі зі зростаючою від часів Великих Реформ кількістю поодиноких праць із історії місцевого управління [3], зокре-ма, і біографічного спрямування [4], це дозволяє очікувати і вітчизняного рене-сансу студій у царині адміністративної історії