Optimal airframe synthesis for gust loads

An optimization capability for sizing airframe structures that are subjected to a combination of deterministic and random flight loads was established. The random vibration environment introduces the need for selecting a statistical process that best describes the random loads and permits computation of the dynamic response parameters of interest. Furthermore, it requires a formulation of design constraints that would minimize the conservativeness in the design and retain computational viability. The random loads are treated as a stationary, homogeneous process with a Gaussian probability distribution. The formulation of the analysis problem, the structure of the optimization programming system and a representative numerical example are discussed

The computation of 15 deg and 10 deg equal area block terrestrial free air gravity anomalies

Starting with the set of 23,355 1 deg x 1 deg mean free air gravity anomalies used in Rapp (1972) to form a 5 deg equal area block terrestrial gravity field, the computation of 15 deg equal area block mean free air gravity anomalies is described along with estimates of their standard deviations. A new scheme of an integral division of a 15 deg block into 9 component 300 n. m. blocks, and each 300 n. m. block being subdivided into 25 60 n.mi. blocks, is used. This insures that there is no loss in accuracy, which would have resulted if proportional values according to area were taken of the 5 deg equal area anomalies to form the 15 deg block anomalies. A similar scheme is used for the computation of 10 deg equal area block mean free air gravity anomalies with estimates of their standard deviations. The scheme is general enough to be used for a 30 deg equal area block terrestrial gravity field

Direct recovery of mean gravity anomalies from satellite to satellite tracking

The direct recovery was investigated of mean gravity anomalies from summed range rate observations, the signal path being ground station to a geosynchronous relay satellite to a close satellite significantly perturbed by the short wave features of the earth's gravitational field. To ensure realistic observations, these were simulated with the nominal orbital elements for the relay satellite corresponding to ATS-6, and for two different close satellites (one at about 250 km height, and the other at about 900 km height) corresponding to the nominal values for GEOS-C. The earth's gravitational field was represented by a reference set of potential coefficients up to degree and order 12, considered as known values, and by residual gravity anomalies obtained by subtracting the anomalies, implied by the potential coefficients, from their terrestrial estimates. It was found that gravity anomalies could be recovered from strong signal without using any a-priori terrestrial information, i.e. considering their initial values as zero and also assigning them a zero weight matrix. While recovering them from weak signal, it was necessary to use the a-priori estimate of the standard deviation of the anomalies to form their a-priori diagonal weight matrix

A GMRT 150 MHz search for variables and transients in Stripe 82

We have carried out a dedicated transient survey of 300 deg2 of the SDSS Stripe 82 region using the Giant Metrewave Radio Telescope (GMRT) at 150 MHz. Our multi-epoch observations, together with the TGSS survey, allow us to probe variability and transient activity on four different time-scales, beginning with 4 h and up to 4 yr. Data calibration, RFI flagging, source finding, and transient search were carried out in a semi-automated pipeline incorporating the SPAM recipe. This has enabled us to produce superior-quality images and carry out reliable transient search over the entire survey region in under 48 h post-observation. Among the few thousand unique point sources found in our 5σ single-epoch catalogues (flux density thresholds of about 24, 20, 16, and 18 mJy on the respective time-scales), we find <0.08 per cent, 0.01 per cent, <0.06 per cent, and 0.05 per cent to be variable (beyond a significance of 4σ and fractional variability of 30 per cent) on time-scales of 4 h, 1 d, 1 month, and 4 yr, respectively. This is substantially lower than that in the GHz sky, where ∼1 per cent of the persistent point sources are found to be variable. Although our survey was designed to probe a superior part of the transient phase space, our transient search did not yield any significant candidates. The transient (preferentially extragalactic) rate at 150 MHz is therefore <0.005 on time-scales of 1 month and 4 yr, and <0.002 on time-scales of 1 d and 4 h, beyond 7σ detection threshold. We put these results in perspective with the previous studies and give recommendations for future low-frequency transient surveys

Structure of a Permethylcyclopentadienyl-μ-tetramethylcyclopentadienylmethylene Scandium Dimer

Bis(η^5-pentamethylcyclopentadienyl)-bis[μ-2,3,4,5-tetramethyl-1-methylene-ScC^1:Sc'(η^5)-cyclopentadienyl]-discandium(III), [Sc_2(C_(10)H_(15))_2-(C_(10)H_(14)_2], M_r = 628.90, triclinic, Pl, ɑ = 8.641 (5), b= 9.478 (2), c = 12.338 (5) Å, ɑ = 75.58 (3), β = 80.39 (4), y = 63.75 (3)º, V = 875.8 (7) Å^3, Z = 1, D_x = 1.19 g cm^(-3), λ(Mo Kɑ}=0.71073 Å, μ = 4.11 cm^(-1), F(000) = 340, room temperature, R (on F) = 0.046 for 2241 reflections with F_o^2 > 3σ(F_o^2). Two bis(pentamethylcyclopentadienyl)scandium molecules are joined across a center of symmetry. One of the Cp^* methyl groups has lost an H atom and that methylene group bonds to the symmetry-related Sc atom: Sc-C6' = 2.282 (5) Å. Other aspects of the bis-Cp^*Sc geometry are normal

Optimal placement of sensors to detect delamination in composite beams

The paper describes an approach for the optimal placement of sensors in composite beam structures for online detection of damage. The ability to identify damage is based on establishing a mapping between the charactgeristics of specific damage mechanisms (location and extent) such as delamination, fiber breakage, and matrix cracking, and strain measurements at the selected sensor locations; a trained neural network is proposed as a tool to generate this mapping. The design problem considered in the present paper was to place the least number of sensors in the structure so that the ability of the neural network to predict the extent and location of damage is not compromised. The optimization problem involved a mix of discrete and integer variables, and a genetic algorithm was used as the search tool. ~TRODUCTION A &quot;smart&quot; structure, instrumented with sensors and actuators, and responding in an intelligent manner to a dyoamically changing environment, is an intriguing concept. The key ingredients in the reafization of such a system include an adequate instrumentation of the structure, ability to rapidly analyze measured data and correlate to the existing state of the system, and to limit adverse structural behavior by providing real-time reaction in response to the evaluated state of the system. The present paper focusses on the use of strain measurements to detect delamination damage in composite beams. The approach can be extended to include other commonly encountered damage mechanisms in composites such as fiber breakage and matrix cracking, and analytical models relating the location and size of damage to strain fields in the structure under an applied load, are presented in (Teboub &amp; Hajela, 1992). Since the damage can be in more than one place, and furthermore, there can be multiple modes of damage present at the same time, the identification space in such a problem is often nonunique. Artificial neural network (ANN) based classifiers present themselves as a logical tool for relating specific strain response to damage type and location. Once trained, these networks can rapidly generalize new strain measurements into an estimated state of the structure, and are therefore ideal for online damage detection systems. An adequate instrumentation of the structure, however, continues to be a pivotal problem. The least number of sensors is clearly desirable from a standpoint of complexity of hardware. However, a sufficient number must be placed to resolve problems of nonunique identification and to have a robust system that is relatively insensitive to partial failures in the sensor array. The problem of optimally locating the least number of sensors that would identify damage over some admissible range of degradation and location, is explored in subsequent sections of this paper. Placement of sensors at some predefined grid in the structure is a discrete optimization problem, and computationally burdensome to handle using traditional branch-and-bound methods in nonlinear programming. The use of a genetic algorithm (Hajela 1993) is adopted in the present work, as this method is naturally amenable to search in a discrete space. Once the placement of the sensors is known, a neural network can be trained to develop the mapping between the characteristics of damage, and the strain measurements at the sensor locations. However, to determine the optimal location of sensors by genetic search requires that a very large number of function evaluations be performed. Such function evaluations would involve determining the strain state for many different sizes and locations of damage, and then varying the number and locations of sensors to find the optimal distribution of the sensors in the structure to success_fully identify various occurrences of damage. This is clearly a computationally intensive procedure, and in the present work, a trained neural network was used as an approximate analysis tool. Note that in optimizing for sensor locations, each strain reading may correspond to a totally different set of sensor locations. A novel hybrid neural network and

A GMRT 150 MHz Search for Variables and Transients in Stripe 82

We have carried out a dedicated transient survey of 300 deg$^2$ of the SDSS Stripe 82 region using the Giant Meterwavelength Radio Telescope (GMRT) at 150 MHz. Our multi-epoch observations, together with the TGSS survey, allow us to probe variability and transient activity on four different timescales, beginning with 4 hours, and up to 4 years. Data calibration, RFI flagging, source finding and transient search were carried out in a semi-automated pipeline incorporating the SPAM recipe. This has enabled us to produce superior-quality images and carry out reliable transient search over the entire survey region in under 48 hours post-observation. Among the few thousand unique point sources found in our 5$\sigma$ single-epoch catalogs (flux density thresholds of about 24 mJy, 20 mJy, 16 mJy and 18 mJy on the respective timescales), we find $<$0.08%, 0.01%, $<$0.06% and 0.05% to be variable (beyond a significance of 4$\sigma$ and fractional variability of 30%) on timescales of 4 hours, 1 day, 1 month and 4 years respectively. This is substantially lower than that in the GHz sky, where $\sim$1% of the persistent point sources are found to be variable. Although our survey was designed to probe a superior part of the transient phase space, our transient sources did not yield any significant candidates. The transient (preferentially extragalactic) rate at 150 MHz is therefore $<$0.005 on timescales of 1 month and 4 years, and $<$0.002 on timescales of 1 day and 4 hours, beyond 7$\sigma$ detection threshold. We put these results in the perspective with the previous studies and give recommendations for future low-frequency transient surveys.Comment: 9 pages, 7 figures. Published in MNRAS; doi: 10.1093/mnras/stz291

Use of an adwanced shear-Sag model to obtain the optimum internal damping in short-fiber composites

The present paper examines a modified shear-lag model for predicting the stress distribution in short fiber reinforced composite materials. The model assumes perfect bonding between the fiber and the matrix materials, and allows for the matrix material to partially sustain axial loads. The stress distribution obtained on the basis of this model is used to predict the internal damping characteristics of the composite material. These characteristics are a function of both the material properties and the geometrical layout of the composite, and are optimized by combining the analytical model with a nonlinear programming optimization algorithm. Representative numerical results are obtained for glass-epoxy and graphite-epoxy composites

A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet

We present new observations of the binary neutron star merger GW170817 at $\Delta t\approx 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $\gtrsim5\sigma$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $\sim 1000$ days post-merger.Comment: Accepted to ApJL. Updated version includes new VLA observations extending through 2018 June

Structure of a Permethylcyclopentadienyl-μ-tetramethylcyclopentadienylmethylene Scandium Dimer

Bis(η^5-pentamethylcyclopentadienyl)-bis[μ-2,3,4,5-tetramethyl-1-methylene-ScC^1:Sc'(η^5)-cyclopentadienyl]-discandium(III), [Sc_2(C_(10)H_(15))_2-(C_(10)H_(14)_2], M_r = 628.90, triclinic, Pl, ɑ = 8.641 (5), b= 9.478 (2), c = 12.338 (5) Å, ɑ = 75.58 (3), β = 80.39 (4), y = 63.75 (3)º, V = 875.8 (7) Å^3, Z = 1, D_x = 1.19 g cm^(-3), λ(Mo Kɑ}=0.71073 Å, μ = 4.11 cm^(-1), F(000) = 340, room temperature, R (on F) = 0.046 for 2241 reflections with F_o^2 > 3σ(F_o^2). Two bis(pentamethylcyclopentadienyl)scandium molecules are joined across a center of symmetry. One of the Cp^* methyl groups has lost an H atom and that methylene group bonds to the symmetry-related Sc atom: Sc-C6' = 2.282 (5) Å. Other aspects of the bis-Cp^*Sc geometry are normal