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Grooteman F.,National Aerospace Laboratory Netherlands
Probabilistic Engineering Mechanics

In this paper, an adaptive directional importance sampling (ADIS) method is presented. The algorithm is based on a directional simulation scheme in which the most important directions are sampled exact and the others by means of a response surface approach. These most important directions are determined by a β-sphere enclosing the most important part(s) of the limit state. The β-sphere and response surface are constantly updated during sampling with information that becomes available from the exact evaluations making the scheme adaptive. Various widely used test problems, representing a broad range of complex limit states that can occur in practice, of which several that pose potential problems to stochastic methods in general, demonstrate the high efficiency, accuracy and robustness of the method. As such, the ADIS method is of particular interest in applications with a low probability of failure and medium number (up to about 40) of stochastic variables, for instance in aircraft and nuclear industry. Source

Wanhill R.J.H.,National Aerospace Laboratory Netherlands
Journal of Failure Analysis and Prevention

This article discusses six cases of ancient silver embrittlement. The diagnostic techniques and results are surveyed, followed by the most likely explanations of embrittlement. There are two basic types of embrittlement: corrosion-induced, and microstructurally induced, which can act synergistically. Corrosion-induced embrittlement takes several forms, including what appears to be intergranular and transgranular stress corrosion cracking. The main intrinsic factor facilitating corrosion-induced embrittlement is retained cold-work in the silver. Microstructural embrittlement is characterized by intergranular fracture, most probably due to low-temperature segregation of lead to the grain boundaries. Knowledge of the details of embrittlement can be essential for optimizing the restoration and conservation of damaged artifacts. © 2011 ASM International. Source

Lorenzoni V.,Siemens AG | Tuinstra M.,National Aerospace Laboratory Netherlands | Scarano F.,Technical University of Delft
Journal of Sound and Vibration

The present study investigates an experimental methodology to determine aeroacoustic emission from vortex-structure interaction by means of Time-resolved Particle Image Velocimetry (TR-PIV). The aeroacoustic investigation is conducted on a rod-airfoil configuration at Re=6000 based on the rod diameter. The time-resolved velocity field obtained from 2D PIV is employed to evaluate the instantaneous planar pressure field by spatial integration of the Navier-Stokes equations under the assumption of 2D incompressible flow. The instantaneous pressure field computed on a control surface approximating that of the physical airfoil is used as source term of Curles aeroacoustic analogy in both a distributed and a lumped formulation to obtain the far-field acoustic prediction. The spanwise coherence function of velocity and pressure fluctuations is determined by means of additional experiments, and is applied to weight the contributions at different frequencies. Results are compared with far-field microphone measurements in terms of spectra and directivity pattern. A good agreement is observed for the tonal component corresponding to the periodic interaction of the Kármán vortices with the airfoil leading edge. The contributions at higher frequencies also show an acceptable agreement when the spanwise coherence is taken into account. © 2012 Elsevier Ltd. Source

Barter S.A.,Defence Science and Technology Organisation, Australia | Molent L.,Defence Science and Technology Organisation, Australia | Wanhill R.J.H.,National Aerospace Laboratory Netherlands
International Journal of Fatigue

A fatigue lifing framework using a lead crack concept, based on years of detailed inspection and analysis of fatigue cracks in many specimens and airframe components, has been developed by the DSTO for metallic primary airframe components. This framework is an important additional tool for determining aircraft component fatigue lives in the Royal Australian Air Force fleet. Like the original Damage Tolerance concept, developed by the United States Air Force, this framework assumes that fatigue cracking begins as soon as an aircraft enters service. However, there are major and fundamental differences. Instead of assuming initial crack sizes and deriving early crack growth behaviour from back-extrapolation of growth data for long cracks, the framework uses data for real cracks growing from small discontinuities inherent to the material and the production of the component. To this end, this paper examines the types of discontinuities that initiate fatigue cracks in typical metallic airframe structures. These discontinuities and the fatigue cracks that have grown from them are taken from coupon, component and full-scale tests, and also from service aircraft, including commercial transport aircraft and high performance military aircraft. © 2011 Elsevier Ltd. All rights reserved. Source

Haverdings H.,National Aerospace Laboratory Netherlands | Chan P.W.,Hong Kong Observatory
Journal of Aircraft

A sophisticated algorithm has been developed by the National Aerospace Laboratory/NLR and Hong Kong Observatory (HKO) to calculate the meteorological parameters crucial for the study of low-level windshear and turbulence, using onboard QAR data. The WINDSTURB program allows batch processing of large amounts of aircraft QAR data to be completed within several minutes. A main feature of WINDSTURB is the use of Kalman filtering and smoothing. It is a process of estimating the state vector x of a dynamical system at a particular stage (or time ti) and its covariance, where x consists of three inertial velocities, three positions, and three accelerometer biases, using measurements of inertial data. The WINDSTURB software will be further refined to cover other aircraft types and expanded to handle missed approaches. The WINDSTURB-calculated F factor and EDR will also be compared more extensively with the estimates of these quantities from ground-based remote-sensing instruments and pilot reports. Source

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