On the quasi-steady aerodynamics of normal hovering flight part II: model implementation and evaluation

This paper introduces a generic, transparent and compact model for the evaluation of the aerodynamic performance of insect-like flapping wings in hovering flight. The model is generic in that it can be applied to wings of arbitrary morphology and kinematics without the use of experimental data, is transparent in that the aerodynamic components of the model are linked directly to morphology and kinematics via physical relationships and is compact in the sense that it can be efficiently evaluated for use within a design optimization environment. An important aspect of the model is the method by which translational force coefficients for the aerodynamic model are obtained from first principles; however important insights are also provided for the morphological and kinematic treatments that improve the clarity and efficiency of the overall model. A thorough analysis of the leading-edge suction analogy model is provided and comparison of the aerodynamic model with results from application of the leading-edge suction analogy shows good agreement. The full model is evaluated against experimental data for revolving wings and good agreement is obtained for lift and drag up to 90° incidence. Comparison of the model output with data from computational fluid dynamics studies on a range of different insect species also shows good agreement with predicted weight support ratio and specific power. The validated model is used to evaluate the relative impact of different contributors to the induced power factor for the hoverfly and fruitfly. It is shown that the assumption of an ideal induced power factor (k = 1) for a normal hovering hoverfly leads to a 23% overestimation of the generated force owing to flapping.     

High power factor operation of resonant converters on the utility line

Operation and characteristics of resonant converters on the utility line are presented. Series-parallel (LCC-type) resonant converter operating with discontinuous current mode and continuous current mode (variable frequency control as well as fixed-frequency) are considered. Design examples are presented. SPICE simulation and experimental results obtained for the designed converters (rated at 150 W) are presented to verify the theory. It is shown that high line power factor (>0.95) and line current total harmonic distortion (THD) of <25% are obtained for the LCC-type converter for a wide load range (from full load to 10% rated load) without any active control, and the switch peak current decreases with the load current. With active line current control, low distortion and zero voltage switching for the entire cycle are realized.     

A numerical analysis of the mechanisms behind plasticity induced crack closure: Application to variable amplitude loadings

The effect of loading parameters on fatigue crack growth has been explained using the concept of crack closure. Plasticity induced crack closure (PICC) is linked to the crack tip plastic deformation, which becomes residual with crack propagation. The objective here is to identify the main mechanisms behind PICC, and for that different loading cases were considered namely overloads and load blocks. An analytical model was used to isolate the effect of residual plastic deformation on PICC, however significant differences were obtained relatively to finite element results. A second mechanism, which is crack tip blunting, was used to explain the transient behaviour observed after overloads and load blocks. For overloads and low–high load sequences there is a sudden increase of crack tip blunting with load increase which explains the sudden decrease of crack opening level. For high–low load sequences there is a sudden decrease of crack tip blunting which enhances the effect of residual plastic wake. Finally, the partial closure concept was tested looking to non-linear crack tip parameters but no evidences of Donald’s effect were found for the material studied.     

Compliance based assessment of stress intensity factor in cracked hollow cylinders with finite boundary restraint: Application to thermal shock part I

The evaluation of mode I stress intensity factor associated with the creep-free thermal shock (TS) of finite length elastically/thermo-elastically restrained cracked hollow cylinders is a problem of interest. Among existing evaluation methodologies, the mechanical weight function approach is often perceived to be an optimal compromise between simplicity and accuracy for and more generalised K_I evaluation. However, to confidently apply a mechanical weight function methodology in such circumstances requires the derivation of different weight functions for each potential boundary restraint configuration, i.e. free, flexible or rigid boundary conditions. In this article, the traditional mechanical weight function philosophy is complimented with an elastic compliance analysis, enabling the mechanical weight function and geometry factors for an equivalent semi-infinite cracked hollow cylinder to be used to evaluate associated with a wide range of finite length elastically/thermo-elastically restrained cracked hollow cylinders. The need for deriving different weight functions is therefore removed and the proposed Compliance Adjusted Weight Function (CAWF) approach becomes more ‘user-friendly’. The targeted cracked hollow cylinders are assumed to exhibit an exterior circumferential edge crack or an exterior circumferential semi-elliptical surface crack.     

Catching-up with supermajors: the technology factor in building the competitive power of national oil companies from developing economies

Abstract

The paper investigates the role of technology in the growing competitive potential of state-controlled national oil companies (NOCs) from developing countries. The technological development of NOCs is analyzed in the context of their increasing rivalry with supermajors, which have dominated the global oil and gas industry for decades. The author reveals the main features of NOCs’ catch-up development amid the dynamic changes in the competitive landscape of the global oil industry. These trends were analyzed against the background of the current phase of the oil industry’s technological evolution. The paper concludes that although rapid technological growth did become a key strategic priority of many NOCs, only very small group of them managed to achieve equal footing with traditional industry leaders. For the majority of the remaining NOCs, the existing limitations related to the policy of their home state did not allow them to narrow the technological gap with global supermajors.     

On the determination of the power spectrum of randomly excited oscillators via stochastic averaging: An alternative perspective

An approximate formula which utilizes the concept of conditional power spectral density (PSD) has been employed by several investigators to determine the response PSD of stochastically excited nonlinear systems in numerous applications. However, its derivation has been treated to date in a rather heuristic, even “unnatural” manner, and its mathematical legitimacy has been based on loosely supported arguments. In this paper, a perspective on the veracity of the formula is provided by utilizing spectral representations both for the excitation and for the response processes of the nonlinear system; this is done in conjunction with a stochastic averaging treatment of the problem. Then, the orthogonality properties of the monochromatic functions which are involved in the representations are utilized. Further, not only stationarity but ergodicity of the system response are invoked. In this context, the nonlinear response PSD is construed as a sum of the PSDs which correspond to equivalent response amplitude dependent linear systems. Next, relying on classical excitation-response PSD relationships for these linear systems leads, readily, to the derivation of the formula for the determination of the PSD of the nonlinear system. Related numerical results are also included.     

On the formulation of closest‐point projection algorithms in elastoplasticity—part I: The variational structure

We present in this paper the characterization of the variational structure behind the discrete equations defining the closest‐point projection approximation in elastoplasticity. Rate‐independent and viscoplastic formulations are considered in the infinitesimal and the finite deformation range, the later in the context of isotropic finite‐strain multiplicative plasticity. Primal variational principles in terms of the stresses and stress‐like hardening variables are presented first, followed by the formulation of dual principles incorporating explicitly the plastic multiplier. Augmented Lagrangian extensions are also presented allowing a complete regularization of the problem in the constrained rate‐independent limit. The variational structure identified in this paper leads to the proper framework for the development of new improved numerical algorithms for the integration of the local constitutive equations of plasticity as it is undertaken in Part II of this work. Copyright © 2001 John Wiley & Sons, Ltd.