Search

An experimental investigation into the use of a bistable winglet to enhance the lift characteristics of a wing transitioning from lower to higher subsonic flow speeds is presented in this paper. The concept centers around the use of a specifically designed composite winglet, manufactured with an unsymmetric layup, which, when increasingly loaded, snaps between two stable states. Initially, during low-speed operation, the winglet is fixed in one stable state that is specifically designed to be cambered, thus enhancing the lift capability of the wing. At higher dynamic pressures, the winglet snaps to a configuration more intuitive and conventional to current winglet design. Results presented in this paper show the concept to be viable at enhancing the lift produced by a swept wing as aerodynamic loading increases before snap-through. During snap-through, however, the absence of any method of controlling the snap-through process generated significant dynamic loading that was transmitted, unhindered, throughout the entire test rig.

DOI: 10.2514/1.27720 This paper investigates a novel method for the control of 'morphing' aircraft. The concept consists of a pair of winglets with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing. The general philosophy behind the concept was that for specific flight conditions such as a coordinated turn, the use of two control devices would be sufficient for adequate control. Computations with a vortex lattice model and subsequent wind-tunnel tests demonstrate the viability of the concept, with individual and/or dual winglet deflection producing multi-axis coupled control moments. Comparisons between the experimental and computational results showed reasonable to good agreement, with the major discrepancies thought to be due to wind-tunnel model aeroelastic effects.

Linear dynamics of Euler-Bernoulli beams with nonviscous non-local damping is considered. It is assumed that the damping force at a given point in the beam depends on the past history of velocities at different points via convolution integrals over exponentially decaying kernel functions. Conventional viscous and viscoelastic damping models can be obtained as special cases of this general damping model. The equation of motion of the beam with such a general damping model results in a linear partial integro-differential equation. Exact closed-form equations of the natural frequencies and mode shapes of the beam are derived. Numerical examples are provided to illustrate the new results. [DOI: 10.1115/1.2712315]

Two frequency response correlation criteria, namely the global shape correlation (GSC) function and the global amplitude correlation (GAC) function, are established tools to quantify the correlation between predictions from a finite-element (FE) model and measured data for the purposes of FE model validation and updating. This paper extends the application of these two correlation criteria to structural health monitoring and damage detection. In addition, window-averaged versions of the GSC and GAC, namely WAIGSC and WAIGAC, are defined as effective damage indicators to quantify the change in structural response. An integrated method of structural health monitoring and damage assessment, based on the correlation functions and radial basis function neural networks, is proposed and the technique is applied to a bookshelf structure with 24 measured responses. The undamaged and damaged states, single and multiple damage locations, as well as damage levels, were successfully identified in all cases studied. The ability of the proposed method to cope with incomplete measurements is also discussed. CE Database subject headings: Structural reliability; Monitoring; Damage assessment; Frequency response; Correlation.

The reliable estimation of a flexible foundation model and the state of unbalance (both amplitude and phase) of a turbogenerator from machine rundown measured vibration data is an active research area. Earlier studies on the estimation of both these quantities used the whole frequency range of the rundown as a single band. However, such an identification may be inaccurate for large flexible foundations having many modes in the rundown frequency range. For reliable identification, the whole frequency range has to be divided into a number of frequency bands and the frequency-dependent foundation models have to be estimated together with the unbalance. This paper combines the unbalance estimation with the split frequency range for the foundation model, and the highlights the limitations observed during estimation of foundation models and the state of unbalance. It is shown that the accuracy of the method may be enhanced by judicious choice of a weighting function. Having established the method in simulation, experimental data from a 3-m long test rig, with four journal bearings, is used to test the method. The approach seems to give reliable estimates of the machine unbalance. [DOI: 10.1115/1.1691441]

The selection of parameters is most important to successful updating of finite element models. When the parameters are chosen on the basis of engineering understanding the model predictions are brought into agreement with experimental observations, and the behavior of the structure, even when differently configured, can be determined with confidence. Physical phenomena may be misrepresented in the original model, or may be absent altogether. In any case the updated model should represent an improved physical understanding of the structure and not simply consist of unrepresentative numbers which happen to cause the results of the model to agree with particular test data. The present paper introduces a systematic approach for the selection and physical realization of updated terms. In the realization process, the discrete equilibrium equation formed by mass, and stiffness matrices is converted to a continuous form at each node. By comparing the resulting differential equation with governing equations known to represent physical phenomena, the updated terms and their physical effects can be recognized. The approach is demonstrated by an experimental example. [DOI: 10.1115/1.1505028]

Model updating often produces sets of equations whose solution are ill-conditioned and extra information must be used to produce a well-conditioned estimation problem. One possibility is to change all the parameters, but to introduce extra constraints, for example by taking the minimum norm solution. This paper takes a different approach, by considering only a subset of the parameters to be in error. The critical decision is then the choice of parameters to include in the subset. The methods of subset selection are outlined and extended to the selection of groups of parameters. The incorporation of side constraints is considered and demonstrated using an experimental example.

This paper considers the equations required to analyze the three-dimensional steady-state configuration of an underwater cable system. Both towed systems and autonomous remotely controlled systems are considered, and the equations used to simulate the cable configuration are given. A sea-buggy example, originally considered by De Zoysa in 1978, is given to demonstrate the solution of the resulting two-point boundary-value problem by a shooting method. The results correct those of Wang et al. in 1993. An example of a towed system using a stranded cable demonstrates the inclusion of cable lift. A simplified analysis of the dependence of the cable tension on the cable length is also given. The optimum length of cable required to product the minimum force on the vehicle is between three and six times the direct range of the vehicle from the ship, for a range of vehicle positions.

Methods to update the parameters of finite element models using measured vibration data usually use the experimentally derived modal model, that is, the system natural frequencies, damping coefficients, and mode shapes. Alternatively the frequency response functions have been used directly to update condensed analytical models and so avoid the sometimes difficult step of deriving the modal model. Previously the authors suggested an algorithm using FRF data that is basically a weighted equation error method based on a reduced order model. This paper investigates the performance of the algorithm for systems with closely coupled or repeated natural frequencies or eigenvalues.