62 research outputs found
Thunder, Indians, and Mythological Birds: School Mascot Changes in Western Wisconsin
No full textIn American society, people identify themselves not just with their creed, race, or ethnicity, but also their sports teams. This tightly knit identity creates a culture within itself that presents the beliefs, norms, and way of life of the people intertwined within it. While most prominent at NCAA Division I and professional levels, this identification also occurs at the local level, such as at high schools and smaller colleges. In some situations schools are forced to change their mascot or team name. This paper will examine an identity crisis at three separate schools in western Wisconsin during the second half of the twentieth and early twenty-first centuries and explain how the schools went about choosing a new mascot or team name. The three different case studies will examine controversial identities, lack of an identity, and popularity creating an identity
Experimental Parameter Identification of Nonlinear Mechanical Systems via Meta-heuristic Optimisation Methods
No full textMeta-heuristic optimisation algorithms are high-level procedures designed to discover near-optimal solutions to optimisation problems. These strategies can efficiently explore the design space of the problems; therefore, they perform well even when incomplete and scarce information is available. Such characteristics make them the ideal approach for solving nonlinear parameter identification problems from experimental data. Nonetheless, selecting the meta-heuristic optimisation algorithm remains a challenging task that can dramatically affect the required time, accuracy, and computational burden to solve such identification problems. To this end, we propose investigating how different meta-heuristic optimisation algorithms can influence the identification process of nonlinear parameters in mechanical systems. Two mature meta-heuristic optimisation methods, i.e. particle swarm optimisation (PSO) method and genetic algorithm (GA), are used to identify the nonlinear parameters of an experimental two-degrees-of-freedom system with cubic stiffness. These naturally inspired algorithms are based on the definition of an initial population: this advantageously increases the chances of identifying the global minimum of the optimisation problem as the design space is searched simultaneously in multiple locations. The results show that the PSO method drastically increases the accuracy and robustness of the solution, but it requires a quite expensive computational burden. On the contrary, the GA requires similar computational effort but does not provide accurate solutions.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Design, Production and Operation
Experimental Characterization of a New Benchmark Structure for Prediction of Damping Nonlinearity
No full textSpacecraft, airplanes, automobiles, machines and civil structures are all constructed from multiple parts joined by bolts, rivets or other fasteners and these joints lead to large uncertainties in the structural stiffness, damping and can even introduce nonlinearity. Even with the best available simulation tools, it is still difficult to predict the effective stiffness and damping of bolted interfaces, and so these parameters are often assumed and updated after tests have been performed. Damping estimates are critical to limit the resonant vibration response of a structure and thus prevent failure. Even so, it remains poorly understood and available methods for modeling damping are inaccurate and computationally expensive. A new benchmark structure has been created that is designed so as to be predictable with current simulation tools. This paper presents a thorough experimental characterization of this new benchmark structure using the Hilbert transform method applied to modally filtered time data. The nonlinear frequency and damping of each mode is characterized for various levels of bolt preload and excitation amplitude. The interfaces of the bolted structure are also characterized in detail by measuring the contact pressure distribution using pressure sensitive film. The resulting data presents a set of well characterized tests that can be used to validate numerical methods that seek to predict the nonlinear behavior of bolted interfaces
An Investigation of Complex Mode Shapes
No full textThis paper presents an investigation of complex mode shape analysis caused by non-linear damping. Nowadays, most academics are accustomed to complex mode shapes, which are a characteristic of most axisymmetric structures. The topic was deeply investigated during the 1980s, sparking the sharpest debates about their physical existence or not. However, after nearly three decades, one question still stands, do we know all about complex mode shapes? This paper takes the dust off this topic again and explores how complex eigenvectors arise when the percentage frequency separation between two mode shapes is the same order of magnitude as the percentage damping. The difference between the past and present investigations relates to the non-linear damping that might arise from joint dynamics under various vibration amplitudes. Hence, the new research question is about the investigation of amplitude-dependent damping on the modal complexity. Why bother? There are several engineering applications in both space and aerospace where axisymmetric structures and joint dynamics can impair the numerical analysis that is currently performed. This paper does not offer any solutions but does expand the research on an unsolved challenge by identifying the questions posed.</p
Challenges in Simulating the Forced Vibration Response of Assembled Space Structures
No full textThis chapter presents an example of how challenging can be the validation of system responses when a product must be certified before being commissioned for a launch or flight. Several divisions of industry dealing with dynamic models and simulations of forced vibrations are discovering how the nonlinear dynamics of joints can lead to inaccuracies in the simulated vibration responses, mainly due to the nonlinear relationships of frequency and damping with the vibration amplitude (which is high for qualification tests). Hence, the example of this chapter refers to analysis of test data gathered from a solar array in qualification tests. It is shown how dissimilar simulated and measured vibration data can be, and how the qualification levels enhance the nonlinear dynamics, often not predicted by dynamic models. Finally, it presents an effort to analyse a cut-out of the solar array in terms of frequency and damping nonlinearity.</p
Nonlinear vibration analysis of uniform and functionally graded beams with spectral Chebyshev technique and harmonic balance method
No full textIn this paper, nonlinear forced vibrations of uniform and functionally graded Euler-Bernoulli Beams with large deformation are studied. Spectral and temporal boundary value problems of beam vibrations do not always have closed-form analytical solutions. As a result, many approximate methods are used to obtain the solution by discretizing the spatial problem. Spectral Chebyshev Technique (SCT) utilizes the Chebyshev Polynomials for spatial discretization and applies Galerkin’s method to obtain boundary conditions and spatially discretized equations of motions. Boundary conditions are imposed using basis recombination into the problem and as a result of this, the solution can be obtained to any linear boundary condition without the need for re-derivation. System matrices are generated with the SCT, and natural frequencies and mode shapes are obtained by eigenvalue problem solution. Harmonic Balance Method (HBM) is used to solve nonlinear equation of motion in frequency domain, with large deformation nonlinearity. As a result, a generic method is constructed to solve nonlinear vibrations of uniform and functionally graded beams in frequency domain, subjected to different boundary conditions
Identification of nonlinear characteristics of an additive manufactured vibration absorber
No full textAdditive manufacturing has become increasingly popular in the last decades and has shown great potential for designing and manufacturing innovative design solutions. Recently it has been demonstrated that additive manufacturing can be used to create monolithic compliant mechanisms that can avoid assembly and relative movement between components, showing considerable advantages in their use in harsh environments (i.e. space applications). In this paper, we explore the possibility of adopting 3D-printed compliant mechanisms as tuned-mass vibration absorbers: the challenge is to identify the characteristics of an equivalent nonlinear oscillator that can be used to assess the performance of the absorber. The experimental and numerical results show that the proposed compliant mechanism offers a complex nonlinear dynamic behaviour and it can effectively act as a vibration absorber for a simple cantilever beam
A novel test rig for the validation of nonlinear friction contact parameters of turbine blade root joints
No full textThe assembly of components into a large-scale engineering system naturally leads to the presence of joints with frictional interfaces. The degree of agreement between numerical models and their experimental counterparts decreases when assemblies based in this kind of interfaces are studied due to the nonlinear dynamic behaviour that joints introduce. This is, for example, the case in turbine blade root joints. The main cause for these deviations are the friction-related nonlinear damping and stiffness effects influencing the dynamic behaviour of the assembly. The experimental measurement of these damping effects poses a challenge due to the presence of the excitation rig itself, which can introduces significant parasitic damping in the system. A free decay measurement is consequently the ideal way to extract the nonlinear behaviour, however, the exciter must be initially in physical contact with the test fixture in order to reach the high excitation amplitudes that lead to macro-slip friction in the fixture joints. The test setup proposed in this paper is developed for a beam on which two blade root designs have been machined at both ends (dog bone). This beam is fitted between two clamps equipped with dovetail roots and pulled into tension to simulate rotational centrifugal loading, thus creating a blade root contact joint at either end of the beam. The novel excitation method excites the beam harmonically with a rigidly connected shaker to macro-slip deflection amplitudes before decoupling from the beam to release it into free decay. This test procedure allows the contactless measurement of the variation in vibrational decay in the beam and the subsequent extraction of the resulting nonlinear frictional behaviour associated with the joints
Application of Geometrically Nonlinear Metamaterial Device for Structural Vibration Mitigation
No full textISSN:2191-5644ISSN:2191-5652ISSN:2191-565
Experimental Analysis of a Nonlinear Piecewise Multi-Degrees of Freedom System
No full textThe dataset contains the experimental dynamic measurements of a nonlinear Multi-Degree of Freedom System featuring hardening and piecewise stiffness characteristics. The system was studied in different ways: a linear analysis was performed with low-amplitude random excitation while a second nonlinear analysis was performed via stepped sine excitation. The nonlinear analysis was repeated with and without the stoppers (which approximate a piecewise stiffness characteristic) to better understand the nature of the system.
All the data are stored in MAT and CSV files which are accessible with MATLAB and Python codes (attached to the dataset)
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