7 research outputs found
Frequency domain modeling of nonlinear end stop behavior in Tuned Mass Damper systems under single- and multi-harmonic excitations
No full textNonsmooth dynamics of a Tuned Mass Damper system with lateral stops are studied using an alternating frequency/time harmonic balancing (AFT-HB) method. To this end, an extremely stiff end stop nonlinearity is considered. The application range of AFT-HB is investigated by including up to 250 harmonics in the external force, as well as in the motion description. Numerical simulations are performed by making use of a Newmark time integration algorithm for numerical verification of the results. The results for single harmonic excitations are further verified with an existing pseudo-arclength path-following tool. Two excitation scenarios are considered: single harmonic- and a wide-spectrum excitation with uniform distribution and random phase correlation between the harmonics. The AFT-HB algorithm is found to accurately reproduce the time integration results, for all considered cases. Finally, insights are gained into the differences between the system responses to single- and multi-harmonic excitations.Accepted Author ManuscriptDynamics of Micro and Nano System
Advances in nonlinear acoustic/elastic metamaterials and metastructures
No full textAcoustic/elastic metamaterials exhibit a wealth of unusual properties conducive to wave manipulation. This review outlines state-of-the-art developments from FPUT chains, granular crystals to nonlinear acoustic metamaterials (NAMs). It mainly discusses key advances made in the domain of NAMs for wave manipulation, vibration control and sound attenuation given the blooming interest in exploring how nonlinearity offers possibilities for discovering novel wave phenomena, principles and properties that potentially go well beyond linear metamaterials and the relevant linear theories. NAMs reveal intriguing wave phenomena, revolutionizing our understanding of wave behavior including the breakdown of reciprocity, stationary invariance and space–time invariance, and have the potential to promote superior engineering performance like ultra-low and ultra-broadband vibration reduction. An overview of present research and further challenges are provided in fields such as calculation methods, amplitude-dependent bandgaps, self-adaptive bands, nonreciprocal wave control, harmonic control, chaotic dynamics, vibration and sound attenuation, practical design, experimental implementation, and practical applications. © The Author(s) 2024
Maximization of the geometric non-linearities of a thin-walled structure in resonance
No full textMicro air vehicles (MAVs) are small aircraft that are promising for search and rescue missions or remote observations of hazardous or inaccessible areas. Currently one of the problems MAVs face is the limited action radius. To increase the action radius, a more energy efficient MAV is needed. This can be achieved by exploiting the resonance of the structure that couples the actuator and the wings. A MAV that brought this concept to practice was designed and manufactured by C.T. Bolsman [1]. The developed prototype uses a flapping wing actuation mechanism producing enough lift to overcome the mass of the resonant structure, but not enough to also lift the actuator. A way to increase lift is to create a more effective flapping wing motion. Investigations carried out within the Department of Precision and Microsystems Engineering (PME) at Delft University of Technology show that a constant velocity of the sweeping (flutter) motion between stroke reversals increases the effectiveness. The function that ideally matches this velocity profile is a triangular-shaped displacement function in time. This work focuses on obtaining a triangular-shaped function for the angular displacement at the end of a resonating beam. This is not a priori granted, because the beam is subject to a harmonic forcing in order to be excited and kept in resonance. Provided that the system operates in its linear regime, the response of the beam, over time, will be a harmonic motion. Yet, by exploiting the geometric non-linearities, it is possible to retrieve a triangular motion as displacement profile, while the input remains a harmonic torque. In this work, the answers are given to the following questions: What causes the geometric non-linearities? Which beam cross-section maximizes the geometric non-linearities? Past work showed that geometric non-linearities arise from the longitudinal stresses induced by the axial non-uniform shortening of the structure over the cross-section. In this work the formulation of the non-linear stiffness is provided, together with the formulation of the uniform torsion stiffness and the stiffness caused by warping. Collectively, they yield the total torsional stiffness of the beam. With this knowledge, the author proposes an analytical model of a beam under torsion and an asymptotic approximation to its solution with the help of a perturbation method, the so-called method of multiple scales. This analytical model is derived for optimization purposes due to the low computational costs associated with the delivered dynamic responses. However, the derived solution of the analytical model is an approximation, so the results are verified with a more reliable, but computational expensive method: the finite element method. Consequently, a finite element code is developed and a beam element that can handle warping and geometric non-linearities is implemented. Next, both models are verified with the finite element program ANSYS. In the last part of the work, the analytical model is used as a basis for the optimization. Some approximations are made to enable the computation of a fine grid of the various crosssectional features and their corresponding degree of triangularity of the response over time. Finally, the optimal cross-sectional features are found. The corresponding dynamic response is, as expected, a triangular-shaped function. Next, the approximations are verified and some recommendations are provided regarding an already manufactured MAV based on torsion. Lastly, recommendations for future investigations, although outside the scope of this project are provided and a point of attention is noted regarding buckling of thin-walled structures.Engineering MechanicsPrecision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
Identification of Parametric Resonances in a Geometrically Exact Model of a Rotating Blade
No full textMuch research has been done in the past couple of decades on the vibrations of rotating structures, such as helicopter and wind turbine blades. This is the consequence of ever increasing standards towards dynamic performance of these systems. Previous investigations have primarily dealt with constant rotational velocity, with a focus on the determination of the linear dynamic response. The rotational speed cannot always be considered constant due to fluctuating external loads or due to specific ramp-up regimes. This non-constant angular velocity brings about terms with time dependent coefficients in the equations of motion which can result in the existence of so-called parametric resonances. These resonance phenomena are quite unknown in this field of application. Nevertheless, they are of major concern to mechanical engineers because it can lead to structural damage due to large amplitude oscillations as a result of parametric resonances. In this thesis a fully geometrically nonlinear beam model is set up to study the fundamental behavior that causes parametric resonances in rotating blades. These equations are solved by means of two numerical approaches, the finite element method and the Galerkin method. The first one is used to obtain the linear modal properties. The Galerkin discretized equations are adopted to examine the linear and non-linear dynamics in time. The linear response is investigated to obtain the regions in which the unstable motions occurs due to parametric excitation. The non-linear response is obtained to show the stable post-critical behavior in these instability regions. Furthermore, it is shown that for specific values of the rotational speed and the parametric excitation frequency, instability occurs in each principal direction.Engineering MechanicsPrecision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
Modeling end-stop nonlinearity of Tuned Mass Dampers in Offshore Wind Turbines
No full textTuned Mass Dampers (TMDs) are passive spring-mass-damper systems that can reduce the fatigue damage in offshore wind turbines, caused by cyclic hydrodynamic loads. The TMD is typically installed within the tower at a high position, where it can attack the first bending mode of the combined structure. In order to calculate the fatigue damage in a wind turbine for many different scenarios, fast simulation tools are of interest. While the dynamics of a wind turbine structure can often be linearized without significant loss of accuracy, a TMD may accompany nonlinearities that cannot easily be discarded. In this work, the nonlinear dynamics of end-stops are studied, which have the purpose of preventing the TMD body from colliding with the tower walls. The resulting problem consists of a multiple degree of freedom structure containing end-stop nonlinearity, subject to hydrodynamic external loads carrying many harmonic components. While time-integration techniques exist that can handle such problems, a frequency domain approach is sought in this work as an alternative, to improve the simulation speed. A harmonic balancing technique has been implemented with an existing finite element model of a wind turbine, and was found to be a promising alternative to time-integration for calculating the internal dynamic loads in the turbine.Mechanical, Maritime and Materials EngineeringPrecision and Microsystems Engineerin
Exploring the dynamics of a vibro-impact capsule moving on the small intestine using finite element analysis
No full textThis is the author accepted manuscript. The final version is available from Springer] via the DOI in this recordThis paper aims to study a realistic finite element (FE)
model to depict the nonlinear dynamics of a vibro-impact capsule moving
on the small intestine for active capsule endoscopy. The FE model takes
both the nonlinear vibro-impact mechanism and the viscoelastic deformation of the small intestine into account. FE results are compared with
the simulation results obtained using non-smooth differential equations
and experimental results. It is found that the FE model can provide a
more realistic prediction of the system in the complex intestinal environment in terms of capsule’s tilted motion and asymmetric distribution of
capsule-intestine contact pressure. In particular, the capsule’s dynamics
is very sensitive to the surface condition of the intestine, so a comprehensive bifurcation analysis is needed for fully understanding its dynamics
under intestinal peristalsis.Engineering and Physical Sciences Research Council (EPSRC)China Scholarship Counci
Water Quality Assessment: A Quali-Quantitative Method for Evaluation of Environmental Pressures Potentially Impacting on Groundwater, Developed under the M.I.N.O.Re. Project
No full textBackground: At global level, the vulnerability of aquifers is deteriorating at an alarming rate due to environmental pollution and intensive human activities. In this context, Local Health Authority ASL Lecce has launched the M.I.N.O.Re. (Not Compulsory Water Monitoring Activities at Regional level) project, in order to assess the vulnerability of the aquifer in Salento area (Puglia Region) by performing several non-compulsory analyses on groundwater samples. This first paper describes the quali-quantitative approach adopted under the M.I.N.O.Re. project for the assessment of environmental pressures suffered by groundwater and determines the number of wells to be monitored in specific sampling areas on the basis of the local potential contamination and vulnerability of the aquifer. Methods: We created a map of the entire Lecce province, interpolating it with a grid that led to the subdivision of the study area in 32 quadrangular blocks measuring 10 km × 10 km. Based on current hydrogeological knowledge and institutional data, we used GIS techniques to represent on these 32 blocks the 12 different layers corresponding to the main anthropic or environmental type of pressures potentially impacting on the aquifer. To each kind of pressure, a score from 0 to 1 was attributed on the basis of the potential impact on groundwater. A total score was assigned to each of the 32 blocks. A higher number of wells was selected to be monitored in those blocks presenting higher risk scores for possible groundwater contamination due to anthropic/environmental pressures. Results: The range of total scores varied from 2.4 to 42.5. On the basis of total scores, the 10 km × 10 km blocks were divided into four classes of environmental pressure (1st class: from 0,1 to 10,00; 2nd class: from 10,01 to 20,00; 3rd class: from 20,1 to 30,00; 4th class: from 30,01 to 42,50). There were 11 areas in the 1st class, 9 areas in the 2nd class, 8 areas in the 3rd class and 4 areas in the 4th class. We assigned 1 monitoring well in 1st class areas, 2 monitoring wells in 2nd class areas, 3 monitoring wells in 3rd class areas and 4 monitoring wells in 4th class areas. Conclusion: The methodology developed under the M.I.N.O.Re. project could represent a useful model to be used in other areas to assess the environmental pressures suffered by aquifers and the quality of the groundwater
