106 results found
Yuan J, Fantetti A, Denimal E, et al., 2021, Propagation of friction parameter uncertainties in the nonlinear dynamic response of turbine blades with underplatform dampers, Mechanical Systems and Signal Processing, Vol: 156, Pages: 1-19, ISSN: 0888-3270
Underplatform dampers are widely used in turbomachinery to mitigate structural vibrations by means of friction dissipation at the interfaces. The modelling of such friction dissipation is challenging because of the high variability observed in experimental measurements of contact parameters. Although this variability is not commonly accounted for in state-of-the-art numerical solvers, probabilistic approaches can be implemented to include it in dynamics simulations in order to significantly improve the estimation of the damper performance. The aim of this work is to obtain uncertainty bands in the dynamic response of turbine blades equipped with dampers by including the variability observed in interfacial contact parameters. This variability is experimentally quantified from a friction rig and used to generate uncertainty bands by combining a deterministic state-of-the-art numerical solver with stochastic Polynomial Chaos Expansion (PCE) models. The bands thus obtained are validated against experimental data from an underplatform damper test rig. In addition, the PCEs are also employed to perform a variance-based global sensitivity analysis to quantify the influence of contact parameters on the variation in the nonlinear dynamic response via Sobol indices. The analysis highlights that the influence of each contact parameter in vibration amplitude strongly varies over the frequency range, and that Sobol indices can be effectively used to analyse uncertainties associated to structures with friction interfaces providing valuable insights into the physics of such complex nonlinear systems.
Fantetti A, Mariani S, Pesaresi L, et al., 2021, Ultrasonic Monitoring of Friction Contacts during Shear Vibration Cycles, Mechanical Systems and Signal Processing, ISSN: 0888-3270
Ondra V, Sever IA, Schwingshackl CW, 2021, Identification of complex non-linear modes of mechanical systems using the Hilbert-Huang transform from free decay responses, Journal of Sound and Vibration, Vol: 495, Pages: 1-26, ISSN: 0022-460X
Modal analysis is a well-established method for analysis of linear systems, but its extension to non-linear structures has proven to be much more problematic. Several competitive definitions of non-linear modes and a variety of experimental methods have been introduced. In this paper, the definition of complex non-linear modes (CNMs) of mechanical systems is adopted and the possibility of their identification from experimental free decay responses using the Hilbert-Huang transform (HHT) is explored. It is firstly discussed that since there are similarities in the definition of intrinsic mode functions obtained using the HHT and reduced order model of slow-flow dynamics based on the CNMs, there is a reason to believe that the HHT can indeed extract the CNMs. This paper, however, presents a new insight into the use of the Hilbert-Huang transform by showing that the amplitude-dependent frequency and damping extracted from a free decay response are only suitable for detection and characterisation of non-linearities, but they cannot be used to quantify the non-linear behaviour by fitting the CNMs even if a model of the system is known. The analytical proof of the HHT cannot be currently formulated due to a limited understanding of its empirical nature. Instead, this unconventional conclusion is supported by a series of numerical studies of conservative and non-conservative non-linear systems with a wide range of parameters. In all cases, a special care is taken to apply the basic HHT only on such signals for which mode separation is possible (no mode-mixing occurs). This eliminates the need for more sophisticated HHT versions and clearly demonstrates the inability of the HHT to extract CNMs even for the simplest cases. In addition to numerical studies, the identification of several non-linear modes is demonstrated experimentally using the free decay responses obtained from the ECL benchmark. It is shown that the HHT is able to successfully extract several non-linear mode
Yuan Y, Jones A, Setchfield R, et al., 2021, Robust design optimisation of underplatform dampers for turbine applications using a surrogate model, JOURNAL OF SOUND AND VIBRATION, Vol: 494, ISSN: 0022-460X
Lasen M, Sun Y, Schwingshackl CW, et al., 2021, Analysis of an Actuated Frictional Interface for Improved Dynamic Performance, Nonlinear Structures & Systems, Publisher: Springer
Yuan J, Schwingshackl C, wong C, et al., 2021, On an improved adaptive reduced order model for the computation of steady state vibrations in large-scale non-conservative system with friction joints, Nonlinear Dynamics, Vol: 103, Pages: 3283-3300, ISSN: 0924-090X
Joints are commonly used in many large-scale engineering systems to ease assembly, and ensure structural integrity and effective load transmission. Most joints are designed around friction interfaces, which can transmit large static forces, but tend to introduce stick-slip transition during vibrations, leading to a nonlinear dynamic system. Tools for the complex numerical prediction of such nonlinear systems are available today, but their use for large-scale applications is regularly prevented by high computational cost. To address this issue, a novel adaptive reduced-order model (ROM) has recently been developed, significantly decreasing the computational time for such high fidelity simulations. Although highly effective, significant improvements to the proposed approach is presented and demonstrated in this paper, further increasing the efficiency of the ROM. An energy-based error estimator was developed and integrated into the nonlinear spectral analysis, leading to a significantly higher computational speed by removing insignificant static modes from the stuck contact nodes in the original reduced basis, and improving the computational accuracy by eliminating numerical noise. The effectiveness of the new approach was shown on an industrial-scale fan blades system with a dovetail joints, showing that the improved adaptive method can be 2–3 times more computationally efficient than the original adaptive method especially at high excitation levels but also effectively improve the accuracy of the original method.
Kosova G, Jin M, Cenedese M, et al., 2021, Nonlinear system identification of a jointed structure using full-field data: Part ii analysis, Pages: 185-188, ISSN: 2191-5644
Mechanical joints have a significant influence on the dynamic response of assembled structures. Due to friction, wear, and non-idealized boundary conditions, joints introduce significant nonlinearity into the dynamics of assembled structures. To better understand and, in the future, tailor the nonlinearities, accurate methods are needed to characterize the dynamic properties of jointed structures. In this research, the response analysis for a beam with a bolted lap joint is studied with the help of several available identification techniques. The experimental setup and data capture are described in Part I of this work, providing high spatial resolution data for a variety of excitation methods. The nonlinear identification of the data is the focus of this paper, aiming to perform nonlinear modal analysis and to localize the nonlinear characteristics of the structure with a series of different approaches.
Lasen M, Sun Y, Schwingshackl CW, et al., 2021, Analysis of an actuated frictional interface for improved dynamic performance, Pages: 227-230, ISSN: 2191-5644
Friction in assembled structures is of great interest due to its ability to reduce the vibration amplitude of critical components. The nonlinear behaviour of a structure depends on a variety of physical parameters. Among these parameters, the contact pressure distribution and the contact area have shown to be critical for the behaviour of the joint and the responses of assembled structures. In most application cases the impact of the interface geometry is not considered as a design parameter, although some attempts have been reported to shape the interface geometry for a specific dynamic response. Taking this idea of designing an interface geometry for a better dynamic performance a step further, the concept presented here propose an actively controlled interface geometry and contact pressure distribution, to change the joint behaviour during a vibration cycle. The concept consists of a device capable of manipulating the shape and pressure of a flexible membrane in contact with a rigid punch, subjected to a normal load and a tangential excitation, via a row of piezoelectric actuators.
Szydlowski MJ, Schwingshackl CW, Rix A, 2021, Distributed Acquisition and Processing Network for Experimental Vibration Testing of Aero-Engine Structures, Pages: 209-212, ISSN: 2191-5644
Detailed vibration testing of large assembled structures, such as aeroengines, leads to significant requirements on data acquisition and processing. This can lead to high system cost and long post processing times, which often limit the amount of data that can be acquired. A novel hardware-software acquisition system combination is proposed here to overcome some of the challenges of large scale data acquisition, based on the idea to distribute the acquisition and data processing load between a network of specialized acquisition nodes. The nodes work in parallel and are independent of each other, while sharing a synchronization clock. Each node has the capability to process the data being acquired on-line. The network allows for testing of novel data analysis methods and its modular nature enables an easy expansion of the system when required.
Smith SA, Brake MRW, Schwingshackl CW, 2020, On the Characterization of Nonlinearities in Assembled Structures, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002
This work refines a recently formalized methodology proposed by D.J. Ewins consisting of ten steps for model validation of nonlinear structures. This work details, through a series of experimental studies, that many standard test setup assumptions that are made when performing dynamic testing are invalid and need to be evaluated for each structure. The invalidation of the standard assumptions is due to the presence of nonlinearities, both known and unrecognized in the system. Complicating measurements, many nonlinearities are currently characterized as constant properties instead of variables that exhibit dependency on system hysteresis and actuation amplitude. This study reviews current methods for characterizing nonlinearities and outlines gaps in the approaches. A brief update to the CONCERTO method, based on the accelerance of a system, is derived for characterizing a system’s nonlinearities. Finally, this study ends with an updated methodology for model validation and the ramifications for modeling assemblies with nonlinearities are discussed.
Brøns M, Kasper TA, Chauda G, et al., 2020, Experimental investigation of local dynamics in a bolted lap joint using digital image correlation, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002
The dynamics of structures with joints commonly show nonlinearity in their responses. This nonlinear behavior can arise from the local dynamics of the contact interfaces. The nonlinear mechanisms at an interface are complicated to study due to the lack of observability within the contact interface itself. In this work, digital image correlation (DIC) is used in combination with a high-speed camera to observe the local motion at the edge of the interface of a bolted lap joint. Results demonstrate that it is possible to use this technique to monitor the localized motion of an interface successfully. It is observed that the two beam parts of the studied lap joint separate when undergoing bending vibrations and that there is a clear asymmetry in the response of the left and the right end of the interface. Profilometry indicates that the asymmetry in the response is due to the mesoscale topography of the contact interface, highlighting the importance of accounting for surface features in order to model the nonlinearities of a contact interface accurately.
Brake MRW, Krack M, Schwingshackl CW, 2020, Special Issue: Tribomechadynamics, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002
Tuzzi G, Schwingshackl CW, Green JS, 2020, Study of coupling between shaft bending and disc zero nodal diameter modes in a flexible shaft-disc assembly, JOURNAL OF SOUND AND VIBRATION, Vol: 479, ISSN: 0022-460X
Heller D, Sever IA, Schwingshackl CW, 2020, A method for multi-harmonic vibration analysis of turbomachinery blades using Blade Tip-Timing and clearance sensor waveforms and optimization techniques, Mechanical Systems and Signal Processing, Vol: 142, ISSN: 0888-3270
A novel concept of investigating blade vibration in turbomachinery is presented on the basis of Blade Tip-Timing (BTT) and clearance sensor waveform analysis methods (BLASMA), with which vibration parameters are determined by global optimization. It is shown that the modulation of the sensor output by blade vibration can offer additional information compared with under-sampled time-of-arrival (TOA) data from traditional BTT applications. The sensor data can not only improve the validity of statements on blade vibration but also lessen the dependence on contact-based strain gauges measurements to produce reference data. A study was conducted to evaluate the merit of sensor waveform analysis with regard to determining asynchronous and synchronous single-harmonic and multi-harmonic blade vibration parameters. At first, waveforms were recorded with capacitive sensors during an experiment conducted on a research compressor. The experimentally measured waveforms were afterwards replicated in a simulator for imitating passing events of rotating and vibrating blades along a single virtual capacitive sensor. Finally, vibration properties, such as amplitudes, frequencies, and phases, are extracted from these waveforms with the help of global optimization methods. An investigation into the error proneness of the methodology is attached.
Haslam AH, Schwingshackl CW, Rix AIJ, 2020, A parametric study of an unbalanced Jeffcott rotor supported by a rolling-element bearing, Nonlinear Dynamics, Vol: 99, Pages: 2571-2604, ISSN: 0924-090X
Rolling-element bearings are widely used in industrial rotating machines, and hence there is a strong need to accurately predict their influence on the response of such systems. However, this can be challenging due to an interaction between the dynamics of the rotor and the bearing nonlinearities, and it becomes difficult to provide a physical explanation for the nonlinear response. A novel approach, combining a Jeffcott rotor supported by a detailed bearing model with the generalised harmonic balance method, is presented, enabling an in-depth study of the complex rotor–stator interaction. This allows the quasi-periodic response of the rotor, due to variable compliance, to be captured, and the impact of clearance, ring and stator compliance, and centrifugal loading of the bearing on the response to be investigated. A strongly nonlinear response was observed due to the bearing, leading to large shifts in frequency as the excitation amplitude was increased, and the emergence of stable and unstable operating regions. The variable compliance effect generated sub-synchronous forcing, which led to sub-resonances when the ball pass frequency coincided with the frequency of one of the modes. Radial clearance in the bearing had by far the largest influence on the unbalance response, the self-excitation due to variable compliance, and the stability. Introducing outer ring compliance was found to slightly soften the system, and centrifugal loading on the bearing elements marginally increased the system’s region of instability, but neither of these effects had a significant impact on the response for the investigated bearing. When the bearing was mounted on a sufficiently compliant stator, the system was found to behave linearly.
Fantetti A, Schwingshackl C, 2020, Effect of friction on the structural dynamics of built-up structures: An experimental study
Frictional contacts are a major source of uncertainty in the correct prediction of the dynamic response of built-up structures. This uncertainty is partially due to a limited understanding of the effects of friction on dynamic responses. Vice versa, dynamic responses can also affect the frictional behaviour of the interfaces in contact. In the present study, the mutual relationships between frictional behaviour and structural dynamics are investigated by means of a high frequency friction rig. The rig is characterised by a simple and localised frictional contact that is needed to accurately measure hysteresis loops. Of course, the rig also has its own dynamic response, and consequently represents an excellent test case to gain a better understanding of the correlation between hysteresis loop shapes and their effect upon the dynamics. Impact hammer tests and shaker tests were performed on the friction rig, and lead to changes in the damping and stiffness of its dynamic response, which were linked to variations in the frictional behaviour of the contact. Furthermore, there was some indication as to how certain resonances of the system might strongly affect the frictional behaviour. In particular, it was observed that full sliding causes excitation of structural modes that in turn lead to distortions in the measured hysteresis loops. These findings confirm the strong relationship between friction and dynamics, thus highlighting the necessity to include a detailed frictional description of contacting interfaces for more accurate modelling of the dynamics of built-up structures.
Mace T, Taylor J, Schwingshackl CW, 2020, A novel technique to extract the modal damping properties of a thin blade, Pages: 247-250, ISSN: 2191-5644
Extracting accurate material modal damping from a specimen can be very challenging due to the potential influence of the excitation and measurement system, and the required support of the specimen. This becomes particularly challenging if large amplitudes are required to determine nonlinear behavior, or large damping is present due to the material under investigation. An improved approach is proposed here to enable large amplitude, single harmonic, free decay damping extraction of a flat test specimen. It is based on a setup that uses the inertia of two large, free hanging end masses to enforce a pinned-pinned boundary condition with predetermined nodal locations, ensuring minimum effect of the support structure on the damping behavior. Excitation is achieved with the help of a removable electromagnetic shaker system that enables large amplitude single harmonic excitation, and smooth transition to a free, single harmonic decay. A Laser Doppler Vibrometer is used to measure the free decay, from which amplitude dependent damping can be obtained for individual modes via logarithmic decrement. Initial results of the approach show its great potential to provide reliable, amplitude dependent parameters with minimum impact of the test setup.
Schwingshackl CW, 2020, Measuring aero-engine pipe vibration with a 3d scanning laser doppler vibrometer, Pages: 101-104, ISSN: 2191-5644
The vibration of accessories in an aero engine represents a major problem in the design, since failing accessories are a source of aero engine shut downs. Understanding the vibration behaviour of the accessories is challenging. Their manufacture, assembly and maintenance introduce a large amount of uncertainty to the actual state of the system and they are often of a very complex shape. This can lead to highly three-dimensional operating deflection shapes, with potentially strong nonlinear dynamic behaviour due to a multitude of joints. The above makes the accurate capture of the vibration response of supply pipes of an aero engine quite challenging. New 3D measurement techniques, such as 3D Scanning Laser Doppler Vibrometers, can help to obtain a detailed map of the complex motion such systems experience in operation, but their tightly curved and small diameter pipes can present many challenges to an experimental setup. This paper will discuss the vibration measurement of a combustion chamber outer casing response with a 3D SLDV system, particularly focusing on some lessons learned during setup.
Zhu YP, Yuan J, Lang ZQ, et al., 2020, The data driven surrogate model based dynamic design of aero-engine fan systems
High cycle fatigue failures of fan blade systems due to vibrational loads are of great concern in the design of aero engines, where energy dissipation by the relative frictional motion in the dovetail joints provides the main damping to mitigate the vibrations. The performance of such a frictional damping can be enhanced by suitable coatings. However, the analysis and design of coated joint roots of gas turbine fan blades are computationally expensive due to strong contact friction nonlinearities and also complex physics involved in the dovetail. In this study, a data driven surrogate model, known as the Nonlinear in Parameter AutoRegressive with eXegenous input (NP-ARX) model, is introduced to circumvent the difficulties in the analysis and design of fan systems. The NP-ARX model is a linear input-output model, where the model coefficients are nonlinear functions of the design parameters of interest, such that the Frequency Response Function (FRF) can be directly obtained and used in the system analysis and design. A simplified fan bladed disc system is considered as the test case. The results show that by using the data driven surrogate model, an efficient and accurate design of aero-engine fan systems can be achieved. The approach is expected to be extended to solve the analysis and design problems of many other complex systems.
Schwingshackl CW, Nowell D, 2020, The Measurement of Tangential Contact Stiffness for Nonlinear Dynamic Analysis, Pages: 165-167, ISSN: 2191-5644
Nonlinear dynamic models for frictional interfaces require a number of input parameters to allow a realistic representation of the contact interface. Interface geometry and static pressure distributions can be obtained reliably from numerical analysis. However, it is also necessary to measure the friction coefficient, and the tangential and normal contact stiffness. The tangential contact stiffness plays a significant role in the dynamic response, but is very challenging to measure. In this paper quasi-static and dynamic experiments developed at the University of Oxford and at Imperial College London respectively, will be compared and discussed. Of particular interest is the dependence of the stiffness on the static normal load and the overall contact area.
Rojas E, Punla-Green S, Broadman C, et al., 2020, A Priori Methods to Assess the Strength of Nonlinearities for Design Applications, Pages: 243-246, ISSN: 2191-5644
One of the greatest challenges to the optimization of assembled systems is a lack of understanding of how jointed interfaces augment system dynamics. Thus, a design tool that can assess the nonlinearity of a joint prior to manufacturing and experimentation will lead to significant savings in qualification testing and improved performance in terms of dynamic properties and failure rates. This paper explores the a priori metric hypothesis for jointed structures, which states that the strength of a nonlinearity (SNL) can be estimated by a metric derived from both the magnitude and uniformity of contact pressure within an interface and the modal strain energy at an interface’s location.
Yuan J, Schwingshackl C, Salles L, et al., 2020, Reduced order method based on an adaptive formulation and its application to fan blade system with dovetail joints
Localized nonlinearities due to the contact friction interfaces are widely present in the aero-engine structures. They can significantly reduce the vibration amplitudes and shift the resonance frequencies away from critical operating speeds, by exploiting the frictional energy dissipation at the contact interface. However, the modelling capability to predict the dynamics of such large-scale systems with these nonlinearities is often impeded by the high computational expense. Component mode synthesis (CMS) based reduced order modelling (ROM) are commonly used to overcome this problem in jointed structures. However, the computational efficiency of these classical ROMs are sometimes limited as their size is proportional to the DOFs of joint interfaces resulting in a full dense matrix. A new ROM based on an adaptive formulation is proposed in this paper to improve the CMS methods for reliable predictions of the dynamics in jointed structures. This new ROM approach is able to adaptively switch the sticking contact nodes off during the online computation leading to a significant size reduction comparing to the CMS based models. The large-scale high fidelity fan blade assembly is used as the case study. The forced response obtained from the novel ROM is compared to the state-of-the-art CMS based Craig-Bampton method. A parametric study is then carried out to assess the influence of the contact parameters on the dynamics of the fan assembly. The feasibility of using this proposed method for nonlinear modal analysis is also characterised.
Pesaresi L, Fantetti A, Cegla F, et al., 2020, On the use of ultrasound waves to monitor the local dynamics of friction joints, Experimental Mechanics, Vol: 60, Pages: 129-141, ISSN: 0014-4851
Friction joints are one of the fundamental means used for the assembly of structural components in engineering applications. The structural dynamics of these components becomes nonlinear, due to the nonlinear nature of the forces arising at the contact interface characterised by stick-slip phenomena and separation. Advanced numerical models have been proposed in the last decades which have shown some promising capabilities in capturing these local nonlinearities. However, despite the research efforts in producing more advanced models over the years, a lack of validation experiments made it difficult to have fully validated models. For this reason, experimental techniques which can provide insights into the local dynamics of joints can be of great interest for the refinement of such models and for the optimisation of the joint design and local wear predictions. In this paper, a preliminary study is presented where ultrasound waves are used to characterise the local dynamics of friction contacts by observing changes of the ultrasound reflection/transmission at the friction interface. The experimental technique is applied to a dynamic friction rig, where two steel specimens are rubbed against each other under a harmonic tangential excitation. Initial results show that, with a controlled experimental test procedure, this technique can identify microslip effects at the contact interface.
Fantetti A, Tamatam, Volvert, et al., 2019, The impact of fretting wear on structural dynamics: experiment and simulation, Tribology International, Vol: 138, Pages: 111-124, ISSN: 0301-679X
This paper investigates the effects of fretting wear on frictional contacts. A high frequency friction rig is used to measure the evolution of hysteresis loops, friction coefficient and tangential contact stiffness over time. This evolution of the contact parameters is linked to significant changes in natural frequencies and damping of the rig. Hysteresis loops are replicated by using a Bouc-Wen modified formulation, which includes wear to simulate the evolution of contact parameters and to model the evolving dynamic behaviour of the rig. A comparison of the measured and predicted dynamic behaviour demonstrates the feasibility of the proposed approach and highlights the need to consider wear to accurately capture the dynamic response of a system with frictional joints over its lifetime.
Brake MRW, Schwingshackl CW, Reuß P, 2019, Observations of variability and repeatability in jointed structures, Mechanical Systems and Signal Processing, Vol: 129, Pages: 282-307, ISSN: 0888-3270
The experimental study of joint mechanics has been limited in its effectiveness due to the high uncertainty associated with assemblies of sub-components. In particular, two categories of uncertainty are variability (the uncertainty in measurements of different, nominally identical parts) and repeatability (the uncertainty in measurements of the same set of parts). As a result, the uncertainty measured is often greater than the nonlinear characteristics being studied (such as amplitude dependent frequency and damping), which makes meaningful experimentation challenging. This paper analyzes the contributors to uncertainty in the form of variability and repeatability in order to make recommendations for methods to reduce the uncertainty and to redesign a joint to improve its dynamics. Experiments are summarized that investigate the role of experimental setup, interface roughness, settling versus wear, interface geometry (both meso-scale and macro-scale), and the structure surrounding the joint. From the results of these studies, recommendations for the measurement of nonlinearities in jointed structures are made.
Tatar A, Schwingshackl CW, Friswell MI, 2019, Dynamic behaviour of three-dimensional planetary geared rotor systems, Mechanism and Machine Theory, Vol: 134, Pages: 39-56, ISSN: 0094-114X
© 2018 Elsevier Ltd A six degrees of freedom dynamic model of a planetary geared rotor system with equally spaced planets is developed by considering gyroscopic effects. The dynamic model is created using a lumped parameter model of the planetary gearbox and a finite element model of the rotating shafts using Timoshenko beams. The gears and carrier in the planetary gearbox are assumed to be rigid, and the gear teeth contacts and bearing elements are assumed to be flexible. The modal analysis results show that torsional and axial vibrations on the shafts are coupled in the helical gearing configuration due to the gear helix angle whereas these vibrations become uncoupled for spur gearing. Mainly, the vibration modes are classified as coupled torsional-axial, lateral and gearbox for the helical gear configuration, and torsional, axial, lateral and gearbox for the spur gear configuration. Modal energy analysis is used to quantify the coupling level between the shafts and the planetary gearbox, highlighting the impact of the gearbox on certain mode families. Gyroscopic effects of the planetary gearbox are found to be of great importance in the gearbox dominated modes.
Armand J, Pesaresi L, Salles L, et al., 2019, A modelling approach for the nonlinear dynamics of assembled structures undergoing fretting wear, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 475, ISSN: 1364-5021
© 2019 The Author(s) Published by the Royal Society. All rights reserved. Assembled structures tend to exhibit nonlinear dynamic behaviour at high excitation levels due to the presence of contact interfaces. The possibility of building predictive models relies on the ability of the modelling strategy to capture the complex nonlinear phenomena occurring at the interface. One of these phenomena, normally neglected, is the fretting wear occurring at the frictional interface. In this paper, a computationally efficient modelling approach which enables considerations of the effect of fretting wear on the nonlinear dynamics is presented. A multiscale strategy is proposed, in which two different time scales and space scales are used for the contact analysis and dynamic analysis. Thanks to the decoupling of the contact and dynamic analysis, a more realistic representation of the contact interface, which includes surface roughness, is possible. The proposed approach is applied to a single bolted joint resonator with a simulated rough contact interface. A tendency towards an increase of real contact area and contact stiffness at the interface is clearly observed. The dynamic response of the system is shown to evolve over time, with a slight decrease of damping and an increase of resonance frequency, highlighting the impact of fretting wear on the system dynamics.
Haslam AH, Schwingshackl CW, Rix AIJ, 2019, Analysis of the Dynamic Response of Coupled Coaxial Rotors, Pages: 53-65, ISSN: 2191-5644
The fundamental dynamics of a single rotor are very well understood, and extensively covered in the literature. However, many rotating machines such as aircraft engines consist of multiple shafts, which are often directly coupled by inter-shaft bearings. This paper aims to provide better insight into the underlying dynamics of such systems by analysing a simple but representative model of a rigid dual-rotor system. The modes and natural frequencies were computed numerically and it was found that the different modes could be classified by the following criteria: (i) relative phase of the motion of each rotor, (ii) whirl direction of the rotors, and (iii) presence of rotational or translational motion. The high-speed mode shapes could also be classified into (i) “static” modes with very low frequencies, (ii) “flat” modes which tend towards constant frequencies, and (iii) “precessional” modes which have a frequency which linearly increases with speed. A parameter study was performed in order to obtain a better understanding of the sensitivity of the modal properties. It was found that increasing the inter-shaft bearing stiffness can raise the natural frequencies of the modes at low speeds as well as the critical speeds, but has less influence at high speeds. The speed ratio influences the whirl direction of the modes and hence plays a crucial role in determining how each mode varies with speed. Since the speed ratio also controls the frequency of excitation from unbalances, it has a particularly profound effect on the critical speeds, and extra ones can arise. The importance of considering the dynamics of the complete system in the design of turbomachinery with multiple-shafts was highlighted.
Sanchez A, Schwingshackl CW, 2019, Low order nonlinear dynamic modelling of fuel supply pipes, Pages: 113-120, ISSN: 2191-5644
In industrial applications the vibration analysis mostly focuses on the main components, while accessories, such as pumps, electronics or pipe work are often neglected. This can lead to failures in these support systems, which can lead to performance reduction, shut down, or in the worst case catastrophe failure of the application due to secondary effects. One such accessory system is the fuel manifold of an aero engine, which delivers the kerosene to the fuel injectors. Previous experimental work has highlighted a strong nonlinear dynamic response of this setup, making an accurate prediction during the design process very challenging. In this study, a low order nonlinear modelling approach for fuel supply pipes, based on linear finite element and implicit nonlinear element analysis, is presented and its advantages and limitations are discussed.
Lawal I, Shah S, Gonzalez-Madrid M, et al., 2019, The effect of non-flat interfaces on system dynamics, Pages: 187-197, ISSN: 2191-5644
Manufactured surfaces are never completely flat due to a variety of reasons including: variability in manufacturing operations, material behavior and achievable geometric tolerances. The curvature of surfaces is a local geometric effect that affects part-to-part variability of jointed surfaces. Joints with different interface geometries behave in unpredictable ways (Brake (2018) The Mechanics of Jointed Structure: Recent Research and Open Challenges for Developing Predictive Models for Structural Dynamics. Sandia National Laboratories, William Marsh Rice University. Springer, Cham). Among the factors that drive the uncertainty in joint performance are frictional micro and macro sliding events, surface tribology effects, residual stress from manufacturing and assembly, loss of bolt pre-load, changes in contact area and the resulting pressure field variation around the joint. The goal of this research is to identify key variables that account for the measured uncertainty in the dynamics of jointed structures, which may have local regions of conformal and non-conformal contact due to variability inherent in the manufacturing process. Using the standard benchmark system of the Brake-Reuβ beam (BRB), recommendations are made for which design parameters require higher tolerances than others to minimize variability in a cost-effective manner. Conformal beams with strong and weak curvature are studied as well as non-conformal (flat vs. curved) beams. Experimental and numerical approaches model and validate the physical behavior of beams to understand primary causes of non-linearity in joints with different interface geometries.
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