1,721,022 research outputs found
Platform centered reduction: A process capturing the essentials for blade-damper coupled optimization
No full textThe purpose of this document is to continue along the line of research of the authors in the direction of developing an attractive tool for designers in the initial design phase of the damping of the turbomachinery blades. In particular, in order to guide their initial choice of a dry friction underplatform damper in the most appropriate way. The paper shows how, to this purpose, certain reasonable simplifications are introduced in the procedure and in the model, leaving the customary full high fidelity computations to the final design verification analysis. The key simplifications here considered are: - the blade neck is modelled with Euler beam finite elements so to speed up the updating of its dimensions during the optimisation process; - the contact forces exerted by the dampers on the two sides of the blade platform are represented by the resultant forces and moments applied to a reference point on the platform, associated to its displacements and rotations; - as an improvement to the model proposed in the paper presented at Turbo Expo 2019, the airfoil is now obtained from a full 3D FE model after a component mode synthesis reduction; this choice is justified by the facts that the airfoil is by large the item with most complex shape and that during the coupled optimization of the damper the airfoil is considered to be of fixed shape. It is shown that the process captures the essentials of the nonlinear dynamics of the blade-damper problem without sacrificing in any way the accuracy of the results. This hybrid model is then employed in the process where the domains of optimal matching between the damper and the blade is searched for by exploring the influence of blade neck thickness (flexibility) and damper mass. Such a purposely simplified process allows a clear identification of relationships between relevant blade features and response with a focus on fatigue life. At the same time, it allows an assessment of the interplay between blade parameters and damper parameters in determining the modal features and the damping capabilities. It is shown how different matching solutions may be identified depending on the expected forcing level on the blade
On the effectiveness of standards application to threshold setting in vibration condition monitoring in industrial machinery
No full textOne of the goals of Industry 4.0 is to minimize unplanned maintenance emergencies and to leverage the efficiency of condition-based, just-in-time labor and repairs. Achieving the benefits of predictive maintenance via an Industry 4.0-style plant requires not only a data capture ecosystem, but also reference values to inform the monitoring system. Effectively estimating the values of monitoring thresholds is not an easy task, especially in those cases where the manufacturer does not provide data, or the machine is not new but the monitoring system has just been installed, therefore a measurement history is not yet available. The purpose of the present work is assess, through the analysis of two separate test cases, whether the monitoring thresholds suggested by different internationally recognized standards may serve as effective starting values in a vibration-based condition monitoring system. While general agreement on the order of magnitude of the thresholds is present in all cases, the differences between the standards allow for conclusions to be drawn on the most convenient format and processing technique to handle the data and to assess the most/least conservative standards. This paper constitutes, to the authors knowledge, the only systematic study in which the recommendations of all available international standards for vibration monitoring of non-rotating parts are applied to the same test cases and compared
Direct measurement of the damping and stiffening capabilities of cylindrical underplatform dampers
Full text linkConvergence-free mapping of non-linear damper-blade performance
Full text linkIn the bladed disks of turbomachinery, the problem arises of finding a damping device that reduces as much as possible the amplitude of the alternating stresses produced by the forced vibrations excited by the gas flow. Dry friction solid underplatform dampers are an established solution. The shape and size of the damper, in association with those of the platform, neck and airfoil, determine the non-linear response curves having as parameter the intensity of external excitation, here synthetically represented by a “proof” excitation force. Of greatest importance are the combinations (frequency, excitation force) that realise the maximum amplitude of forced oscillation (measured, for example, at the blade tip) and the maximum value of the amplitude of the variable stress produced by the vibrations at a critical point of the blade. Since the design of the damper-blade coupling is High-Cycle Fatigue driven, this stress amplitude is taken as a reference and related to the value of the excitation force. This can finally take the well-known form of the damper performance curve. Especially in the case of parametric explorations concerning shape, size and contact parameters, the current approach to this non-linear response problem, based on iterative convergence, is numerically prohibitive unless one uses special search techniques, such as the surrogate models of various types that are favoured today. An alternative computational process is presented here, that of integrating PCR (Platform Centred Reduction) with a new approach called Amplitude Layered (excitation) Force Mapping. This process reduces the amount of computation by up to three orders of magnitude compared to standard techniques and is a winning alternative to surrogate models
The effect of friction damping on the dynamic response of vibrating structures: an insight into model validation
No full textDry friction is widely used in turbomachinery in the form of under platform dampers to limit resonant vibration and avoid high-cycle fatigue failures of the blades. Most test rigs that are used to investigate the behavior of dampers aim to evaluate their performance by reduction in blade vibration amplitude. This approach is insufficient to understand local nonlinearities of the contact and influence of blade dynamics on UPDs behavior. A newly developed test rig provides the user with an unprecedented set of information: it measures contact forces and relative displacements between dampers and blade together with the overall blade response. This controlled environment, together with a state-of-the-art numerical model of the test rig, is used to provide an insight into the subject of model validation. The presented experimental and numerical study of the damper is used to highlight the relevance of an accurate representation of the constraints induced by friction contacts and to discuss the adequacy of state-of-the-art contact models
Numerical and Experimental Stability Investigation of a Parametrically Excited Cantilever Beam at Combination Parametric Resonance
Full text linkBackground The presence of parametric excitation in dynamic structures, caused by friction, crack, varying compliance, electromagnetic field, etc. may generate unbounded responses. In the literature there exist several numerical analyses of systems affected by parametric excitation, while experimental studies are less frequent. Objective The goal of the paper is to create a demonstrator of a parametrically excited system, whose stability can be modified through a controlled physical parameter. This work also investigates the applicability of the recently developed stability analysis method named Jacobian Based Approach (JBA). Methods This paper studies a simple experimental set-up comprising of a cantilever beam mounted on a spring with time - varying stiffness, achieved through the use of an electromagnet. The test rig allows measuring directly the magnetic force without any preknowledge of the values of electrical parameters. Results obtained from the test rig are compared with numerical results obtained from the Finite Element model. In this study, Hill's method and JBA are employed to obtain the stability plot highlighting the regions of parametric instabilities. Results Good agreement is found between experimental and numerical data and the presence of unstable behavior is verified through the use of the well - known Hill's method and the JBA. Furthermore, this study demonstrates that the stability plot, highlighting the unstable regions, computed by JBA is in complete agreement with the one obtained by Hill's method. Conclusions It is shown how the parametric instability can be triggered through the regulation of a simple physical parameter, i.e. the gap between the electromagnet and the beam. The numerical model analyzed by the ad - hoc technique proposed by the authors i.e. JBA has been proven to have predictive capabilities in studying a system under parametric excitation and could be a potential substitution for state-of-the-art stability analysis techniques such Hill's method
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Design of a dynamic damper for a high precision numerical control machine
No full textThe paper presents the design of a dynamic damping device to reduce the vibration amplitude of a Cartesian robot prototype used for high-precision Computer Numerical Control (CNC) machining under high accelerations. Firstly, a Finite Element (FE) model of the entire machine was created in the MSC Apex CAE environment. Modal and frequency response analyses were performed, considering the machine in its most critical configuration. The analyses showed that the relevant vibration modes are the first two, i.e. column bending he proposed solution foresees a mounting on the column top, cylindrically shaped springs working in shear mode, thus making the stiffness independent of the angular coordinate, and a mass ratio (μ) of about 1% for compactness. The damper shifted the resonance frequencies of both modes and reduced the highest vibration amplitude by 50%
The Present State of “Solid” under-Platform Damper Mechanics at AERMEC - POLITO
No full textThe numerical simulation of friction-damped blades in turbomachinery applications requires knowledge of contact parameters to be introduced in contact models. The equilibrium and kinematics of dampers are dominated by phenomena occurring at the two interfaces between the damper and the corresponding blade platforms. A precise knowledge of parameter values is vital to ensure trustworthy predictions of blades vibration frequency and of the available amount of amplitude damping. Test rigs developed at the AERMEC lab. during the last decade are presented, and reasons for their improvements are explained. These test rigs have the primary purpose to allow direct observations of contact forces and displacements, and the ensuing estimate of contact parameters. The reliability of the measurements and of their processing is demonstrated through a worked out example
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