1,720,952 research outputs found
Stability Analysis of a Laval Rotor on Hydrodynamic Bearings by Numerical Continuation: Investigating the influence of rotor flexibility, rotor damping and external oil pressure on the rotordynamic behaviour
No full textSelf-excited vibrations in Laval rotors supported by hydrodynamic bearings, know as oil whirl, induce friction losses and noise, or may even damage the system. This transient dynamic behaviour is caused by the stable and unstable equilibria in the system. To study this phenomenon, this research uses numerical continuation. This method determines the equilibria in the system, their stability and the bifurcations into other equilibria. The sensitivity of these properties with respect to the parameters of the system are studied. The parameters under consideration are the system geometry, rotor stiffness, rotor damping and external oil supply. For this purpose analytic models with the use of short bearing theory and Guembel boundary conditions are made of a rigid, flexible and damped rotor. It extends the results from literature beyond the complications in the model due to the use of numerical continuation, which others did not acknowledge. The novel contribution of the rotor including the damping of the shaft showed to be essential to detect the same dynamic behaviour seen by experimental results from literature. With the addition of the oil feed pressure, the results are strongly improved and are shown to predict the dynamic behaviour of a high speed Laval rotor with reasonable accuracy.Mechanical, Maritime and Materials EngineeringPrecision and Microsystems EngineeringEngineering Mechanic
Towards robust design optimization of automotive turbocharger rotor-bearing systems
No full textIn the competitive automotive market, the performance of turbochargers is constantly being pushed towards their theoretical optimum. One of the key components of the turbocharger is the rotor-bearing system, which determines the friction losses and noise output and furthermore affects the overall turbocharger efficiency, reliability and cost. In order to fulfil the demands of the automotive market, developing methods to optimize the rotor-bearing system is the focus of this study, where particular attention is paid to taking into account the product-to-product variations that are inevitable in cost-effective mass-produced parts, as well as the variations in turbocharger operating conditions.First, a model of the rotor-bearing system was developed to predict the rotordynamic response over the operating range. The model is constructed in a step-by-step fashion, starting with a simple test case: a Laval rotor supported by plain journal bearings. As the behavior of the rotor-bearing system varies over its rotation speed range, run-up simulations were performed by a time-transient multi-physical model. In this model, several sub-models are coupled: a rotordynamic sub-model, a thermo-hydrodynamic submodel and a thermal network model. Once a satisfactory correlation was found between numerical simulation results and measurement results, the test case progressed to a Laval rotor with floating ring bearings instead of plain journal bearings. Correspondingly, the bearing model was extended to include the dynamics of the floating ring and its two oil films. The resulting run-ups showed a response consisting of a critical speed, an oil whirl and an oil whip. Analysis of a turbocharger rotor-bearing system was subsequently performed, showing a more complex response, consisting of multiple critical speeds and the co-existence of sub-synchronous whirling modes. The effect of the rotor-bearing operating conditions, unbalance configuration, the thrust bearing and the bearing cylindricity were investigated. Most of the trends are correctly predicted by the model, however the correlation between measurement results and simulation results was clearly inferior to the case of the Laval rotor, most likely due to the uncertainties in the actual turbocharger geometry and the actual unbalance distribution.Lastly, an optimization of a Laval rotor-bearing system was performed. The resulting robust optimum design ensures optimum rotor-bearing performance, even at the most severe operating conditions and even if all manufacturing tolerances represent the worst case scenario. Particularly the uncertainties in rotor unbalance and oil supply temperature were found to have a significant influence on the optimum design.<br/
Global Robust Optimization of Computationally Expensive Systems: A Lavel Rotor Suspended by Fluid Film Bearings
No full textKriging based methods enable the deterministic and robust optimization of computationally expensive systems. With a limited amount of function evaluations the optima are found in an iterative process with expected improvement as infill sampling criteria. Outputs of computer models can be stochastic and/or the models do not always succeed to perform the analysis. For the latter case, a problem is said to be affect by a hidden constraint. Regression Kriging can be included in the methods to deal with the stochastic model outputs. A new method is introduced to handle the hidden constraint in both deterministic and robust optimization. The methods are used to optimize a validated model of a Laval rotor suspended by plain journal bearings. To capture the non-linear behaviour of the self-excited vibrations, a computationally expensive time-transient run-up analysis needs to be performed. The output of this model is stochastic and the model fails to perform a run-up for some combinations of model inputs. The most influential control variables and uncertainties are indicated with an efficient global sensitivity study and used to optimize the system. With the extension of the deterministic and robust optimization, the optima are successfully obtained and can be compared.Mechanical, Maritime and Materials EngineeringPrecision and Microsystems Engineering (PME)Engineering Mechanic
On the stiffness of linear roller bearings: On the stiffness of linear roller bearings
No full textThis thesis creates and validate a model for predicting the static stiffness of a linear crossed roller bearing as a function of its key design variables within a range of 20%. Linear crossed roller bearings play an essential role in the high-tech industry, this kind of bearing is used for precision applications such as microscopes, optical systems, and semiconductor production equipment. The performance of all these systems is related to the ability to produce and control a certain motion of a body. Bearings facilitate this motion, they are designed such that the resistance in the direction of movement is minimal. Generally, in all other directions, the bearing should be as stiff as possible as a high bearing stiffness is beneficial for controlling the motion. Stiffness is beneficial as it contributes to achieving a high control bandwidth and minimizes sensitivity to external forces or vibrations. Mechatronic systems have to meet very strict targets in terms of positioning accuracy and settling times. The dynamics of mechatronic systems is dominantly dependent on the performance of their bearings. Nowadays, the performance of a machine is predicted before building the physical machine through virtual prototyping. Virtual prototyping is becoming increasingly important: an accurate model in an early stage of a development project minimizes the risk of not achieving specifications in a later stage. For accurate modelling of precision machines using linear cross roller bearings, reliable data on the stiffness characteristics of these bearings is required. This thesis performs a step-by-step validation of models. Starting with an empiric determination of the load-stiffness relation for an individual cylindrical roller-rail contact as this is not affected by the uncertainties present in bearing assemblies. This is followed by an analysis of the tangential effects that come into play when a roller is loaded at 45°. Once the roller-rail model at 45° is validated, this empirical roller-rail model is used to develop a model that predicts the stiffness of linear roller bearing assemblies
Thermo-Hydrodynamic Analysis of a Plain Journal Bearing on the Basis of a New Mass Conserving Cavitation Algorithm
No full textAccurate prediction of cavitation is an important feature in hydrodynamic bearing modeling. Especially for thermo-hydrodynamic modeling, it is crucial to use a mass-conservative cavitation algorithm. This paper introduces a new mass-conserving Reynolds cavitation algorithm, which provides fast convergence and easy implementation in finite element models. The proposed algorithm is based on a variable transformation for both the pressure and mass fraction, which is presented in the form of a complementary condition. Stabilization in the streamline and crosswind direction is provided by artificial diffusion. The model is completed by including a simple and efficient thermal model and is validated using the numerical values of a reference plain journal bearing experiment under steady-state conditions. In addition, a transient analysis is performed of a journal bearing subjected to a harmonic load. It is shown that the proposed cavitation algorithm results are in good agreement with the reference measurement results. Moreover, the algorithm proves to be stable and requires only a small number of iterations to convergence in the Reynolds-based finite element model.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Towards accurate prediction of unbalance response, oil whirl and oil whip of flexible rotors supported by hydrodynamic bearings
No full textJournal bearings are used to support rotors in a wide range of applications. In order to ensure reliable operation, accurate analyses of these rotor-bearing systems are crucial. Coupled analysis of the rotor and the journal bearing is essential in the case that the rotor is flexible. The accuracy of prediction of the model at hand depends on its comprehensiveness. In this study, we construct three bearing models of increasing modeling comprehensiveness and use these to predict the response of two different rotor-bearing systems. The main goal is to evaluate the correlation with measurement data as a function of modeling comprehensiveness: 1D versus 2D pressure prediction, distributed versus lumped thermal model, Newtonian versus non-Newtonian fluid description and non-mass-conservative versus mass-conservative cavitation description. We conclude that all three models predict the existence of critical speeds and whirl for both rotor-bearing systems. However, the two more comprehensive models in general show better correlation with measurement data in terms of frequency and amplitude. Furthermore, we conclude that a thermal network model comprising temperature predictions of the bearing surroundings is essential to obtain accurate predictions. The results of this study aid in developing accurate and computationally-efficient models of flexible rotors supported by plain journal bearings
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