Papers by Fabrizio Stefani
Data for the Assessment of Stress Integration Schemes for Large-Deformation Finite Element Analysis
Files "results_comp_ext_repository.xlsx", "results_ext_rot_repository.xlsx", ... more Files "results_comp_ext_repository.xlsx", "results_ext_rot_repository.xlsx", "results_simple_shear_repository.xlsx", "results_uniaxial_extension_repository.xlsx" gather the results (stress predictions) of four tests, aimed to check stress integration algorithms in four corresponding deformation paths: "extension-compression", "extension-rotation", "simple shear", "uniaxial extension". They are chosen in order to include the most common motions of the structure particles, i.e. relative motions that yield extension, compression and shear combined with possible rigid rotations of the body. Particularly, the rotation motion is simulated in order to test the incremental objectivity or, better, the priority on rigid motion or weak objectivity of the algorithms. The exact analytical solutions of the three tests are known from literature. In all of the tests, a unit square (1 x 1 m2) with unit thickness undergoes a deformation path under plane stress conditions. It is made of linear elastic material with Young's modulus E = 1 Pa and Poisson ratio nu = 0. A fixed number of time steps N equal to 50 (the highest employed in the reference papers) is chosen. The simulated "time" parameter t ranges between 0 and 1 in "extension-compression" as well as "extension-rotation" tests, whereas it raises from 0 to 0.9 in "simple shear" test. Therefore, the corresponding time steps t are equal to 1/50 and 9/500, respectively. The various simulation models include different stress integration methods labelled as described in the paper "Choice of the Stress Integration Scheme for Accurate Large-Deformation Finite Element Analysis" in revision for the publication on Mechanics Research Communications journal. The files include a data sheet and a corresponding plot for each category of algorithms (classical update-Lagrange algorithms, CC methods, and commercial software). The data are organized in columns, where a stress component is given for each stress update method. In the file "cc methods_validation_repository.xlsx" the simple shear te [...]
Aqueous dispersions of gelled collagen particles. Part 2: numerical technique for tribological properties
Industrial Lubrication and Tribology, Sep 1, 2006
PurposeTo propose a new numerical elastohydrodynamic (EHD) analysis method suited to cinematic pa... more PurposeTo propose a new numerical elastohydrodynamic (EHD) analysis method suited to cinematic pairs lubricated with aqueous dispersions of collagen gelled media.Design/methodology/approachIn comparison with traditional lubricants, these media are characterized by an apparent viscosity that increases with film thickness. Hence, the Reynolds equation has been numerically solved taking into account the new rheological law. The apparent viscosity of 4 percent collagen gel dispersions, as measured at a fixed shear rate of 100 l/s, is in the range 2‐5 Pa s, when film thickness varies between 20 and 200 μm. These experimental data have been fitted using a power law. The proposed analysis method has been applied to a 180° partial bearing, which rigid journal rotates in a flexible sleeve, made up by a resin shell.FindingsThe results of this tribological analysis have been compared with those concerning traditional isoviscous fluids which viscosity has been set equal to the viscosity of the gelled media when the film thickness is constant, i.e. in the unloaded bearing. The minimum film thickness calculated has been turned out equal to 66.8 μm in the former case, and 27.6 μm in the latter case. Furthermore, when the cinematic pair is lubricated with the aqueous gel media, film thickness distribution over the bearing surface is more uniform, active film region is wider and, consequently, peak pressure is lower than when the isoviscous lubricant is employed.Research limitations/implicationsFor lightly loaded journal bearings, the dependence between apparent viscosity and the shear rate in the lubricant film has been neglected, so that the apparent viscosity has been considered changing only with the film thickness.Originality/valueThe paper introduces a new numerical EHD analysis method for use with cinematic pairs lubricated with aqueous dispersions of collagen gelled media.
Evaporative cooling of sessile water microdrops measured with atomic force microscope cantilevers
Journal of Micromechanics and Microengineering, Aug 20, 2008
E3S web of conferences, 2019
The aim of the present work is to design a test rig suited to investigate the dynamic interaction... more The aim of the present work is to design a test rig suited to investigate the dynamic interaction between rotor and hydrodynamic journal bearings in micro gas turbines (microGT), i.e. with reference to small bearings (diameter in the order of ten millimeters). Particularly, the device is capable of measuring the journal location. Therefore, the journal motion due to rotor vibrations can be displayed, in order to assess performance as well as stiffness and damping of the bearings. The new test rig is based on Bently Nevada Rotor Kit (RK), but substantial modifications are carried out. Indeed, the relative radial clearance of the original RK bearings is about 2/100, while it is in the order of 1/1000 in industrial bearings. Therefore, the same RK bearings are employed in the new test rig, but a new shaft has been designed in order to reduce the original clearance. The new shaft enables us to study the bearing behaviour for different clearances, as it is equipped with interchangeable journals. The experimental data yielded by the new test rig are compared with numerical results. These are obtained by means of a suitable finite element (FEM) code developed by our research group. It allows the Thermo Elasto-HydroDynamic (TEHD) analysis of the bearing in static and dynamic conditions. In the present paper, bearing static performances are analysed in order to assess the reliability of the journal location predictions by comparing numerical and experimental results. Such comparisons are presented for both large and small clearance bearings of original and modified RK, respectively. Good agreement is found only for the modified RK equipped with small clearance bearings (relative radial clearance equal to 8/1000). Nevertheless, rotor alignment is quite difficult with small clearance bearings and a completely new test rig is designed for future experiments.
InTech eBooks, May 31, 2017
Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearin... more Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearings are a key factor in their design and development. The aim of the present research work is the development of the support system for a typical microturbine intended for power generation. To this goal, the present chapter defines the typical requirements of the machine and, afterward, describes the different technologies available to develop the support system of a reliable microturbine. Conventional (rolling element and oil-film) supports and cutting-edge (magnetic, aerodynamic, and aerostatic) bearings are reviewed. Particularly, their suitability to the operating conditions is compared by means of a literature review and elaboration of the relevant data. By analyzing all this information, a new concept for the design of a micro-GT support system is devised. Instead of using a single type of bearing as usual, the new system includes different types in order to take advantage of the best characteristics of each one and, simultaneously, to minimize the effects of the relevant flaws. The innovative support system requires a suitable bearing arrangement, which is compared with the conventional ones. The conceptual design of the innovation is completed by a discussion of its advantages, drawbacks, and prospective improvements.
Tribology International, Mar 1, 2009
A finite-element approach to thermoelastohydrodynamic lubrication analysis is developed by extend... more A finite-element approach to thermoelastohydrodynamic lubrication analysis is developed by extending a previous mass-and energy-conserving algorithm to include wall-convection boundary conditions, groove-mixing theory, and thermo-mechanical deformations. To this end, the cross-film-averaged energy equation is coupled with the heat conduction equations relevant to the bearing sleeve and the journal by fitting the temperature profile across the film thickness with a fourth-order polynomial. A finite-element condensation technique is used to reduce the unknowns in heat conduction equations in the bush and in the journal to the temperatures of the sleeve surface and journal axis, respectively. Applied to the analysis of steadily loaded journal bearings, the proposed method shows good agreement with published experimental results and incurs low computational cost.
Removal notice to “Hysteresis and torsional-lateral vibration coupling in a complex shaft line supported by hydrodyanamic journal bearings” [Mech. Syst. Signal Process. 181 (2022) 109505]
Mechanical Systems and Signal Processing, Feb 1, 2023
A Nonlinear Structure Based Elastohydrodynamic Analysis Method for Connecting Rod Big End Bearings of High Performance Engines
Journal of tribology, Oct 1, 2004
Big end bearings of connecting rods for high performance automotive engines are quite compliant a... more Big end bearings of connecting rods for high performance automotive engines are quite compliant and exhibit relatively large deformations at high rotation speed. Nowadays numerical simulation provides good estimate of tribological parameters for bearings operating in severe, but not extreme elastohydrodynamic lubrication conditions. At very high rotation speed regimes, reliable previsions are achieved only if an effort to increase the accuracy of structural modeling is accomplished. A finite element model able to account for the nonlinear friction controlled sliding phenomena occurring at the junction surface between the cap and the body of the rod has been developed. A new method to solve the coupled equations governing the interaction between the hydrodynamic fluid model and the nonlinear structural model of the connecting rod bearing is presented, together with a simple application example.
Advances in Tribology, Dec 4, 2020
e aim of the present research is to characterize both experimentally and numerically journal bear... more e aim of the present research is to characterize both experimentally and numerically journal bearings with low radial clearances for rotors in small-scale applications (e.g., microgas turbines); their diameter is in the order of ten millimetres, leading to very small dimensional clearances when the typical relative ones (order of 1/1000) are employed; investigating this particular class of journal bearings under static and dynamic loading conditions represents something unexplored. To this goal, a suitable test rig was designed and the performance of its bearings was investigated under steady load. For the sake of comparison, numerical simulations of the lubrication were also performed by means of a simplified model. e original test rig adopted is a commercial rotor kit (RK), but substantial modifications were carried out in order to allow significant measurements. Indeed, the relative radial clearance of RK4 RK bearings is about 2/100, while it is around 1/1000 in industrial bearings. erefore, the same original RK bearings are employed in this new test rig, but a new shaft was designed to reduce their original clearance. e new custom shaft allows to study bearing behaviour for different clearances, since it is equipped with interchangeable journals. Experimental data obtained by this test rig are then compared with further results of more sophisticated simulations. ey were carried out by means of an in-house developed finite element (FEM) code, suitable for thermoelasto-hydrodynamic (TEHD) analysis of journal bearings both in static and dynamic conditions. In this paper, bearing static performances are studied to assess the reliability of the experimental journal location predictions by comparing them with the ones coming from already validated numerical codes. Such comparisons are presented both for large and small clearance bearings of original and modified RKs, respectively. Good agreement is found only for the modified RK equipped with small clearance bearings (relative radial clearance 8/1000), as expected. In comparison with two-dimensional lubrication analysis, three-dimensional simulation improves prediction of journal location and correlation with experimental results.
FEM Applied to Hydrodynamic Bearing Design
InTech eBooks, Apr 26, 2011
Industrial Lubrication and Tribology, Apr 8, 2014
Purpose-The purpose of this paper was to conceive a fast method to verify design and performance ... more Purpose-The purpose of this paper was to conceive a fast method to verify design and performance of elliptical pocket journal bearings. Design/methodology/approach-The computer-aided verification of pocket journal bearings is performed by means of a suitable finite element analysis method. Findings-The results of sample analyses indicate that the machining tolerances are very influential on elliptical pocket bearing performances, and they must be included among the input data. Originality/value-Although elliptical pocket bearings are widely used in turbomachineries, the influence of their design on performance has not been specifically investigated. A lot of works about tribological models are available, but few of them focus on their application to bearing design at the industrial level.
THD Analysis of Journal Bearings: A Finite Element Algorithm for Groove Mixing Conditions
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, Feb 3, 2023
The use of computational structural models that include geometrical non-linearity in many applica... more The use of computational structural models that include geometrical non-linearity in many application cases may require high reliability in prediction of displacements. Nevertheless, large differences up to 60% on maximum total displacement have been found among results of static large-deformation analyses performed by means of the major commercial software packages in a simple benchmark study with linear material properties. In order to investigate the causes of such disagreement, the present work compares different finite element formulations including well-established stress update schemes. The various formulations are tested, and results are compared in three test cases. Rodriguez stress update algorithms have shown the best performance among methods reported in literature. Finally, the cause of the large differences found in the predictions of commercial codes is identified. It is linked to the energetic inconsistency of some stress update methods in the simulation of extension/compression loading conditions. Such inaccuracy is reproduced analytically by formulating and integrating the corresponding inconsistent constitutive equations. The identified problem is very important for designers, as it affects almost all the static simulations, which are the most common type of large-deformation analyses and usually involve extension/compression loading.
Application of a Finite Element Quasi-3D Solution of the Energy Equation to the TEHD Analysis of Dynamically Loaded Journal Bearings
Volume 3: Dynamic Systems and Controls, Symposium on Design and Analysis of Advanced Structures, and Tribology, 2006
Taking advantage of the quasi-three-dimensional (quasi-3D) solution of the energy equation in the... more Taking advantage of the quasi-three-dimensional (quasi-3D) solution of the energy equation in the lubricant film presented in a previous work, a new method for analyzing the lubrication of dynamically loaded bearings is proposed. Cavitation is taken into account with a mass-conserving formulation of the hydrodynamic problem, where viscosity variations through the oil thickness are considered. The heat exchanged by the lubricant film with the bearing and the shaft is computed. Hence the thermo-elastic expansion of both the journal and the sleeve can be considered, together with the mechanical deformations of the bearing.
Aqueous dispersions of gelled collagen particles. Part 1: experimental characterization of stiffness and rheological properties
Industrial Lubrication and Tribology, Sep 1, 2006
ABSTRACT Purpose – To propose a new numerical elastohydrodynamic (EHD) analysis method suited to ... more ABSTRACT Purpose – To propose a new numerical elastohydrodynamic (EHD) analysis method suited to cinematic pairs lubricated with aqueous dispersions of collagen gelled media. Design/methodology/approach – In comparison with traditional lubricants, these media are characterized by an apparent viscosity that increases with film thickness. Hence, the Reynolds equation has been numerically solved taking into account the new rheological law. The apparent viscosity of 4 percent collagen gel dispersions, as measured at a fixed shear rate of 100 l/s, is in the range 2-5 Pa s, when film thickness varies between 20 and 200 μm. These experimental data have been fitted using a power law. The proposed analysis method has been applied to a 180° partial bearing, which rigid journal rotates in a flexible sleeve, made up by a resin shell. Findings – The results of this tribological analysis have been compared with those concerning traditional isoviscous fluids which viscosity has been set equal to the viscosity of the gelled media when the film thickness is constant, i.e. in the unloaded bearing. The minimum film thickness calculated has been turned out equal to 66.8 μm in the former case, and 27.6 μm in the latter case. Furthermore, when the cinematic pair is lubricated with the aqueous gel media, film thickness distribution over the bearing surface is more uniform, active film region is wider and, consequently, peak pressure is lower than when the isoviscous lubricant is employed. Research limitations/implications – For lightly loaded journal bearings, the dependence between apparent viscosity and the shear rate in the lubricant film has been neglected, so that the apparent viscosity has been considered changing only with the film thickness. Originality/value – The paper introduces a new numerical EHD analysis method for use with cinematic pairs lubricated with aqueous dispersions of collagen gelled media.
REMOVED: Hysteresis and torsional-lateral vibration coupling in a complex shaft line supported by hydrodyanamic journal bearings
Mechanical Systems and Signal Processing
Energies
To address the increasing interest towards more environmentally friendly naval transportation and... more To address the increasing interest towards more environmentally friendly naval transportation and the introduction of IMO2020 restrictions on pollutant emissions onboard ships, the present work details the preliminary design of a mini gas turbine engine, i.e., a gas turbine engine with an output power up to 5 MW, for onboard energy generation. In comparison to conventional propulsion systems, gas turbine units benefit from known compactness, which can be further enhanced by employing single-stage uncooled radial machines, according to similar works in the field. As such, the present paper aims to set up a complete procedure that allows a reliable and fast (i.e., requiring a limited computational effort) preliminary design of one-stage centrifugal compressors and radial turbines operating at a high pressure ratio via the use of classical one-dimensional theory. The aerodynamic design outputs in terms of forces and torques are then used to perform a preliminary mechanical design of th...
International Journal of Thermal and Environmental Engineering, 2016
In the last years micro gasturbines for cogeneration power plants have been proved to be a promis... more In the last years micro gasturbines for cogeneration power plants have been proved to be a promising technical solution for the distributed production of electricity and heat. In order to enhance the machine efficiency and make it competitive with internal combustion engines, it is necessary to improve the efficiency of each component of the GT: in the first part of the paper the focus will be put on the combustor, the recuperator and the bearings. Combustor and bearings are cutting edge technology and for them a brief description of the design procedure has been reported. The recuperator plays a key role in the cycle efficiency, since the reduction of the pinch point temperature difference can strongly enhance the overall efficiency.
E3S Web of Conferences, 2019
The aim of the present work is to design a test rig suited to investigate the dynamic interaction... more The aim of the present work is to design a test rig suited to investigate the dynamic interaction between rotor and hydrodynamic journal bearings in micro gas turbines (microGT), i.e. with reference to small bearings (diameter in the order of ten millimeters). Particularly, the device is capable of measuring the journal location. Therefore, the journal motion due to rotor vibrations can be displayed, in order to assess performance as well as stiffness and damping of the bearings. The new test rig is based on Bently Nevada Rotor Kit (RK), but substantial modifications are carried out. Indeed, the relative radial clearance of the original RK bearings is about 2/100, while it is in the order of 1/1000 in industrial bearings. Therefore, the same RK bearings are employed in the new test rig, but a new shaft has been designed in order to reduce the original clearance. The new shaft enables us to study the bearing behaviour for different clearances, as it is equipped with interchangeable j...
Bearing Technology
Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearin... more Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearings are a key factor in their design and development. The aim of the present research work is the development of the support system for a typical microturbine intended for power generation. To this goal, the present chapter defines the typical requirements of the machine and, afterward, describes the different technologies available to develop the support system of a reliable microturbine. Conventional (rolling element and oil-film) supports and cutting-edge (magnetic, aerodynamic, and aerostatic) bearings are reviewed. Particularly, their suitability to the operating conditions is compared by means of a literature review and elaboration of the relevant data. By analyzing all this information, a new concept for the design of a micro-GT support system is devised. Instead of using a single type of bearing as usual, the new system includes different types in order to take advantage of the best characteristics of each one and, simultaneously, to minimize the effects of the relevant flaws. The innovative support system requires a suitable bearing arrangement, which is compared with the conventional ones. The conceptual design of the innovation is completed by a discussion of its advantages, drawbacks, and prospective improvements.
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Papers by Fabrizio Stefani