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Orientation of orthotropic material properties in a femur FE model: a method based on the principal stresses directions.

Most work done on bone simulation has modeled the tissue as inhomogeneous and isotropic even though it is a recognized anisotropic material. Some recent investigations have included orthotropic behavior in bone finite elements (FE) models; however the problem regarding the orientation of these properties along the irregular bone anatomy remains. In this work, a procedure to orientate orthotropic properties in a proximal femur FE model using the directions of the principal stresses produced by a physiological load scheme was developed. Two heterogeneous material models, one isotropic and one orthotropic, were employed to test their influence on the mechanical behavior of the bone model. In the developed orthotropic material, the mechanical properties are aligned with the highest principal stress produced from the successive application of a multi load scenario corresponding to 10%, 30% and 45% of the gait cycle. A solid match between anatomical structures in the proximal femur and the corresponding directions of the main principal stress of the elements of the model suggests that the developed methodology works accurately. The differences found in the stress distributions were small (maximum 7.6%); nevertheless the changes in the strain distributions were important (maximum 27%) and located in areas of clinical relevance.

Application of a Reduced Order Modeling Technique for Mistuned Bladed Disk-Shaft Assemblies

This paper presents a reduced-order modeling technique, based on a component mode synthesis method specifically tailored for bladed disks, that allows the resulting low-order model to be attached to a shaft. Mistuning is included in the bladed disk model and the shaft is modeled using uniaxial finite elements according to the rotordynamic approach. The proposed formulation is applied to an example finite element model of a bladed disk, for both tuned and mistuned blades. Comparisons are made between the reduced model and the full finite element solution for free and forced responses in order to assess the methodology. The forced response amplitudes of the blades are found to vary significantly with the inclusion of a flexible shaft. This work suggest that stage independent analyses might not be adequate for predicting the global dynamic response of rotating assemblies of turbomachines.Copyright

Reduced Order Model for a Two Stage Gas Turbine Including Mistuned Bladed Disks and Shaft Interaction

A number of previous works have suggested that in some cases the interaction between shaft and bladed disk modes could significantly modify the dynamics of the whole assembly i.e. the bladed disks mounted on a flexible shaft. This paper presents the application of a previously published reduced-order modeling technique to the dynamical modeling of a real two stage gas turbine, including the bladed disks and the shaft. In the resulting reduce order model, mistuning is included in the bladed disk models and the shaft is modeled using beam finite elements according to the classical rotordynamic approach. Generation of finite element parent model for the real turbine is presented and discussed as well as simplifications used in order to generate the reduced order model. Comparisons are made between the reduced model and the full finite element solution for free response frequencies and mode shapes in order to assess the methodology and to evaluate the impact of simplifying hypothesis considered in model generation. Finally, this work also shows interaction between shaft modes and bladed disk modes, therefore confirming that stage independent analysis might not be adequate for predicting the global dynamic response of some turbomachinery rotors.Copyright

A Preliminary Study on the Interaction Between Slug Flow and Vortex Induced Vibration in Fatigue Life of Submarines Pipelines

Flow induced vibrations is a very important subject that should be carefully considered when facilities to transport hydrocarbons from marine sub-sea to shore line are designed. In particular, interaction between internal pipeline flow and external marine currents on horizontal, inclined or vertical (i.e. risers) pipelines need to be considered to properly estimate their fatigue life. Between these two form of excitations a coupling of resonances may appear, thus producing fatigue life predictions different from those calculated when each excitation is considered separately. In this paper a fluid-structural model for analyzing the dynamical behavior of a riser subjected to simultaneous internal slug flow and an external marine current uniform flow was developed. The fluid model was based on the classical wake oscillator model. A numerical model based on the finite difference method was implemented for the structure. Different tests cases were modeled to analyze the riser dynamics for a typical field case, when marine current velocity is constant. Preliminary results show that, in some cases of practical interest, the dynamical behavior of the riser could be influenced by the slug flow and the predicted fatigue life may be different from those predicted when each flow is considered separately. Additionally, results show that study of different slug flow scenarios may be simplified considering a monophasic flow whose density is equivalent to that of the average slug flow density.Copyright

Fatigue Life Prediction Due to Slug Flow in Extra Long Submarine Gas Pipelines

Some offshore production fields require transporting of production fluids through very long submarines pipelines without a previous separation process. In the case of gas production, condensate will appear in the pipeline due to the pressure losses and low temperatures. For some production conditions a slug flow pattern may then develop in the pipeline, and because of the irregular sea bottom profile, there may be pipe unsupported spans of even hundreds of meters long. Therefore, slugs traveling in the pipeline will act as moving loads for the unsupported pipe, producing a dynamic response that in some cases might reduce the fatigue life of the pipeline. In this work, a finite element (FE) model of a pipeline transporting slugs has been developed and used to assess the fatigue life of a horizontal pipeline. Slug hydrodynamic characteristics have been obtained using Taitel & Barnea’s model. The structural FE model is based in Bernoulli beam elements where slugs, once they have been geometrically characterized, are input as moving loads traveling in the pipeline. The system dynamic response was calculated for different spans conditions and slugs characteristics corresponding to different gas-liquid ratios typical from gas field production conditions. Once a steady state condition was obtained in the dynamic response, mean and alternating stress levels were obtained for each analyzed case and introduced in fatigue formulae to obtain the fatigue life of the pipeline. Results show that for some production conditions and free span longitudes, fatigue life of pipeline may experience important reductions due to slug flow. These free spans are obviously most likely to happen in extra long submarines pipelines.Copyright

Stress Field Around a Large Opening in a Pressure Vessel During a Major Repair

It is a regular practice in the oil industry to modify mechanical equipment to incorporate new technologies and to optimize production. In the case of pressure vessels, it is occasionally required to cut large openings in their walls in order to have access to the interior part of the equipment for executing modifications. This cutting process produces temporary loads, which were obviously not considered in the original mechanical design. Up to now, there is not a general purpose specification for approaching the assessments of stress levels once a large opening in a vertical pressure vessel has been made. Therefore stress distributions around large openings are analyzed on a case-by-case basis without a reference scheme. This work studies the distribution of the von Mises equivalent stresses around a large opening in FCC Regenerators during internal cyclone replacement, which is a frequently required practice for this kind of equipment. A finite element parametric model was developed in ANSYS, and both numerical results and illustrating figures are presented.Copyright

Determination of converge parameters for Monte Carlo experiments in the simulation of the failure of bone tissue

The Monte Carlo method is widely used in the field of biomechanics to study the variability of diverse parameters, like tissues properties, magnitude and direction of loads, kinematic of joints, among others. In particular, the failure of bone tissue, which is the target of this investigation, has been extensively studied; however, it is common to find in the literature realizations of Monte Carlo experiments with arbitrary sample size, or with a convergence criterion for which a statistically valid confidence level, or interval, is not defined. These strategies lead to results with presumed low, but unknown uncertainty. One option to address this problem is the acceptable shifting convergence band rule which, if appropriately configured and applied, serves as a convergence criterion with an implicit confidence level. However, in order to ensure a desired confidence level, it is necessary to determine the correct parameters for the method. As the typical biomechanical simulation is very time consuming, it is not advisable to calculate these parameters with the full model. Therefore, it is recommended to run a Monte Carlo experiment with a simpler, faster to simulate, model that is probabilistically similar to the full model. In this work, a pilot experiment is developed in order to compute the parameters required to stop the Monte Carlo simulation of the failure of bone tissue, with a desired confidence level. Two different failure criteria are applied, one with two and the other with three input probabilistic variables. Also, the variation of the convergence parameter with the desired precision of the mean is explored. Results led to determine suitable parameters for the different combinations of desired confidence level, precision of the mean and failure criterion. It was also found that when three input variables were involved, or when a three significant digits precision of the mean was required, the number of trials needed to attain convergence was greater than when two inputs variables were involved or when two significant digits precision was required.

Stress and Strain Fields in the Hemispherical Head of an FCC Regenerator During the Lifting Maneuver in Cyclone Replacement

The Oil and Gas industry is constantly seeking for improvements in the design of mechanical equipment. Each refining process is the subject of continuous research, which is frequently addressed in the revisions of the corresponding standard. Nevertheless, particular technologies such as the Fluid Catalytic Cracking Units (FCCU) are not governed by any International Standard but by designs developed and patented by specialized licensors. The implementation of new designs requires special studies of the original equipment in order to assess the feasibility of the related works and the required provisions to accomplish the revamp. This work studies the stress and displacement fields occurring in the hemispherical head of an FCC regenerator during the lifting maneuver for a typical cyclone replacement.A parametric finite element model was developed and stress and total displacement charts are presented as a function of diameters and thicknesses of hemispherical heads commonly found in the industry. Sensitivity analyses are presented with respect to a variation of ±15% of the applied loads and the size of the plenum chamber. Therefore, the results shown in this work present a reference framework for the replacement of cyclones in FCC regenerators when removing their hemispherical heads.Copyright

Forced Response of a Mistuned Industrial Impeller With Two Different Blade Geometries, via ROM

Numerous works have presented techniques for obtaining reduced order models (ROMs) of mistuned bladed disks. Most of these works focus in only one rotor’s stage, though some also include several stages and even the rotor shaft. However, to the authors’ knowledge, the ROM techniques available in the literature consider only one blade geometry by stage, thus making impossible their use for the case of impellers with two or more different blade geometries.This paper shows an adaptation of a previously published reduced order modeling technique in order to allow its application to the case of industrial compressor impellers incorporating two or more different blade geometries (main and splitter blades). The technique is based on Craig and Bampton’s component mode synthesis and it permits to introduce different mistuning patterns for each blade geometry while the disk is considered as a cyclic symmetric structure.The proposed technique is applied to an industrial compressor impeller in order to assess its precision and validity. Generation of finite element parent model for the real compressor via laser scanning is presented and discussed as well as simplifications used in order to generate the impeller’s ROM. Validation is carried out by comparison of predictions for the forced response of the tuned and mistuned impeller, obtained by means of the ROM and the finite element parent model. Results show that the ROM properly represents the dynamic features of the parent model in the frequency range of interest, with minimal computational cost. Furthermore, the ROM properly captures the localization phenomenon when it occurs.© 2012 ASME

Harmonic Response of a Piping System Considering Pipe - Support Friction via HFT Method

The harmonic response of piping systems is in general estimated considering linear behavior, i.e. neglecting pipe-support dry friction, shocks between adjacent pipes or between pipe and supports, unidirectional supports, gaps, etc. Under this hypothesis, harmonic analysis is straight forward for a forcing frequency and not specially demanding in terms of computational cost. On the contrary, if any nonlinearity is taken into account, integration in time is required until the system achieves a steady-state solution, which demands dealing with initial conditions, intensive computation and long calculation times. In the particular case of support friction, vibration amplitudes calculated using the linear assumption may be well overestimated for some systems or conditions, thus misguiding designers and analysts, which traduces in excessive cost in system construction. In this work, the Hybrid Frequency-Time method (HFT) is used to calculate the steady state amplitudes of a piping system subjected to harmonic excitations and considering pipe-support friction. Comparisons between a full integration scheme and the proposed methodology are presented and discussed for a typical system. Results show that the HFT method is a valid practical option to estimate the harmonic response of a piping system when considering support friction.Copyright