A crucial part of building reliable models for the design of under-platform dampers for turbine blades resides in the appropriate description of the contact conditions, both in the normal and in the ...tangential direction.
The aim of this paper is to determine to what extent microslip due to the combined non-linearities along the normal and the tangent of non-conforming contact surfaces influences the damper behavior. The ultimate goal is to determine whether introducing these features in the contact model would improve the performance of numerical routines used at the blade-damper design stage. In order to explore this problem, a purposely developed contact model is tuned on a single-contact test and then included in the numerical model of a curved-flat damper to simulate its cylindrical interface. The damper numerical routine is then validated against the results from an experimental device purposely developed to test the dynamics of a damper loaded between moving platforms.
It is shown that the validated numerical routine featuring the newly introduced contact model predicts, in comparison with the standard contact model (where partial slip and normal approach non-linearity are not considered), a lower dissipated energy by an amount that would not be justifiable to neglect.
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•A contact model featuring microslip and non-linearity in the normal direction is proposed.•The model is used to represent contacts of under-platform dampers for turbine blades.•Numerical results match accurate measurements on damper.•Disregarding microslip leads to overestimating the damper dissipative capability.•Microslip should be modeled at the damper design stage to ensure a robust design.
•Proposing a new microslip contact model considering the time-variable normal load.•Mechanics of a laboratory under-platform damper is analysed by the new model.•Numerical results are validated by ...experimental results.•The effect of micro-slip on under-platform mechanics is identified and highlighted.
In turbine blade systems, under-platform dampers are widely used to attenuate resonant vibrations and prevent high cycle fatigue failure depending on friction contact. To effectively predict dynamic response of blade-damper system, a numerical model of friction contact between damper and blade is required. The majority of existing friction contact model either cannot reproduce micro-slip behavior or include too many contact parameters, which restricts their application. The proposed model, a modification of the IWAN model, not only describes the tangential micro/gross-slip and normal contact/separation motions, but also has the same number of parameters as the classical gross-slip model. A group of microslip-dominated experimental results for a laboratory crossed curve-flat under-platform damper are shown, and a specially developed procedure is used to determine the elastic contact parameters in order to validate the proposed model. It is shown that the proposed model is able to correctly represent the damper’s energy dissipation under low vibration amplitudes. The relative difference of energy dissipation between experiment and simulation by the proposed model is about 1.15%, far less than 24% according to the gross-slip model. Furthermore, the relative simple expression of the model is fully compatible with the current state-of-the-art in the numerical analysis of resonance damping of bladed disks for axial flow turbo-machinery.
The focus of this paper is on the direct experimental investigation of underplatform dampers, friction damping devices used on turbine bladed disks to limit resonant vibrations. Given the nonlinear ...and highly complex nature of dry friction, model validation of bladed disks with underplatform dampers still relies heavily on experimental verification, which is typically performed using the Frequency Response Function.
This paper is the authors’ most recent effort to increase the understanding of friction and damper mechanics by gathering experimental evidence directly at the blade platform/damper interface.
A new test rig for frequencies and contact pressures in line with real working conditions has been developed. The geometry of the test rig and its technical features are described in detail in the paper. The dedicated experimental evidence thus obtained is used to estimate (not tune) contact parameters. The influence of frequency, centrifugal load and of the flat-on-flat contact interface morphology is highlighted. Results are compared with those gathered using similar dampers on a different test rig. Contact parameters are then be fed to a state-of-the-art numerical code for the prediction of the dynamic response of friction damped systems. The experimental-numerical comparison is used as validation and as a basis to investigate the simulation sensitivity to contact parameters variability.
A method called PCR (Platform Centered Reduction) is designed to more effectively perform complex iterative and nonlinear calculations required for the dynamic response of turbine blades damped by ...dry friction contacts between rigid dampers and airfoil-to-neck platform. The key feature of PCR is to represent all nonlinear forces on the blade platform by means of only six degrees of freedom at a point located within the platform volume, regardless of the number of damper–platform contact elements. Despite reducing the effort and computational time by more than one order of magnitude, the method proves to be fully accurate by a check against the corresponding nonlinear Finite Elements (FE) calculation. It is also shown that the limit exciting force, indicating the upper capability to dampen vibrations, can be calculated with a simple linear modal analysis. In order to search for the best blade–damper match, the preferred graph represents relevant bending stresses on the airfoil against excitation forces. A detailed application of the method concerns two significantly different blade sizes, by varying parameters such as neck length and damper centrifugal force. Finally, it is emphasized that a final check by a complete FE analysis is still possible as a purely linear solution fed by sets of contact forces previously determined through the PCR at any desired frequency and excitation.
In turbomachinery, the perfect detuning of turbine blades in order to avoid high cycle fatigue damage due to resonant vibration is often unfeasible due to the high modal density of bladed disks.
To ...obtain reliable predictions of resonant stress levels of turbine blades, accurate modeling of friction damping is mandatory.
Blade root is one of the most common sources of friction damping in turbine blades; energy is dissipated by friction due to microslip between the blade and the disk contact surfaces held in contact by the centrifugal force acting on the blade.
In this paper, a method is presented to compute the friction forces occurring at blade root joints and to evaluate their effect on the blade dynamics. The method is based on a refined version of the state-of-the-art contact model, currently used for the nonlinear dynamic analysis of turbine blades.
The refined contact model is implemented in a numerical solver based on the harmonic balance method able to compute the steady-state dynamic response of turbine blades
The proposed method allows solving the static and the dynamic balance equations of the blade and of the disk, without any preliminary static analysis to compute the static loads acting at the contact interfaces.
Medication errors are one of the most common causes of negative events affecting patient safety all over the world. Scientific literature divides the factors that contribute to the occurrence of ...harmful events into factors related to the characteristics of the healthcare workers and factors related to the organization of the drug management process. The aim of the study was to examine the knowledge, attitudes and behaviours related to medication errors among Italian and Maltese nurses.
Cross-sectional survey of nurses working in Intensive Care settings in Italian and Maltese hospitals was conducted. A valid and reliable questionnaire used in previous studies was adapted for online use. Despite improved reporting, The Strengthening the Reporting of Observational Studies in Epidemiology was used.
Findings showed good psychometric properties and reliability. MANOVA demonstrated significant differences in nurses' perception of the pharmacist presence during medication process and of the use of computerized provider order entry. MANOVA also demonstrated significant differences in the control of vital parameters and the application of the 8 right.
These findings support the contention that knowledge, attitude and behaviour of nurses is similar across different contexts in different countries wherein nurse training is harmonised and regulated through a transnational directive.
Friction contacts are often used in turbomachinery design as passive damping systems. In particular, underplatform dampers are mechanical devices used to decrease the vibration amplitudes of bladed ...disks.
Numerical codes are used to optimize during designing the underplatform damper effectiveness in order to limit the resonant stress level of the blades. In such codes, the contact model plays the most relevant role in calculation of the dissipated energy at friction interfaces. One of the most important contact parameters to consider in order to calculate the forced response of blades assembly is the static normal load acting at the contact, since its value strongly affects the area of the hysteresis loop of the tangential force, and therefore the amount of dissipation.
A common procedure to estimate the static normal loads acting on underplatform dampers consists in decoupling the static and the dynamic balance of the damper. A preliminary static analysis of the contact is performed in order to get the static contact/gap status to use in the calculation, assuming that it does not change when vibration occurs.
In this paper, a novel approach is proposed. The static and the dynamic displacements of the system (bladed disk+underplatform dampers) are coupled together during the forced response calculation. Static loads acting at the contacts follow from static displacements and no preliminary static analysis of the system is necessary.
The proposed method is applied to a numerical test case representing a simplified bladed disk with underplatform dampers. Results are compared with those obtained with the classical approach.
We aimed to evaluate quality of sleep and self-perception risk of medication errors, in a significantly-sized sample of nurses in Italy, using a web survey.
An anonymous questionnaire about ...self-perception of quality of sleep (Pittsburgh Sleep Quality Index - PSQI) and risk of medication errors (based on the 7 R-rule), was made up and delivered by social media, i.e., Facebook and Instagram. Risk of medication errors was intended as near misses, i.e., accidents that do not cause the patient harm.
A poor quality of sleep stated by PSQI score >5, was present in 87.9% of subjects and the risk of medication errors during the last shift was reported in 76% of them. However, more than half of nurses' sample (60.1%) reported a good or excellent self-perception quality of sleep. Risk of medication errors was associated with poor quality of sleep and it was independently associated with short resting time after night shift and bad self-perception quality of sleep (OR 3.165, 95% CI 1.468-6.827, p=0.003).
Absence of proper resting is crucial on performance even if nurses perceived a good quality sleep. The relationship between shift work, poor sleep quality, and risk of medication errors represents crucial point for all health professionals' community, and web-survey represents a valuable information in order to capture the risk of medication errors. Health care organizations should encourage such a type of research in order to show a more proactive approach towards patient safety.
•The Base-Cycle completely characterizes a damper-platform pair.•The Base-Cycle is dependent on damper shape and platform proportions.•The Base-Cycle for a case chosen as reference case can be scaled ...for size and force.•The Base-Cycle is an input to the vibrational response of a damped blade in Mode 1.
This paper presents a general geometric theory of in-plane motion of underplatform dampers as a function of blade neck deflection and the resulting platform kinematics. In-plane treatment is compatible with Mode 1 blade vibration, in which the rigid-body platform rotation associated with pure neck bending largely predominates over other rotations. The method applies when the amplitude of platform vibrational rotation reaches at least (or is greater than) the minimum value required to have, at both ends of the forward and reverse half-cycles, the onset of full slip in the last contact element still in stick.
In this context, the first objective of this paper is to present a general geometric theory for damper kinematics and damper-platform forces, for both the In-Phase and Out-of-Phase blade vibrations and is then developed in detail for the In-Phase case. The concept of equivalent virtual parallel geometry is introduced, which transforms a case with nonnegligible inclination between adjacent blades into one, more tractable, valid for a very large (ideally infinite) number of blades. The equations are developed for the In-Phase case according to a formulation whose structure is particularly suited to highlighting the properties of the system, thus facilitating both its solution and interpretation.
The second objective is to employ the equations in a piecewise linear algorithm to pre-compute a cycle as a sequence of a few finite linear sections in the number, per half-cycle, of Jenkins-type contact elements, so as to avoid both the time-step integration of the damper cycle used in the HBM-AFT approach and its repetition at each iteration of the solving system.
The concept of a force Base-Cycle is introduced for the contact forces, unique for a given damper shape and its set of contact parameters, along with a moment Base-Cycle for the platform-damper system, an in-plane moment on the platform, generated by the contact forces, function of platform rotation.
It is shown that such Base Cycles uniquely characterize a damper shape so that, once produced for a convenient Reference Case, they can be scaled linearly without recalculation according to size, radial force, platform proportions, and proportional variation of contact stiffness.
To better illustrate the procedure, a practical example is developed in parallel with the theory, concluding with indications on how the moment Base-Cycle can be used in a reduced version of the Platform Centered Reduction, proposed in recent papers, meant to be a utility tool to support the initial damper design phase.
The use of chemical devices for domestic oral hygiene in periodontal patients has led to new treatment strategies aiming primarily at a control of infection. Over the last few years, carvacrol and ...thymol (CT) have been subjected to many scientific and medical studies. The purpose of the present study was to assess the effect of CT on the red complex bacteria using Polymerase Chain Reaction (PCR) for microbiological analysis. Five patients with a diagnosis of chronic periodontitis in the age group >25 years, were selected. None of these patients had received any surgical or non-surgical periodontal therapy and demonstrated radiographic evidence of moderate bone loss. After scaling and root planning, patients received a CT gel to be used at home. Four non-adjacent sites in separate quadrants were selected in each patient for monitoring, based on criteria that the sites localize chronic periodontitis. Microbial analysis (MA) was analyzed at baseline and at day 15. SPSS program was used for statistical purposes and a paired samples correlation was performed at the end of the observation period. Although an absolute reduction was observed among the studied bacteria (i.e. Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Fusobacterium nucleatum, Campylobacter rectus and Total bacteria loading) none reach a statistical significant value. The present study demonstrated that CT gel has a small impact on oral biofilm. Additional studies are needed to detect the efficacy of CT gel.
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