scholarly journals Modal Tests and Analysis of a Radial Impeller at Rest: Influence of Surrounding Air on Damping

2012 ◽  
Author(s):  
C. Gibert ◽  
L. Blanc ◽  
P. Almeida ◽  
X. Leblanc ◽  
J.-P. Ousty ◽  
...  
Keyword(s):  
Ambient Pressure ◽  
Damping Ratio ◽  
Flexural Mode ◽  
Aerodynamic Damping ◽  
Basic Fluid ◽  
Modal Density ◽  
Single Piece ◽  
Direct Interpretation ◽  
Turbomachinery Blades ◽  
High Modal Density

HCF risk assessment for turbomachinery blades requires the prediction of vibratory levels, which in turn requires fine damping quantification. This issue is especially sensitive for structures with low structural damping such as monobloc centrifugal compressor disks (blisks). The material composing blisks and aero-dynamic flow both contribute to damping phenomena. A strategy for non-aerodynamic damping characterization is to perform experiments in vacuum. This paper focuses on the use of modal tests in vacuum to estimate material damping under non-rotating conditions. Experiments are performed on an isolated impeller manufactured from a single piece in a vacuum chamber at different air pressure levels ranging from 10 mbar to 1 bar. Strong dependency of damping ratios on pressure can be found on the first flexural mode, leading to two types of application. Firstly, measurements enable assessing the validity of extrapolations of non-aerodynamic damping from measurements sometimes performed under less thorough vacuum conditions. Basic fluid-structure interaction models are used to interpret and quantify the evolution of modal quantities when air is progressively removed. Secondly, vacuum measurements can give frequency response functions (FRFs) with much greater separation between resonance peaks. In this study, the damping ratio found in vacuum condition are 3% of these at ambient pressure corresponding to a magnitude 30dB higher at resonance peaks. This contrasts with in-air measurements on cyclic symmetry structures, like blisks, with high modal density that make the direct interpretation of FRFs and their modal analysis more difficult.

Power Flow and Energy Sharing between an Oscillator and a Continuous Receiver Structure

2011 ◽  
Vol 189-193 ◽  
pp. 1914-1917
Author(s):  
Lin Ji
Keyword(s):  
High Frequency ◽  
Power Flow ◽  
Energy Analysis ◽  
Statistical Energy Analysis ◽  
Coupled Subsystems ◽  
Modal Density ◽  
Vibration Problems ◽  
Energy Sharing ◽  
High Modal Density ◽  
Vibration Modeling

A key assumption of conventional Statistical Energy Analysis (SEA) theory is that, for two coupled subsystems, the transmitted power from one to another is proportional to the energy differences between the mode pairs of the two subsystems. Previous research has shown that such an assumption remains valid if each individual subsystem is of high modal density. This thus limits the successful applications of SEA theory mostly to the regime of high frequency vibration modeling. This paper argues that, under certain coupling conditions, conventional SEA can be extended to solve the mid-frequency vibration problems where systems may consist of both mode-dense and mode-spare subsystems, e.g. ribbed-plates.

Influence of Internal Structures of High Modal Density on Shell’s Radiation

1997 ◽  
Author(s):  
Lionel Oddo ◽  
Bernard Laulagnet ◽  
Jean-louis Guyader
Keyword(s):  
Cylindrical Shell ◽  
Sound Radiation ◽  
Numerical Results ◽  
Structural Damping ◽  
Radiation Spectra ◽  
Modal Density ◽  
Internal Structures ◽  
High Modal Density

Abstract The aim of this paper is to study the sound radiation by a cylindrical shell internally coupled with mechanical structures of high modal density. The model is based on a mobility technique. The numerical results show a smoothing of the cylinder’s velocity and radiation spectra associated with an increase of the apparent damping. The use of the S.E.A. method allows us to calculate an additional structural damping of the shell, equivalent to the effect of the internal structures.

Aeroelastic Flutter Investigation and Stability Enhancement of a Transonic Axial Compressor Rotor Using Casing Treatment

2017 ◽  
Author(s):  
Kirubakaran Purushothaman ◽  
Sankar Kumar Jeyaraman ◽  
Ajay Pratap ◽  
Kishore Prasad Deshkulkarni
Keyword(s):  
Flow Separation ◽  
Aerodynamic Force ◽  
Damping Ratio ◽  
Axial Compressor ◽  
Casing Treatment ◽  
Aerodynamic Damping ◽  
Compressor Rotor ◽  
Aeroelastic Flutter ◽  
Blade Tip ◽  
Transonic Axial Compressor

This study discusses in detail the aeroelastic flutter investigation of a transonic axial compressor rotor using computational methods. Fluid structure interaction approach is used in this method to evaluate the unsteady aerodynamic force and work done of a vibrating blade in CFD domain. Energy method and work per cycle approach is adapted for this flutter prediction. A framework has been developed to estimate the work per cycle and aerodynamic damping ratio. Based on the aerodynamic damping ratio, occurrence of flutter is estimated for different inter blade phase angles. Initially, the baseline rotor blade design was having negative aerodynamic damping at part speed conditions. The main cause for this flutter occurrence was identified as large flow separation near blade tip region due to high incidence angles. The unsteadiness in the flow was leading to aerodynamic force fluctuation matching with natural frequency of blade, resulting in excitation of the blades. Hence axially skewed slot casing treatment was implemented to reduce the flow separation at blade tip region to alleviate the onset of flutter. By this method, the stall margin and aerodynamic damping of the test compressor was improved and flutter was avoided.

Simplified Forced Response HCF Assessment of Turbomachinery Blades

2009 ◽  
Author(s):  
Hans Ma˚rtensson ◽  
Johan Forsman ◽  
Martin Eriksson
Keyword(s):  
High Speed ◽  
Damping Ratio ◽  
Tangential Force ◽  
Forced Response ◽  
Fe Model ◽  
Compressor Blades ◽  
Design Data ◽  
Modal Force ◽  
Turbomachinery Blades ◽  
Compressor Performance

A method is proposed for HCF-analysis that is suitable for use in early design stages of turbomachinery blades. Quantitative measures of the risk for later encountering HCF life limiting vibrations are the goal for the development. The novelty of the system is the unique and rational way all design data are processed resulting in a mode risk priority listing. The method makes extensive use of FE calculated modal analyses and simple assumptions on the modal force and damping. The modal force is taken proportional to the tangential force on the blade over the operating range. This choice is made because the tangential force is known early on from the compressor performance map, and gives a reasonable scaling with the operating point. Crossings occurring at low speed get a lower force than at high speed. The system damping used is a constant critical damping ratio. Using a modal force and damping along with the FE model forced response amplitude can be directly computed at resonance crossings inside operating envelope. The modal force calculated this way can be compared to the force amplitude needed to reach the fatigue limit in a Haigh diagram. Using the Haigh diagram this way allows modes with localized high stresses, so-called hot spots, to be highlighted. Taking the ratio of the forces gives a ranking value that can be used to compare risk. Details of the technique along with example applications to compressor blades are presented in the paper. It is found that many mode crossings can be excluded as low risk this way and that a rational way of prioritizing is achieved.

Nonlinear Parametric Reduced-Order Model for the Structural Dynamics of Hybrid Electric Vehicle Batteries

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Jauching Lu ◽  
Kiran D'Souza ◽  
Matthew P. Castanier ◽  
Bogdan I. Epureanu
Keyword(s):  
Hybrid Electric Vehicle ◽  
Vibration Response ◽  
Point Of View ◽  
Reduced Order Models ◽  
Structural Variations ◽  
Order Model ◽  
Reduced Order ◽  
Modal Density ◽  
Battery Packs ◽  
High Modal Density

Battery packs used in electrified vehicles exhibit high modal density due to their repeated cell substructures. If the excitation contains frequencies in the region of high modal density, small commonly occurring structural variations can lead to drastic changes in the vibration response. The battery pack fatigue life depends strongly on their vibration response; thus, a statistical analysis of the vibration response with structural variations is important from a design point of view. In this work, parametric reduced-order models (PROMs) are created to efficiently and accurately predict the vibration response in Monte Carlo calculations, which account for stochastic structural variations. Additionally, an efficient iterative approach to handle material nonlinearities used in battery packs is proposed to augment the PROMs. The nonlinear structural behavior is explored, and numerical results are provided to validate the proposed models against full-order finite element approaches.

scholarly journals Development of a hybrid SmEdA/SEA model for predicting the power exchanged between low and high modal density subsystems

2021 ◽  
Vol 263 (3) ◽  
pp. 3824-3832
Author(s):  
Guang Zhu ◽  
Laurent Maxit ◽  
Nicolas Totaro ◽  
Alain Le Bot
Keyword(s):  
Power Flow ◽  
The Other ◽  
Mode Shapes ◽  
Distribution Analysis ◽  
Coupled Subsystems ◽  
Modal Density ◽  
Spectrum Density ◽  
Energy Equipartition ◽  
Cavity System ◽  
High Modal Density

Statistical modal Energy distribution Analysis (SmEdA) was developed from classical Statistical Energy Analysis (SEA). It allows computing power flow between coupled subsystems from the deterministic modes of uncoupled subsystems without assuming the SEA modal energy equipartition. SmEdA is well adapted in mid-frequency when the subsystems have not a very high modal density. However, for some systems e.g. the plate-cavity system, one subsystem can exhibit a low modal density while the other one a high one. The goal of the paper is then to propose an extension of SmEdA formulation that allows describing one subsystem by its deterministic modes, and the other one as a diffuse field statistically supposing modal energy equipartition. The uncertain subsystem is then characterized by sets of natural frequencies and mode shapes constructed based on Gaussian Orthogonal Ensemble matrix and the cross-spectrum density of a diffuse field, respectively. This formulation permits not only the computation of mean noise response but also the variance generated by the uncertainties and furthermore without bringing in much computation. It is demonstrated that the obtained analytical results from the proposed hybrid SmEdA/SEA are consistent with numerical results computed by FEM with an appropriate degree of uncertainty.

Aerodynamic Damping Analysis for Radial Turbine Featuring a Multi-Channel Casing Design

2020 ◽  
Author(s):  
Ahmed Farid Hassan ◽  
Tobias Müller ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Damping Ratio ◽  
Full Model ◽  
Aerodynamic Damping ◽  
Radial Turbine ◽  
Damping Behavior ◽  
Ansys Cfx ◽  
Partial Admission ◽  
Time Marching ◽  
Opening And Closing ◽  
Spiral Casing

Abstract Radial turbine featuring a Multi-channel Casing (MC) is a new design under investigation for enhancing the turbine controllability. The idea behind this new design is to replace the traditional spiral casing with a MC, which allows controlling the mass flow by means of opening and closing control valves in each channel. The arrangement of the closed and opened channel is called the admission configuration, while the ratio between the counts of the open channels to the total number of channels is called the admission percentage. Among several aspects, when applying different admission configurations, the aerodynamic damping during resonant excitation is considered during the design of the turbine. The present study aims at investigating the effect of different MC admission configurations on the aerodynamic damping as an extension to an aerodynamic forcing study, which already assessed the different forcing patterns associated with these different admission configurations. Due to the asymmetry of the flow in circumferential direction resulting from the different partial admission configurations, the computational model is solved as full 3D time-marching, unsteady flow using ANSYS CFX in a one-way fluid-structure analysis. Two different modeling approaches have been considered in this study to investigate their capability of predicting the damping ratio for different MC admission configurations: a) the conventional isolated rotor approach and b) a full model consisting of the rotor and its casing. The results show that the casing affects the aerodynamic damping behavior, which can only be captured by the full model. Furthermore, the damping ratios for all different admission configurations have been calculated using the full stage model.

Friction Damping of Hollow Airfoils: Part II—Experimental Verification

1998 ◽  
Vol 120 (1) ◽  
pp. 126-130 ◽  
Author(s):  
Y. M. EL-Aini ◽  
B. K. Benedict ◽  
W.-T. Wu
Keyword(s):  
Essential Element ◽  
Forced Response ◽  
Friction Damping ◽  
Modal Density ◽  
Speed Range ◽  
Manufacturing Tolerances ◽  
Hollow Cavity ◽  
Microslip Friction ◽  
High Modal Density ◽  
Damping Model

The use of hollow airfoils in turbomachinery applications, in particular fans and turbines, is an essential element in reducing the overall engine weight. However, state-of-the-art airfoil geometries are of low aspect ratio and exhibit unique characteristics associated with plate like modes. These modes are characterized by a chordwise form of bending and high modal density within the engine operating speed range. These features combined with the mistuning effects resulting from manufacturing tolerances make accurate frequency and forced response predictions difficult and increase the potential for High Cycle Fatigue (HCF) durability problems. The present paper summarizes the results of an experimental test program on internal damping of hollow bladelike specimens. Friction damping is provided via sheet metal devices configured to fit within a hollow cavity with various levels of preload. The results of the investigation indicate that such devices can provide significant levels of damping, provided the damper location and preload is optimized for the modes of concern. The transition of this concept to actual engine hardware would require further optimization with regard to wear effects and loss of preload particularly in applications where the preload is independent of rotational speed. Excellent agreement was achieved between the experimental results and the analytical predictions using a microslip friction damping model.

Aeroelastic Flutter Analysis of Linear Cascade Blades: STC5

2017 ◽  
Author(s):  
Kirubakaran Purushothaman ◽  
Sankar Kumar Jeyaraman ◽  
Sasikanta Parida ◽  
Kishore Prasad Deshkulkarni
Keyword(s):  
Test Data ◽  
Aerodynamic Force ◽  
Damping Ratio ◽  
Pressure Coefficient ◽  
Axial Compressor ◽  
Sensitivity Study ◽  
Standard Test ◽  
Test Configuration ◽  
Linear Cascade ◽  
Aerodynamic Damping

Study of aerodynamic flow and aeroelastic stability in vibrating blades of cascade is the main objective of this study. Standard test configuration (STC-5) was chosen for this study as it involves transonic flow regime in compressor blade cascades. CFD analysis were carried out for 11 test cases of STC-5 configuration and pressure coefficient values were compared with test data. The range of incidence angles vary from 2° to 10° and reduced frequency varies from 0.14 to 1.02. Inflow Mach number was fixed at 0.5 and Reynolds number was fixed at 1.4 × 106. Analysis of vibrating blades and comparison of test data results of axial compressor with linear cascade stator blades of fifth standard configuration at high subsonic speed is compared with CFD results. While doing this vibration of only the center blade is concerned when all the other blades in the cascade are fixed. Fluid structure interaction approach is used here to evaluate the unsteady aerodynamic force and work done for a vibrating blade in CFD domain. Energy method and work per cycle approach is adapted for aerodynamic damping prediction. A framework has been developed to estimate the work per cycle and aerodynamic damping ratio. Final sensitivity study was carried out to evaluate the influence of blade incidence and frequency on blade damping values.

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