Influence of Intake on Fan Blade Flutter

2014 ◽  
Author(s):  
Mehdi Vahdati ◽  
Nigel Smith ◽  
Fanzhou Zhao
Keyword(s):  
Correct Prediction ◽  
Pressure Waves ◽  
Blade Vibration ◽  
Reflected Waves ◽  
Reflection Model ◽  
Flutter Analysis ◽  
Flutter Boundary ◽  
Blade Flutter ◽  
Fan Blade ◽  
Insight Into

The main aim of this paper is to study the influence of upstream reflections on flutter of a fan blade. To achieve this goal, flutter analysis of a complete fan assembly with an intake duct and the downstream OGVs (whole LP domain) is undertaken using a validated CFD model. The computed results show good correlation with measured data. Due to space constraints, only upstream (intake) reflections are analyzed in this paper. It will be shown that the correct prediction of flutter boundary for a fan blade requires modeling of the intake and different intakes would produce different flutter boundaries for the same fan blade. However, the ‘blade only’ and intake damping are independent and the total damping can be obtained from the sum of the two contributions. In order to gain further insight into the physics of the problem, the pressure waves created by blade vibration are split into an upstream and a downstream traveling wave in the intake. The splitting of the pressure wave allows one to establish a relationship between the phase and amplitude of the reflected waves and flutter stability of the blade. By using this approach, a simple reflection model can be used to model the intake effects.

Influence of Intake on Fan Blade Flutter

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Mehdi Vahdati ◽  
Nigel Smith ◽  
Fanzhou Zhao
Keyword(s):  
Correct Prediction ◽  
Blade Vibration ◽  
Reflected Waves ◽  
Reflection Model ◽  
Flutter Analysis ◽  
Flutter Boundary ◽  
Blade Flutter ◽  
Computational Fluid Dynamics Cfd ◽  
Fan Blade ◽  
Insight Into

The main aim of this paper is to study the influence of upstream reflections on flutter of a fan blade. To achieve this goal, flutter analysis of a complete fan assembly with an intake duct and the downstream outlet guide vanes (OGVs) (whole low pressure (LP) domain) is undertaken using a validated computational fluid dynamics (CFD) model. The computed results show good correlation with measured data. Due to space constraints, only upstream (intake) reflections are analyzed in this paper. It will be shown that the correct prediction of flutter boundary for a fan blade requires modeling of the intake and different intakes would produce different flutter boundaries for the same fan blade. However, the “blade only” and intake damping are independent and the total damping can be obtained from the sum of the two contributions. In order to gain further insight into the physics of the problem, the pressure waves created by blade vibration are split into an upstream and a downstream traveling wave in the intake. The splitting of the pressure wave allows one to establish a relationship between the phase and amplitude of the reflected waves and flutter stability of the blade. By using this approach, a simple reflection model can be used to model the intake effects.

On the Importance of Engine-Representative Models for Fan Flutter Predictions

2017 ◽  
Author(s):  
Sina Stapelfeldt ◽  
Mehdi Vahdati
Keyword(s):  
Inlet Temperature ◽  
Blade Vibration ◽  
Cfd Models ◽  
Numerical Predictions ◽  
Reduced Frequency ◽  
Aerodynamic Properties ◽  
Acoustic Liner ◽  
Flutter Boundary ◽  
Fan Blade ◽  
Flutter Margin

This paper examines the factors which can result in discrepancies between rig tests and numerical predictions of the flutter boundary for fan blades. Differences are usually attributed to the deficiency of CFD models for resolving the flow at off-design conditions. This work was initiated as a result of inconsistencies between the flutter prediction of two rig fan blades, called here Fan F1 and Fan F2. The numerical results agreed well with the test data in terms of flutter speed and nodal diameter for both fans. However, they predicted a significantly higher flutter margin for F2 than for Fan F1, while rig tests showed that the two blades had similar flutter margins. A new set of flutter computations for both blades using the whole LP domain (intake, fan, OGV and ESS) was therefore performed. The new set of computations considered the effects of the acoustic liner and mistuning for both blades. The results of this work indicate that the previous discrepancies between CFD and tests were due to: 1. Differences in the effectiveness of the acoustic liner in attenuating the pressure wave created by the blade vibration as a result of differences in flutter frequencies between the two fan blades. 2. Differences in the level of unintentional mistuning of the two fan blades due to manufacturing tolerances. In the second part of this research, the effects of blade misstaggering and inlet temperature on aerodynamic damping were investigated. The data presented in this paper clearly show that manufacturing and environmental uncertainties can play an important role in the flutter stability of a fan blade. They demonstrate that aeroelastic similarity is not necessarily achieved if only aerodynamic properties and the traditional aeroelastic parameters, reduced frequency and mass ratio, are maintained. This emphasises the importance of engine-representative models, in addition to an accurate and validated CFD code, for the reliable prediction of the flutter boundary.

Mechanisms for Wide-Chord Fan Blade Flutter

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Mehdi Vahdati ◽  
George Simpson ◽  
Mehmet Imregun
Keyword(s):  
Flow Separation ◽  
Pressure Wave ◽  
Geometric Features ◽  
The Third ◽  
Reflected Wave ◽  
Flutter Analysis ◽  
Parametric Studies ◽  
Blade Flutter ◽  
Stall Flutter ◽  
Fan Blade

This paper describes a detailed wide-chord fan blade flutter analysis with emphasis on flutter bite. The same fan was used with three different intakes of increasing complexity to explain flutter mechanisms. Two types of flutter, namely, stall and acoustic flutters, were identified. The first intake is a uniform cylinder, in which there are no acoustic reflections. Only the stall flutter, which is driven by flow separation, can exist in this case. The second intake, based on the first one, has a “bump” feature to reflect the fan’s forward pressure wave at a known location so that detailed parametric studies can be undertaken. The analysis revealed a mechanism for acoustic flutter, which is driven by the phase of the reflected wave. The third intake has the typical geometric features of a flight intake. The results indicate that flutter bite occurs when both stall and acoustic flutters happen at the same speed. It is also found that blade stiffening has no effect on aero-acoustic flutter.

Optical Determination of Rotating Fan Blade Deflections

1977 ◽  
Vol 99 (2) ◽  
pp. 204-209 ◽  
Author(s):  
H. Stargardter
Keyword(s):  
Mode Shape ◽  
Shape Description ◽  
Compressor Blades ◽  
Blade Vibration ◽  
Reflected Light ◽  
Flutter Analysis ◽  
Optical Determination ◽  
Fan Blade ◽  
A New Technique

Measurement of flutter motion for rotating fan and compressor blades is necessary to verify mode shape analysis and assure an accurate description of the deflection and twist distribution required for stability prediction. The static deflection of blades caused by centrifugal and gas loads also needs to be measured to improve the accuracy of performance analysis. This paper presents a new technique for making these measurements with small blade-mounted mirrors that reflect laser light once per revolution. For steady operation, in the absence of blade vibration, each mirror reflects a light beam to project a repeating spot on a display screen, once for every revolution of the rotor. However, when the blades are fluttering the reflected light moves from its stationary position and during successive revolutions describes the blade motion as a lissajous pattern. Vibration amplitude, phase, and frequency are discussed and related to analysis. Limits in accuracy and the importance of precise mode shape description for flutter analysis are presented.

Flutter of Swept Fan Blades

1985 ◽  
Vol 107 (2) ◽  
pp. 394-398
Author(s):  
R. E. Kielb ◽  
K. R. V. Kaza
Keyword(s):  
Detrimental Effect ◽  
Structural Model ◽  
Aerodynamic Loads ◽  
Unsteady Aerodynamic ◽  
Aeroelastic Analysis ◽  
Flutter Analysis ◽  
Blade Flutter ◽  
Fan Blade ◽  
Element Technique ◽  
Nonuniform Beam

The purpose of the research presented in this paper is to study the effect of sweep on fan blade flutter by applying the analytical methods developed for aeroelastic analysis of advanced turboprops. Two methods are used. The first method utilizes an approximate structural model in which the blade is represented by a swept, nonuniform beam. The second method utilizes a finite element technique to conduct modal flutter analysis. For both methods, the unsteady aerodynamic loads are calculated using two-dimensional cascade theories that are modified to account for sweep. An advanced fan stage is analyzed with 0, 15, and 30 deg of sweep. It is shown that sweep has a beneficial effect on predominantly torsional flutter and a detrimental effect on predominantly bending flutter. This detrimental effect is shown to be significantly destabilizing for 30 deg of sweep.

Mechansims for Wide-Chord Fan Blade Flutter

2009 ◽  
Author(s):  
Mehdi Vahdati ◽  
George Simpson ◽  
Mehmet Imregun
Keyword(s):  
Flow Separation ◽  
Pressure Wave ◽  
Geometric Features ◽  
The Third ◽  
Reflected Wave ◽  
Flutter Analysis ◽  
Parametric Studies ◽  
Blade Flutter ◽  
Stall Flutter ◽  
Fan Blade

This paper describes a detailed wide-chord fan blade flutter analysis with emphasis on flutter bite. The same fan was used with three different intakes of increasing complexity to explain flutter mechanisms. Two types of flutter, namely stall flutter and acoustic flutter, were identified. The first intake is a uniform cylinder for which there are no acoustic reflections. Only stall flutter, driven by flow separation, can exist in this case. The second intake, based on the first one, has a ‘bump’ feature to reflect the fan’s forward pressure wave at a known location so that detailed parametric studies can be undertaken. The analysis revealed a mechanism for acoustic flutter, which is driven by the phase of the reflected wave. The third intake has the typical geometric features of a flight intake. The results indicate that flutter bite occurs when both stall and acoustic flutter happen at the same speed. It is also found that blade stiffening has no effect on aero-acoustic flutter.

On the Importance of Engine-Representative Models for Fan Flutter Predictions

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Sina Stapelfeldt ◽  
Mehdi Vahdati
Keyword(s):  
Inlet Temperature ◽  
Pressure System ◽  
Blade Vibration ◽  
Cfd Models ◽  
Numerical Predictions ◽  
Reduced Frequency ◽  
Aerodynamic Properties ◽  
Acoustic Liner ◽  
Flutter Boundary ◽  
Fan Blade

Discrepancies between rig tests and numerical predictions of the flutter boundary for fan blades are usually attributed to the deficiency of computational fluid dynamics (CFD) models for resolving flow at off-design conditions. However, as will be demonstrated in this paper, there are a number of other factors, which can influence the flutter stability of fan blades and lead to differences between measurements and numerical predictions. This research was initiated as a result of inconsistencies between the flutter predictions of two rig fan blades. The numerical results agreed well with rig test data in terms of flutter speed and nodal diameter (ND) for both fans. However, they predicted a significantly higher flutter margin for one of the fans, while measured flutter margins were similar for both blades. A new set of flutter computations including the whole low-pressure system was therefore performed. The new set of computations considered the effects of the acoustic liner and mistuning for both blades. The results of this work indicate that the previous discrepancies between CFD and tests were caused by, first, differences in the effectiveness of the acoustic liner in attenuating the pressure wave created by the blade vibration and second, differences in the level of unintentional mistuning of the two fan blades. In the second part of this research, the effects of blade mis-staggering and inlet temperature on aerodynamic damping were investigated. The data presented in this paper clearly show that manufacturing and environmental uncertainties can play an important role in the flutter stability of a fan blade. They demonstrate that aeroelastic similarity is not necessarily achieved if only aerodynamic properties and the traditional aeroelastic parameters, reduced frequency and mass ratio, are maintained. This emphasizes the importance of engine-representative models, in addition to accurate and validated CFD codes, for the reliable prediction of the flutter boundary.

A Numerical Investigation of Aeroacoustic Fan Blade Flutter

2003 ◽  
Author(s):  
X. Wu ◽  
M. Vahdati ◽  
A. I. Sayma ◽  
M. Imregun
Keyword(s):  
Research Effort ◽  
Pressure Rise ◽  
Acoustic Properties ◽  
Pressure Amplitude ◽  
Ongoing Research ◽  
Blade Flutter ◽  
Fan Blade ◽  
Underlying Mechanisms ◽  
Flutter Margin

This paper reports the results of an ongoing research effort to explain the underlying mechanisms for aeroacoustic fan blade flutter. Using a 3D integrated aeroelasticity method and a single passage blade model that included a representation of the intake duct, the pressure rise vs. mass flow characteristic of a fan assembly was obtained for the 60%–80% speed range. A novel feature was the use of a downstream variable-area nozzle, an approach that allowed the determination of the stall boundary with good accuracy. The flutter stability was predicted for the 2 nodal diameter assembly mode arising from the first blade flap mode. The flutter margin at 64% speed was predicted to drop sharply and the instability was found to be independent of stall effects. On the other hand, the flutter instability at 74% speed was found to be driven by flow separation. Further post-processing of the results at 64% speed indicated significant unsteady pressure amplitude build-up inside the intake at the flutter condition, thus highlighting the link between the acoustic properties of the intake duct and fan blade flutter.

Flutter of Perforated Metallic Plates Repaired with Cross-Ply Composite Patches

2009 ◽  
Vol 417-418 ◽  
pp. 709-712
Author(s):  
Ali Amin Yazdi ◽  
Jalil Rezaeepazhand
Keyword(s):  
Laminated Composite ◽  
Material Anisotropy ◽  
Perforated Plates ◽  
Closed Form Solutions ◽  
Finite Element Software ◽  
Composite Patch ◽  
Numerical Studies ◽  
Flutter Analysis ◽  
Flutter Boundary ◽  
Metallic Plates

This study investigates the application of laminated composite patches for enhancement of flutter behavior of perforated metallic plates repaired with an external composite patch. Due to material anisotropy and discontinuity in geometry involved in flutter analysis of repaired plates, closed form solutions are practically unobtainable. Numerical studies using commercial finite element software were conducted to investigate the effects of variation in lamination parameters on the flutter boundary of perforated plates repaired with cross-ply composite patches. Both ply-level and sub-laminate level configurations are investigated. Presented results illustrate that flutter boundaries of perforated plates can be changed by choosing proper stacking sequence for composite patches.

Robust Flutter Analysis of an Airfoil with Flap Freeplay Uncertainty

2008 ◽  
Vol 33-37 ◽  
pp. 1247-1252 ◽  
Author(s):  
Zhi Chun Yang ◽  
Ying Song Gu
Keyword(s):  
Control Surface ◽  
Two Dimensional ◽  
Advanced Technique ◽  
Worst Case ◽  
Wing Model ◽  
Freeplay Nonlinearity ◽  
Flutter Analysis ◽  
Flutter Boundary ◽  
Nominal Parameter ◽  
Flutter Margin

Modern robust flutter method is an advanced technique for flutter margin estimation. It always gives the worst-case flutter speed with respect to potential modeling errors. Most literatures are focused on linear parameter uncertainty in mass, stiffness and damping parameters, etc. But the uncertainties of some structural nonlinear parameters, the freeplay in control surface for example, have not been taken into account. A robust flutter analysis approach in μ-framework with uncertain nonlinear operator is proposed in this study. Using describing function method the equivalent stiffness formulation is derived for a two dimensional wing model with freeplay nonlinearity in its flap rotating stiffness. The robust flutter margin is calculated for the two dimensional wing with flap freeplay uncertainty and the results are compared with that obtained with nominal parameter values. It is found that by considering the perturbation of freeplay parameter more conservative flutter boundary can be obtained, and the proposed method in μ-framework can be applied in flutter analysis with other types of concentrated nonlinearities.

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