Improving the Flutter Margin of an Unstable Fan Blade

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Sina Stapelfeldt ◽  
Mehdi Vahdati
Keyword(s):  
Steady Flow ◽  
Leading Edge ◽  
Measured Data ◽  
Optimization Approach ◽  
Speed Range ◽  
Final Design ◽  
Flutter Boundary ◽  
Fan Blade ◽  
Flutter Margin ◽  
Stagger Angle

The aim of this paper is to introduce design modifications that can be made to improve the flutter stability of a fan blade. A rig fan blade, which suffered flutter in the part-speed range and for which good quality measured data in terms of steady flow and flutter boundary is available, is used for this purpose. The work is carried out numerically using the aeroelasticity code AU3D. Two different approaches are explored: aerodynamic modifications and aero-acoustic modifications. In the first approach, the blade is stabilized by altering the radial distribution of the stagger angle based on the steady flow on the blade. The re-staggering patterns used in this work are therefore particular to the fan blade under investigation. Moreover, the modifications made to the blade are very simple and crude, and more sophisticated methods and/or an optimization approach could be used to achieve the above objectives with a more viable final design. This paper, however, clearly demonstrates how modifying the steady blade aerodynamics can prevent flutter. In the second approach, flutter is removed by drawing bleed air from the casing above the tip of the blade. Only a small amount of bleed (0.2% of the total inlet flow) is extracted such that the effect on the operating point of the fan is small. The purpose of the bleed is merely to attenuate the pressure wave that propagates from the trailing edge to the leading edge of the blade. The results show that extracting bleed over the tip of the fan blade can improve the flutter margin of the fan significantly.

Improving the Flutter Margin of an Unstable Fan Blade

2018 ◽  
Author(s):  
Sina Stapelfeldt ◽  
Mehdi Vahdati
Keyword(s):  
Steady Flow ◽  
Leading Edge ◽  
Measured Data ◽  
Optimization Approach ◽  
Speed Range ◽  
Final Design ◽  
Flutter Boundary ◽  
Fan Blade ◽  
Flutter Margin ◽  
Stagger Angle

The aim of this paper is to introduce design modifications which can be made to improve the flutter stability of a fan blade. A rig fan blade, which suffered from flutter in the part-speed range and for which good quality measured data in terms of steady flow and flutter boundary is available, is used for this purpose. The work is carried out numerically using the aeroelasticity code AU3D. Two different approaches are explored; aerodynamic modifications and aero-acoustic modifications. In the first approach, the blade is stabilized by altering the radial distribution of the stagger angle based on the steady flow on the blade. The re-staggering patterns used in this work are therefore particular to the fan blade under investigation. Moreover, the modifications made to the blade are very simple and crude and more sophisticated methods and/or an optimization approach could be used to achieve the above objectives with a more viable final design. This paper, however, clearly demonstrates how modifying the steady blade aerodynamics can prevent flutter. In the second approach, flutter is removed by drawing bleed air from the casing above the tip of the blade. Only a small amount of bleed (0.2% of the total inlet flow) is extracted such that the effect on the operating point of the fan is small. The purpose of the bleed is merely to attenuate the pressure wave which propagates from the trailing edge to the leading edge of the blade. The results show that extracting bleed over the tip of the fan blade can improve the flutter margin of the fan significantly.

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.

Numerical Analysis of Fan Transonic Stall Flutter

2014 ◽  
Author(s):  
Mizuho Aotsuka ◽  
Takeshi Murooka
Keyword(s):  
Navier Stokes ◽  
Test Results ◽  
Rotating Speed ◽  
Speed Range ◽  
Boundary Layer Interaction ◽  
Detached Shock Wave ◽  
Long Time ◽  
Flutter Boundary ◽  
Stall Flutter ◽  
Fan Blade

This paper describes numerical investigation of fan transonic stall flutter, especially focused on flutter bite. A transonic stall flutter occurs in high loaded condition at part rotating speed. A region of the transonic stall flutter occasionally protrudes to an operating line at narrow rotational speed range. This protrusion of flutter boundary is called flutter bite. In that case, it is necessary to re-design the blade for securing sufficient operating range. The re-design process might require some compromise on performance and/or weight, and takes long time. So it is important to understand the mechanism of the flutter bite. Two types of fan blade, one has a flutter bite and another dose not, are numerically studied with a 3D Navier Stokes CFD code. Numerical results show agreement with rig test results for the fans in qualitative sense. Detailed flow fields reveal that a detached shock wave and separation due to the shock boundary layer interaction play significant role for the flutter stability.

Composite Inlays Increase Flutter Resistance of Turbine Engine Fan Blades

1976 ◽  
Author(s):  
W. Troha ◽  
K. Swain
Keyword(s):  
Engine Performance ◽  
Leading Edge ◽  
High Strength ◽  
Turbine Engine ◽  
Performance Improvements ◽  
Response Characteristics ◽  
Vibrational Characteristics ◽  
Flutter Boundary ◽  
Fan Blade ◽  
Strength And Stiffness

A method for improving turbine engine blade flutter stability was recently demonstrated on a TF41 fan rig and engine. The concept uses the high strength and stiffness properties of B/SiC-titanium composite, bonded into the leading edge tip section of a titanium fan blade, to control the blade vibrational characteristics. Comparisons are made between the blade response characteristics with and without the composite inlay for blade natural frequency, untwist due to rotational speed, and shift in flutter boundary. Engine performance improvements due to shroud removal are also shown.

Numerical Study on Aeroelastic Instability for a Low Speed Fan

2016 ◽  
Author(s):  
Kuen-Bae Lee ◽  
Mark Wilson ◽  
Mehdi Vahdati
Keyword(s):  
Numerical Study ◽  
Pressure Ratio ◽  
Three Dimensional ◽  
Computational Domain ◽  
Low Speed ◽  
Acoustic Liners ◽  
Significant Difference ◽  
Flutter Boundary ◽  
Fan Blade ◽  
Flutter Margin

Over recent years engine designs have moved increasingly toward low specific thrust cycles to deliver significant specific fuel consumption (SFC) improvements. Such fan blades are more loaded than conventional fan blades and therefore can be more prone to aerodynamic and aeroelastic instabilities. The aim of this paper is to analyse the flutter stability of a low speed/low pressure ratio fan blade. By using a validated CFD model (AU3D), three dimensional unsteady simulations are performed for a modern low speed fan rig for which extensive measured data are available. The computational domain contains a complete fan assembly with an intake duct and the downstream OGVs (whole LP domain). Flutter simulations are conducted over a range of speeds to understand flutter characteristics of this blade. Only the 1F (first flap) mode is considered in this work. Measured rig data obtained by using the same fan set but with two different intakes showed a significant difference in the flutter boundary for the two intakes. AU3D computations were performed for both intakes and were used to explain this difference between the two intakes, and showed that intake reflections play an important role in flutter of this blade. In the next phase of this work, two possible modifications for increasing the flutter margin of the fan blade were explored: 1. Changing the mode shape of the blade 2. Using acoustic liners in the casing The results show that it is possible to increase the flutter margin of the blade by either decreasing the ratio of the twisting to plunging motion in 1F mode or by introducing deep acoustic liners in the intake. The liners have to be deep enough to attenuate the flutter pressure waves and hence influence the stability. The results indicate the importance of reflection in flutter stability of the fan blade, and clearly show that intake duct needs to be included in flutter study of any fan blade.

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.

scholarly journals Secondary Flow Control in Low Aspect Ratio Vanes Using Splitters

2016 ◽  
Author(s):  
Christopher Clark ◽  
Graham Pullan ◽  
Eric Curtis ◽  
Frederic Goenaga
Keyword(s):  
Kinetic Energy ◽  
Aspect Ratio ◽  
Secondary Flow ◽  
Current Practice ◽  
Leading Edge ◽  
Horseshoe Vortex ◽  
Secondary Flows ◽  
Optimization Approach ◽  
Flow Strength ◽  
Low Aspect Ratio

Low aspect ratio vanes, often the result of overall engine architecture constraints, create strong secondary flows and high endwall loss. In this paper, a splitter concept is demonstrated that reduces secondary flow strength and improves stage performance. An analytic conceptual study, corroborated by inviscid computations, shows that the total secondary kinetic energy of the secondary flow vortices is reduced when the number of passages is increased and, for a given number of vanes, when the inlet endwall boundary layer is evenly distributed between the passages. Viscous computations show that, for this to be achieved in a splitter configuration, the pressure-side leg of the low aspect ratio vane horseshoe vortex, must enter the adjacent passage (and not “jump” in front of the splitter leading edge). For a target turbine application, four vane designs were produced using a multi-objective optimization approach. These designs represent: current practice for a low aspect ratio vane; a design exempt from thickness constraints; and two designs incorporating splitter vanes. Each geometry is tested experimentally, as a sector, within a low-speed turbine stage. The vane designs with splitters geometries were found to reduce the measured secondary kinetic energy, by up to 85%, to a value similar to the design exempt from thickness constraints. The resulting flowfield was also more uniform in both the circumferential and radial directions. One splitter design was selected for a full annulus test where a mixed-out loss reduction, compared to the current practice design, of 15.3% was measured and the stage efficiency increased by 0.88%.

Development and Assessment of a Total Resistance Optimized Bow for the AE 36

1994 ◽  
Vol 31 (02) ◽  
pp. 149-160
Author(s):  
Donald C. Wyatt ◽  
Peter A. Chang
Keyword(s):  
Experimental Data ◽  
Optimization Procedure ◽  
Wave Resistance ◽  
Scale Model ◽  
Optimization Approach ◽  
Nonlinear Constraints ◽  
Total Resistance ◽  
Hull Form ◽  
Final Design ◽  
Resistance Data

A numerically optimized bow design is developed to reduce the total resistance of a 23 000 ton ammunition ship (AE 36) at a speed of 22 knots. An optimization approach using slender-ship theory for the prediction of wave resistance is developed and applied. The new optimization procedure is an improvement over previous optimization methodologies in that it allows the use of nonlinear constraints which assure that the final design remains within practical limits from construction and operational perspectives. Analytic predictions indicate that the AE 36 optimized with this procedure will achieve a 40% reduction in wave resistance and a 33% reduction in total resistance at 22 knots relative to a Kracht elliptical bulb bow design. The optimization success is assessed by the analysis of 25th scale model resistance data collected at the David Taylor Research Center deepwater towing basin. The experimental data indicate that the optimized hull form yields a 51% reduction in wave resistance and a 12% reduction in total resistance for the vessel at 22 knots relative to the Kracht bulb bow design. Similarly encouraging results are also observed when comparisons are made with data collected on two other conventionally designed AE 36 designs.

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.

scholarly journals Redesigning a compressor inlet guide vane for 0 to 60 degree stagger angles

2021 ◽  
Author(s):  
Stephanie Waters
Keyword(s):  
Fuel Consumption ◽  
Guide Vane ◽  
Leading Edge ◽  
Aircraft Engine ◽  
Inlet Guide Vane ◽  
Pressure Loss Coefficient ◽  
Compressor Inlet ◽  
Curvature Distribution ◽  
Stagger Angle ◽  
Total Pressure Loss Coefficient

This report's objective is to reduce the total pressure loss coefficient of an inlet guide vane (IGV) at high stagger angles and to therefore reduce the overall fuel consumption of an aircraft engine. IGVs are usually optimized for cruise where the stagger angle is approximately 0 degrees. To reduce losses, four different methodologies were tested: increasing the leading edge radius, increasing the camber, creating a "drooped nose", and creating an "S" curvature distribution. A baseline IGV was chosen and modified using these methodologies to create 10 new IGV designs. CFX was used to perform a CFD analysis on all 11 IGV designs at 5 stagger angles from 0 to 60 degrees. Typical missions were analyzed and it was discovered that the new designs decreased the fuel consumption of the engine. The IGV with the "S" curvature and thicker leading edge was the best and decreased the fuel consumption by 0.24%.

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