Flutter Stability Research Articles

Computational study of intake duct effects on fan flutter stability

Computational study of intake duct effects on fan flutter stability

Grating effect on flutter instability

Flutter stabilization of girders should be one of the major subjects for long-span bridges. In the former studies by authors, various geometric-shaped sections have been studied from the viewpoint of flutter stabilization, such as elliptical section, rhombus section, trapezoidal section and so on. Recently, the aerodynamic characteristics of grating-installed bridge decks have been focused for future super long-span bridges. However, systematic and detailed studies on the efficiency of grating on flutter stabilization are inadequate. For example, it remains unsolved yet in terms of flutter characteristics in relation with their opening ratio (OR) and their location of grating installed on bridge deck. The aim of this paper is to clarify the influence of the opening ratio and the location of grating for plate-like bridge deck on flutter instabilities based upon their aerodynamic derivatives and flutter analysis. A series of wind tunnel tests have been conducted for various grating conditions, such as changing the OR in the range of 0%, 20%, 40%, 60%, 80% and 100%, and their locations. Furthermore, a “step-by-step” analysis is applied to clear the role of each aerodynamic derivative on flutter instability and flutter mechanism. Depending on the grating conditions on the bridge deck, peculiar flutter type was observed in addition to the conventional torsional branch-coupled flutter. It was the hybrid branches-coupled flutter, that is with both heaving- and torsional-branch instability at a certain high-reduced velocity range. These particular flutter instabilities were discussed according to their aerodynamic derivatives.

Aeroelastic stability analysis of a bird-damaged aeroengine fan assembly

Bird strike is a major consideration when designing fan blades for large-diameter aeroengines. Current methods rely on impact tests and structural optimisation but it is highly desirable to have predictive numerical models to assess the aerodynamic and aeroelastic stability of bird-damaged fan assemblies. The aim of this paper is to present such a methodology and to study a representative case. The particular fan assembly under investigation contained two consecutive blades with unequal impact damage, the so-called heavy-damage and medium-damage blades. A detailed finite element analysis of the dynamic behaviour revealed that the vibration modes were significantly different from those of the tuned assembly. The twin modes were found to be split into single modes, some with highly distorted modeshapes, the so-called rogue modes. A nonlinear viscous flow analysis revealed truly unsteady effects and time-accurate aeroelasticity analyses with vibratory blade motion were undertaken to investigate the flutter stability. The computational domain included both a whole-annulus fan assembly and an intake duct and the resulting mesh contained approximately 2,200,000 grid points. The investigation was conducted for two points on the compressor characteristic, the first one corresponding to higher mass flow/lower pressure ratio and the second one to lower mass flow/higher pressure ratio. At the higher mass flow point, the flow separation was restricted to the immediate surrounding passages and the forcing onto the downstream blades was relatively small. However, a rotating stall event was observed for the lower mass flow point and the subsequent unsteady aerodynamic forces on the blade were high. At both mass flow settings, the flutter stability of the damaged fan assembly was predicted to be worse than that of the undamaged reference assembly.

Improving the Wind Stability of Suspension Bridges during Construction

The wind stability of suspension bridges in the construction phase can often be more problematic than in the final state. In this paper, numerical wind stability analyses are performed for the erection process of both state-of-the-art (the Hoga Kusten Bridge) and hypothetical 3,000-m-span box-girder suspension bridges. Measures to improve the wind stability response in the erection phase are discussed. In particular, it is seen that the provision of an eccentric windward mass does not cause any significant improvement on the flutter stability limits for the analyzed case, but the selection of an alternative nonsymmetric erection sequence or the use of stationary aerodynamic appendages are seen to be more effective. Comparatively, assuming that the design wind speed for the construction stage is lower than for the final state, the response of the 3,000-m-span bridge is less problematic in the construction phase than in the final state.

Flutter stabilization and heaving-branch flutter

Abstract This paper aims to clarify the flutter mechanism for super-long span bridges. A series of wind tunnel tests are conducted against the fundamental girder sections, such as flat diamond section, twin rectangular cylinder and rectangular cylinder with vertical plates installed at the mid-chord points. The former two sections indicate relatively stable aerodynamic properties against the conventional coupled flutter, namely for torsional branch. The main reason for flutter stabilization of flat diamond section is the flow separations at two different points on the side-surface of the body. Regarding twin rectangular cylinder, the opening space and the turbulent flow induced in the upstream side has a large contribution to the flutter characteristics. On the other hand, rectangular cylinder with VP indicates coupled flutter for heaving branch. Mainly based upon aerodynamic derivatives and flutter analysis, this paper clarifies that the extremely small value of A ∗ 2 can cause this type of flutter instabilities.

Effect of modal damping on bridge aeroelasticity

Abstract Estimation of modal damping is always a critical first step in the analytical estimation of the flutter and buffeting reponses of long-span suspended bridges. The impact of under or over estimation of modal damping on the aerodynamic and aeroelastic behavior of suspended bridges is not clear. This issue surfaced when a seismic retrofit of the Golden Gate birdge using viscous dampers was proposed. The affect of these viscous dampers on the flutter and buffeting response of the Golden Gate bridge needed investigation. This paper considers the affect of additional damping imposed by these viscous dampers on the aeroelastic and aerodynamic response to wind-induced excitation. This mechanical damping was introduced into the aeroelastic system as additional modal damping. A multi-mode flutter and buffeting analysis was performed for several levels of modal damping. The results of the analysis revealed that additional damping decreased the buffeting response whereas the effect in flutter stability was more complex. At different levels of damping not only did the critical flutter velocity change but also the angle of attack at which it occurred.

Flutter Stability of Very Long Suspension Bridges

As longer suspension bridges are being built and planned around the world, many feasibility problems arise. Experiences related with preliminary studies on the Gibraltar Bridge are presented. Flutter is considered to be the main feasibility problem for a very-long-span bridge, and it is analyzed in this paper for span lengths ranging from 1,600 to 6,000 m. As conventional deck designs present unacceptably low critical velocities, several alternative suspension schemes are analyzed to determine their qualities to prevent flutter instability. These systems use a variable number of main cables (from one to four) and hangers arranged in such a way as to produce the highest stiffening effects on the statical system. Flutter is analyzed by means of a multimodal finite-element-based method, and critical conditions are defined by observation of the U-f diagrams. The crossed-hangers system is chosen as the simplest way to improve flutter stability, although the results of this study indicate that it is useful only for very long spans.

Aeroelastic Analysis of Advanced Geometry Tiltrotor Aircraft

A tiltrotor model is formulated to predict the dynamics and aeroelastic stability of composite-coupled advanced geometry rotors. Advanced geometry rotor blades are modeled with arbitrary sweep, anhedral, pretwist and planform taper variation with span. The effect of advanced geometry blades on the tiltrotor response and whirl flutter stability in airplane mode is studied. Blade tip sweep and tip droop increase the flutter speed while forward tip sweep, tip anhedral and planform taper decrease it. Predicted wing beam mode damping correlates satisfactorily with test data.

Fluid Force and Flutter Stability of a Pitching Hydrofoil in Subcavitation Region.

The torsional flutter boundary of a pitching hydrofoil was experimentally investigated in the subcavitation region. Unsteady lift and moment were measured using load cells installed in a torsional vibration apparatus. The type and the degree of separation on the hydrofoil exert much influence on the unsteady fluid force and the flutter boundary in the subcavitation region. In the absence of separation, no unstable range of the flutter exists in the noncavitation or the subcavitation region. In the noncavitation region, the flutter boundary extends to the higher reduced frequency side as the angle of attack increases. In the subcavitation region, these flutter boundaries remain until the cavity length reaches the mid-chord. However, the flutter boundary shifts towards the higher reduced frequency side, keeping the width of the unstable range almost constant as the cavity length extends from the mid-chord to the full chord.

The Influence of Composite Induced Couplings on Tiltrotor Whirl Flutter Stability

The Influence of Composite Induced Couplings on Tiltrotor Whirl Flutter Stability

An application of the ICED‐ALE methodology to integrated non‐linear aeroelasticity analyses

Presents a finite element/volume method for non‐linear aeroelasticity analyses of turbomachinery blades. The method uses an Arbitrary Lagrangian‐Eulerian (ALE) kinematical description of the fluid domain, in which the grid points can be displaced independently of the fluid motion. In addition, it employs an iterative implicit formulation similar to that of the Implicit‐continuous Eulerian (ICE) technique, making it applicable to flows at all speeds. A deforming mesh capability that can move the grid to conform continuously to the instantaneous shape of an aeroelastically deforming body without excessive distortion is also included in the algorithm. The unsteady aerodynamic loads are obtained using inviscid Euler equations. The model for the solid is general and can accommodate any spatial or modal representation of the structure. Determines the flutter stability of the system by studying the aeroelastic time response histories which are obtained by integration of the coupled equations of motion for both the fluid and the structure. Develops and demonstrates in 2D the formulation, which includes several corrections for better numerical stability. The cases studied include NACA64A006 and NACA0012 aerofoils and the EPFL Configuration 4 cascade. Finds the results from the numerical indicate good overall agreement with other published work and hence demonstrates the suitability of an ICED‐ALE formulation for turbomachinery applications.

Flutter stability of a detuned cascade in subsonic compressible flow

A mathematical model is developed to predict the unsteady aerodynamics of a detuned two-dimensional flat plate cascade in subsonic compressible flow. Aerodynamic detuning is introduced by nonuniform circumferential spacing and chordwise offset. Combined aerodynamic-structural detuning is accomplished by replacing alternate airfoils with splitter blades. A torsion mode stability analysis that considers aerodynamic and combined aerodynamic-structural detuning is developed by combining the unsteady aerodynamic model with a single degreeof-freedom structural model. The effect of these detuning techniques on flutter stability is then demonstrated by applying this model to a baseline unstable 12-bladed rotor and detuned variations of this rotor. This study demonstrates that detuning is a viable passive flutter control technique. Nomenclature A = cascade A airfoil index a — freestream speed of sound B - cascade B airfoil index CL = lift coefficient CM - moment coefficient C, = ratio of chord length of cascade B to cascade A CA = chord length of cascade A CB = chord length of cascade B ea = elastic axis location k - reduced frequency a>c/W n = airfoil index os - chordwise offset of cascade B relative to cascade A

Prediction of Flutter Stability Using Aeroelastic Frequency Response Functions

This paper deals with the flutter stability of turbomachinery blading using aeroelastic frequency response functions (FRFs) which are obtained by inverting the dynamic stiffness matrix of the aeroelastic system. The structural model is a lumped parameter representation while the aerodynamic model is based on linearized 2D cascade theories for subsonic and supersonic flows. The advantages of identifying the aeroelastic modes via a rational fraction modal analysis of the aeroelastic FRFs rather than using a standard complex eigensolution are discussed in some detail. It is found that significant natural frequency and mode shape differences can exist between the structural and aeroelastic systems, a characteristic which must be considered duly when predicting the flutter stability. The consequences of changing the elastic axis position were discussed in the case of a 12-bladed disk and it was found that flutter occurred mainly in torsion for the system studied. Finally, the effects of random and alternate mistuning on flutter stability were also investigated using the same model and it was found that mistuning had a stabilizing effect in some, but not all, cases.

A Linearized Euler Analysis of Unsteady Transonic Flows in Turbomachinery

A computational method for efficiently predicting unsteady transonic flows in two-and three-dimensional cascades is presented. The unsteady flow is modeled using a linearized Euler analysis whereby the unsteady flow field is decomposed into a nonlinear mean flow plus a linear harmonically varying unsteady flow. The equations that govern the perturbation flow, the linearized Euler equations, are linear variable coefficient equations. For transonic flows containing shocks, shock capturing is used to model the shock impulse (the unsteady load due to the harmonic motion of the shock). A conservative Lax–Wendroff scheme is used to obtain a set of linearized finite volume equations that describe the harmonic small disturbance behavior of the flow. Conditions under which such a discretization will correctly predict the shock impulse are investigated. Computational results are presented that demonstrate the accuracy and efficiency of the present method as well as the essential role of unsteady shock impulse loads on the flutter stability of fans.

Flutter of Hybrid Laminated Flat Panels with Simply Supported Edges in Supersonic Flow

Flutter of hybrid laminated flat panels in supersonic flow is studied by using first order shear deformation theory in conjunction with the assumed mode method. Both the quasi-static approximation and piston theory are used for aerodynamic force calculations at supersonic speeds. The flutter stability boundaries are determined by using the frequency coalescence criterion with the quasi-static approximation and Movchan-Krumhaar's criterion with the piston theory aerodynamics. Numerical calculations are presented for hybrid laminates consisting of graphite, Kevlar and glass fibres in an epoxy matrix. The effects of hybridization, shear deformation, ply orientation and aspect ratio are studied. The critical dynamic pressure parameter of a hybrid laminate lies between the values for laminates made with all plies of higher stiffness and with all plies of lower stiffness, respectively. The role of aerodynamic damping is found to be particularly important in determining the aeroelastic stability boundaries of laminated composite panels. Shear flexibility reduces the critical dynamic pressure parameter, but the reduction is insignificant for thin panels.

Aeroelastic stability of cascades in turbomachinery

State-of-the-art prediction of the aeroelastic stability of cascades in axial-flow turbomachines is reviewed. The first main chapter of the article presents a comprehensive formulation of the two- and three-dimensional classical (unstalled) flutter problem of tuned and mistuned rotor blade rows and bladed disc assemblies. Within the framework of linearized analysis, a complete and generalized theory in modal form is outlined, comprising the various formulations of the cascade flutter problem distributed in fragments throughout the literature. Brief outlines are also made of recent advances in unsteady aero-dynamic methods for turbomachinery aeroelastic applications. The second main chapter contains a parametric study of the classical flutter stability characteristics of compressor and turbine cascades in subsonic and supersonic flow. Stability boundaries and dominant trends in flutter behaviour are outlined, and the significant effects of blade mistuning on the aeroelastic stability of turbomachine bladings are highlighted.

Parametric Study of the Flutter Stability of a Semi-Rigid 3-D Wing-With-Engine Nacelle Model in Subsonic Flow

A parametric investigation is performed of the aeroelastic flutter stability behaviour of a semi-rigid 3-D wing-with-engine nacelle model in subsonic flow. The system under investigation is a wind tunnel model that was flutter tested some years ago. It consists of a swept-back half wing with a pylon-mounted engine nacelle, representative of modern large transport aircraft, and is elastically restrained at its wing root, so that it may execute decoupled (rigid body) rolling and pitching oscillations about two orthogonal axes. For this binary aeroelastic system, first the equations of motion and then the aeroelastic stability equations are set up in terms of generalized coordinates. In addition to the basic wind tunnel model configuration, two artificial configurations with other positions of the rotation axes and corresponding mode shapes are investigated. For the computation of the motion-induced generalized airloads, a panel technique is used for both the wing and the engine nacelle that is replaced by an annular wing. Numerical results are presented for several systematic parameter variations and Mach numbers, where special emphasis is placed on the effects of the motion-induced unsteady airloads acting on the engine nacelle, the position of the rotation axes, and the frequency ratio of the two modes in roll and pitch. Moreover, a comparison is made with some wind tunnel test results.

Supersonic turbomachine rotor flutter control by aerodynamic detuning

A mathematical model is developed to analyze the flutter stability characteristics of an aerodynamically detuned rotor operating in a supersonic inlet flowfield with a supersonic axial component. Alternate-blade aerodynamic detuning is considered, accomplished by alternating the circumferential spacing of adjacent rotor blades. The unsteady aerodynamics are determined by developing an influence coefficient technique which is appropriate for both aerodynamically tuned and detuned rotor configurations. The effects of this detuning on the flutter stability characteristics of supersonic axial flow rotors are then demonstrated by applying this model to baseline 12-bladed rotors. Results show that, dependent on the specific blade row and flowfield geometry, alternate blade aerodynamic detuning is a viable flutter control mechanism for supersonic through-flow rotors.

The Effect of Steady Aerodynamic Loading on the Flutter Stability of Turbomachinery Blading

An aeroelastic analysis is presented that accounts for the effect of steady aerodynamic loading on the aeroelastic stability of a cascade of compressor blades. The aeroelastic model is a two-degree-of-freedom model having bending and torsional displacements. A linearized unsteady potential flow theory is used to determine the unsteady aerodynamic response coefficients for the aeroelastic analysis. The steady aerodynamic loading was caused by the addition of (1) airfoil thickness and camber and (2) steady flow incidence. The importance of steady loading on the airfoil unsteady pressure distribution is demonstrated. Additionally, the effect of the steady loading on the tuned flutter behavior and flutter boundaries indicates that neglecting either airfoil thickness, camber, or incidence could result in nonconservative estimates of flutter behavior.

Aerodynamically forced response and flutter of structurally mistuned bladed disks in subsonic flow

To analyze the aeroelastic characteristics of structurally mistuned rotors, both flutter and forced response, a structural dynamics model appropriate for flexible bladed disks is developed utilizing flexible disk theory and small perturbation gust and motion-induced unsteady aerodynamic analyses. This model is then used to investigate the effects of structural mistuning on rotor aeroelasticity. Mistuning is found to be beneficial for flutter stability, with the motion-induced unsteady aerodynamics and thus the stability somewhat dependent on the far field acoustic wave behavior generated by the blade motion, as indicated by the acoustic resonance conditions. With regard to forced response, mistuning is shown to be generally detrimental. Nomenclature