Flutter Oscillations Research Articles

Effect of impedance boundary-controlled casing treatment on performance of a fan subjected to inlet swirls

An experimental investigation is conducted to evaluate the performance and the stalling process of a fan subjected to inlet swirls, as well as the effectiveness of an Impedance Boundary-Controlled (IBC) Casing Treatment (CT) on the stall margin recovery. An operating cycle is proposed based on the hysteresis effect of harmonic flap oscillation of airfoils and parallel compressor theory to explain the pressure characteristic of the fan under twin swirl inlets. Twin swirls are observed to reduce the stall margin of the fan, and the circumferential location where the spike is detected turns to the intersection area of the twin swirl. The IBC CT is proven to extend the stall margin of the fan for 12.7%–22.3% when subjected to inlet swirls with an efficiency loss of around 1%. The IBC CT helps to reduce the size of the operating cycle of the fan by redistributing the blade loading and adding the system damping to dissipate the perturbation energy.

A RARE CASE OF TYPE A AORTIC DISSECTION PRESENTING AS ACUTE STROKE: A DIAGNOSTIC CHALLENGE

Abstract A 60–year–old woman presented to the emergency department with acute left hemiplegia, not chest pains. Past medical history was significant for family history of aortic aneurysm, arterial hypertension. At brain–CT was reported hypodensity areas in posterior arm of the right internal capsule and in right tempero–polar lobe, confirmed on brain–MRI, and compatible with acute ischemic injury. Echocolor–dopper of epiaortic vessels documentated ulcerated plaque into the right common carotid artery (CCA). Selective catheterization found focal dissection obstructing the flow, treated with self–expanding stent. On echocardiography evalutation for suspiction of TAAD: dilatation of ascending aorta with flap oscillation, severe arotic regurgitation, pericardial effusion (Fig. 1). CT–Angiography was performed with evidence of type A dissection extending superiorly to the right CCA and inferiorly to the iliac bifurcation. For coma state, the patient was mechanically ventilated, and seriate CTs showed new ischemic lesions and hemorrhagic infarction of previous. Because of contraindication for cardiac surgery due neurological state, the patient was referred at medical therapy as bridge to hybrid aortic arc substitution at resolution of intracranial hemorrhages. In the next days there was progressive improvement neurological status with restoration of sensory and autonomic breathing with hemodynamic stability. Aortic dissection (AD) is defined as a disruption of the medial layer leading to the formation of two lumens separated by an intimomedial flap. AD is a critical diagnosis to make due to life–threatening outcomes and its non–specific presentation as in the case of our patient where the management of AD was delayed due to an unusual initial clinical presentation. Multimodality imaging approach has been fundamental for correct diagnosis and risk stratification. A limited number of studies have reported cases with stroke as the first symptom with concomitant TAAD. Concomitant intracerebral hemorrhage and TAAD is a very rare presentation and the optimal timing of surgery in these patients is unknown. In our opinion, patients who presents acute stroke should be carefully evaluated using multimodal imaging to exclude TAAD before administering reperfusion therapy. In addition, performing CAS before surgery for TAAD is challenging but might be a valid treatment option to maintain good brain perfusion and improve prognosis, but further investigations are needed.

Transonic Flutter of Skin Panels Pinned at the Center

This work is a numerical and experimental study of transonic panel flutter. Two configurations were explored: clean and pinned geometry. The former is a panel clamped on all sides, while the configuration with pin also includes an additional constraint in the middle of the panel. This case study was inspired by the skin-panel design used for Skylon, a single-stage-to-orbit concept vehicle. In Skylon, a series of discrete supports are used to stiffen thinner skin panels to maximize the payload; thus, there is a need to explore the dynamic stability of this configuration also in the transonic regime. To this end, a panel clamped on all sides and with a pin in the center was tested in the transonic wind tunnel, where time-accurate displacement measurements were performed using laser sensors. From the analysis of experiments and numerical simulations, transonic flutter appeared to be a one-way type of interaction, where the aerodynamic pressure characteristic is not affected by the structure deformation. This is due to small panel displacements, typically two orders of magnitude smaller than the panel thickness. Higher natural modes, with a frequency close to the aerodynamic fluctuating pressure, are directly excited. Generally, the additional constraint in the center does not seem to add significant benefits in terms of dynamic stability in the transonic regime. A numerical parametric study showed that for external Mach numbers ranging approximately between 0.8 and 1.2, flutter oscillation amplitude is the largest for low Mach speeds.

Performance characterization and modeling of an oscillating surge wave energy converter

Testing wave energy converters in the ocean could be expensive and complex, which necessitates the use of numerical modeling. However, accurately modeling the response of wave energy converters with high-fidelity simulations can be computationally intensive in the design stage where different configurations must be considered. Reduced-order models based on simplified equations of motion can be very useful in the design, optimization, or control of wave energy converters. Given the complex dynamics of wave energy converters, accurate representation, and evaluation of relative contributions by different forces are required. This effort is concerned with a performance characterization of the hydrodynamic response of an oscillating surge wave energy converter that is based on a reduced-order model. A state-space model is used to represent the radiation damping term. Morison’s representation of unsteady forces is used to account for the nonlinear damping. Wave tank tests are performed to validate simulations. A free response simulation is used to determine the coefficients of the state-space model. Torque-forced simulations are used to identify the coefficients of the nonlinear damping term for different amplitudes and wave frequencies. The impact of varying these coefficients on the response is investigated. An assessment of the capability of the model in predicting the hydrodynamic response under irregular forcing is performed. The results show that the maximum error is 3% when compared with high-fidelity simulations. It is determined that the nonlinear damping is proportional to the torque amplitude and its effects are more pronounced as the amplitude of the flap oscillations increases.

Two-dimensional pressure field imaging of an elastic panel executing post-flutter oscillations

Two dimensional pressure field was acquired at 10 kHz to study the post flutter oscillations of a thin elastic panel placed beneath a Mach 2.5 turbulent boundary layer. The panel was made of aluminum and is secured to the mounting fixture using a collection of rivets, which resulted in a boundary condition that was between both ideally clamped and pinned boundaries. Direct comparison of the mean and unsteady pressure fields were made for the panel executing post flutter oscillations and oscillations away from the flutter boundary. Whereas the mean pressure fields were largely similar during and away from post-flutter oscillations, the unsteady pressure fields showed a significant increase in the pr.m.s. during post flutter oscillations. The spectral content of the pressure oscillations and panel oscillations revealed that the tonal aeroelastic frequency dominate the post flutter oscillations. This tonal frequency was determined to lie at the close vicinity of the (2,1) panel elastic mode. The r.m.s. panel deflection field during post flutter oscillations were also reconstructed from the unsteady pressure fields and the reconstructed panel deflection also corresponded to the (2,1) elastic mode.Further coherence and cross-correlation analyses provided insights into possible mechanisms that control the transition of the panel oscillations away from the flutter boundary. The analyses suggest that the transition away from the flutter boundary is possibly initiated by the decoherence of the organized pressure field during the post flutter region by the turbulent boundary layer.

Bio-inspired variable-stiffness flaps for hybrid flow control, tuned via reinforcement learning

A bio-inspired, passively deployable flap attached to an airfoil by a torsional spring of fixed stiffness can provide significant lift improvements at post-stall angles of attack. In this work, we describe a hybrid active–passive variant to this purely passive flow control paradigm, where the stiffness of the hinge is actively varied in time to yield passive fluid–structure interaction of greater aerodynamic benefit than the fixed-stiffness case. This hybrid active–passive flow control strategy could potentially be implemented using variable-stiffness actuators with less expense compared with actively prescribing the flap motion. The hinge stiffness is varied via a reinforcement-learning-trained closed-loop feedback controller. A physics-based penalty and a long–short-term training strategy for enabling fast training of the hybrid controller are introduced. The hybrid controller is shown to provide lift improvements as high as 136 % and 85 % with respect to the flapless airfoil and the best fixed-stiffness case, respectively. These lift improvements are achieved due to large-amplitude flap oscillations as the stiffness varies over four orders of magnitude, whose interplay with the flow is analysed in detail.

Coupled fluid-structure simulations and experiments of DBD plasma actuator for damping of plate vibrations

In the present work, an experimental and numerical study has been performed to evaluate the suitability of the dielectric barrier discharge plasma actuators to reduce free vibrations of plates. Two configurations composed of two different plexiglass plates and identical actuators were manufactured. Experimental campaigns, in terms of laser doppler anemometer velocity measurements, flow visualization and proper orthogonal decomposition were carried out to characterize the plasma actuator and to validate the numerical models. The plasma-induced virtual body force for the numerical analysis has been calibrated based on the experimental body force and the induced flow field. Then numerical studies of the plasma effect on the flow around an oscillating plate was performed. An experimental modal analysis has been also performed to understand the vibration characteristics of the plate and to support the numerical study. A reduced-order model of the system, based on the harmonic oscillator, was developed to properly design an open loop controller (system natural frequency, amplitude and timing of the attenuation force). Eventually, the work demonstrates that the control strategy based on the plasma actuation is effective in reducing the plate vibrations. However, a deeper study and a more sophisticated control strategy would be required to significantly mitigate the flutter oscillations of airfoils in real applications.

Numerical investigations of the flow control effect on a thick wind turbine airfoil using deformable trailing edge flaps

Numerical investigations have been carried out regarding the aerodynamic performance of deformable trailing edge flaps (DTEFs) to reduce the unsteady loads on the FFA-W3-241 airfoil under dynamic stall conditions. Simulations were conducted within the framework of OpenFOAM using Reynolds-averaged Navier-Stokes (RANS) approach with the SST k-ω turbulence model and dynamic mesh technique. The control effects of the DTEF size and deflection angle on the stationary airfoil were evaluated through comparative calculations. Then, comprehensive simulations were conducted to gain a deeper insight into the aerodynamic characteristics of a combined simultaneous airfoil-DTEF oscillating motion. Effects of the flap size, oscillation amplitude, frequency, and phase shifts on the airfoil lift and drag hysteresis loops have been analyzed. When the DTEFs oscillate in the same frequency as the airfoil pitch motion and with the phase shift angle of π, the dynamic load can be effectively alleviated. Under this condition, with the appropriate arrangement of flap size and oscillation amplitude, reduction of the lift fluctuations during dynamic motion can be achieved by a maximum of 42.78%, and the corresponding lift-to-drag ratio can also be regulated within a small variation range at a relatively high level.

Using scientific machine learning for experimental bifurcation analysis of dynamic systems

Augmenting mechanistic ordinary differential equation (ODE) models with machine-learnable structures is a novel approach to create highly accurate, low-dimensional models of engineering systems incorporating both expert knowledge and reality through measurement data. Our exploratory study focuses on training universal differential equation (UDE) models for physical nonlinear dynamical systems with limit cycles: an aerofoil undergoing flutter oscillations and an electrodynamic nonlinear oscillator. We consider examples where training data is generated by numerical simulations, whereas we also employ the proposed modelling concept to physical experiments allowing us to investigate problems with a wide range of complexity. To collect the training data, the method of control-based continuation is used as it captures not just the stable but also the unstable limit cycles of the observed system. This feature makes it possible to extract more information about the observed system than the open-loop approach (surveying the steady state response by parameter sweeps without using control) would allow. We use both neural networks and Gaussian processes as universal approximators alongside the mechanistic models to give a critical assessment of the accuracy and robustness of the UDE modelling approach. We also highlight the potential issues one may run into during the training procedure indicating the limits of the current modelling framework.

Active flow control of a wing section in stall flutter by dielectric barrier discharge plasma actuators

This paper investigates the potential of using an active flow control technique to modify stall flutter oscillations of a NACA (National Advisory Committee for Aeronautics) 0015 wing section. Wind tunnel experiments have been performed with a test-rig that provides the elastic degree of freedom in pitch. Measurements of the clean airfoil are taken at preset angles of θ0=6°−12°, and for Reynolds numbers of Rec=6.2×104−1.25×105, which reveal the dependency of the stall flutter oscillations to Rec and θ0. Then, flow control experiments are carried out at θ0=10° and Rec=1.04×105. Two dielectric barrier discharge plasma actuators have been employed simultaneously to exert dual-point excitation to the baseline flow. It is shown that during the upstroke half-cycle, plasma actuation postpones the dynamic stall of the airfoil and increases the maximum pitch angle of the stall flutter cycle. On the downstroke, dual-point excitation effectively improves the rate of pitching moment recovery and leads to flow reattachment at a larger pitch angle. Normalized excitation frequencies Fex=fex/fw (where fw is the wake mode frequency of the stalled airfoil) ranging from 0.1 up to 3 have been examined. Among the controlled cases, excitation with Fex=0.6 and Fex=2.2 provides the largest and smallest pitch amplitude, respectively, and the case of Fex=3 demonstrates the most impact on flow reattachment. Finally, it has been concluded that the employed control strategy is effectively capable of modifying the dynamic stall process and associated pitching moment. However, a more sophisticated control strategy would be required to significantly mitigate the stall flutter oscillations.

The effect of external perturbations on nonlinear panel flutter at low supersonic speed

We studied two-dimensional nonlinear flutter oscillations of a flat elastic panel in a low supersonic flow, where several panel eigenmodes are simultaneously unstable, and resulting nonlinear oscillations are governed by the nonlinear interaction of growing modes. The objective of our work was to study the dependence of limit cycle oscillations of the panel on an external disturbance. To analyse possible limit cycle oscillations, we first investigated the effect of the spatial distribution and amplitude of the initial excitation loading. We show that, depending on the initial perturbation, different modes of limit cycle oscillations can develop, in which the cyclic stress amplitude differs by an order of magnitude. Second, we investigated the effect of a short strong perturbation applied to the developed limit cycle oscillations and show the possibility of switching the limit cycle mode from high- to low-energy oscillations, thus providing the ability to control oscillations of the fluttering panel.

Lift enhancement by a flapped trailing edge at low Reynolds number: A frequency response approach

The capability of an oscillating trailing edge flap to enhance the lift generated by a rigid NACA-0012 airfoil at 2.1×104 Reynolds number is investigated. Experiments are performed at two angles of attack, namely α0=0∘ that represents a fully attached flow and α0=10∘ that represents a near-stall flow condition. The presented data and analysis cover sinusoidal oscillations of the flap at five reduced frequencies between k=0.02 and 0.12, and three oscillating amplitudes δA=5∘, 8° and 10° relative to the airfoil’s angle of attack. Enhancement to the mean lift, lift-to-drag ratio, and unsteady lift dynamics are discussed. The validity of quasi-steady formulation of Leishman’s model in predicting the lift is quantitatively assessed in light of the measurements. The unsteady lift dynamics generated by flap oscillations are modeled by constructing frequency response functions with the quasi-steady lift as the input and unsteady lift as the output. The experimentally determined gain variations are compared with analytically-derived gain functions. The effects of rate and amplitude of flap oscillations on lift cycle variations are examined using aerodynamic hysteresis response.

On the performance improvement of a flutter based energy harvester by introducing additional wake field

ABSTRACT This work experimentally investigates dynamics of flutter oscillation of a plane structure under the presence of an additional wake field and is motivated to improve performance of a flutter based energy harvester. Flutter instability is a potential future source to harvest abundant ambient energy, although, such harvesters are still in developing stage due to requirement of a flow velocity higher than the available ambient wind speed. The present work primarily proposes a viable means to reduce the critical velocity of flutter and so enhance the efficiency of the flutter-based energy harvester. Here, two groups of experiments are conducted. First, the effect of wake from an upstream body on the flutter is studied, and then, energy harvesting performance, using piezoelectric elements, are analyzed. Moreover, to understand flow dynamics, numerical studies are also performed. Tests are performed in a wind tunnel and the vibrations are visualized by a high-speed camera and sensed by piezoelectric transducer. The results obtained confirm a notable reduction of critical velocity, an upstream bluff body of 20 mm reduces the critical velocity by 38% when compared with the original flutter speed of sheet alone. Inline to this, a higher output of 1.46 V/6.81 µW than 1.42 V/6.46 µW is observed. Computational study shows that circulation over the plane structure plays an important role and its downstream extension relates to the level of reduction of critical velocity.

Complex damping influences on the oscillatory/static instability characteristics of heated panels in supersonic airflow

The aim of this study is to investigate the complex damping influences on the oscillatory/static instability characteristics of heated panels in supersonic airflow. Firstly, by assuming a constant, uniform thermal loading and adopting the piston theory, the panel aeroelastic governing equation is obtained. After deriving the panel buckling and vibration modes, the reduced order model can be built and adopted to investigate the system primary instability in the modal coordinates. Then, introducing the modal damping coefficients ratio η>0, the critical parameters of the panel flutter oscillation are theoretically evaluated based on the non-conservative energy balance principle, thus the system instability characteristics can be investigated. The results show that the system oscillatory and static buckling instability characteristics are significantly regulated by the thermal loading and modal damping. For the oscillatory instability, there exists the damping paradox, which is associated with the system energy dissipation efficiency, and can be quantitatively evaluated by the ratio 2η/1+η. The system static buckling instability characteristics is also affected by the modal damping, and this damping destabilization is clarified based on Hamiltonian energy conservation law. The results agree well with that obtained by Routh-Hurwitz criteria, and lead an in-depth understanding of the complex role played by the damping within the non-conservative dissipative systems.

Routes to Synchronization in a pitch–plunge aeroelastic system with coupled structural and aerodynamic nonlinearities

The present study aims to investigate the synchronization characteristics of a pitch–plunge aeroelastic system subjected to coupled structural free-play and dynamic stall-induced aerodynamic nonlinearities. The semi-empirical Leishman–Beddoes (LB) aerodynamic model is used to capture the dynamic stall phenomena at large angles-of-attack (AoA). A bifurcation analysis, considering the flow-speed as the control parameter, reveals that the free-play nonlinearity is primarily responsible for the dynamical transition in the presence of the attached flow. In this case, a pitch and plunge frequency coalescence takes place through an aperiodic to periodic signature transition. Furthermore, by tracking the flow specific variables estimated using the LB model, it is observed that the onset of the dynamic stall plays a pivotal role in the bifurcation scenario at high flow speed regimes, marking the presence of stall flutter. The framework of synchronization is used to discern the response dynamics at low speed and high speed flow regime by estimating the pitch and plunge modal frequencies and their corresponding phase components. The routes to synchronization are identified to be through phase-locking (frequency coalescence) and through suppression of the plunge mode by the dominance of pitching frequency in the regimes before and after the onset of dynamic stall, respectively. Finally, it is shown that the different synchronization routes can possibly be a useful tool to identify the presence of stall flutter oscillations in a coupled aeroelastic system.

Thermoelastic non-linear flutter oscillations of rectangular plate

In this paper the stability of a rectangular plate in a supersonic flow in the presence of a temperature field, inhomogeneous across the thickness, is investigated. The temperature field inhomogeneity across the thickness leads to the buckling of the plate, and this buckled state is considered as the initial unperturbed state. Governing equations and boundary conditions are obtained for the unperturbed and perturbed states. The corresponding linear analysis leads to the stability conditions of unperturbed state of the thermal-fluid-elastic structure system. The stability boundary is determined as function of the flow speed, the middle surface temperature, and the temperature gradient normal to the plane. It is shown that the temperature field and the flow speed have interdependencies and regulate the stability process. It is also shown that the temperature field significantly affects the critical flutter speed. The nonlinear stability is also investigated, both cubic geometrical non-linearity and aerodynamic non-linearity of both quadratic and cubic types are considered. Compared with the case without thermal field, this study shows for the first time the existence of new types of “amplitude-frequency” limit cycles.

Experimental Study on Heat Transfer Enhancement by Using Textile Flap Oscillation

This paper experimentally studies a new method to enhance heat transfer in which a soft textile flap flutters and slaps a hot plate surface, due to fluid-structure interaction. The self-sustained periodic slapping of the textile flap against the heat transfer surface disrupts the boundary layers and thus, increases the heat transfer between the air and plate surface. Heat transfer coefficients in the presence of textile flaps of different shapes and sizes were measured while oscillatory motion of the flaps was recorded. The results show that rectangular flaps provide consistent heat transfer enhancement with increases in heat transfer coefficient observed of up to 42%, from 113 W·m−2·K−1 in the absence of a textile flap, to 161 W·m−2·K−1 in the presence of a rectangular textile flap, at a wind speed of 18.5 m·s−1. It is found that the Nusselt number in the presence of rectangular flaps varies with the flap oscillation frequency in a nonlinear fashion and that the maximum heat transfer coefficient generally occurs when the dimensionless flow-induced oscillation frequency is in the range of 0.22 to 0.32. These results show the potential of using self-sustained oscillating textile flaps to enhance the convective heat transfer in various heat exchangers.

Nonlinear aeroelastic analysis of airfoil section under stall flutter oscillations and gust loads

As the desire for more flexible aircraft structures increases, so does the need to incorporate the intrinsic nonlinearities in their aeroelastic analyses. In that manner, it becomes essential to fully characterize the aeroelastic behavior and to investigate the effects that common external disturbances, such as atmospheric gust turbulence, might induce. In this work, an augmentation of the semi-empirical Beddoes–Leishman dynamic stall model to include gust loads is presented. This augmented dynamic stall model is coupled with the typical section equations of motion to yield a complete aeroelastic framework for gust-induced stall flutter analyses. The stability boundary of a classic 2-DOF typical section is investigated, and a stochastic gust response study reveals that depending on the turbulence intensity, the probabilities of reaching aeroelastic instability can be quite high even for airspeeds well below the one for linear flutter.

Three-dimensional nonlinear dynamics of prestressed active filaments: Flapping, swirling, and flipping.

Initially straight slender elastic filaments or rods with constrained ends buckle and form stable two-dimensional shapes when prestressed by bringing the ends together. Beyond a critical value of this prestress, rods can also deform off plane and form twisted three-dimensional equilibrium shapes. Here, we analyze the three-dimensional instabilities and dynamics of such deformed filaments subject to nonconservative active follower forces and fluid drag. We find that softly constrained filaments that are clamped at one end and pinned at the other exhibit stable two-dimensional planar flapping oscillations when active forces are directed toward the clamped end. Reversing the directionality of the forces quenches the instability. For strongly constrained filaments with both ends clamped, computations reveal an instability arising from the twist-bend-activity coupling. Planar oscillations are destabilized by off-planar perturbations resulting in twisted three-dimensional swirling patterns interspersed with periodic flipping or reversal of the swirling direction. These striking swirl-flip transitions are characterized by two distinct timescales: the time period for a swirl (rotation) and the time between flipping events. We interpret these reversals as relaxation oscillation events driven by accumulation of torsional energy. Each cycle is initiated by a fast jump in torsional deformation with a subsequent slow decrease in net torsion until the next cycle. Our work reveals the rich tapestry of spatiotemporal patterns when weakly inertial strongly damped rods are deformed by nonconservative active forces. Taken together, our results suggest avenues by which prestress, elasticity, and activity may be used to design synthetic macroscale pumps or mixers.

Nonlinear flutter response of cylindrical shell in thermal field and supersonic gas flow

The work is devoted to the investigation of flutter oscillations and stability of closed cylindrical shell in supersonic gas flow and placed in an inhomogeneous temperature field. It is assumed that supersonic gas flows on the outside of the shell with an unperturbed velocity U, directed parallel to the cylinder generatrix. Under the action of an inhomogeneous temperature field, the shell bulges out; this deformed state is accepted as unperturbed, and the stability of this state is studied. The main nonlinear equations and relations describing the behavior of the examined system are derived. The formulated boundary value problem is solved using Galerkin method. The joint influence of the flow and the temperature field on the relation between the amplitude and frequency of nonlinear oscillations of a cylindrical shell is studied. The critical velocity values are calculated from the corresponding linear system and are given in tables. The numerical results show that: a) the surrounding flow significantly affects the nature of the investigated relation; b) it is impossible to excite steady-state flutter oscillations (the silence zone) up to a certain value of the oscillations frequency; c) the dependence of the amplitude on the oscillations frequency can be either multi-valued or single-valued.