Aeroelastic Phenomena Research Articles

Study of Aeroelastic Phenomena of the Hull and Thin-Walled Structures of Unmanned Aircraft at High Supersonic Speeds

Study of Aeroelastic Phenomena of the Hull and Thin-Walled Structures of Unmanned Aircraft at High Supersonic Speeds

A Numerical Study of a Rotor Induced Flow Based on a Finite-State Dynamic Wake Model.

A Helicopter rotor undergoes unsteady aerodynamic loads ruled by the aeroelastic coupling between the elastic blades and the dynamic wake induced by rotary wings. Modeling the dynamic interaction between the structural and aerodynamic fields is a key point to understand aeroelastic phenomena associated with rotor stability, flow induced vibration and noise generation, among others. In this study, we address the Generalized Dynamic Wake Model, which describes the inflow velocity field at the rotor disk as a superposition of a finite number of induced flow states. It is a mature model that has been validated based on experimental data and numerically investigated from an eigenvalue problem formulation, whose eigenvalues and eigenvectors provide a deeper insight on the dynamic wake behavior. The paper extends the results presented in the literature to date in order to support physical interpretation of inflow states drawn from the finite-state wake model for flight conditions varying from hover to edgewise flight. The discussion of the wake model mathematical formulation is also oriented towards practical engineering applications to fill a gap in the literature.

Математична модель фюзеляжних коливань на трансзвукових швидкостях польоту

Ensuring the safety of supersonic aircraft flights and aerospace systems in the transonic range of M flight numbers still remains an urgent scientific and applied problem. This is caused by the peculiarities of the aerodynamic surfaces flow by inhomogeneous (transonic) air and is due to the emergence of various aeroelastic phenomena in these flight modes and the current lack of a generally accepted model of transonic flutter, even for aerodynamic control surfaces. Based on a joint analysis of the conditions for the formation of shock waves on the surface of the aerodynamic profile, changes in the parameters of supersonic flow across the Prandtl-Meyer expansion fan, and the hypothesis of "dynamic curvature of the aerodynamic profile", the approximate laws of interaction of elastic bending vibrations of the fuselage with fluctuations in shock waves were obtained. The obtained regularities are used to substantiate a mathematical model for estimating excited forces and excited bending moments of the fuselage. The analysis of the obtained mathematical model confirms the theoretical possibility of the appearance of fuselage forms of transonic flutter in supersonic aircraft, which was observed in the flight experiment and which is due to the interaction of shock waves with the angular velocity of the fuselage elastic bending vibrations. With the accepted in the article input geometrical data of a fuselage aerodynamic surfaces’ profile, the maximum possible values of fuselage bending moments are obtained using the developed mathematical model. The obtained mathematical model can be used for a preliminary approximate assessment of the transonic flutter fuselage forms characteristics in supersonic aircraft and aerospace systems.

Aerodynamic Investigation of a Composite Low-Speed Fan for UHBR Application

Abstract A composite fan stage representative of a modern Ultra High Bypass Ratio (UHBR) architecture has been investigated experimentally on a novel test facility at Ecole Centrale de Lyon. These measurements show indications for strong overloading of the tip region resulting in extensive blockage of the blade passage. The performance of the fan is analyzed with extensive instrumentation including radial profiles upstream and downstream of the rotor. Unsteady pressure measurements help to interpret the flow structure in the tip region. The results are presented across a range of operating points on the design speedline. At the stability limit, the machine suffers from non-synchronous vibrations, which result from small-scale aerodynamic disturbances propagating between the leading edges. Detailed analysis of the occurring waveforms is presented for two operating speeds. In order to further analyze the observed phenomena, a numerical study has been conducted using the Reynolds-averaged Navier–Stokes (RANS) solver elsA. The results of steady calculations are discussed in comparison with the detailed experiments. Unsteady simulations near the stability limit accurately predict the aerodynamic disturbances observed during non-synchronous vibrations (NSV). The obtained results are unusual for typical state-of-the-art transonic fans, as they show the same behavior as high-pressure compressor front stages, dominated by a blockage caused by tip-leakage flow. Even though flutter is not observed, the observed non-synchronous vibration mechanism is a critical aeroelastic phenomenon which is of great interest for future designs of low-speed fans.

Automation of a Thin-Layer Load-Bearing Structure Design on the Example of High Altitude Long Endurance Unmanned Aerial Vehicle (HALE UAV)

In the aerospace industry, thin-layer composites are increasingly used for load-bearing structures. When designing such composite structures, particular attention must be paid to the development of an appropriate geometric form of the structure to increase the structure’s load capacity and reduce the possibility of a loss of stability and harmful aeroelastic phenomena. For this reason, the use of knowledge-based engineering support methods is complicated. Software was developed to propose and quickly evaluate a thin-layer load-bearing structure using generative modeling methods to facilitate development of the initial concept of an aerospace load-bearing structure. Finite Element Method (FEM) analysis verifies and improves such structures. The most important contributions of the paper are a methodology for automating the design of ultralight and highly flexible aircraft structures with the use of generative modelling, proposing and verifying the form of generative models for selected fragments of the structure, especially wings, and integration of the use of generative models for iterative improvement of structures using low- and middle-fidelity methods of numerical verification.

Experimental determination of the resistance of a single-axis solar tracker to torsional galloping

One of the most efficient designs of solar trackers for photovoltaic panels is the single-axis tracker, which holds the panels along a torque tube that is driven by a motor at the central section. These trackers have evolved to become extremely slender structures due to mechanical optimization against static load and the need of cost reduction in a very competitive market. Owing to the corresponding decrease in mechanical resistance, some of these trackers have suffered aeroelastic instability even at moderate wind speeds, leading to catastrophic failures. In the present work, an analytical and experimental approach has been developed to study that phenomenon. The analytical study has led to identify the dimensionless parameters that govern the motion of the panel-tracker structure. Also, systematic wind tunnel experiments have been carried out on a 3D aeroelastic scale model. The tests have been successful in reproducing the aeroelastic phenomena arising in real-scale cases and have allowed the identification and a close characterization of the phenomenon. The main results have been the determination of the critical velocity for torsional galloping as a function of tilt angle and a calculation methodology for the optimal sizing of solar tracker shafts.

Predicting the Flutter of the Fan Row in the Aircraft Engine

The aeroelasticity of the cascades of blades is considered to be the most burning and important problem, because it effects the reliability of the flow channels of turbine machines. The main complexity of the investigation of aeroelastic phenomena that occur in turbine machines consists in the need of the simulation of the interaction of the two physical environments (liquid and elastic). One of the approaches to the computation of aeroelastic phenomena (the flutter) is the nonstationary aerodynamic analysis that studies nonstationary flows in turbine machines independently of their origin and over the past decades it covered the way from the linear theory to the solution of Navier -Stokes equations. The second approach to the investigation of the aeroelasticity of the cascades of blades of turbine machines is related to the studies of the motion of blades under the action of aerodynamic forces induced by the nonuniformity of the main flow and independent of the motion of the blades. These approaches ignore the influence of vibrating blades on the main gas flow. Within the framework of individual problems considered in scientific papers the problem of self-excited vibrations (the flutter) that are the most complicated and least studied phenomena falls out of consideration. Based on the analysis of the contemporary state of the problem dealing with the turbine machine aeroelasticity and available methods of the flutter prediction we can draw a conclusion that the most promising approach to the investigation of the aeroelastic behavior of the blade row of the turbine machine is the approach that is based on the three-dimensional model of the nonstationary dynamics and the modal analysis of the blade motion ( the method of the solution of the coupled aeroelastic problem). This method of the solution of the coupled problem of the nonstationary aerodynamics and elastic vibrations of the blades will enable the prediction of the amplitude-&-frequency spectrum of blade vibrations in the three-dimensional gas flow, including forced self-excited vibrations in order to increase the reliability of the cascades of blades of turbine machines. Using the developed numerical method, we analyzed the aeroelastic behavior of the fan row of the aircraft engine in different modes. The computation data confirmed the appearance of nonstationary modes in the fan operation. The research done allows us to give recommendations on the elimination of the aeroelastic phenomena, i.e. the flutter (by changing the fan blade geometry).

Constructive Aerodynamic Interference in a Network of Weakly Coupled Flutter-Based Energy Harvesters

Converting flow-induced vibrations into electricity for low-power generation has received growing attention over the past few years. Aeroelastic phenomena, good candidates to yield high energy performance in renewable wind energy harvesting (EH) systems, can play a pivotal role in providing sufficient power for extended operation with little or no battery replacement. In this paper, a numerical model and a co-simulation approach have been developed to study a new EH device for power generation. We investigate the problem focusing on a weakly aerodynamically coupled flutter-based EH system. It consists of two flexible wings anchored by cantilevered beams with attached piezoelectric layers, undergoing nonlinear coupled bending–torsion limit cycle oscillations. Besides the development of individual EH devices, further issues are posed when considering multiple objects for realizing a network of devices and magnifying the extracted power due to nonlinear synergies and constructive interferences. This work investigates the effect of various external conditions and physical parameters on the performance of the piezoaeroelastic array of devices. From the viewpoint of applications, we are most concerned about whether an EH can generate sufficient power under a variable excitation. The results of this study can be used for the design and integration of low-energy wind generation technologies into buildings, bridges, and built-in sensor networks in aircraft structures.

Motion-induced vortex shedding and lock-in phenomena of a rectangular section

The nonlinear vortex-induced vibration of a rectangular section with an aspect ratio B/D = 6 is investigated in this study, aiming at explanation of the phenomenon of double lock-in ranges. First, numerical results by two-dimensional CFD simulation are compared with experimental results; then flow field characteristics, aerodynamic loadings and structural motion properties are presented and discussed; finally, an energy-trapping-based model for motion stability is brought forward, based on which the observed aeroelastic phenomena are discussed. The present study shows the motion-induced lock-in range is able to be explained qualitatively with the proposed principle describing the free-oscillation stability. Within the motion-induced lock-in range, the 1-DOF system can experience two motion components. The dominance of the free vibration fueled by feedback lift can dwarf or even eliminate the vortex-shedding-induced motion. Further, it is demonstrated that the phase angle between the lift and motion velocity, instead of the load amplitude, dominates the motion stability.

Comparison of international wind codes for across wind response of concrete chimneys

Lift force arises due to the shedding of vortices in the wake of structures. Flexible structures such as the chimney tend to experience aero-elastic phenomenon during the vortex-induced across wind vibration caused due to vortex induced lift forces. While rigorous theoretical analysis including the aero-elastic effect is desired for the validation of the design of the chimney and for performing the reliability analysis, the initial design of the chimney for wind induced forces is routinely performed using code provisions. Almost all the countries have their codes for considering the wind loads on chimneys. For the design of tall chimneys for across wind forces, different codes differ in their approaches and provide different code provisions. The objective of the present paper is to compare seven major international code methods for the design of reinforced concrete chimneys for across wind forces and to present the basis of the formulations of their code provisions. They include the CICIND-2011, ACI 307-08, NBCC, IS 4998:2015, AS/NZ 1170.2:2011, Euro code method-I (EN 1991-1-4) and Euro code method-II (EN 1991-1-4). It is shown that the equivalent static formulas provided by different codes follow either Vickery and Basu method or Ruscheweyh’s method. Further, how different code formulae are derived from these two formulations is shown while comparing different codes. The comparison of different important code provisions is illustrated and evaluated with the help of three numerical example problems.

Advanced aeroelastic robust stability analysis with structural uncertainties

Robust flutter analysis described in this paper is based on the robust control theory framework. Therefore, a time-domain linear fractional transformation representation of the perturbed aeroelastic system is modeled. Then, the robust stability is analyzed by means of the structured singular value mu, which is defined as an alternative measure of robustness. Robust flutter analysis deals with aeroelastic (or aeroservoelastic) stability analysis taking structural dynamics, aerodynamics and/or unmodeled system dynamics uncertainties into account. Flutter is a well-known dynamic aeroelastic instability phenomenon caused by an interaction between structural vibrations and unsteady aerodynamic forces, whereby the level of vibration may trigger large amplitudes, eventually leading to catastrophic failure of the structure. The primary motivation of the robust flutter analysis is that this method allows the computation of the worst-case flutter velocity which can support, for example, the flight test program by a valuable robust flutter boundary. This paper addresses the issue of an approach for aeroelastic robust stability analysis with structural uncertainties with respect to physical symmetric and asymmetric stiffness perturbations on the wing structure by means of tuning beams.

Aeroelastic Vibration Measurement Based on Laser and Computer Vision Technique

The new aeronautical structures have become more flexible and light weight to reduce energy consumption, emissions, and noise, and to operate at high altitudes for long periods in the air, such as those required in the NASA Helios project. The increased structural flexibility of these aircraft has reignited concerns related to aeroelastic instabilities, such as flutter. Improving the techniques and methods used in aircraft certification flights is an important concern of the aeronautical community, because current standards and procedures do not provide recommendations and guidelines for aircraft with a high degree of flexibility. The techniques traditionally used in commercial airplanes cannot be used in this new aircraft concept, because they have a high degree of non-linearity in their flight dynamics. Current research indicates an increasing awareness about the importance of vision in the monitoring of UAV structural health. This work presents a new methodology to measure natural frequencies of aeronautical structures using a computer vision system. We also discusses new approaches to sense and acquire vibration data on aeroelastic certification flights test. These new approaches aim to reduce both the time required to identify the aeroelastic phenomenon and the size of the hardware that must be boarded on the aircraft, thus minimizing the risks and costs of the vibration tests. The advance of computer vision systems enables the use of cameras as a motion tracker sensor with millimeter precision and accuracy. Non-contact sensors are suitable for flutter analysis because they do not interfere with the dynamics of the aircraft. Therefore, this new methodology is able to process the obtained images and provide the user with the data about movements in a ready to use vector, at a reasonable cost. Using the data provided by this methodology the natural frequencies of the first bending modes were identified. This new methodology can be a user-friendly tool to support the Brazilian National Civil Aviation Agency - ANAC program called iBR 2020, which aims to certify small aircraft.

Analysis of a piecewise linear aeroelastic system with and without tuned vibration absorber

The dynamics of a two-degrees-of-freedom (pitch–plunge) aeroelastic system is investigated. The aerodynamic force is modeled as a piecewise linear function of the effective angle of attack. Conditions for admissible (existing) and virtual equilibria are determined. The stability and bifurcations of equilibria are analyzed. We find saddle-node, border collision and rapid bifurcations. The analysis shows that the pitch–plunge model with a simple piecewise linear approximation of the aerodynamic force can reproduce the transition from divergence to the complex aeroelastic phenomenon of stall flutter. A linear tuned vibration absorber is applied to increase stall flutter wind speed and eliminate limit cycle oscillations. The effect of the absorber parameters on the stability of equilibria is investigated using the Liénard–Chipart criterion. We find that with the vibration absorber the onset of the rapid bifurcation can be shifted to higher wind speed or the oscillations can be eliminated altogether.

Steady-state aeroelasticity of a ram-air wing for airborne wind energy applications

In this paper we present a computational approach to simulate the steady-state aeroelastic deformation of a ram-air kite for airborne wind energy applications. The approach is based on a computational fluid dynamics (CFD) solver that is two-way coupled with a finite element (FE) solver. All components of the framework, including the meshing tools and the coupling library, are available in open source. The flow around the wing is described by the steady-state Reynolds-averaged Navier-Stokes (RANS) equations closed by an SST turbulence model. The FE model of the cellular membrane structure includes a wrinkling model and uses dynamic relaxation to find the deformed steady-state shape. Each simulation comprises four distinct steps: (1) generating the FE mesh of the design geometry, (2) pre-inflation of the wing, applying a uniform pressure on the inside, (3) generating the CFD mesh around the pre-inflated wing, and (4) activating the exterior flow and two-way coupling iterations. We first present results for the aerodynamics of the pre-inflated rigid ram-air wing and compare these to similar results for a leading edge inflatable (LEI) tube kite. Both wings are characterized by a high anhedral angle and low aspect ratio which induce spanwise flows that reduce the aerodynamic performance. The comparison shows a better performance for the LEI wing which can be attributed to its higher aspect ratio. The aeroelastic deformation of the ram-air wing further improves the aerodynamic performance, primarily because of the increasing camber which in turn increases the lift force. A competing aeroelastic phenomenon is the formation of bumps near the leading edge which increase the drag.

Non-synchronous vibration in axial compressors: Lock-in mechanism and semi-analytical model

Non-Synchronous-Vibration (NSV) in high-speed turbomachinery compressors is an aeroelastic phenomenon which can have devastating consequences, including loss of rotor blades. Despite extensive research over the past two decades its underlying mechanisms are not yet understood. This paper aims to explain the physical mechanisms causing NSV in a modern transonic compressor rotor. Referring to previous experimental results and using validated computational fluid dynamics (CFD), a parametric study is performed in order to characterize the aerodynamic disturbance causing NSV, and to understand the lock-in mechanism between the fluid and the structure seen during NSV. The results show that the process is driven by aerodynamics in the tip region. Under highly throttled conditions, the tip leakage flow blocks the passage and causes the disturbance, which is characterised as a vorticity fluctuation, to propagate circumferentially in the leading edge plane. It is found that the propagation speed of the disturbance is determined by the mean flow conditions and only its phase is periodically modulated through interaction with oscillating blades. This is the mechanism facilitating lock-in. Based on these findings a semi-analytic model is developed and calibrated with the numerical results. The model is capable of simulating the lock-in process and correctly predicts unstable vibration modes. It can therefore be used to identify critical operating conditions and develop mitigation measures early in the design.

Time-domain non-linear aeroelastic analysis via a projection-based reduced-order model

ABSTRACTThe aeroelastic phenomenon of limit-cycle oscillations (LCOs) is analysed using a projection-based reduced-order model (PROM) and Navier–Stokes computational fluid dynamics (CFD) in the time domain. The proposed approach employs incompressible Navier–Stokes CFD to construct the full-order model flow field. A proper orthogonal decomposition (POD) of the snapshot matrix is conducted to extract the POD modes and corresponding temporal coefficients. The POD modes are directly projected to the incompressible Navier–Stokes equation to reconstruct the flow field efficiently. The methodology is applied to a plunging cylinder and an aerofoil undergoing LCOs. This scheme decreases the computational time while preserving the capability to predict the flow field accurately. The ROM is capable of reducing the computational time by at least 70% while maintaining the discrepancy within 0.1%. The causes of LCOs are also investigated. The scheme can be used to analyse non-linear aeroelastic phenomena in the time domain with reduced computational time.

Revisiting the Fundamentals of Control Surface Reversal Including Nonlinear Effects

Control surface reversal is a classical aeroelastic phenomenon commonly investigated during aircraft development. It occurs under static equilibrium between the elastic restoring torque and the aerodynamic moment on the system. Besides the equilibrium, it depends on an additional condition defined through the lift force. As discussed in the literature, this second condition can be considered by two different approaches for a linear model. In this context, this paper discusses the reversal considering typical nonlinearities. The two different approaches are compared. The findings demonstrate that the influence of nonlinearities is correctly considered when reversal is defined as when the lift due to control surface rotation is zero. Also, it is concluded that nonlinear reversal cannot be determined by employing the generalized reversal condition often discussed in the classical linear problem.

A Method for Flutter Detection by Infrared Imaging

The performance enhancement of aircraft coupled with the development of increasingly lightweight and flexible materials has led designers to use smaller structural safety factors along the time, which can make aerodynamic surfaces more susceptible to aeroelastic phenomena, including flutter. This kind of occurrence must be carefully investigated by ground and flight tests during aircraft development and certification, which requires suitable instrumentation in order to predict the occurrence of unwanted vibrations. The sensors in this kind of application must be less intrusive as possible, in order to not modify the dynamic or aerodynamic behavior of the system. This work proposes the use of infrared imaging as a tool for flutter detection, analyzing the suitability of the technique for this application. For this purpose, a literature review was performed by presenting infrared technology concepts; then, some preliminary tests were performed in a structure to predict flutter characteristics, and finally wind tunnel tests were executed in the same structure, validating this technique and highlighting its positive points and points that need improvement.

Numerical simulations of flutter mechanism for high-speed wide-chord transonic fan

Flutter is a self-excited aeroelastic instability phenomenon that can cause blade failure and lead to safety accidents. This paper investigates the first bending mode flutter behavior of a high-speed wide-chord transonic fan under different operating conditions. The aim is to reveal the relationship between the flow structure and blade flutter stability at different inter-blade phase angles (IBPA). Two methods are used to model the traveling wave propagation along the circumferential direction: the influence coefficient method (ICM) and the traveling wave method (TWM). The former is used to calculate the aerodynamic damping at different IBPA with fewer computational resources, and the latter is used to analyze the relationship between the flow and flutter stability. The results suggest that the most unstable behaviors occur with two nodal diameters (ND) at three representative conditions: near stall, peak efficiency, and choke. The aerodynamic damping distribution of the blade surface is concentrated primarily at the shock wave, separation, and large vibration amplitude regions, whose effects on the blade flutter stability are greatly impacted by the IBPA. When the blade IBPA is small and positive, the shock and separation areas in the suction side reduce the blade flutter stability. This shows that the IBPA strongly affects the unsteady pressure fluctuations, including the direction and shape of the pressure wave propagation, and whether the unsteady pressure can propagate downstream (cut-on) and result in different aerodynamic damping distributions on the blade surface.

Design and Fabrication of Vortex Bladeless Turbine

ABSTRACT - In present situation, India is one of the top growing economies. The various sectors contributing to this,need electricity for its functioning. Non-renewable resources being depleted day by day, importance is given to develop power from renewable sources of energy like wind, solar, hydro energy etc. In the year 2017-2018 the total utility power generated in India is 1,303,493 GWh and captive power generated is 183,000 GWh making a total of 1,486,493 GWh. Out of 1,303,493 GWh, 52,666 GWh(4% utility power) is generated using wind power. The aim of this project is to utilize wind power to its maximum potential to generate electricity. The region of high speed wind is limited and the area required for installation of conventional windmill is high due to the wake effect. Research is done to find new innovative methods that can operate under optimum wind conditions, under less area by minimizing wake effect and provide an efficient output. One such technology is Bladeless turbine that provides a quiet, safe, simple and efficient alternative to the conventional bladed turbines. Bladeless turbine is not actually a turbine, since it does not rotate. This new approach captures wind energy based on the phenomenon of aeroelastic resonance. Harnessing energy from the vortexes, a process called vortex shedding or Vortex Street. This causes the device to oscillate with little movement which is perfect to be placed anywhere without lubricants and without disturbing wildlife. Aeroelastic resonance phenomenon is usually considered as a problem but this has been used as basic technology for power generation. Bladeless turbines are also the only ones with almost no harmful effects on the environment. Another major advantage of this turbine is that it has less moving parts, thereby reducing losses to a minimum. This is a new age turbine with improved performance that is economic, eco-friendly and less complex with wear prone transmission being eliminated.