Aerodynamic Excitation Research Articles

Horizontal tail buffeting characteristics at wing vortex flow impact

Modern, high-agility aircraft configurations often suffer from tail buffeting at subsonic speeds and medium to high angles of attack. This structural dynamic excitation through the unsteady flow field can result in heavy structural damage and degraded handling qualities. A flexible wind tunnel half model was developed at the Chair of Aerodynamics and Fluid Mechanics of the Technical University of Munich in cooperation with the Department of Acoustics and Vibration of Airbus Defence and Space. To provide enough flexibility, the wing and horizontal tail plane (HTP) are 3D-printed from polylactide (PLA). The model is used to experimentally analyze buffeting and to validate computational buffeting prediction. The objective of the present work is to examine aeroelastic phenomena with the modular designed flexible wind tunnel model. The measurement campaign takes place at the Göttingen type wind tunnel A. The model is equipped with various sensors. For unsteady pressure measurements on the surface of the wing and the HTP, piezo-resistive Kulite pressure transducers are installed on the wing and on the HTP. In addition, the flow field is described on the basis of numerical simulation results. For analyzing the structural response resulting from buffeting, miniature accelerometers are installed at the tips of the wing and the HTP. Strain gauges are used for calculating bending strains. As a reference case, a fully aluminum model is equipped correspondingly, but without strain gauges. Dominant frequencies corresponding to the structural eigenmodes can be identified and are excited in the PLA-setup (Buffeting). The unsteady pressure fluctuations on the surfaces act as the aerodynamic excitation input (Buffet). The measured tip accelerations of wing and tail are compared to simulation results with a one-way coupling CFD-CSM simulation.

Assessing Low Frequency Flow Noise Based on an Experimentally Validated Modal Substructuring Strategy Featuring Non-Conforming Grids

<div class="section abstract"><div class="htmlview paragraph">The continuous encouragement of lightweight design in modern vehicles demands a reliable and efficient method to predict and ameliorate the interior acoustic comfort for passengers. Due to considerable psychological effects on stress and concentration, the low frequency contribution plays a vital rule regarding interior noise perception. Apart other contributors, low frequency noise can be induced by transient aerodynamic excitation and the related structural vibrations. Assessing this disturbance requires the reliable simulation of the complex multi-physical mechanisms involved, such as transient aerodynamics, structural dynamics and acoustics. The domain of structural dynamics is particularly sensitive regarding the modelling of attachments restraining the vibrational behaviour of incorporated membrane-like structures. In a later development stage, when prototypes are available, it is therefore desirable to replace or update purely numerical models with experimental data. To this end, an original strategy has therefore been developed to estimate the vibro-acoustic response due to aerodynamic excitation based on a modal coupling approach. The incorporated acoustic and structural modes can be obtained either from numerical models or through experimental modal analysis. The presented work begins with the principles of modal vibro-acoustic substructuring and the practical workflow employed. Its applicability is demonstrated by an experimentally validated automotive structure subject to transient aerodynamic load.</div></div>

Simulation and experimental investigation on vibration of a radial turbine wheel using a two-way FSI approach

When turbine blades of a radial turbocharger are subjected to an unsteady aerodynamic load excitation force, high cycle fatigue of the turbine blades will occur when the excitation force reaches a certain level. The fatigue of the radial turbine wheel is affected by both static and dynamic stresses, and high cycle fatigue occurs when the dynamic stress reaches a certain amplitude. In this study, a two-way fluid–structure interaction (FSI) analysis method was used to predict the dynamic excitation force and dynamic response of the turbine wheel, which can precisely predict the forced vibration of the turbine wheel under the condition of rotor–stator interaction. A resonance intersection point was excited by the stator (turbine housing vortex tongue) of the radial turbocharger as the operating boundary condition, and the aerodynamic excitation caused by the rotor–stator interaction and the vibration amplitude of the turbine blade was predicted and analysed. A turbine blade dynamic measurement system (tip timing) was used to measure the dynamic deformation of turbine blades for turbochargers in real time. The simulation and measurement results showed that the errors of the predicted vibration displacements and measured mistuning blades were within the acceptable range, and the two-way FSI analysis method can precisely predict the forced vibration of the model.

Non-synchronous blade vibration analysis of a transonic fan

This study numerically investigates the aeromechanic behavior of a transonic fan model with a flat tip-leading-edge on the NASA rotor 67 test case. Single-passage unsteady calculations at a near stall operating point of 82% design speed show that the dominant frequencies of mass flow were not the harmonics of the rotor rotational frequency. A full-annulus fluid–structure interaction analysis was subsequently carried out to examine the unsteady flows and their interactions with blade vibrations. The results show that the modal displacement of the backward traveling seventh nodal diameter of the second torsion mode grew exponentially, which reveals that the blade vibration was non-synchronous. The vibration pattern indicates that the aerodynamic mode was resonant with the structural vibration mode. Around the rotor tip, the circumferential vortical propagation induced by interactions among the main flow, tip leakage flow, and tip clearance vortex was the source of aerodynamic excitation. To clarify the mechanism of the non-synchronous vibration, the coupling between aerodynamic disturbance and structural response, i.e., aliasing, was summarized. The frequency spectra of the fluctuating pressure show that an aerodynamic Backward Traveling Wave (BTW) was co-aliased to a structural BTW due to the propagation of the circumferential vortex. The correlation between the frequency and free convective speed of the aerodynamic disturbance determined the directions of aliasing.

Acoustic Feedback and Its Impact on Fan Flutter in Short Aeroengine Intakes

The authors have investigated pressure waves being emitted by the structural motion of a rotor blade and their impact on fan flutter sensitivity. This impact results from an acoustic feedback loop caused by reflection phenomena at the intake lip and may lead to a specific form of aeroelastic instability. Harmonic balance simulations of the fan/intake system over a wide operating range and blade eigenfrequencies were carried out. A significant impact on the rotor aerodynamic damping was observed as compared with the isolated rotor analysis due to the presence of the intake. This influence was either stabilizing or destabilizing, depending mainly on the phase relation between the emitted and reflected unsteady pressure fields. Computational fluid dynamics (CFD) data were analyzed accordingly, taking advantage of two-dimensional wave split as well as three-dimensional in-duct modal decomposition techniques. By deriving a simple analytical model based on eigenvalue analysis, a landscape showing regions of aerodynamic damping and excitation could be drawn over the investigated frequency–mass flow range. This analytical model and its verification with CFD data can help in identifying destabilizing areas at an early design stage, and hence support design decisions regarding flutter stability or give indications where analysis with higher fidelity is required.

Two-Dimensional Investigation of the Fundamentals of OGV Buffeting

The increased demands of compact modern aero engine architectures have highlighted the problem of outlet guide vane (OGV) buffeting in off-design conditions. This structural response to aerodynamic excitations is characterised by increased vibration, risking structural fatigue. Investigations focused on understanding, mitigation and avoidance are therefore of high priority. OGV buffet is a type of transonic buffet caused by unsteady shock movement, but the exact parameters driving it are not fully understood. To try and understand them, this paper examines the buffet of a quasi-2D OGV geometry. Parametric studies of the incidence angle and inlet Mach number were performed. Forcing frequencies for both studies were found to be close to the experimentally detected frequency of vibration in the first bow mode, which demonstrates that buffet is driven by quasi-2D flow features. Increasing the inlet Mach number increased the dominant forcing frequency, whereas increasing the incidence yielded little change. Profiles of unsteady pressure amplitudes were shown to smoothly increase in magnitude with an increasing incidence and inlet Mach number.

Vibration characteristics of axial compressor blade under complex loads

In this study, 3D models of a compressor blade for an aero engine with three different levels of surface accuracy (with 4, 6 and 10 section profiles) were obtained via reverse engineering by fitting point-cloud data. Then, finite element and flow-field models of the blade were established and validated. The vibration characteristics of the blade under typical operating conditions were analyzed by considering the interaction between the aerodynamic load by airflow and the centrifugal load by blade rotating. The results showed that, under complex centrifugal and aerodynamic loads, the three blade models differed significantly in their high-order dynamic frequencies, and the modal frequencies that were dominated by a torsional mode of vibration were sensitive to the aerodynamic load. Hence, considering both the aerodynamic excitation and the centrifugal load increased the accuracy of the numerical computation of the blade vibration characteristics. Moreover, a comparison of the computational results from the three blade models showed that models B6 and B10 differed only insignificantly, and B6 decreased the complexity of the modeling process while satisfying the required computational accuracy. Thus, model B6 is the best option for engineering analysis and computation.

Wind-structure interaction analysis of empty cylindrical silos for various slenderness ratio

Wind is the flow of air within the earth’s atmosphere, which affects any structure greatly. Studies on the effect of wind loading on shell structures is an important field of structural research. Many shell structures, such as, chimneys, silos, tanks etc. are an integral part of the advancement of the industry and agriculture. Such shell structures are vulnerable against aerodynamic excitation and can be subjected to structural failure due to vortex-induced vibration, ovalling, buckling, rupture, over-turning etc. The present study focuses on the static fluid–structure interaction analysis to assess the wind pressure and wind-induced deformation of cylindrical silo shells. Cylindrical silos are a type of shell structure for storing granular materials, such as grains, wheat, cement, coal, fly-ash etc. Cylindrical silos very much susceptible to radial deformation and buckling failure under wind loading in empty state. In the present study the authors have conducted the wind-structure interaction studies on intermediate to tall (slenderness ratio range: 1.5 to 4.0) cylindrical silo shells for terrain category 2, as per IS-875: part 3, 2015. The silos are flat topped with uniform thickness and fixed at the base, free at the top. The logarithmic law is used to simulate wind velocity profile. The circumferential mean wind pressure coefficient distribution for each silo, is calculated and compared with Indian standard. This study is conducted for smooth flow with very low turbulence intensity for simplicity. This study exhibits that, with the increase in slenderness ratio from 1.5 to 4.0, the magnitude of negative pressure coefficients decreases, however no significant increase can be seen for positive pressure coefficients. The sway of the silos increases with the increment in slenderness ratio. The radial deformation is dominant in the intermediate portion and the nature of deformation is typical for each silo. Based on the nature of radial deformation the critical locations can be identified, which need to be strengthened to resist wind-induced deformation. It is also seen that the presence of flat roof provides significant strengthening at the top of the silos against wind-induced radial deformation.

Wind induced oscillation on a tall circular storage shell for various terrains

The wind is an important natural phenomenon, which adversely affects different structures, such as high-rise buildings, chimneys, tanks, silos, bridges etc. The effect of wind on tall circular storage shell structures becomes critical, especially when it is in empty condition. Wind induced radial, meridional and axial stresses can cause ovalling, buckling, rupture, overturning etc. on circular storage shells. Terrain, topography, surface roughness, wind velocity, etc. are some of the important factors which influences the wind pressure distribution on circular storage shells. Also, the wind-induced oscillation may lead to structural failure of such shells. Significantly, a small number of studies are there on the effect of these factors on the aerodynamic excitation of circular storage shells. These available scopes and the importance of the studies on circular shell under aerodynamic excitation draws the attention of the present authors to investigate in this field. In the present study the authors have conducted computational fluid dynamics analysis, fluid–structure interaction analysis and modal analysis on a tall circular storage shell with slenderness ratio 4.0 considering four different terrains as per IS 875, part 3, 2015. These terrains are open terrain (terrain 1), open terrain with well scattered low to mid-rise objects (terrain 2), terrain with numerous closely spaced low and mid-rise objects (terrain 3) and terrain with closely spaced tall objects (terrain 4). The atmospheric boundary for each terrain is simulated using power law. The effect of terrain variation is assessed through the variation in the mean, and fluctuating wind pressure coefficient distribution around the circumference of the tall shell. It is observed that the mean pressure coefficients decreases and the fluctuating pressure coefficients increases in the side region of the shell, with the change in terrain conditions from terrain 1 to terrain 4. The highest and lowest pressure fluctuations are observed in the side region and leeward region, respectively. The wind-induced oscillation of the tall shell in relation to the fluctuating pressure coefficients have been assessed and the deformation of the tall storage shell for six consecutive mode shapes have been highlighted for terrain 4, where the distribution of fluctuating pressure coefficients is highest in magnitude.

The Computational Method of Estimating Vibration Stress Levels for GTE Compressor Blades

At present, the process of designing a GTE involves a large amount of computational modeling. With the help of computational modeling, it is possible to predict a behavior of an engine part during engine operations before conducting experimental studies. For example, the numerical dynamic behavior analysis of compressor blades and prediction of dynamic stress levels during fluctuations in free modes are urgent problems. A high level of dynamic stress in the compressor blades in resonant modes can break a blade and stop an engine. In this paper, we propose a simple vibration stress estimation method for the compressor blades based on the calculation of natural frequencies and vibration forms. The method is based on a comparative analysis and scaling of stresses by the value of the total potential or kinetic energy. This estimation method is valid for local changes in the blade geometry, which do not lead to changes in the natural frequencies and vibration forms of the blades, assuming that the geometry change does not change the level of the aerodynamic excitation of the blade or its damping. At the stage of development or revision of the blade, a large number of variants of the blade geometry needs to be analyzed in order to reduce dynamic stresses. The proposed vibration stress estimation method has shown its high efficiency in developing and refining the geometry of the compressor blade. The vibration stress estimation method was tested using the rotor blade of a high-pressure compressor. As a result of the experimental study of the rotor blade, a high level of vibration stresses exceeding the permissible level was found for natural frequencies and vibration forms. To reduce the vibration stresses, measures were proposed to modify the geometry of the blade. For the modified blade geometry, the vibration stress estimation was performed with a prediction of the vibration stress values based on the manifested vibration forms. In order to verify the estimated vibration stress change, an experimental study of the modified blade was conducted. The vibration stress estimation method for the compressor blades was successfully verified.

Numerical Study on Vibration Response and Fatigue Damage of Axial Compressor Blade Considering Aerodynamic Excitation

Axial compressor blades with a deformed initial torsion angle caused by aerodynamic excitation resonated at the working speed and changed the rule of fatigue damage accumulation. The fatigue life of a blade has a prediction error, even causing serious flight accidents if the effect of torque causing damage deterioration of the blade fatigue life is neglected. Therefore, in this paper, a uniaxial non-linear fatigue damage model was modified using the equivalent stress with torsional shear stress, and the proposed fatigue model including the torsional moment was used to study the compressor blade fatigue life. Then, the blade numerical simulation model was established to calculate the vibration characteristics under complex loads of airflow excitation and a rotating centrifugal force. Finally, the blade fatigue life under actual working conditions was predicted using the modified fatigue model. The results show that the interaction between centrifugal and aerodynamic loads affects the natural frequency, as the frequencies in modes dominated by bending deformation decreased whereas those dominated by torsional deformation increased. Furthermore, the blade root of the suction surface showed stress concentration, but there is an obvious difference of stress distribution and amplitude between the normal stress and the equivalent stress including torsional shear stress. The additional consideration of the torsional shear stress decreased the predicted fatigue life by 4.5%. The damage accumulation rate changes with the loading cycle, and it accelerates fast for the last 25% of the cycle, when the blade fracture may occur at any time. Thus, the aerodynamic excitation increased the safety factor of blade fatigue life prediction.

Effects of a Single Blade Incidence Angle Offset on Adjacent Blades in a Linear Cascade

The paper presents a numerical and experimental investigation of the effect of incindence angle offset in a two-dimensional section of a flat blade cascade in a high-speed wind tunnel. The aim of the current work is tp determine the aerodynamic excitation forces and approximation of the unsteady blade-loading function using a quasi-stationary approach. The numerical simulations were performed with an in-house finite-volume code built on the top of the OpenFOAM framework. The experimental data were acquired for regimes corresponding to the numerical setup. The comparison of the computational and experimental results is shown for the static pressure distributions on three blades and upstream and downstream of the cascade. The plot of the aerodynamic moments acting on all five blades shows that the adjacent blades are significantly influenced by the angular offset of the middle blade.

An Efficient Approach for Predicting Resonant Response with the Utilization of the Time Transformation Method and the Harmonic Forced Response Method

Resonant response of turbomachinery blades can lead to high cycle fatigue (HCF) if the vibration amplitudes are significant. Therefore, the dangerousness assessment of the resonance crossing is important. It requires accurate predictions of the aerodynamic excitation, damping, and response, which will consume immense computational costs. The novel aspect of this study is the development of an efficient approach, which incorporates the time transformation (TT) method to predict the aerodynamic excitations and the harmonic forced response method to obtain the response levels. The efficiency and accuracy of this method were evaluated by comparing with traditional methods for the resonance crossing excited by upstream wake in a 1.5 multistage compressor. For the aerodynamic excitation, discrepancies of ±2% at the mean pressure and ±25% at the harmonic pressure in most areas expect for the blade root were observed, but the calculation time required by the TT method was only 5% of that by the time-marching method. Moreover, response levels with the same aerodynamic forces were compared between the harmonic forced-response and transient dynamic methods. Small differences in the displacement and stress variables were observed; the relative deviation was smaller than 2% with only 1% computing time compared with the transient method, indicating the high accuracy and efficiency of the efficient approach.

Study on aerodynamic excitation of radial turbine blades with vaneless volute at low excitation order

High cycle fatigue (HCF) of radial turbine blades is one of key issues of turbochargers and it determines the reliability of the device. Most of researches about HCF of radial turbine blades focus on vaned turbines, whereas few investigations have been carried out on vaneless turbines which are widely applied in automobile turbochargers. This paper studies the blade vibration mechanism of a vaneless radial turbine at low excitation order via one-way fluid–structure interaction simulation method validated by experiments. The results surprisingly demonstrate that blade vibration response does not increase consistently with turbine load. Instead, a non-monotonic correlation is observed between vibration response and turbine load. The energy analysis is employed to understand the phenomenon and the results manifest that the variation of energy imposed on blade suction surface is the direct reason for the non-monotonic correlation. Detailed flow field analysis further suggests that the non-monotonic trend of energy on suction surface is caused by the interaction of two flow structures at different turbine loads: separation vortex at blade leading edge and tip leakage vortex caused by pressure difference between pressure surface/suction surface. The study of blade excitation in this research can enlighten the new design of vaneless turbine with both good performance and high reliability.

A unified equivalent circuit and impedance analysis method for galloping piezoelectric energy harvesters

In the past decade, galloping-based energy harvesters (GPEH) connected with various interface circuits have been developed and analytical models have been built. However, the power performances of these advanced structures and circuits are always treated separately, and a general model is missing to gain insights at a system level. To tackle this issue, this paper proposes a unified analysis framework for GPEHs. Its results are consistent with validated (but disconnected) results in the literature. The method provides an integrated view of the physics of linear GPEHs in multiple domains at the system level, and elucidates the similarities and differences among power behaviors of GPEHs connected with various interface circuits. The framework is based on two major elements: an equivalent circuit that represents the entire system, and an equivalent impedance that represents the interface circuit. Firstly, the electromechanical system is linearized and modeled in the electrical domain by an equivalent self-excited circuit with a negative resistive element representing the external aerodynamic excitation, and a general load impedance representing the interface circuit. Then, a closed-form, analytical expression of the harvested power is obtained based on the Kirchhoff’s Voltage Law, from which the optimal load, maximum power, power limit, and critical electromechanical coupling (minimum coupling to reach the power limit) are determined. In this unified analysis, the exact type of energy harvesting interface circuit is not assumed. After that, the power characteristics of a GPEH connected with five representative interface circuits are analytically derived and discussed, by using the particular equivalent impedance of the interface circuit of interest. It is shown that they are subjected to the same power limit. However, the critical electromechanical coupling depends on the type of circuit. Throughout the discussions, impedance plots are used to illustrate the relationship between the internal system characteristics and external load impedance, facilitating the understanding of system power behavior.

On the Efficiency of a Piezoelectric Energy Harvester under Combined Aeroelastic and Base Excitation.

A flutter-type, nonlinear piezoelectric energy harvester was tested in various combinations of aerodynamic and harmonic base excitation to study its power output and efficiency. The commercial polyvinylidene fluoride film transducer LDT1-028K was used in 33 excitation mode. The aerodynamic excitation was created by a centrifugal fan and the base excitation by a cone speaker. The excitations were produced by varying independently the mean airflow velocity and the frequency of base vibration. A capacitive load was used to store the harvested energy. A line laser was employed along with long exposure photography and high-speed video, for the visualization of the piezo film’s mode shapes and the measurement of maximum tip deflection. The harvested power was mapped along with the maximum tip deflection of the piezo-film, and a process of optimally combining the two excitation sources for maximum power harvesting is demonstrated. The energy conversion efficiency is defined by means of electrical power output divided by the elastic strain energy rate of change during oscillations. The efficiency was mapped and correlated with resonance conditions and results from other studies. It was observed that the conversion efficiency is related to the phase difference between excitation and response and tends to decrease as the excitation frequency rises.

MULTIDISCIPLINARY DESIGN OPTIMIZATION OF A LOW-NOISE AND EFFICIENT NEXT-GENERATION AERO-ENGINE FAN

Abstract Due to the increasing bypass ratios of modern engines, the fan stage is increasingly becoming the dominant source of engine noise. Accordingly, it is becoming more and more important to develop not only efficient but also quiet fan stages. In this paper the noise emission of a fan for an aero-engine with bypass ratio of 19 is reduced within a multidisciplinary design optimization (MDO) by means of a hybrid noise prediction method while at the same time optimizing the aerodynamic efficiency. The aerodynamic performance of each configuration in the optimization is evaluated by stationary Reynolds-Averaged Navier- Stokes (RANS) simulations. These stationary flow simulations are also used to extract the aerodynamic excitation sources for the analytical fan noise prediction. The resulting large database of the optimization provides new insights into which extent an MDO can contribute to the design of both quiet and efficient fan stages. In addition to that the hybrid approach of numerical flow solutions and analytical description of the noise sources enables to understand the noise reduction mechanisms. In particular, the influence of rotor blade loading on the aerodynamic efficiency and the noise sources as well as the potential of configurations with a comparatively low number of outlet guide vanes (OGV) is explored.

First models for structural energy transmission decoupling in the high frequency regime under aerodynamic excitations

In the wind tunnel facility, a test structure is often used for measuring its vibrational response to the aerodynamic excitation. A support is needed to sustaining the structure and it is mandatory that this support does not influence the vibrational energy to be measured. To this aim, the maximum amount of energy decoupling between the structure and the support is desired. This work is focused around a quick method to estimate this decoupling by using simplified models for the Turbulent Boundary Layer (TBL) excitation and for the structural response. Specifically, the Equivalent Rain-on-the-roof excitation is invoked with a Statistical Energy Analysis model for the structure. Some simple design rules are proposed and based on little information leading to foresee the difference of vibrational velocity levels between the two structural systems. A simplified test-case is used for the first investigations and a complex structure is finally conceived thinking to vibroacoustic measurements in a large wind tunnel facility. Although some results are largely expected, the global approach is promising.

Rub-Impact Dynamics of Shrouded Blades under Bending-Torsion Coupling Vibration

Shroud devices which are typical cyclic symmetric structures are widely used to reduce the vibration of turbine blades in aero engines. Asymmetric rub-impact of adjacent shrouds or aerodynamic excitation forces can excite the bending-torsion coupling vibration of shrouded blades, which will lead to complex contact motions. The aim of this paper is to study the rub-impact dynamic characteristics of bending-torsion coupling vibration of shrouded blades using a numerical method. The contact-separation transition mechanism under complex motions is studied, the corresponding boundary conditions are set up, and the influence of moments of contact forces and aerodynamic excitation forces on the motion of the blade is considered. A three-degree-of-freedom mass-spring model including two mass blocks with the same size and shape is established to simulate the bending-torsion coupling vibration, and the dynamic equations of shrouded blades under different contact conditions are derived. An algorithm based on the fourth-order Runge–Kutta method is presented for simulations. Variation laws of the forced response characteristics of shrouded blades under different parameters are studied, on the basis of which the method to evaluate the vibration reduction characteristics of the shrouded blade system when the motion of the blade is chaotic is discussed. Then, the vibration reduction law of shrouded blades under bending-torsion coupling vibration is obtained.

Integration of electromagnetic finite element models in a\xa0multibody simulation to evaluate vibrations in direct-drive generators

This paper introduces a novel electromechanical model for calculating electromagnetic excited structural vibrations and structure borne acoustics for gearless wind turbines. Therefore, the wind turbine model structure is explained and a drivetrain model is derived to investigate the drivetrain decoupled from the aerodynamic excitations. The drivetrain model is fed with results from an electromagnetic finite element model of the generator considering air gap width changes and the wind turbine torque and speed characteristics. Furthermore, an exemplary ramp-up of the drivetrain is simulated. It can be seen, that generator structure oscillations are excited during certain rotational speeds, which may be relevant for the acoustic behavior of the turbine.