Abstract: Abstract
Numerical simulations of the flow about a NACA 0012 airfoil in low speed stall conditions are presented. The focus is set on the propagation of turbulence in the wake of the airfoil. The calculations were performed at a Mach number of 0.3, a Reynolds number of 6 million and an angle of attack close to maximum lift. The unstructured finite-volume solver TAU, which is developed by the German Aerospace Centre, was used to simulate the unsteady flow in the vicinity of the airfoil and its wake up to two chord lengths downstream. Hybrid turbulence models are used to represent the attached part of the boundary layer with the Reynolds-averaged Navier–Stokes equations (RANS) and the detached regions with large eddy simulation (LES). This enables a direct resolution of the large-scale turbulent movement in the separated regions and the wake behind the airfoil. The simulations were conducted on different meshes with increasing resolution. The capability of two hybrid RANS–LES models of detached eddy simulation type is investigated in representing the turbulent characteristics of the far wake. The influence of the mesh properties and the modeling approach is discussed. Hence the pressure spectra in the wake are compared to experimental data. Due to good agreement of the improved DDES results with the measurement data, the wake of the airfoil is evaluated over a long time period. The development of pressure and velocity spectra along the wake is discussed. PubDate: 2013-06-01
Abstract: Abstract
In this paper, the first experimental results of tests with a new model support designed for simulations of high frequency and high amplitude pitching maneuvers are presented. A generic missile model with a blunt nose was used for which static test data from earlier experiments with a conventional model support were existing. At the beginning, static tests were done for comparison purposes to judge the influence of the new setup (model support, sting for the model, pivot arm) on the measured forces and moments. Afterwards, dynamic tests with sine oscillations at frequencies of
$f_{\rm a}=0.05\ldots 4\,\hbox{Hz}$
and angles of attack varying between
$\alpha = 0^{\circ} \ldots 45^{\circ}$
were performed. Beside the qualification of this new test rig, the tests were used to study the “Phantom Yaw Effect” and to prove an interactive method of its control. This phenomenon is characterized by unwanted yawing moments resulting from asymmetric vortices which can occur on slender bodies at high angles of attack. In the tests, a lee-vortex control device with symmetrically arranged longitudinal slot nozzles producing “air-jet strakes” was demonstrated to decrease the yawing moments both under static and dynamic conditions. PubDate: 2013-06-01
Abstract: Abstract
This paper presents an overall methodology for the development and assessment of multifunctional fuel cell system architectures in the context of More-Electric-Aircraft. A system design method which assists all stages of the system design process is being introduced. Starting with the required functions, it supports the task to design a fuel cell system, which is assessed based on reference flight missions. This paper focusses on the sizing and optimization procedure as part of the overall system
design
method. The developed procedure makes use of so-called behavior models, which own the property of being self-sizing. Highlighting its advantages, this approach is compared to a “classical” one. Finally, a showcase application of the overall system design method illustrates its capability. PubDate: 2013-06-01
Abstract: Abstract
Environment protection is now considered as a key point by the population and has become a political and economical stake as related in Kyoto protocol signature which defines objectives for greenhouse effect gases reduction. Based on greenhouse effect gases like carbon dioxide (CO2) and increasing temperature, European Union has implemented an Emission Trading System to give each country emission CO2 targets and a trade tool to share tons of CO2. The European Union has fixed specific objectives to aeronautic section via the Advisory Council for Aviation Research and innovation in Europe. Helicopter community is implementing qualitative tools called environmental metrics to measure the achieved progress. Environmental acoustic metrics have been defined based on noise levels and colors, related to ICAO limitation levels and in the frame of European research project Cleansky. Similarly, environmental metrics for CO2 emission have also been defined based on fuel consumption values and colors. The more the helicopter is environment friendly, the greener the metric. In anticipation of environmental requirements strengthening, some helicopter manufacturers have launched research studies on environmental technologies like Friendcopter project to develop low acoustic level flight procedures based on HELENA computations, or engine manufacturers have invested in technological research projects to improve engine performance and environment impact. In addition, the helicopter product full life cycle is globally considered under the international standard ISO 14001 for environment management as well as human health and environment protection in comparison to REACH European regulation. The reader of the article will certainly notice that the challenge is now: “Towards greener helicopters”. PubDate: 2013-05-16
Abstract: Abstract
State-of-the-art of advanced composite manufacturing uses autoclave devices to ensure the overall curing of the polymer resins for large complex carbon–fibre–polymer composites (CFRP) helicopter structures. A homogeneous temperature distribution on the part surfaces is the key to obtain high and constant quality parts. Local heat transfer coefficients are, therefore, important monitoring factors of autoclave technology processes. Many researchers focussed on the exothermic curing reaction inside the composites, but the surrounding temperature field (inclusive air and mould temperature field) was often neglected. Thus, a thermal uniformity at the surface of the part was often not foreseeable. Due to the 3D arrangements of the parts in the autoclave, together with the design of the tools themselves, strong turbulences and velocity differences are created inside the industrial autoclave, affecting the heat transfer mechanisms at the surface of CFRP parts. Therefore, the temperature field is non-uniform in the spatial and temporal domains. This non-uniform distribution of part and mould temperatures has an important impact on the local degree of cure and on residual stresses in the composite parts. To better understand the process and to optimise the curing cycles, virtual autoclaves have been successfully implemented by MET together with Eurocopter. The validation phase showed very good agreement between measured and simulated temperature fields. On this basis, different internal arrangements of parts for helicopter airframes have been investigated leading to the development of practical solutions to reduce the cycle time, and improve the quality of the parts. Examples of application will be presented together with industrial benefits. This simulation methodology provides Eurocopter with a strong tool for the improvement of composites part quality and productivity. The article content was presented at the Eucomas Conference on February 7, 2012 in Hamburg, Germany. PubDate: 2013-04-24
Abstract: Abstract
Crashworthiness research for transport aircraft fuselage structures is important particularly with regard to the increasing ratio of carbon fibre reinforced plastic (CFRP) in primary structures. Although today’s aluminium fuselage structures offer sufficient crashworthiness purely due to the ductile behaviour of metal, the brittle behaviour of a CFRP fuselage structure implicates the need for special crash devices to avoid uncontrolled failure with little energy absorption. Hence, a specific crash design has to be developed for a CFRP fuselage structure. A numerical methodology was developed to investigate potential crash concepts on fuselage section level and to design appropriate crash devices. The essential of this so-called kinematics model is the potential to define the behaviour of structural crash devices by characteristic input curves using macro elements in the frames, vertical struts and the sub-cargo structure. By varying the load–deformation characteristics of the crash devices, different crash scenarios, in the sense of alternative crash sequences, can be defined, assessed and compared to each other. Different potential crash scenarios for narrow-body fuselage structures were analysed using this modelling approach. The final output characteristic of the macro elements in the frame structure represents the basis for an experimental investigation of energy absorbing frame bending mechanisms. A test setup for four point flexural test configurations was developed to investigate such concepts on the generic frame level. The setup was used to analyse the failure behaviour of frame components made of pure CFRP as well as hybrid CFRP/titanium laminates. Four quasi-static as well as four dynamic frame bending tests were conducted to investigate the energy absorbing failure mechanisms of such hybrid frame structures. PubDate: 2013-04-05
Abstract: Abstract
Within this paper, the guided Lamb wave propagation in thin honeycomb sandwich panels is studied. The Lamb waves are excited by thin piezoelectric (PZT) patch actuators glued to the surface of the plate, and the signals are received by similar PZT sensor patches. Such actuator and sensor systems can be used for a cost-effective online health monitoring of structures. In homogeneous plates, Lamb waves propagate with symmetrical and anti-symmetrical modes. However, the propagation in heterogeneous media is not as clear and depends on the exciting frequency, the material properties, and the geometry of the structure. In this paper, the influence of the geometrical properties of honeycomb plates on the Lamb wave propagation is studied. For this purpose detailed 3-D finite element calculations are performed, which result in very time consuming computations. Consequently, also simplified models are developed to reduce the computing time without losing the quality of the results. To this end, the honeycomb core material is replaced by a homogeneous layer with orthotropic mechanical properties. The homogenized properties are calculated numerically using a homogenization technique based on the representative volume element method. The comparison of the results received with the two different approaches has shown that the simplified models are in a good agreement with the extended models for a certain range of exciting frequencies and geometrical properties only. The wave propagation on the top and bottom surfaces is also compared in order to show how deep the waves can travel inside the structure. PubDate: 2013-04-01
Abstract: Abstract
Structural Health Monitoring based on Lamb waves, a type of ultrasonic guided waves, is a promising method for in-service inspection of composite structures. In this study, mode selective actuators are developed to excite a particular Lamb wave mode in quasi-isotropic CFRP (carbon fibre-reinforced polymer) plates. Different manufacturing technologies based on monolithic piezoceramics and piezocomposites are described. The actuators are based on interdigital transducer design in order to control the frequency as well as the wavelength of the desired mode within the excitation. To determine the wavelength of the desired Lamb wave mode, experimental dispersion diagrams of the CFRP plates are measured using air-coupled ultrasonic scanning technique. The dispersion diagrams show the A
0 and S
0 mode in a frequency range of 25–400 kHz. A mode selective actuator is designed to amplify the A
0 mode and to attenuate the S
0 mode at 40 kHz. Within experimental tests the actuator and circular piezoceramic sensors are applied on a CFRP plate in order to determine the mode selectivity. These tests are accompanied by 2D finite element simulations. On the basis of simulations and experimental test the influence of different parameters such as number and width of electrode segments, excitation signals and apodization is investigated. The results show that a mode selectivity of the A
0 mode in CFRP plates can be achieved by the designed actuator. PubDate: 2013-04-01
Abstract: Abstract
Accurate knowledge of the dispersion relations of guided waves in plates is important for the efficient use of Lamb wave-based damage-detection methods. In this paper, we introduce a method which aims at automatically extracting the dispersion curves from laser vibrometer measurement data in an easy and robust manner. This method works by Fourier transforming the measurement data into the wavenumber domain and then applying the matrix pencil method by Hua and Sarkar to extract the wavenumber-dependent frequencies. As an additional result, we are able to experimentally detect backward propagating waves in aluminium plates. PubDate: 2013-04-01
Abstract: Abstract
Wave propagation in plates are multi-modal and dispersive by nature. Their behavior is highly dependent on the material properties. In homogeneous isotropic plates Lamb modes can be grouped into symmetric and anti-symmetric modes and they are decoupled from the shear modes. Due to the material isotropy, Lamb wave’s propagation behavior is not dependent on the propagation direction. This property can be used to find analytical solutions for the field of displacements in the frequency domain. In this paper, series of numerical simulations on the Lamb wave propagation in homogeneous isotropic are presented. The concept of the membrane carrier wave is used together with integral transforms in the space domain, and analytical expressions are found for the response of a homogeneous isotropic plate under different load regimes. We considered line forces and axisymmetric loads applied to the plate. The procedure can be applied to other types of load distributions. PubDate: 2013-04-01
Abstract: Abstract
The use of scanning laser-vibrometry for the investigation of various phenomena of Lamb wave propagation and interaction for structural health monitoring purposes in CFRP-structures is presented. Different measuring modes and data processing are introduced to characterise piezoactuators and their wave fields, measure velocity dispersion and direction-dependent attenuation and describe the interactions with defects. PubDate: 2013-04-01
Abstract: Abstract
For the analysis of wave propagation at high frequencies, the spectral finite element method is under investigation. In contrast to the conventional finite element method, high-order shape functions are used. They are composed of Lagrange polynomials with nodes at the Gauss–Lobatto–Legendre points. The Gauss–Lobatto–Legendre integration scheme is applied in order to obtain a diagonal mass matrix. The resulting system equations can be solved efficiently. In the numerical examples, spectral finite elements with shape functions of different order are applied to a plane strain problem. The numerical examples cover structures without and with stiffness discontinuities. It is shown that the results agree well with analytical and experimental solutions. PubDate: 2013-04-01
Abstract: Abstract
The development of a structural health monitoring (SHM) strategy based on a PZT phased array system is proposed. The objective is to increase the low signal-to-noise ratio (SNR) compared to PZT networks for Lamb wave-based SHM systems. This is achieved by constructive interference—beamforming—of the different waves generated by the different transducers in the array. By carefully selecting and changing the delays in between the actuation of consecutive transducers in the array, a generated wave front can be steered to different selected directions in the plane of the plate component being scanned. By increasing the amplitude of generated waves, through beamforming, potential damage reflected waves present also an increased amplitude and higher SNR, facilitating their assessment in sensor signals and consequently damage detection. The developed automatic system was designed based on the use of the fast propagating fundamental symmetric Lamb wave mode (S
0). The accuracy of the method is strongly dependant on a precise multiple actuation system and particularly in the accuracy at which the diminutive time delays are introduced in between the actuation of the different array elements. This problem was addressed by developing a dedicated multiple actuation system. The developed system merits were explored particularly and more importantly considering the application of the fast propagating and dispersive fundamental Lamb wave mode, requiring alternative actuation strategies with respect to conventional NDI systems based on non-dispersive body waves. This is done also to considerably decrease the threshold for minimum detectable damage dimensions which can be verified by conventional NDI systems and by similar state-of-the-art SHM systems, in an attempt to benefit a subsequent damage repair and/or prognosis procedure. Tests using the developed system were performed with the successful and repeatable detection of the 1 mm damages applied cumulatively into both aluminum and composite plates, subjected to different boundary conditions. Damages were simulated by surface and through thickness holes and cuts with different orientations. Finally, a network and phased array were also applied to a more complex composite panel with embedded fiber Bragg grating (FBG) optical sensors. An FBG interrogation technique, based in a tunable laser and photo-detector, was developed. The laser is tuned just before test execution and it is not changed during scans. With no moving components, this technique does not impose intrinsically a maximum sampling frequency. At the same time, the influence of temperature and operational induced strains (both static and due to low frequency vibration) are eliminated. This technique proved to be robust and of great potential for a future aircraft implementation. PubDate: 2013-04-01
Abstract: Abstract
Lamb wave propagation in plates is multi-modal, dispersive and highly dependent on the material properties. In homogeneous isotropic plates, Lamb modes can be grouped into symmetric and anti-symmetric modes, and they are decoupled from the shear modes. Due to the material isotropy, Lamb wave propagation behavior is independent from the propagation direction. This property can be used to find analytical solutions for the field of displacements in the frequency domain. However, in composite plates having anisotropic material properties, Lamb wave propagation behavior depends on the propagation direction. For this reason, the complexity of Lamb wave propagation modes increases, and no direct analytical solutions are available. Thus, numerical methods need to be used. In this paper, the semi-analytical finite element (SAFE) method is applied as it is suitable for both isotropic homogeneous plates and anisotropic composite laminated plates. Dispersion curves for these complex materials are calculated using SAFE. The effect of obstacles on the reflection and transmission of Lamb waves is considered. SAFE is also used for 2D point force response analysis. By applying the 2D point response analysis for perfectly bonded strip actuators, mode tuning behavior is calculated for general composite plates. A method to consider plate edge reflections in the 2D force response analysis is also presented. PubDate: 2013-04-01
Abstract: Abstract
Piezoelectric induced ultrasonic Lamb waves can easily be used for the development of structural health monitoring systems for aircraft and other thin-walled structures. The reduction of the excited wave amplitudes depends mainly on the traveling distance and the material damping, especially in composite materials which restricts the maximum scanning distance between a piezoelectric actuator and a sensor. But there are several possibilities to increase the scanning range of Lamb waves. In the present paper, the focus is on the influence of the adhesive layer and the resonances of the actuator to increase the amplitudes of the excited wave. The objective is to excite Lamb waves with higher amplitudes without increasing the electrical energy for the wave excitation. In the paper, a numerical optimization is proposed which aims at increasing the wave amplitudes by optimizing sensor parameters and the excitation frequency. It has been found that small changes in the geometry of the piezoelectric actuator patch and the use of an optimized excitation frequency elevate the amplitudes of waves significantly. PubDate: 2013-04-01
Abstract: Abstract
The present paper aims at a numerical concept for simulating pneumatic active flow control devices in aeronautical flows. The numerical approach simulates the effect of vortex generator jets (VGJ’s) via chimera grids using Reynolds-averaged Navier–Stokes equations (RANS) and zonal detached-Eddy simulation (DES). The numerical results with active flow control over the flat plate and two-element DLR-F15 high-lift airfoil are validated against the experiments at chord-based Reynolds number upto
$Re_x=7.67 \times 10^6$
and
$Re_x=2.0 \times 10^6$
for flat plate and airfoil, respectively. The flat plate results showed that the use of DES model predicts the peak vorticity, turbulent transport, and flow dynamics of the vortex quite well at the expense of computational effort compared to the RANS approach. Numerical Unsteady RANS (URANS) simulations showed a positive effect of the VGJ’s on the lift curve that lead to abrupt leading edge stall, whereas wind tunnel data that included tunnel side-wall effects indicate a more gentle stalling process. The zonal DES computations confirmed the stalling behaviour observed with URANS. At the highest angle of attack both approaches show growing cross flow separations that result from the common flow up behaviour between the longitudinal vortices. PubDate: 2013-03-31
Abstract: Abstract
This paper is concerned with the optimization of fire resistance, impact properties and fast curing of CFRP. The investigation is part of the CFRP fuselage project. Nanoscale additives selected from the most promising materials are used to optimize fire resistance and impact properties. The challenge is to disperse the particles thoroughly and to determine the optimal particle concentrations. The optimum has to be determined taking into consideration the expected effect and the resulting material properties. Standard fire and mechanical tests are therefore carried out together with a thermogravimetric analysis, a simple thermal material process, which is used for fast comparison of the fire properties of the material. These results are compared and the related behaviour of the fire properties can be shown for standard fire tests and the TGA. It can be shown that the impact properties of both tough and hard particles improve the impact resistance of CFRP. Rubber particles as well as CoreShell particles are particularly well-suited as toughening agents and improve the CAI values by up to 28 %. A comprehensive knowledge of the behaviour of the curing resins is necessary for reduction of the curing time of the RTM-resin, which passes through different material stages. The most effective time reduction results from the omission of several heating phases during the hardening process of a component. Neat resin test specimens are therefore produced and the mechanical properties are examined. One of the challenges is the proper filling of the RTM-component, which depends amongst other things on the resin pot life and viscosity while the resin is liquid. Standard test panels are therefore produced from different promising resins using adjusted cure cycles and finally the mechanical properties are tested. PubDate: 2013-03-26
Abstract: Abstract
A refined beam model with hierarchical features is in this work extended to the static aeroelastic analysis of lifting surfaces made of metallic and composite materials. The refined structural one-dimensional (1D) theory is based on the Carrera Unified Formulation and it permits to take into account any cross-section deformation, including warping effects. The vortex lattice method is employed to provide aerodynamic loadings along the two in-plane wing directions (wing span and wing cross-section). Applications are obtained by developing a coupled aeroelastic computational model which is based on the finite element method. The accuracy of the proposed 1D model is shown by a number of applications related to various wings made of metallic and composite materials. The effect of the cross-section deformation is evaluated on the aeroelastic static response and divergence of the considered wings. The need of higher-order expansions is underlined as well as the limitations of beam results which are based on classical theories. Comparison with results obtained by existing plate/shell aeroelastic models shows that the present 1D model could result less expensive from the computational point of view with respect to shell cases. The beneficial effects of aeroelastic tailoring in the case of wings made of composite anisotropic materials are also confirmed by the present analysis. PubDate: 2013-02-27
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