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Zhang X.F.,NRC Institute for Aerospace Research | Hodson H.,University of Cambridge
Journal of Turbomachinery | Year: 2010

The effects of Reynolds numbers and the freestream turbulence intensities (FSTIs) on the unsteady boundary layer development on an ultra-high-lift low-pressure turbine airfoil, so-called T106C, are investigated. The measurements were carried out at both Tu =0.5% and 4.0% within a range of Reynolds numbers, based on the blade chord and the isentropic exit velocity, between 100,000 and 260,000. The interaction between the unsteady wake and the boundary layer depends on both the strength of the wake and the status of the boundary layer. At Tu=0.5%, both the wake's high turbulence and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface of the airfoil. Since the wake turbulence cannot induce transition before separation on this ultra-high-lift blade, the negative jet of the wake has the opportunity to induce a rollup vortex. At Tu=4.0%, the time-mean separation on the suction surface is much smaller. With elevated FSTI, the turbulence in the wake just above the boundary layer is no longer distinguishable from the background turbulence level. The unsteady boundary layer transition is dominated by the wake's negative jet induced boundary layer variation. © 2010 by ASME.

Poirel D.,Royal Military College of Canada | Yuan W.,NRC Institute for Aerospace Research
Journal of Fluids and Structures | Year: 2010

Experimental observations of self-sustained pitch oscillations of a NACA 0012 airfoil at transitional Reynolds numbers were recently reported. The aeroelastic limit cycle oscillations, herein labelled as laminar separation flutter, occur in the range 5.0×104≤Rec≤1.3×105. They are well behaved, have a small amplitude and oscillate about O{middle tilde}=0°. It has been speculated that laminar separation leading to the formation of a laminar separation bubble, occurring at these Reynolds numbers, plays an essential role in these oscillations. This paper focuses on the Rec=7.7×104 case, with the elastic axis located at 18.6% chord. Considering that the experimental rig acts as a dynamic balance, the aerodynamic moment is derived and is empirically modelled as a generalized Duffing-van-der-Pol nonlinearity. As expected, it behaves nonlinearly with pitch displacement and rate. It also indicates a dynamically unstable equilibrium point, i.e. negative aerodynamic damping. In addition, large eddy simulations of the flow around the airfoil undergoing prescribed simple harmonic motion, using the same amplitude and frequency as the aeroelastic oscillations, are performed. The comparison between the experiment and simulations is conclusive. Both approaches show that the work done by the airflow on the airfoil is positive and both have the same magnitude. The large eddy simulation (LES) computations indicate that at O{middle tilde}=0°, the pitching motion induces a lag in the separation point on both surfaces of the airfoil resulting in negative pitching moment when pitching down, and positive moment when pitching up, thus feeding the LCO. © 2010.

Liao M.,NRC Institute for Aerospace Research
Engineering Fracture Mechanics | Year: 2010

This paper presents a study on dislocation theory based short/small crack modeling, and its application for short crack growth life analysis on 2024-T351 aluminum specimens. The dislocation theory was applied to determine the crack tip opening displacement (CTOD) of a microstructurally short crack by taking into account the effects of microstructural features, such as grain size, orientation, and grain boundary. The CTOD was then used as the parameter for calculating the short crack growth rate. In this work, an existing CTOD model was modified for estimating the short crack life of 2024-T351 single edge notch tension (SENT) specimens, which were tested in an Advisory Group for Aerospace Research and Development (AGARD) program [Newman JC, Edward PR. Short-crack growth behavior in an aluminum alloy: an AGARD cooperative test programme. AGARD-R-732, NATO, Advisory Group for Aerospace Research and Development; 1988]. The analytical results matched the test results reasonably well. Crown Copyright © 2009.

Dubourg L.,Institute Maupertuis | Merati A.,NRC Institute for Aerospace Research | Jahazi M.,NRC Institute for Aerospace Research
Materials and Design | Year: 2010

This paper focuses on the results of process optimisation and mechanical tests that were used to ascertain the feasibility of using friction stir welding (FSW) to join stringers to skin. The effects of process parameters on weld quality of 1.5-mm 7075-T6 stringers lap-joined on 2.3-mm 2024-T3 skins were investigated. Advancing and retreating side locations on the joint configuration were alternated to determine optimal design arrangements. The effects of travel and rotation speeds on weld quality and defect generation were also investigated. Weld quality was assessed by optical microscopy and bending tests. It was found that: (i) the increase of the welding speed or the decrease of the rotational speed resulted in a reduction of the hooking size and top plate thinning but did not eliminated them, (ii) double pass welds by overlapping the advancing sides improved significantly the weld quality by overriding the hooking defect, and (iii) change of the rotational direction for a counter clockwise with a left-threaded probe eliminated the top sheet thinning defect. Subsequently, FSW lap joints were produced using optimum conditions and underwent extensive mechanical testing program. Several assembly configurations including discontinuous and continuous welds as well as single and double pass welds were produced. The results obtained for cyclic fatigue performance of FSW panels are compared with riveted lap joints of identical geometry. S-. N curves, bending behaviour, failure locations and defect characterisation are also discussed. It was found that: (i) the tensile strength of FSW joints approached that of the base material but with a significant reduction in the fatigue life, (ii) the probe plunge and removal locations served as the key crack nucleation sites in specimens with discontinuous welds, and (iii) double pass welds with overlapping advancing sides showed outstanding fatigue life and very good tensile properties. The present work provided some valuable insight into both the fabrication and application of FSW on stringer/skin lap joints. © 2010 Elsevier Ltd.

Li G.,NRC Institute for Aerospace Research
Journal of Mechanics of Materials and Structures | Year: 2010

In this paper, the adhesive stresses in unbalanced bonded single-strap butt joints are theoretically studied. Mathematical difficulties in the analysis of high order differential equations were solved and closedform solutions for both the adhesive peel and shear stresses have been successfully developed. In the proposed solutions the adherends and doublers can be different in material and thickness. Peak stresses are located at the bonded overlap edges, especially at the inner edges. In addition, two-dimensional geometrically nonlinear finite element analyses were carried out to study the adhesive stresses in two different bonded butt joints. One was a special butt joint case with the adherends and doubler of identical material and thickness, and the other was a general butt joint case with different adherends and doubler. Good agreement in the adhesive stresses between the closed-form solutions and finite element results has been achieved. The single-strap butt joint actually consists of two single-lap joints; thus, the adhesive stress solutions can be further applied to unbalanced single-lap joints.

Vakanski A.,Ryerson University | Mantegh I.,NRC Institute for Aerospace Research | Irish A.,NRC Institute for Aerospace Research | Janabi-Sharifi F.,Ryerson University
IEEE Transactions on Systems, Man, and Cybernetics, Part B: Cybernetics | Year: 2012

The main objective of this paper is to develop an efficient method for learning and reproduction of complex trajectories for robot programming by demonstration. Encoding of the demonstrated trajectories is performed with hidden Markov model, and generation of a generalized trajectory is achieved by using the concept of key points. Identification of the key points is based on significant changes in position and velocity in the demonstrated trajectories. The resulting sequences of trajectory key points are temporally aligned using the multidimensional dynamic time warping algorithm, and a generalized trajectory is obtained by smoothing spline interpolation of the clustered key points. The principal advantage of our proposed approach is utilization of the trajectory key points from all demonstrations for generation of a generalized trajectory. In addition, variability of the key points' clusters across the demonstrated set is employed for assigning weighting coefficients, resulting in a generalization procedure which accounts for the relevance of reproduction of different parts of the trajectories. The approach is verified experimentally for trajectories with two different levels of complexity. © 2012 IEEE.

Cao X.,NRC Institute for Aerospace Research | Jahazi M.,NRC Institute for Aerospace Research
Materials and Design | Year: 2011

The effects of tool rotational speed and probe length on lap joint quality of friction stir welded 2-mm AZ31B-H24 magnesium alloy were investigated in terms of welding defects, microstructure and mechanical properties. Tensile shear load initially increases with increasing tool rotational speed but decreases with further increase. However, the tensile shear load always increases with increasing probe length. The highest shear strength is obtained using a set of welding parameters resulting in a downward hooking defect at the maximum stress location of the top sheet. Sound lap joints with low distortion, lack of cavity, minor kissing bond and preferable hooking defects, and high tensile shear strength were successfully obtained, indicating the great potential of friction stir welding technique for magnesium alloys. © 2010.

Ghinet S.,NRC Institute for Aerospace Research | Atalla N.,Université de Sherbrooke
Computers and Structures | Year: 2011

The paper describes the modeling of thick composite laminate and sandwich plates and beams with linear viscoelastic treatments. A discrete laminate model (DLM) is described, validated and compared to numerical spectral finite elements method (SFEM), finite element method (FEM) and experimental results. The DLM approach assumes each layer as thick laminate with orthotropic orientation, rotational inertia and transversal shearing, membrane and bending deformations. First order shear deformation theory is used. The equation of motion is developed following a wave approach based on discrete layer description. It handles symmetrical and asymmetrical layouts of unlimited number of transversal incompressible layers. Next, dilatational (symmetric mode) motion along the core's thickness is considered to complete the DLM solution when applied to the case of symmetric sandwich structures with soft and thick core. The model is compared to a second approach employing spectral finite elements. The latter handles composite laminated plates and beams with orthotropic orientation. It is shown that both approaches estimate accurately the propagating wave solutions of laminated structures. Using these solutions, the input mobility and the mechanical impedance are computed. The two models are successfully compared to classical finite elements as well as to experimental results for different boundary conditions. Moreover, the equivalent damping loss factor of composite laminate plates with viscoelastic treatment is addressed and the influence of the heading direction is discussed. © 2011 Elsevier Ltd. All rights reserved.

Galeote B.,NRC Institute for Aerospace Research
AIAA Atmospheric and Space Environments Conference 2010 | Year: 2010

Various methods and types of instrumentation have been used to measure the particle size distribution of ice crystals. They all have their strengths and weaknesses which affect the results. The National Research Council of Canada (NRC) has conducted a study to develop a technique to measure ice particulates using a particle image velocimeter (PIV) in a shadowgraph or backlighting configuration. Due to the inherent capability of the PIV system, using this technology for ice crystal particle measurement has some specific advantages in accomplishing this task. This paper describes the setup, methodologies used in the analysis and the experiments performed to calibrate the system. © 2010 by Her Majesty the Queen in Right of Canada.

At very low Froude number, buoyancy instabilities of round laminar jet diffusion flames disappear (except for small tip oscillations referred to as flickering) and those flames look stable and smooth. This study examines the contributions of the different phenomena in the flow dynamics that may explain this effect. It is observed that, at ultra-low Froude/Reynolds numbers, the material influenced by buoyancy is the plume of the flame and not the flame itself (reaction zone) that is short. Therefore, the vorticity creation zone does not profit from the reaction neighbourhood promoting a sharp gradient of density. Expansion and stretch are also important as they push vorticity creation terms more inside the flame and closer to the burner rim compared to moderate Froude flames. In these latter, the vorticity is continuously created around the flame reaction zone, along its developed height and closer to the vertical direction (in average). Crown Copyright © 2009.

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