1,721,132 research outputs found
Modelling the limit cycle oscillations of flat plate wings using inextensible plate theory and the vortex lattice method
No full textA closed form state-space model of the nonlinear aeroelastic response of thin cantilever flat plates is derived using a combination of Inextensible thin plate theory and a linearized continuous time vortex lattice aerodynamic model. The modal-based model is solved for the amplitude and period of the limit cycles of the flat plates using numerical integration. The resulting predictions are compared to theoretical predictions obtained using Von Karman thin plate theory for an identical flat plate. It is shown that the aeroelastic model predicts the linear flutter conditions and nonlinear response of the plates with reasonable accuracy and the Limit Cycle Oscillation (LCO) amplitude, calculated from the inextensible plate theory, has an initial curvature very similar to the one obtained during of experimental test on similar plates, contrary to the amplitude predictions of the Von Karman model. This striking feature is very encouraging for future experimental and numerical work
Experimental Investigation of Space Debris Separation in Cold Hypersonic Flow Using a Free-flight Measurement Technique
No full textThe aerodynamic forces and aerothermal loads experienced by spacecraft components during a fragmenting re-entry likely influence their trajectories, demisability, and, as a consequence, the resulting ground casualty probability.
Due to the complexity of this phenomenon, Design for Demise tools often simplify the dispersion of the fragments and the interactions between the components, considering their trajectories as independent (i.e. neglecting any mutual interaction) as soon as a structural limit triggers the fragmentation event.
Studying the interaction of proximal bodies and clusters is required to develop improved separation models that can advance demisability predictions.
The present doctoral thesis experimentally examines aerodynamic interactions between separating space debris objects in cold hypersonic flow conditions.
The investigations are executed in the VKI Longshot wind tunnel at Mach 12 and Mach 14 flow conditions.
The newly manufactured Mach 14 nozzle is commissioned via a flow characterization campaign, revealing crucial parameters for the design of free-flight experiments, which are, hence, used throughout the work.
In free-flight testing methodology, the test model is exposed to the flow without structural constraints; hence, it performs an unrestricted flight driven by its aerodynamic properties, the freestream conditions, and gravity.
The aerodynamic forces and moments are then derived from the model's motion in the freestream.
In short-duration facilities, the capabilities of free-flight testing techniques proved superior to balance measurements since they allow unperturbed flowfields in the close vicinity of the test articles, and optical-based methods permit the analysis of multiple models simultaneously.
A state-of-the-art free-flight testing technique has been developed in the frame of this doctoral work.
The methodology employs two high-speed cameras to record via the Schlieren system of the wind tunnel a side view and via a backlighting setup a top view perspective of the experiments.
A contour-matching-based algorithm is developed to identify the position and attitude of one to multiple test articles at the same time, which may even present significant overlaps.
Six degrees-of-freedom flight trajectories are derived from such analyses, allowing the computation of the aerodynamic forces that drove the motion.
The thesis presents a macroscopic to microscopic analysis of space debris separations via particular experimental campaigns investigating the behavior of fragment clusters, two separating proximal bodies, and the interaction of a typical debris object with a two-dimensional curved shock wave.
Experiments with clusters and two-body tandem configurations of spherical and cuboid fragments confirm a high degree of dependency on the shape of the objects.
Testing with cubes presented significantly (~40%) higher mean separation velocities, larger extrema, and more substantial test-to-test variability.
The interference of an annular ring with the two-dimensional shock wave was a microscopic-scale study, coupling the flow visualization, the flight trajectory, and the measured aerodynamic coefficients.
During the most interesting test, a significant flow-normal velocity is induced even at the early stage of the interaction, which could promote a shock wave surfing configuration and contribute to object dispersal in a real fragmentation scenario.
Overall, the experimental observations conducted in the framework of this thesis suggest a strong influence on the bodies’ initial arrangement and the individual orientations, which are also responsible for the induced lift, that must have an essential contribution to the spread
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
AERODYNAMIC AND AEROELASTIC MODELING OF HORIZONTAL AXIS WIND TURBINES USING PANEL METHODS
No full textWind energy is one of the most reliable renewable energy sources and constitutes
a viable alternative to fossil and nuclear fuels for the generation of
electricity. Over the last couple of decades the increasing demand for wind
energy has resulted in increasingly large and sophisticated wind turbines. Accurate
but efficient aerodynamic and aeroelastic modelling at the design stage
has become a key issue. The surface panel and vortex lattice methods are efficient
aerodynamic modelling tools that are routinely employed in design
calculations by the aerospace industry. They constitute a good compromise
between fidelity and computational cost in the preliminary design and optimization
phase. However, these approaches have not been widely adopted for
wind turbine modelling due to their inability to represent separated flow.
The main objective of this thesis is the development of a 3D unsteady viscous-inviscid
interaction technique that couples panel methods to a boundary layer
solution and can be used to model separated flow over the blades of a wind
turbine rotor. The technique is based on a quasi-3D, quasi-steady integral
boundary layer solution, coupled to a 3D unsteady surface panel method by
means of a two-way interaction scheme. The boundary layer solution results
in an estimate of the separation line on the suction surface of the blade. A
separated shear layer made up of doublet panels is shed from this line and
allowed to propagate freely at the local flow velocity, exactly like the wake shed
at the trailing edge. Aerodynamic pressure and load predictions obtained from
this method are validated through comparison to experimental measurements
from the NREL phase VI wind turbine.
The thesis also describes the development of a complete methodology for the
unsteady aeroelastic and aeroservoelastic modeling of horizontal axis wind
turbines at the design stage. The methodology is based on the implementation
of unsteady aerodynamic modeling, advanced control strategies and
nonlinear finite element calculations in the Siemens LMS Samcef for Wind
Turbines design package. The aerodynamic modelling is carried out by means
of the unsteady Vortex Lattice Method. The complete methodology is used
to perform full aeroservoelastic simulations of a 2MW prototype wind turbine
model.DYNAWIN
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Continuation of Higher Order Harmonic Balance Solutions for Nonlinear Aeroelastic Systems
No full textpeer reviewedThe Harmonic Balance method is a very useful tool for characterizing and predicting the response of nonlinear dynamic systems undergoing periodic oscillations, either self-excited or due to harmonic excitation. The method and several of its variants have been applied to nonlinear aeroelastic systems over the last two decades.
This paper presents a detailed description of several Harmonic Balance methods and a continuation framework allowing the methods to follow the response of dynamic systems from the bifurcation point to any desired parameter value, while successfully negotiating further fold bifurcations. The continuation framework is described for systems undergoing sub-critical and super-critical Hopf bifurcations as well as a particular type of explosive bifurcation.
The methods investigated in this work are applied to a nonlinear aeroelastic model of a Generic Transport Aircraft featuring polynomial or freeplay stiffness nonlinearity in the control surface. It is shown that high order Harmonic Balance solutions will capture accurately the complete bifurcation behavior of this system for both types of nonlinearity. Low order solutions can become inaccurate in the presence of numerous folds in the Limit Cycle Oscillation branch but can still yield practical engineering information at a fraction of the cost of higher order solutions. Time domain Harmonic Balance schemes are shown to be more computationally expensive than the standard Harmonic Balance approach
Approximate Numerical Continuation for Aeroelastic Systems Undergoing Aperiodic Limit Cycle Oscillations
No full textThis paper presents a modified numerical continuation approach for predicting the bifurcation
behaviour of aeroelastic systems undergoing aperiodic limit cycles oscillations. Such
oscillations can occur due to a number of nonlinear functions. Here, backlash nonlinearity in
the aileron stiffness for a Generic Transport Aircraft is considered. It is shown that classical
numerical continuation will fail due to the aperiodic nature of the limit cycles and the inability
to perform period scaling and phase fixing. An alternative, approximate numerical continuation
method is proposed, based on longer numerical integration sequences and a heuristic method
for determining the period of the limit cycle oscillations. The approach is applied successfully
to a simulated aeroelastic model of the Generic Transport Aircraft with backlash
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