Browsing by Author "Van Zyl, Louwrens H"

Now showing 1 - 12 of 12
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    Development of a sine-dwell ground vibration test (GVT) system
    (2006-02-27) Van Zyl, Louwrens H; Wegman, Erik J
    Knowledge of the natural modes of vibration of a structure is required to solve or avoid vibration and flexibility problems in industrial, automotive, aerospace and civil engineering applications. All new aircraft must undergo a flutter clearance to ensure that it will be free from flutter within the intended operating envelope. Long-span bridges are also subject to flutter, and high-rise buildings can oscillate severely in high winds. Vibrations in industrial installations are also quite common and are often due to the unfortunate matching of an excitation frequency and a natural frequency of the installation. The methods of determining the natural modes of a structure are continually evolving, and this paper describes one GVT system with some novel features
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    DLM for T-Tails
    (2007-06) Van Zyl, Louwrens H
    This paper describes the extension of the DLM to account for effects that are critical to the modelling of T-tail flutter. The boundary condition is made more general to account for yaw/dihedral and sideslip/dihedral coupling and the calculation of forces is generalised to account for lateral load due to roll, and rolling moment due to yaw, yaw rate and sideslip. In addition, the steady load and the quadratic components of the mode shapes are taken into account in the calculation of generalised forces
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    Flutter clearance techniques and tools for gliders
    (2024-08) Van Zyl, Louwrens H
    The flutter clearance of gliders pose some special challenges. Modal frequencies are low, making suspension of the glider for ground vibration testing challenging. The low frequencies also make large amplitudes of excitation desirable. In flutter flight testing space for excitation systems and telemetry systems is limited.
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    Framework for T-tail flutter analysis
    (2011-06) Van Zyl, Louwrens H
    The flutter analysis of T-tail aircraft poses challenges that are unique to this configuration, including the fact that the unsteady air loads are dependent on the steady load distribution and static deformation of the aircraft. In particular, the trim load on the horizontal stabilizer and the static deformation of the horizontal stabilizer, an induced dihedral effect, are significant. These effects are now well understood and accurate analyses can be made for a given set of conditions of incidence angle, elevator deflection and deformation. This paper considers the process required to perform the flutter analyses of a T-tailed aircraft.
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    Holistic approach to flutter clearance using classical methods
    (2006-09) Van Zyl, Louwrens H
    Flutter clearance is usually regarded as a two step process: flutter analysis (including ground vibration testing, aerodynamic analysis and equation of motion solution), and flutter flight testing. The problem with this approach is that there are often significant discrepancies between the predicted damping and frequency trends and the trends measured during flutter flight testing. The discrepancies, when they become apparent during flight testing, cast doubt on the accuracy and validity of the analysis and hamper processing of the flight test data. By employing relatively simple techniques in combination, the analyst can be much better prepared for the flutter flight tests, enabling him to process the data efficiently despite the usual discrepancies and to better judge whether the discrepancies indicate serious deficiencies in the flutter analysis.
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    Hybrid finite-volume-ROM approach to non-linear aerospace fluid-structure interaction modelling
    (2011-06) Mowat, AGB; Malan, AG; Van Zyl, Louwrens H; Meyer, JP
    A fully-coupled partitioned fluid-structure interaction (FSI) scheme is developed for sub- and transonic aeroelastic structures undergoing non-linear displacements. The Euler equations, written in an Arbitrary Lagrangian Eulerian (ALE) coordinate frame, describe the fluid domain while the structure is represented by a quadratic modal reduced order model (ROM). A Runge-Kutta dual-timestepping method is employed for the fluid solver, and three upwind schemes are considered viz. AUSM+ -up, HLLC and Roe schemes. The HLLC implementation is found to offer the superior balance between efficiency and robustness. The developed FSI technology is applied to modelling nonlinear flutter, and the quadratic ROM demonstrated to offer dramatic improvements in accuracy over the more conventional linear method.
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    Integration of the supersonic kernel function
    (American Institute of Aeronautics and Astronautics, 1994-11) Van Zyl, Louwrens H
    The article discusses ways in which the integrals resulting from a zero-order discontinuous pressure distribution can be arranged in such a way that they can be solved by either normal quadrature or curve fitting followed by analytical integration is shown. This ability amplifies the panel method for unsteady supersonic flow and is essential to model the discontinuities that occur in reality, e.g., at the supersonic leading or trailing edges and control surface hinge lines.
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    Modifications to 25m2 target-aligned research Heliostat mirror panels
    (2009-09) Roos, TH; Rossouw, F; Van Zyl, Louwrens H
    Several heliostat mirror panels of the CSIR 25m2 target-aligned heliostat suffered corrosion-related failure, prompting a panel redesign. Two test samples of the new design were subjected to mechanical and thermal cycling tests in an attempt to simulate accelerated life loading, as well as simulated hail testing. While the samples survived all tests without failure, the results are not fully conclusive as the mechanical cycling and hail test sample was manufactured of 4mm instead of the design 3mm mirror sheet. The test results nonetheless suggest that a sample made of 3mm mirror sheet would successfully survive the tests, and preparations for such tests are underway. Conclusive thermal cycling tests require a reduced scale mirror panel to be manufactured and tested in an environmental test chamber.
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    A pragmatic approach to including complex natural modes of vibration in aeroelastic analysis
    (2015-09) Van Zyl, Louwrens H
    Aeroelasticity is often described as the study of the interaction of inertial, elastic and aerodynamic forces that occur when an elastic body is exposed to a fluid flow (Wikipedia). The aim of a flutter analysis is to determine the speed above which structural vibrations will grow exponentially and potentially cause structural failure. On the one hand it is necessary to model how the structure would respond to forces applied to it, and on the other hand it is necessary to model what aerodynamic forces would be generated due to the movement of the structure. This presentation concerns mainly the structural dynamic component of the aeroelastic problem, and specifically the structural damping forces (which is usually not mentioned in the definition of aeroelasticity).
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    T-tail flutter analysis using Edge
    (2011-11) Van Zyl, Louwrens H; Gledhill, Irvy MA
    A grid was generated with the base of the fin, i.e. the hinge line, on the axis of a cylindrical domain with a diameter of 2m. Due to the axial symmetry of the setup, rotation of the T-tail should not change the pressure distribution on the T-tail. In reality, due to some a-symmetry in the grid, this was not quite achieved. The analyses consisted of a steady solution, followed by a prescribed, sine-squared, disturbance of 0.03 radians (corresponding to 10 mm lateral displacement at the fin top), followed by a coupled time-domain simulation.
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    Unsteady panel method for complex configurations including wake modeling
    (American Institute of Aeronautics and Astronautics, 2008-01) Van Zyl, Louwrens H
    The calculation of unsteady air loads is an essential step in any aeroelastic analysis. The subsonic doublet lattice method (DLM) is used extensively for this purpose due to its simplicity and reliability. The body models available with the popular implementations of the DLM are however not very versatile in terms of geometries that can be modeled. The ZONA6 code offers a versatile surface panel body model including a separated wake model, but uses a pressure panel method for lifting surfaces. This paper describes the development of an unsteady subsonic wing-body code based on the DLM, with a source panel representation of bodies, a separated wake flow model based on the ZONA6 approach and an approximate method of images to model wing-body interference. This approach provides greater flexibility in body shapes that can be modeled while retaining the reliability of the DLM for lifting surfaces.
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    Use of eigenvectors in the solution of the flutter equation
    (American Institute of Aeronautics and Astronautics, 1993-07) Van Zyl, Louwrens H
    The use of eigenvectors to assign eigenvalues to modes for the p-k formulation of the flutter equation is described. The procedure has the potential to overcome some of the problems of the determinant iteration procedure to solve the flutter equation. Advantages of the proposed procedure include the possibility of using a general eigenvalue routine capable of solving repeated eigenvalues, effective distinguishing of eigenvalues, initial frequency values requirement used only for the first calculation of reduced frequency at each speed, and reduced computation time.