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  • 1.
    Lokatt, Mikaela
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Aeroelastic flutter analysis considering modeling uncertainties2017In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 74, p. 247-262Article in journal (Refereed)
    Abstract [en]

    A method for efficient flutter analysis of aeroelastic systems including modeling uncertainties is presented. The aerodynamic model is approximated by a piece-wise continuous rational polynomial function, allowing the flutter equation to be formulated as a set of piece-wise linear eigenproblems. Feasible sets for eigenvalue variations caused by combinations of modeling uncertainties are computed with an approach based on eigenvalue differentials and Minkowski sums. The method allows a general linear formulation for the nominal system model as well as for the uncertainty description and is thus straightforwardly applicable to linearized aeroelastic models including both structural and aerodynamic uncertainties. It has favorable computational properties and, for a wide range of uncertainty descriptions, feasible sets can be computed in output polynomial time. The method is applied to analyze the flutter characteristics of a delta wing model. It is found that both structural and aerodynamic uncertainties can have a considerable effect on the damping trends of the flutter modes and thus need to be accounted for in order to obtain reliable predictions of the flutter characteristics. This indicates that it can be beneficial to allow a flexible and detailed formulation for both aerodynamic and structural uncertainties, as is possible with the present system formulation.

  • 2.
    Lokatt, Mikaela
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    On a Viscous-Inviscid Interaction Model for Aeronautical Applications2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The work presented in this thesis concerns the development of a viscous-inviscid interaction model for prediction of viscous flow properties in aeronautical applications. The inviscid model is based on an existing potential flow model and the thesis thus focuses on the development of a viscous model based on the three-dimensional integral boundary layer equations. The model is to be applicable to complex geometries with unstructured meshes and this requirement, in combination with the fairly complex character of the integral boundary layer equations, sets high standards for the discretization. In order to arrive at a stable and well-conditioned scheme a number of topics related to the formulation and discretization of the integral model are analyzed and discussed. These include the challenge of finding a discretization which ensures flow conservation on curved surfaces and provides a stable and well-conditioned discretization for mixed-hyperbolic systems of conservation laws. Different coupling strategies for the viscous and inviscid models are discussed and analyzed and so are the singularities in the integral boundary layer equations in separated flow regions. The predictions of the resulting viscous-inviscid coupling scheme are validated by comparison to experimental measurements as well as to predictions from other numerical models. The currently developed coupled model is found to provide reasonably accurate predictions of viscous flow properties in laminar as well as turbulent flow regions while being stable and convergent in separated flow regions.

  • 3.
    Lokatt, Mikaela
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    On Aerodynamic and Aeroelastic Modeling for Aircraft Design2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The work presented in this thesis was performed with the aim of developing improved prediction methods for aerodynamic and aeroelastic analysis to be used in aircraft design. The first part of the thesis concerns the development of a viscous-inviscid interaction model for steady aerodynamic predictions. Since an inviscid, potential flow, model already is available, the main focus is on the development of a viscous model consisting of a three-dimensional integral boundary layer model. The performance of the viscous-inviscid interaction model is evaluated and it is found that the accuracy of the predictions as well as the computational cost appear to be acceptable for the intended application. The presented work also includes an experimental study aimed at analyzing steady and unsteady aerodynamic characteristics of a laminar flow wing model. An enhanced understanding of these characteristics is presumed to be useful for the development of improved aerodynamic prediction models. A combination of nearly linear as well as clearly nonlinear aerodynamic variations are observed in the steady as well as in the unsteady experimental results and it is discussed how these may relate to boundary layer properties as well as to aeroelastic stability characteristics. Aeroelastic considerations are receiving additional attention in the thesis, as a method for prediction of how flutter characteristics are affected by modeling uncertainties is part of the presented material. The analysis method provides an efficient alternative for obtaining increased information about, as well as enhanced understanding of, aeroelastic stability characteristics.

  • 4.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    A study of unsteady aerodynamic loads on a natural laminar flow wing model2017Manuscript (preprint) (Other academic)
  • 5.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Experimental validation of a viscous-inviscid interaction model2017Manuscript (preprint) (Other academic)
  • 6.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Finite-Volume Scheme for the Solution of Integral Boundary Layer EquationsManuscript (preprint) (Other academic)
  • 7.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Finite-volume scheme for the solution of integral boundary layer equations2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 132, p. 62-71Article in journal (Refereed)
    Abstract [en]

    An unstructured-mesh finite-volume formulation for the solution of systems of steady conservation laws on embedded surfaces is presented. The formulation is invariant to the choice of local tangential coordinate systems and is stabilized by a novel up-winding scheme applicable also to mixed-hyperbolic systems. The formulation results in a system of non-linear equations which is solved by a quasi-Newton method. While the finite volume scheme is applicable to a range of conservation laws, it is here implemented for the solution of the integral boundary layer equations, as a first step in developing a fully coupled viscous-inviscid interaction method. For validation purposes, integral boundary layer quantities computed using a minimal set of three-dimensional turbulent integral boundary layer equations are compared to experimental data and an established computer code for two-dimensional problems. The validation shows that the proposed formulation is stable, yields a well-conditioned global Jacobian, is conservative on curved surfaces and invariant to rotation as well as convergent with regard to mesh refinement.

  • 8.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Robust Viscous-Inviscid Interaction Scheme for Application on Unstructured MesheManuscript (preprint) (Other academic)
  • 9.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Robust viscous-inviscid interaction scheme for application on unstructured meshes2017In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 145, p. 37-51Article in journal (Refereed)
    Abstract [en]

    A coupled viscous-inviscid interaction scheme combining the continuity equation for potential flow with the three-dimensional integral boundary layer equations is presented. The inviscid problem is discretized by a finite-element approach whereas an upwind-biased finite-volume scheme is employed for the boundary layer equations. The discretization is applicable to unstructured tetrahedral-triangular meshes and results in a sparse system of non-linear equations which is solved by a Newton-type method. The mathematical reasons for the singularities commonly associated with the integral boundary layer equations in separated flow regions are analyzed and the connection between the mathematical singularities and the numerical ill-conditioning is discussed. It is shown that, by a suitable choice of closure relations, it is possible to obtain a boundary layer model free from numerical ill-conditioning in separated flow regions. The accuracy of the coupled viscous-inviscid model is investigated in a number of test cases including transitional and mildly separated flow over two different natural laminar flow airfoils and three-dimensional flow over a swept wing. It is concluded that the coupled method is able to provide reasonably accurate predictions of viscous and inviscid flow field quantities for the investigated cases.

1 - 9 of 9
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