A structure-coupled CFD method for time-marching flutter analysis

2007

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294

205

SUMMARYA multivariate interpolation scheme, using radial basis functions, is presented, which results in a completely unified formulation for the fluid-structure interpolation and mesh motion problems. The method has several significant advantages. Primarily, all volume mesh, structural mesh, and flow-solver type dependence is removed, and all operations are performed on totally arbitrary point clouds of any form. Hence, all connectivity and user-input requirements are removed from the computational fluid dynamics-computational structural dynamics (CFD-CSD) coupling problem, as only point clouds are required to determine the coupling. Also, it may equally well be applied to structured and unstructured grids, or structural and aerodynamic grids that intersect, again because no connectivity information is required. Furthermore, no expensive computations are required during an unsteady simulation, just matrix-vector multiplications, since the required dependence relations are computed only once prior to any simulation and then remain constant. This property means that the method is both perfectly parallel, since only the data relevant to each structured block or unstructured partition are required to move those points, and totally independent from the flow solver. Hence, a completely generic 'black box' tool can be developed, which is ideal for use in an optimization approach. Aeroelastic behaviour of the Brite-Euram MDO wing is analysed in terms of both static deflection and dynamic responses, and it is demonstrated that responses are strongly dependent on the exact CFD-CSD interpolation used. Mesh quality is also examined during the motion resulting from a large surface deformation. Global grid quality is shown to be preserved well, with local grid orthogonality also being maintained well, particularly at and near the moving surface, where the original orthogonality is retained.

Section: Discussion Of Available Methodsmentioning

confidence: 99%

Unified fluid–structure interpolation and mesh motion using radial basis functions

Rendall

2007

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294

205

“…This work has already begun in both the full non-linear scheme [46][47][48][49], and the ROM. The latter provides the possibility that the entire aeroservoelastic system can be modelled using a continuation method, and examined holistically in a way not attainable through conventional CFD-CSD analysis [50].…”

Section: Discussionmentioning

confidence: 99%

A comparison of full non‐linear and reduced order aerodynamic models in control law design using a two‐dimensional aerofoil model

Taylor

Fenwick

et al. 2005

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SUMMARYThe prediction of the flutter boundary of an aircraft is a necessary but time consuming process, particularly as for the most realistic results a time accurate simulation of the interaction between the non-linear aerodynamic and structural forces is required. Extension of the flight envelope by the design of active control laws to suppress flutter further increases the demands on computational time, to presently unrealistic levels. Use of a reduced order model (ROM) derived from, and in place of, the full non-linear aerodynamics greatly reduces the time required for calculation of aerodynamic forces. However, this is necessarily accompanied by some loss in accuracy, and hence the method must be verified by comparison with results obtained by the full aerodynamic model before it may be used with confidence.Such a comparison is presented here, using a two-dimensional aerofoil and control surface combination as a test case. Active control of the deflectable surface is used to attempt to increase the flutter speed across the complete Mach range, feedback control being achieved by gains acting on heave and pitch proportional and differential signals, interpreted as a hinge moment demand. Full non-linear and reduced order aerodynamic models are then used to obtain optimum control law gain for flutter suppression. The results demonstrate that the ROM accurately predicts the open loop flutter boundary, gives a good approximation to the increase in flutter speed that may be produced by gain optimization, and produces a similar response given the identical gain values in each system for a significantly reduced cost.

“…1189 monolithic and partitioned approaches have been proposed to cope with the multi-disciplinary nature of the problem. The monolithic approach uses a tailored aero-structural solver [1, 2], while the partitioned approach favours the coupling of existing structural and fluid solvers [3][4][5][6][7][8][9]. The partitioned approach is used almost as standard, including within the CFD Group at the University of Bristol [6-9], because it allows the use of existing software and grids, but synchronization of the structure and fluid at each time level must be done iteratively [6,7,9].However, the partitioned approach raises a difficult question in terms of information transfer.…”

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confidence: 99%

“…However, the space between the meshes is not governed by any physical laws. It is a component of neither mesh and is only a space across which the forces and deflections must be transferred.Numerous approaches have been considered for this problem, including a variety of weighting methods [10][11][12][13][14], the inverse isoparametric mapping [15], the constant volume tetrahedra (CVT) transform [8,[16][17][18][19] and the boundary element method [20]. These are fully discussed by Rendall and Allen [21, 22] and de Boer et al [23].…”

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confidence: 99%

“…These are fully discussed by Rendall and Allen [21, 22] and de Boer et al [23]. A common restriction of [8,[10][11][12][13][14][15][16][17][18][19][20] is that the transformation often depends on some connectivity relationship between the two meshes, and this is not normally explicit, often requiring user input. A more general approach has recently been developed by the authors [21,22] where, through the use of radial basis functions (RBFs) [24][25][26], this problem may be solved explicitly on totally arbitrary point clouds, in any form, so that the need to specify any connectivity information is removed entirely.…”

mentioning

confidence: 99%

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Improved radial basis function fluid–structure coupling via efficient localized implementation

Rendall

2009

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SUMMARYPrevious work by the authors has developed a universal interpolation scheme, using radial basis functions (RBFs), which results in a unified formulation for robust fluid-structure interpolation and high-quality mesh motion. The method has several significant advantages. Primarily, all volume mesh, structural mesh, and flow-solver-type dependence is removed entirely, as all operations are performed on totally arbitrary point clouds of any form. Hence, all connectivity requirements are removed from both the coupling and mesh motion problems. Furthermore, only matrix-vector multiplications are required during unsteady simulation because dependence relations are computed once prior to any simulation and then remain constant. This property means that the method is both perfectly parallel and totally independent from the flow-solver. However, the full method is expensive, since the dependence matrix between two sets of points is N points 1 × N points 2 . The fluid-structure coupling behaviour can also be influenced by parameters used in the interpolation. To alleviate these difficulties a more efficient form of the RBF fluid-structure coupling is presented, which also greatly reduces the interpolation parameter influence. A pointwise form of the partition of unity approach is developed that localizes the interpolation, with results presented for static aeroelastic simulations of the Brite-Euram multi-disciplinary optimization wing using a very fine mesh containing 58 000 surface points. It is shown that a 58× reduction in data size is achieved, and equally importantly the interpolation has a much smaller influence on final aeroelastic results.