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Dr. Xavier Tricoche

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Computational Fluid Dynamics (CFD) has become an essential tool in various engineering fields. In aeronautics it is a key element in the design of modern aircrafts. The performances of today's computers combined with the increasing complexity of physical models yields numerical simulations that accurately reproduce the flow structures observed in practical experiments and permit to study their impact on flight stability. Yet, to fully exploit the huge amount of information contained in typical data sets engineers require powerful post-processing techniques that allow insight into the results of their large-scale computation.

Flow visualization aims at addressing this challenge by offering intuitive and effective depictions of interesting flow patterns. Unfortunately, many problems remain that limit the usefulness of existing methods in practical applications. Our recent work has focused on the design of new visualization techniques suitable for large-scale CFD simulations. Special emphasis was put on critical flight situations that lead to turbulent and vortical flows as well as complex and structurally involved phenomena like flow recirculation and vortex breakdown.

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Among usual techniques to explore general three-dimensional flows, stream surfaces are especially appealing. They are the natural extension of streamlines and provide a better understanding of depth and spatial relationships. The standard method for stream surface integration is Hultquist's advancing front algorithm that propagates a polygonal front along the flow, while accounting for divergence and convergence by adapting the front resolution. However the application of this method to vortical flows is faced with strong limitations, mainly because of the folding behavior induced by swirling motion and the strong variations in flow velocity that produce shearing effects. In this context we improve on Hultquist's original scheme in several ways. First we use a more accurate integration scheme to properly handle high curvature and strong spatial variation of the vector field. We also control the front advancement with respect to arc length, which permits to deal with shearing flow and to ensure well-shaped triangles in the surface tessellation. Moreover we incorporate additional control mechanisms in the front refinement strategy that specifically address folding and recirculation pattern and avoid creases. Our method yields smooth, high-quality surfaces, even for very complex flow features [1][2].

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Recirculation bubbles are prominent examples of such features. They characterize the vortex breakdown phenomenon that has a dramatic impact on the flight stability of high-performance aircrafts like delta wings flying at low speed and high angle of attack. Such configurations typically occur during takeoff and landing. Therefore vortex breakdown is a critical issue that stands in the way of an industrial implementation of these high-speed designs. Nowadays, CFD computations allow for numerical analysis and investigation of the patterns present in flows undergoing vortex breakdown. However, the extreme structural intricacy of these features is poorly visualized using standard approaches. Vortex core extraction schemes are typically unable to extract the skeleton of the associated swirling motion. Stream surfaces are more robust and provide a general idea of the overall shape of the bubble but they also lead to cluttered pictures inside the bubble and do not permit a precise analysis of the flow behavior in the recirculation region.

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To address this deficiency we introduce a new approach that compounds two kinds of visualization techniques [3]. We combine a topology-based flow visualization method and volume rendering. In that way we achieve representations that both convey subtle flow structures and provide an intuitive understanding of their spatial context and associated physical properties. Moving cutting planes are used that smoothly travel along prescribed trajectories. The latter can be either obtained automatically by standard feature extraction schemes or provided by the user to explore a particular region. The associated evolution of the projected vector field topology is tracked along the way. This allows us to detect and visualize essential properties of the flow.

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Our application of volume rendering is based on the concept of multidimensional transfer functions. This methodology proves extremely useful in permitting the simultaneous and coherent depiction of multiple flow-derived scalar fields, traditionally used to analyze vortices.

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Combined with the structural information obtained by moving cutting plane this enhances the visualization and facilitates the interpretation of both the geometry and the physical properties of complex fluid flow data.

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See publications:

C. Garth, X. Tricoche, T. Salzbrunn, T. Bobach, G. Scheuermann. "Surface Techniques for Vortex Visualization," Proceedings of Joint EUROGRAPHICS - IEEE TCVG Symposium on Visualization (VisSym '04), Constance, Germany, 2004.

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X. Tricoche, C. Garth, T. Bobach, G. Scheuermann, M. Ruetten. "Accurate and Efficient Visualization of Flow Structures in a Delta Wing Simulation," In Proceedings of 34th AIAA Fluid Dynamics Conference and Exhibit, Portland, OR., June, 2004.

Xavier Tricoche, Christoph Garth, Gordon Kindlmann, Eduard Deines, Gerik Scheuermann, Markus Rütten, Charles Hansen. "Visualization of Intricate Flow Structures for Vortex Breakdown Analysis," In Proceedings of IEEE Visualization 2004, pp. (accepted). 2004.

Versions Available: [PDF]

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