A comprehensive study on interpolation schemes used in overset grid techniques is here presented. Based on a literature review, numerous schemes are implemented, and their robustness, accuracy, and performance are assessed. Two code verification exercises are performed for this purpose: a 2D analytical solution of a laminar Poiseuille steady flow; and an intricate manufactured solution of a turbulent flow case, characteristic of a boundary layer flow combined with an unsteady separation bubble. For both cases, the influence of grid layouts, grid refinement, and time-step is investigated. Local and global errors, convergence orders, and mass imbalance are quantified. In terms of computational performance, strong scalability, cpu timings, load imbalancing, and domain connectivity information (DCI) overhead are reported. The effect of the overset-grid interpolation schemes on the numerical performance of the solver, that is, number of nonlinear iterations, is also scrutinized. The results show that, for a second order finite volume code, once diffusion is dominant (low Reynolds number), interpolation schemes higher than second order, for example, least squares of degree 2, are needed not to increase the total discretization errors. For convection dominated flows (high Reynolds numbers), the results suggest that second order schemes, for example, nearest cell gradient, are sufficient to prevent overset grid schemes to taint the underlying discretization errors. In terms of performance, by single-process and parallel communication optimization, the total overset-grid overhead (with DCI done externally to the CFD code) may be less than 4% of the total run time for second-order schemes and 8% for third-order ones, therefore empowering higher-order schemes and more accurate solutions.
SummaryThe purpose of this study was to describe downward gait transitions in dressage horses using wavelet analysis spectrum of dorsoventral acceleration. An accelerometric device fixed at the sternum measured the dorsoventral and longitudinal acceleration of the running horse. Eighteen horses trained for dressage at the national level were recorded during dressage tests. Nine variables were calculated from a Morlet's wavelet analysis at canter-trot, canter-walk, canter-halt, trot-walk, trot-halt and walk-halt transition. These variables quantified the transition duration, the energy and the frequency of the dorsoventral acceleration during the gait transition. The variables distinguished each type of transition (P<0.05). It was found that the transition duration increased and variation of energy and frequency were minimised with dressage training. Keywords:horse, acceleration, gait, wavelet analysis
Increased graphical capabilities of contemporary computer hardware make ray tracing possible for a much wider range of applications. In science, and numerical fluid mechanics in particular, visual inspections still play a key role in both understanding flows, predicted by computational fluid dynamics, exhibiting features observable in real-life, such as interfaces or smoke, and when comparing such flows against experimental observations. Usually, little attention is paid to the visualisation itself, unless when the render is used solely for its eye-catching appearance. In this work, we argue that the use of ray tracing software can help make comparisons between computational and experimental fluid dynamics more robust and meaningful, and that, in some cases, it is even a necessity. Several visualisation problems which can be overcome through application of this methodology are discussed, and the use of ray tracing is exemplified for several common test cases in the maritime field. Using these examples the benefits of ray tracing are shown, and it is concluded that ray tracing can improve the reliability of scientific visual comparisons.
Sliding Grids (SG) and Overset Grids (OG) are two CFD methods for discretizing the domain with several sub-grids, with the potential of enabling complex body motion in unsteady simulations. Their fundamental difference lies on the sub-grid placement, fitted into each other (SG) or overlapped (OG), which ends up impacting the information transfer mechanism that couples them. In the present work Verification procedures are applied to a novel wind turbine flow manufactured solution, followed by the aerodynamic analysis of the rotor of the NREL 5MW wind turbine. The studies aim at assessing the impact of these methods on the simulation errors, namely in terms of interpolation schemes and resulting mass imbalance, and to compare them directly performing the same task, to fulfill a gap perceived in the literature. The results suggest that SG tend to be more convenient to be used in meshes incorporating the rotor geometry, while potentially being similar to OG in terms of accuracy. This convenience consderation also derives from convergence issues with OG, which are thought to be linked with the rotating domain’s interface being too close to the turbine’s geometry. Moreover, higher than second order interpolation schemes are found to minimize the mass imbalance introduced by these methods and consequent pressure fluctuations.
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