De Nayer, Guillaume

© 2019 The Authors Within this study the influence of the interface description for partitioned Fluid–Structure Interaction (FSI) simulations is systematically evaluated. In particular, a Non-Uniform Rational B-Spline (NURBS)-based isogeometric mortar method is elaborated which enables the transfer of fields defined on low-order and isogeometric representations of the interface along which the FSI constraints are defined. Moreover, the concept of the Exact Coupling Layer (ECL) using the proposed isogeometric mortar-based mapping method is presented. It allows for smoothing fields that are transferred between two standard low-order surface discretizations applying the exact interface description in terms of NURBS. This is especially important for highly turbulent flows, where the artificial roughness of the low-order faceted FSI interfaces results in spurious flow fields leading to inaccurate FSI solutions. The approach proposed is subsequently compared to the standard mortar-based mapping method for transferring fields between two low-order surface representations (finite volume method for the fluid and finite element method for the structure) and validated on a simple lid-driven cavity FSI benchmark. Then, the physically motivated 3D example of the turbulent flow around a membranous hemisphere (Wood et al., 2016) is considered. Its behavior is predicted by combining the large-eddy simulation technique with the isogeometric analysis to demonstrate the usefulness of the isogeometric mortar-based mapping method for real-world FSI applications. Additionally, the test case of a bluff body significantly deformed in an eigenmode shape of the aforementioned hemisphere is used. For this purpose, both “standard” low-order finite element discretizations and a smooth IGA-based description of the structural surface are considered. This deformation is transferred to the fluid FSI interface and the influence of the interface description on the fluid flow is analyzed. Finally, the computational costs related to the presented methodology are evaluated. The results suggest that the proposed methodology can effectively improve the overall FSI behavior with minimal effort by considering the exact geometry description based on the Computer-Aided Design (CAD) model of the FSI interface.

© Springer Nature Switzerland AG 2019. The dimensioning of lightweight structures under wind loads strongly depends on realistic flow conditions. Two different setups have to be distinguished. For a long-term analysis such as dynamic fatigue, temporally and spatially correlated velocity distributions are required as inflow conditions for a large-eddy simulation (LES) to mimic a realistic physical setup (Wood et al, Flow Turbul Combust 97(1):79–119, 2016, [9]).

Lightweight shell structures have gained significant popularity in the engineering design as they offer high load carrying capacity for low cost. While classical Finite Element models of such structures have been extensively used in practice, the shell formulations for Isogeometric Analysis (IGA), particularly for Kirchhoff-Love shell structures [1], have demonstrated benefits arising from the smooth geometry description and the highly accurate solutions that are enabled. Moreover, the direct use of CAD-models for the structural simulations is possible based e.g. on the Isogeometric B-Rep Analysis (IBRA) [2]. Herein the application of IBRA to Fluid-Structure Interaction (FSI) simulations of wind turbines is demonstrated, thus naturally extending IBRA to coupled multiphysics problems.

The present paper is the numerical counterpart of a recently published experimental investigation by Wood et al. (2018). Both studies aim at the investigation of instantaneous fluid–structure interaction (FSI) phenomena observed for an air-inflated flexible membrane exposed to a turbulent boundary layer, but looking at the coupled system based on different methodologies. The objective of the numerical studies is to supplement the experimental investigations by additional insights, which were impossible to achieve in the experiments. Relying on the large-eddy simulation technique for the predictions of the turbulent flow, non-linear membrane elements for the structure and a partitioned algorithm for the FSI coupling, three cases with different Reynolds numbers (Re=50,000, 75,000 and 100,000) are simulated. The time-averaged first and second-order moments of the flow are presented as well as the time-averaged deformations and standard deviations. The predictions are compared with the experimental references data solely available for 2D planes. In order to better comprehend the three-dimensionality of the problem, the data analysis of the predictions is extended to 3D time-averaged flow and structure data. Despite minor discrepancies an overall satisfying agreement concerning the time-averaged data is reached between experimental data in the symmetry plane and the simulations. Thus for an in-depth analysis, the numerical results are used to characterize the transient FSI phenomena of the present cases either related to the flow or to the structure. Particular attention is paid to depict the different vortex shedding types occurring at the top, on the side and in the wake of the flexible hemispherical membrane. Since the fluid flow plays a significant role in the FSI phenomena, but at the same the flexible membrane with its eigenmodes also impacts the deformations, the analysis is based on the frequencies and Strouhal numbers found allowing to categorize the different observations accordingly.

Copyright © 2016 John Wiley & Sons, Ltd. The present work introduces an efficient technique for the deformation of block-structured grids occurring in simulations of fluid–structure interaction (FSI) problems relying on large-eddy simulation (LES). The proposed hybrid approach combines the advantages of the inverse distance weighting (IDW) interpolation with the simplicity and low computational effort of transfinite interpolation (TFI), while preserving the mesh quality in boundary layers. It is an improvement over the state-of-the-art currently in use. To reach this objective, in a first step, three elementary mesh deformation methods (TFI, IDW, and radial basis functions) are investigated based on several test cases of different complexities analyzing not only their capabilities but also their computational costs. That not only allows to point out the advantages of each method but also demonstrates their drawbacks. Based on these specific properties of the different methods, a hybrid methodology is suggested that splits the entire grid deformation into two steps: first, the movement of the block-boundaries of the block-structured grid and second, the deformation of each block of the grid. Both steps rely on different methodologies, which allows to work out the most appropriate method for each step leading to a reasonable compromise between the grid quality achieved and the computational effort required. Finally, a hybrid IDW-TFI methodology is suggested that best fits to the specific requirements of coupled FSI-LES applications. This hybrid procedure is then applied to a real-life FSI-LES case. Copyright © 2016 John Wiley & Sons, Ltd.

© 2016, Springer Science+Business Media Dordrecht. The objective of the present paper is to provide a detailed experimental and numerical investigation on the turbulent flow past a hemispherical obstacle (diameter D). For this purpose, the bluff body is exposed to a thick turbulent boundary layer of the thickness δ = D/2 at Re = 50,000. In the experiment this boundary layer thickness is achieved by specific fences placed in the upstream region of the wind tunnel. A detailed measurement of the upstream flow conditions by laser-Doppler and hot-film probes allows to mimic the inflow conditions for the complementary large-eddy simulation of the flow field using a synthetic turbulence inflow generator. These clearly defined boundary and operating conditions are the prerequisites for a combined experimental and numerical investigation of the flow field relying on the laser-Doppler anemometry and a finite-volume Navier-Stokes solver for block-structured curvilinear grids. The results comprise an analysis on the unsteady flow features observed in the vicinity of the hemisphere as well as a detailed discussion of the time-averaged flow field. The latter includes the mean velocity field as well as the Reynolds stresses. Owing to the proper description of the oncoming flow and supplementary numerical studies guaranteeing the choice of an appropriate grid and subgrid-scale model, the results of the measurements and the prediction are found to be in close agreement.