In book: Direct and Large-Eddy Simulation VIII, Publisher: Springer Netherlands, Editors: Hans Kuerten, Bernard Geurts, Vincenzo Armenio, Jochen Fröhlich, Dec 31, 2010
A structure placed in a fluid flow is always affected by the pressure and shear forces acting on ... more A structure placed in a fluid flow is always affected by the pressure and shear forces acting on the surface leading to structural deformations or deflections. Partially these can be neglected and such a rigid body assumption strongly reduces the complexity of a numerical simulation setup. However, in many circumstances this assumption does not hold and fluid–structure interaction (FSI) becomes of major interest. Technical applications are numerous such as artificial heart valves, lightweight roofage or tents. Therefore, a need for appropriate numerical simulation tools exists for such coupled problems and this is the objective of the present study.
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Papers by Michael Breuer
The development of an advanced methodology to investigate extreme events within a fluid-structure interaction (FSI) framework based on
large-eddy simulation developed and validated at the institute (Breuer et al., 2012, De Nayer et al., 2014) is the topic of the present contribution. The synthetic turbulence inflow generator based on the digital filter method is extended to generate distinctive wind gusts of different shapes and amplitudes based on either deterministic (Gaussian, 1-cosine, Mexican hat shape) or stochastic methods. The injection of these wind gusts into the flow domain implies a short but brutal change of the total mass inflow, which has to be corrected so that the incompressible solver does not diverge. In order to evaluate the method in the FSI context, a test case based on a rigid structure is considered in a first phase, while simulations with flexible structures will be carried out later. The flow around the wall-mounted cube of Martinuzzi (1992) is selected and slightly modified. The effects of different forms and amplitudes of wind gusts on the resulting fluid forces acting on the rigid structure are investigated.
The numerical results are produced by a finite-volume Navier-Stokes solver for block-structured curvilinear grids. A fine wall-resolving mesh is applied resulting from a preliminary study. An additional analysis is conducted to select a suitable subgrid-scale model.
The final investigation includes a profound analysis on the unsteady flow features observed in the vicinity of the hemisphere like the horseshoe vortex, the recirculation area, the hairpin structure or the vortex shedding processes. A detailed discussion of the time-averaged flow field comprising the mean velocity field as well as the Reynolds stresses is provided. Owing to the proper description of the oncoming flow and the additional numerical studies guaranteeing the choice of an appropriate grid and subgrid-scale model, the experimental and numerical results are found to be in close agreement.
- First, the test case should be geometrically simple which is realized by a classical cylinder flow configuration extended by a flexible plate structure attached to the backside of the cylinder.
- Second, clearly defined operating and boundary conditions are a must and put into practice by a constant inflow velocity and channel walls. The latter are also evaluated against a periodic setup relying on a subset of the computational domain.
- Third, the model to describe the material behavior under load (denoted material model in the following) should be widely used. Although a rubber plate is chosen as the flexible structure, it is demonstrated by additional structural tests that a classical St. Venant-Kirchhoff material model is sufficient to describe the material behavior appropriately.
- Fourth, the flow should be in the turbulent regime. Choosing water as the working fluid and a medium-size water channel, the resulting Reynolds number of Re = 30,470 guarantees a sub-critical cylinder flow with transition taking place in the separated shear layers.
- Fifth, the benchmark results should be underpinned by a detailed validation process.
For this purpose two dynamic structural tests were carried out experimentally and numerically in order to evaluate an appropriate model to describe the material behavior and to check and evaluate the material parameters of the rubber (Young's modulus, damping). This preliminary work has shown that the St. Venant-Kirchhoff material law is sufficient to describe the deflection of the flexible structure.
After these structural tests, complementary numerical and experimental investigations with flow around the cylinder-plate configuration were performed. Based on optical contactless measuring techniques (particle-image velocimetry and laser distance sensor) the phase-averaged flow field and the structural deformations were determined. These data were compared with corresponding numerical predictions relying on large-eddy simulations and a recently developed semi-implicit predictor-corrector FSI coupling scheme. Both results were found to be in close agreement showing a quasi-periodic oscillating flexible structure in the first swiveling FSI mode with a corresponding Strouhal number of about St_FSI = 0.11.
What are the differences between the previous case and the present one? For the previous configuration (FSI-PfS-1a, UFR 2-13) the flexible structure deforms in the first swiveling mode inducing only moderate structural displacements by an instability-induced excitation. In contrast, the new case denoted FSI-PfS-2a is
- a movement-induced excitation;
- with significantly larger deformations of the flexible structure;
- in the second swiveling mode.
In order to achieve these more challenging features of the flow and the structure, the previous test case http://qnet-ercoftac.cfms.org.uk/w/index.php/UFR_2-13 (FSI-PfS-1a) is slightly modified: A 2 mm thick flexible plate is clamped behind the fixed cylinder. However, this time a rear mass is added at the extremity of the flexible structure. Moreover, the material (para-rubber) is less stiff than in FSI-PfS-1a. The Reynolds number is again Re = 30,470.
The three-dimensional fluid velocity results show shedding vortices behind the structure, which reaches the second swiveling mode with a frequency of about 11.25 Hz corresponding to a Strouhal number of St = 0.179. Providing phase-averaged flow and structure measurements, precise experimental data for coupled computational fluid dynamics (CFD) and computational structure dynamics (CSD) validations are available for this new benchmark case. The test case possesses four main advantages:
(i) The geometry is rather simple;
(ii) Kinematically, the rotation of the front cylinder is avoided;
(iii) The boundary conditions are well defined;
(iv) Nevertheless, the resulting flow features and structure displacements are challenging from the computational point of view.
Besides these experimental investigations detailed predictions based on LES are available. Particular attention has been paid to the computational model and the numerical set-up. Special seven-parameters shell elements are applied to precisely model the flexible structure. Structural tests are carried out to approximate the optimal structural parameters. A fine and smooth mesh for the flow calculation has been generated in order to correctly predict the wide range of different flow structures presents near and behind the flexible rubber plate. In accordance with the measurements, phase-averaging is applied to the numerical results allowing a detailed comparison with the phase-averaged experimental data. Both are found to be in close agreement exhibiting a structure deformation in the second swiveling mode with similar frequencies and amplitudes. Finally, a sensitivity study is carried out to show the influence of different physical parameters (e.g. Young’s modulus) and modeling aspects (e.g. subgrid-scale model) on the FSI phenomenon.