In the last decade, the demand for the prediction of complex multi-physics prob-lems such as fluid-structure interaction (FSI) has strongly increased. For the development and improvement of appropriate numerical tools several test case swere designed in order to vali-date the numerical res ults based on experimental reference data [4, 12, 13, 8, 9, 10 ]. Since FSI problems often occur inturbulent flows also in the experiments similar conditions have to be provided. Inthetest - case FSI - PfS- 1a [7] presented in the first contribution to this session, acylinder is used with anattached flexible rubber plate. Theresulting FSI problem is nearly two - dimensionalregarding the phase -averaged flow and thes tructure deformations. Theac - tualtestcase FSI - PfS -3a is the reasonable further development step of this two - -dimensional flow, Which now alsoleads to a significant three - dimensional structure deformation. Thecy linder is replaced by a truncated cone .Similar to FSI - PfS - 1a [7] arubber plate is attachedat the backside. This geometrical setup is exposed to aconstant flowat Re = 32 ,000 which hisin the subcritical regime. Dueto the linearly increasing diameter of the cone the alternating eddies in the wake even become larger resulting incorre - spondingly increasing structural displacements. Owingt o thes echallenging flow and structure effects, this benchmark will be the next step for validating FSI predictions for real applications. The experiments are performed in a water channel with clearly defined and controllable bound - ary and operating conditions. Formeasuring the flowa two - dimensional mono - particle - image velocimetry (PIV) system isapplied. In order to characterize the three - dimensional behaviorof the flow, phase - averaged PIV measurements are performed at three different planes. The structural deformations are measured along a line on the structure surface with atime - resolved laser distance sensor. The resulting FSI problem shows a quasi - periodic deformation behavior so that a phase averaging of the results is reasonable. Byphase - averaging turbulent fluctua-tions are averaged out and thus a comparison with corresponding numerical simulations basedon LES [3] and RANS [12, 13] approaches is possible.

This paper presents phase-resolved Particle-Image Velocimetry measurements on fluid-structure interactions in a water channel, performed at a Reynolds number of 30,470. The structure model consists of a fixed rigid cylinder with a deformable polymer plate behind it and is implemented into a vertical closed circuit tunnel at inlet velocities of 1.38 m/s. Phase-resolved measurements triggered by a precise laser distance sensor were performed to capture averaged velocity fields at several moments of the structure deflection, enabling comparisons to numerical fluid-structure interaction simulations currently under investigation.

In the last decade, the demand for the prediction of complex multi-physics problems such as fluid-structure interaction (FSI) has strongly increased. These FSI phenomena can be found in many industry-related applications, e.g., in the design of aircrafts, wind turbines or heart valves to mention only a few. In present and future applications with complex multi-physics couplings, the numerical prediction of FSI problems is an important and valuable engineering tool in the design, life cycle analysis and prototyping. Due to enhanced numerical algorithms and the strong increase of the computational power in the last decades, it is now feasible to simulate real-world FSI problems. Thus, a variety of numerical models are available or are currently in development to predict FSI applications. To evaluate and improve these complex non-linear computations, experimental studies are highly necessary. In order to provide reliable data for the validation and evaluation of coupled Computational Fluid Dynamics (CFD) and Computational Structure Dynamics (CSD) tools, different experimental test configurations (benchmarks) were developed and studied. In the present thesis four series of test configurations exposed to turbulent flows are investigated using precise experimental measurement techniques and complementary numerical predictions. In each case the flexible structures are excited by vortex shedding. The shed vortices move downstream and start to interact with the flexible structures leading to a bidirectional self-excited fluid-structure interaction. Due to the oscillating behavior of this flow phenomenon and the resulting pressure distribution along the fluid-structure interface, the flexible structures start to deflect. The experimental investigations are performed in a water channel (Göttingen type) allowing contactless data acquisition systems for the flow (2D particle image velocimetry and 3D particle tracking techniques) and structural measurements (2D laser line triangulation). Due to cycle-to-cycle variations of the structural deflections owing to chaotic irregular fluctuations of the turbulent flow field, the flow measurements are phase-averaged to obtain representative data. For all test configurations the system response as a function of the inflow velocities is analyzed. Different swiveling states of the flexible structures are identified and assigned to already known excitation mechanisms. Furthermore, for each test case the flow and structural behavior at a well-chosen inflow velocity is extensively investigated. Here, the coupled system is experimentally determined in form of two-dimensional flow fields and two-dimensional structural deformations representing the data base for the validation purpose. The first test case series FSI-PfS-1x consists of a fixed rigid cylinder with an attached flexible plate. Three different elastic materials are applied to similar working conditions producing deflections in the order of the front cylinder diameter in the first and second swiveling mode. The second series FSI-PfS-2x uses the same configuration but applies an additional steel weight attached at the trailing edge of the flexible plate and a fixed or rotational mount of the front cylinder. This modification increases the inertia of the system and enables even larger structural deformations in the second swiveling mode. Regarding these first two test cases, both configurations are generating almost two-dimensional structural system responses. To develop a three-dimensional test case, the third benchmark FSI-PfS-3x replaces the circular cylinder by a tapered cylinder. As a consequence the flow and the resulting structural responses are of three-dimensional kind in the first swiveling mode. The last test series FSI-PfS-4x is application-oriented and addresses well-known vortex-induced vibrations which are common in technical applications like heat exchangers consisting of tube bundles. Here, a long circular cylinder is fixed at one channel wall and is free at the opposite site. Two different configurations, a single flexible cylinder and a 3x3 arrangement are studied. The crossflow configuration causes large deflections of the free cylinder tip and high swiveling frequencies. The complementary numerical studies are carried out with the multi-physics software environment of ANSYS 14.0 using the FSI coupling interface between ANSYS CFX for the fluid flow and ANSYS Mechanical for the structure deformation. Due to the turbulent flow conditions a RANS turbulence model is used to predict the vortex shedding from the cylinder and the fluid flow in the wake of the structure. The numerical predictions lead to satisfactory results for the two-dimensional test cases and less reasonable results for the three-dimensional test configurations. For the three dimensional numerical simulations limitations of the cell numbers and difficult boundary conditions for the structure lead to non-physical results.

The investigation of the bidirectional coupling between a fluid flow and a structure motion is a growing branch of research in science and industry. Applications of the so-called fluid-structure interactions (FSI) are widespread. To improve coupled numerical FSI simulations, generic experimental benchmark studies of the fluid and the structure are necessary. In this work, the coupling of a vortex-induced periodic deformation of a flexible structure mounted behind a rigid cylinder and a fully turbulent water flow performed at a Reynolds number of Re=30. 470 is experimentally investigated with a planar particle image velocimetry (PIV) and a volumetric three-component velocimetry (V3V) system. To determine the structure displacements a multiple-point laser triangulation sensor is used. The three-dimensional fluid velocity results show shedding vortices behind the structure, which reaches the second swiveling mode with a frequency of about 11.2. Hz corresponding to a Strouhal number of St=0.177. 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 denoted FSI-PfS-2a. 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. In addition to the flow field and displacement data a PIV-based force calculation method is used to estimate the lift and drag coefficients of the moving structure. © 2013 Elsevier Ltd.

Objectives: The objective of the present paper is to provide a challenging and well-defined validation test case for fluid-structure interaction (FSI) in turbulent flow to close a gap in the literature. The following list of requirements are taken into account during the definition and setup phase. First, the test case should be geometrically simple which is realized by a classical cylinder flow configuration extended by a flexible 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 material model 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 test case results should be underpinned by a detailed validation process.Methods: For this purpose complementary numerical and experimental investigations were carried out. 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.Outcome: 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 StFSI = 0.11. © 2014 Elsevier Ltd.

This contribution presents a complementary numerical/experimental investigation on a new fluid-structure interaction (FSI) test case denoted FSI-PfS-1. In comparison to previously suggested FSI benchmark cases (see Gomes and Lienhart (2006, 2010)) the present configuration is less challenging from the computational point of view. The reasons are versatile: Owing to a fixed cylindrical front body the mechanical system has less degrees of freedom. Furthermore, the swiveling structure is consisting of a unique material without an additional rear weight. Finally, the thickness of the flexible structure is 50 times larger than the very thin structure used in previous investigations. The structural model was installed in a water tunnel and operated in the subcritical turbulent regime at a Reynolds number of Re = 3 104. Based on optical measuring techniques the phase-averaged flow field as well as the deformation of the structure were experimentally determined. Additionally, the FSI test case was predicted by a partitioned semi-implicit predictor-corrector coupling scheme applying the large-eddy simulation technique. The contribution presents a first comparison concerning the phase-resolved flow field and the structure deformation. The swiveling motion of the flexible structure found in the experiment is predicted in reasonable agreement. Finally, an outlook is given about all issues of the benchmark case which need further evaluations and improvements.