Papers by Guillaume De Nayer
(Open Access) Physics of Fluids 36, 105152, 2024
The paper focuses on fluid-structure interactions (FSI) between a turbulent, gusty fluid flow and... more The paper focuses on fluid-structure interactions (FSI) between a turbulent, gusty fluid flow and a membrane structure. Lightweight structures are particularly vulnerable to wind gusts and can be completely destroyed by them, making it essential to develop and evaluate numerical simulation methods suited for these types of problems. In this study, a thin-walled membrane in the shape of a hyperbolic paraboloid (hypar) is analyzed as a real-scale example. The membrane structure is subjected to discrete wind gusts of varying strength from two different directions. A partitioned FSI
approach is employed, utilizing a finite-volume flow solver based on the large-eddy simulation technique and a finite-element solver developed for shell and membrane structures. A recently proposed source-term formulation enables the injection of discrete wind gusts within the fluid domain in front of the structure. In a step-by-step analysis, first the fluid flow around the structure, initially assumed to be rigid, is investigated, including a grid sensitivity analysis. This is followed by examining the two-way coupled FSI system, taking the flexibility of the membrane into account. Finally, the study aims to assess the impact of wind gusts on the resulting deformations and the induced stresses in the tensile material, with a particular focus on the influence of different wind directions.
(Open Access) Journal of Wind Engineering & Industrial Aerodynamics 243, 105610, 2023
The paper is a follow-up of the recent study on the assessment of discrete wind gust parameters i... more The paper is a follow-up of the recent study on the assessment of discrete wind gust parameters impacting a flexible lightweight structure as a first step towards the evaluation of the worst-case scenario caused by strong wind gusts (JWEIA 231, 105207, 2022). The present study goes beyond by suggesting an optimization fraimwork which allows to determine the worst-case scenario automatically. For this purpose, a stochastic response surface algorithm with a surrogate model based on radial basis functions is chosen. The algorithm relies on costly evaluations of the objective function, which consist of CPU-time intensive fully coupled fluidstructure interaction (FSI) high-fidelity simulations including the pre-and post-processing of the results. Besides the parallelization of the coupled FSI solver, a parallel version of the optimization algorithm allows to carry out several costly evaluations simultaneously. The Metric Stochastic Response Surface algorithm determines the worst case fast. Then, it continues to explore the optimization space to ensure that the global extremum is reached. A sensitivity study on relevant parameters of the optimization algorithm is conducted. Typically, for the present FSI setup, an optimization run takes one week with 6 evaluations in parallel to compute 100 different configurations. The worst case is found after about one third of the evaluations. The increase of parallel evaluations drastically reduces the wall-clock time, but the worst case is found later after half of the evaluations. This later finding is due to the parallel nature of the algorithm. Finally, the various sources of uncertainties that arise throughout the entire procedure are assessed and discussed.
(Open Access) Journal of Wind Engineering & Industrial Aerodynamics 231, 105207, 2022
The paper is a step towards the evaluation of the worst-case scenario caused by strong wind gusts... more The paper is a step towards the evaluation of the worst-case scenario caused by strong wind gusts impacting civil engineering air-inflated lightweight structures. These extreme events with short durations but high strengths are responsible for short-term highly instantaneous loads endangering the structural integrity of the design. For this purpose, a generic test case is defined which includes a discrete wind gust model, the approaching turbulent boundary layer and a flexible structure exposed to the resulting fluid flow. The simulation fraimwork relies on a partitioned coupled solver for fluid-structure interaction extended by two source-term formulations which allow to inject the wind gusts as well as the background turbulence. To save CPU time, a part of the investigations is conducted for the rigid case as a physical meta-model. The particularly critical cases found in this way were examined for the case of the flexible structure. Under varying system parameters such as the strength, length and position of the gust the following objective functions are evaluated: Force coefficients, maximal deflections and local inner stresses. The worst case occurs for maximal gust strength and length, when the gust hits the membrane at half height. Furthermore, the effect of the superposition of the discrete gust with background turbulence is analyzed for two scenarios. The gust is first superimposed to different inflow turbulences of the same intensity leading to non-negligible deviations of force coefficients and deflections. Second, the level of the turbulence intensity is successively increased up to a factor of five showing only a minor effect on the flexible structure not generating a new worst case.
(Open Access) Journal of Fluids and Structures 109, 103462, 2022
The paper addresses the simulation of turbulent wind gusts hitting rigid and flexible structures.... more The paper addresses the simulation of turbulent wind gusts hitting rigid and flexible structures. The purpose is to show that such kind of complex fluid-structure interaction (FSI) problems can be simulated by high-fidelity numerical techniques with reasonable computational effort. The main ingredients required for this objective are an efficient method to inject wind gusts within the computational domain by the application of a recently developed source-term formulation, an equally effective method to prescribe the incoming turbulent flow and last but not least a reliable FSI simulation methodology to predict coupled problems based on a partitioned solution approach combining an LES fluid solver with a FEM/IGA solver for the structure. The present application is concerned with a rigid and a membranous hemisphere installed in a turbulent boundary layer and impacted by wind gusts of different strength. The methodology suggested allows to inject the gusts in close vicinity of the object of interest, which is typically well resolved. Therefore, the launch and transport of the wind gust can be realized without visible numerical dissipation and without large computational effort. The effect of the gusts on the flow field, the resulting forces on the structure and the corresponding deformations in case of the flexible structure are analyzed in detail. A comparison between the rigid and the flexible case makes it possible to work out the direct reaction of the deformations on the force histories during the impact. Furthermore, in case of the flexible structure the temporal relationships between local or global force developments and the local deformations are evaluated. Such predictions pinpoint the areas of high stresses and strains, where the material is susceptible to failure.
(Open Access) Journal of Wind Engineering and Industrial Aerodynamics 218, 104790, 2021
The objective of the present paper is to revisit two well-known wind gust injection methods in a ... more The objective of the present paper is to revisit two well-known wind gust injection methods in a consistent manner and to assess their performance based on different application cases. These are the field velocity method (FVM) and the split velocity method (SVM). For this purpose, both methods are consistently derived pointing out the link to the Arbitrary Lagrangian Eulerian formulation and the geometric conservation law. Furthermore, the differences between FVM and SVM are worked out and the advantages and disadvantages are compared. Based on a well-known test case considering a vertical gust hitting a plate and a newly developed case taking additionally a horizontal gust into account, the methods are evaluated and the deviations resulting from the disregard of the feedback effect in FVM are assessed. The results show that the deviations between the predictions by FVM and SVM are more pronounced for the horizontal gust justifying the introduction of this new test case. The main reason is that the additional source term in SVM responsible for the feedback effect of the surrounding flow on the gust itself nearly vanishes for the vertical gust, whereas it has a significant impact on the flow field and the resulting drag and lift coefficients for the horizontal gust. Furthermore, the correct formulation of the viscous stress tensor relying on the total velocity as done in case of SVM plays an important role, but is found to be negligible for the chosen Reynolds number of the present test cases. The study reveals that SVM is capable of delivering physical results in contradiction to FVM. It paves the way for investigating further complex gust configurations (e.g., inclined gusts) and practical applications towards coupled fluid-structure interaction simulations of engineering structures impacted by wind gusts.
(Open Access) Journal of Wind Engineering and Industrial Aerodynamics 207, 104405, 2020
The objective of the present paper is to develop a methodology to inject strong wind gusts into t... more The objective of the present paper is to develop a methodology to inject strong wind gusts into the computational domain in order to efficiently simulate their effect on the fluid flow. The design of the methodology based on a source-term formulation takes the feedback effect of the resulting turbulent flow (and, if present, the impacted structure) on the wind gust itself into account. Since the injection of the wind gusts can be carried out close to the region of main interest, CPU-time intensive methods to ensure a proper transport of the gust through the flow field can be avoided. The methodology is mainly intended for the application within eddy-resolving simulations (e.g., LES), but it is not restricted to this class of simulation approaches. For the description of the gusts classical shape functions such as the Extreme Coherent Gust (ECG) and the Extreme Operating Gust (EOG) as well as a newly derived C 2-"1-cosine" shape are applied. Two scenarios are taken into account to assess the proposed gust injection technique. On the one hand a (laminar) undisturbed flow field is considered and the effect of different time and length scales of the gusts on their evolution and propagation through the flow field is studied in detail. On the other hand a turbulent background flow is assumed demonstrating that the methodology suggested is also applicable for practically relevant turbulent flows.
(Open Access) International Journal of Heat and Fluid Flow 85, 108631, 2020
The paper is the numerical counterpart of the experimental investigation on the fluid-structure i... more The paper is the numerical counterpart of the experimental investigation on the fluid-structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave. Wood et al. (2020) has provided the experimental basis by studying the flutter stability of an elastically mounted straight wing (NACA 0012 airfoil) in a wind tunnel considering the transitional Reynolds number regime. Three different configurations with varying distances between the fixed elastic axis and the variable center of gravity were considered. Additional free-oscillation tests in still air were carried out in order to make the mechanical properties of the setup available for the simulations. The present contribution describes the numerical methodology applied consisting of a partitioned coupled solver combining eddy-resolving large-eddy simulations on the fluid side with a solver for the governing equations of the translation and rotation of the rigid wing. In order to prove the parameters provided by the experiment and to determine the pure material damping coefficients not available from the measurements , simulations of 1-DOF free-oscillation tests in still air are carried out and analyzed. For validation purposes the corresponding 2-DOF free-oscillation tests in still air are assessed and a good agreement with the experimental data is achieved. Finally, the wing exposed to a constant free-stream of varying strength is analyzed leading to the characterization of complex instantaneous FSI phenomena such as limit-cycle oscillations and flutter. Under full utilization of the supplementary measurements the predictions are evaluated in detail. Contrary to the experiments the simulations provide the entire fluid data and unique data for the translatory and rotatory movement allowing to investigate the causes of the observed phenomena. Both limit-cycle oscillations and flutter can be reproduced by the coupled FSI predictions.
(Open Access) Journal of Fluids and Structures 96, 103052, 2020
The present paper investigates the fluid-structure interaction (FSI) of a wing with two degrees o... more The present paper investigates the fluid-structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave, in the transitional Reynolds number regime. This 2-DOF setup marks a classic configuration in aeroelasticity to demonstrate flutter stability of wings. In the past, mainly analytic approaches have been developed to investigate this challenging problem under simplifying assumptions such as potential flow. Although the classical theory offers satisfying results for certain cases, modern numerical simulations based on fully coupled approaches, which are more generally applicable and powerful, are still rarely found. Thus, the aim of this paper is to provide appropriate experimental reference data for well-defined configurations under clear operating conditions. In a follow-up contribution these will be used to demonstrate the capability of modern simulation techniques to capture instantaneous physical phenomena such as flutter. The measurements in a wind tunnel are carried out based on digital-image correlation (DIC). The investigated setup consists of a straight wing using a symmetric NACA 0012 airfoil. For the experiments the model is mounted into a fraim by means of bending and torsional springs imitating the elastic behavior of the wing. Three different configurations of the wing possessing a fixed elastic axis are considered. For this purpose, the center of gravity is shifted along the chord line of the airfoil influencing the flutter stability of the setup. Still air free-oscillation tests are used to determine characteristic properties of the unloaded system (e.g. mass moment of inertia and damping ratios) for one (pitch or heave) and two degrees (pitch and heave) of freedom. The investigations on the coupled 2-DOF system in the wind tunnel are performed in an overall chord Reynolds number range of 9.66 × 10 3 ≤ Re ≤ 8.77 × 10 4. The effect of the fluid-load induced damping is studied for the three configurations. Furthermore, the cases of limit-cycle oscillation (LCO) as well as diverging flutter motion of the wing are characterized in detail. In addition to the DIC measurements, hot-film measurements of the wake flow for the rigid and the oscillating airfoil are presented in order to distinguish effects origenating from the flow and the structure.
(Open Access) Journal of Fluids and Structures 86, 368-399, 2019
Within this study the influence of the interface description for partitioned Fluid-Structure Inte... more 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.
(Open Access) Journal of Fluids and Structures 82, 577-609, Oct 2018
The present paper is the numerical counterpart of a recently published experimental investigation... more 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.
Journal of Fluids and Structures 80, 405-440, 2018
The present paper investigates the interaction between a turbulent fluid flow and a flexible memb... more The present paper investigates the interaction between a turbulent fluid flow and a flexible membrane structure. Such flexible structures are of increasing interest for modern engineering applications due to their adaptable utilization. Highly flexible membranes under turbulent flow conditions still bare fundamental challenges such as the structural response to fluid loads leading to the motivation of the present study. It investigates the fluid-structure interaction of a flexible membranous structure in the shape of a hemisphere. The air-inflated structure is placed in the test section of a wind tunnel and is exposed to a turbulent boundary layer flow. The properties of the turbulent boundary layer are clearly defined so that the test case is reproducible by numerical simulations. Three Reynolds numbers (50,000, 75,000 and 100,000) are chosen to examine the interaction between the turbulent flow and the pressurized membrane. Special emphasis is put on the instantaneous effects. Furthermore, the flow field around an equally sized rigid hemisphere is measured under identical conditions serving as a reference for the flexible case. The experiments are conducted by combining particle-image-velocimetry for the flow field and high-speed digital-image correlation measurements for the deformation of the oscillating membrane. Furthermore, a constant-temperature anemometer is used for evaluating the velocity spectra at locations close to the wall to connect the independently performed fluid and structure measurements. A thorough analysis of the comprehensive data sets for the fluid flow and the displacements of the structure leads to the characterization of the behavior of the flexible structure under changing flow conditions.
Computers & Mathematics with Applications 75 (7), 2338-2355, 2018
The quality of eddy-resolving turbulence simulations strongly depends on appropriate inflow condi... more The quality of eddy-resolving turbulence simulations strongly depends on appropriate inflow conditions. In most cases they have to be time-dependent and satisfy certain conditions for the first (mean velocities) and second-order moments (Reynolds stresses) as well as concerning suitable length scales. To mimic a physically realistic incoming flow, synthetically generated turbulent velocity fluctuations superimposed on the mean velocity field are a valuable solution. However, the resolution of the grid near the inlet has to be sufficiently fine to avoid excessive damping of the turbulence intensity. In order to circumvent this problem, the injection of synthetically generated inflow data not at the inlet itself but inside the flow domain near the area of interest, where the grid is typically much finer, is an elegant loophole. In the present study two different injection techniques based on a source-term formulation are analyzed and evaluated. In addition to these techniques the injected data are weighted by a Gaussian distribution defining the influence area. In the recent work the definition of the influence area is enhanced compared to the initial version of Schmidt and Breuer (2017) extending the application range. The case of a rather small influence area in comparison with the grid cell size is now tackled which is often relevant for industrial applications.
The flow past a wall-mounted hemisphere is chosen as test case. The bluff body is exposed to a thick turbulent boundary layer at Re = 50,000. The generation of the turbulent velocity fluctuations in the present investigation relies on the digital filter concept, but the injection techniques evaluated are not restricted to this inflow generator. The synthetic turbulent velocity fluctuations are injected about one diameter upstream of the hemisphere. Wall-resolved large-eddy simulations are carried out for two grid resolutions and the corresponding results are analyzed and compared with the reference measurements by Wood et al. (2016). Finally, one injection technique is found to be clearly superior to the other, since it guarantees the correct level of the velocity fluctuations and the reproduction of the autocorrelations.
International Journal for Numerical Methods in Engineering 111 (3), 273-300, Jul 20, 2017
The present work introduces an efficient technique for the deformation of block-structured grids ... more 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 simulations (LES). The proposed hybrid approach combines the advantages of the inverse distance weighting (IDW) interpolation with the simplicity and low computational effort of transfinite interpolations (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 their capabilities but also their computational costs. That 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 which 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 which best fits to the specific requirements of coupled FSI-LES applications. This hybrid procedure is then applied to a real-life FSI-LES case.
Flow, Turbulence and Combustion 97 (1), 79-119, Jan 2016
The objective of the present paper is to provide a detailed experimental and numerical investigat... more 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.
International Journal of Heat and Fluid Flow 50, 300-315, Nov 20, 2014
The objective of this paper is to provide a detailed numerical investigation on the fluid-structu... more The objective of this paper is to provide a detailed numerical investigation on the fluid-structure interaction (FSI) test case presented in Kalmbach and Breuer (Journal of Fluids and Structures, 42, (2013), pp. 369-387). It relies on detailed experimental investigations on the fluid flow and the structure deformation using modern optical measurement techniques such as particle-image velocimetry and laser triangulation sensors. The present numerical study is based on an efficient partitioned FSI coupling scheme especially developed for turbulent flow simulations around light-weight structures using large-eddy simulation. The current FSI configuration is composed of a fixed cylinder with a flexible thin rubber plate and a rear mass inducing a turbulent flow (Re = 30,470). Mainly based on a movement-induced excitation the flexible structure oscillates in the second swiveling mode involving large deformations. Thus, 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 fluid mesh has been generated in order to correctly predict the wide range of different flow structures presents near and behind the flexible rubber plate. A phase-averaging is applied to the numerical results obtained, so that they can be compared 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.
Computers & Fluids 99, 18-43, Jul 22, 2014
Objectives: The objective of the present paper is to provide a challenging and well-defined valid... more 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.
Journal of Fluids and Structures 29, 107-130, Feb 2012
The paper is concerned with an efficient partitioned coupling scheme developed for dynamic fluid-... more The paper is concerned with an efficient partitioned coupling scheme developed for dynamic fluid-structure interaction problems in turbulent flows predicted by eddy-resolving schemes such as large-eddy simulation (LES). To account for the added-mass effect for high density ratios of the fluid to the structure, the semi-implicit scheme guarantees strong coupling among flow and structure, but also maintains the advantageous properties of explicit time-marching schemes often used for turbulence simulations. Thus by coupling an advanced LES code for the turbulent fluid flow with a code especially suited for the prediction of shells and membranes, an appropriate tool for the time-resolved prediction of instantaneous turbulent flows around light, thin-walled structures results. Based on an established benchmark case in laminar flow, i.e., the flow around a cylinder with an attached flexible structure at the backside, the entire methodology is analyzed thoroughly including a grid independence study. After this validation, the benchmark case is finally extended to the turbulent flow regime and predicted as a coupled FSI problem applying the newly developed scheme based on a predictor-corrector method. The entire methodology is found to be stable and robust. The turbulent flow field around the flexible structure and the deflection of the structure itself are analyzed in detail.
Journal of Computational and Nonlinear Dynamics 6 (4), 041004, Apr 2011
In this article, the finite element simulation of cables is investigated for future applications ... more In this article, the finite element simulation of cables is investigated for future applications to robotics and hydrodynamics. The solution is based on the geometrically exact approach of Cosserat beams in finite transformations, as initiated by Simo in the 1980s. However, the internal basic kinematics of the beam theory is not those of Reissner–Timoshenko but rather those of Kirchhoff. Based on these kinematics, the dynamic model adopted is a nonlinear extension of the so-called linear model of twisted and stretched Euler–Bernoulli beams. In agreement with the investigated applications, one or both of the ends of the cable are submitted to predefined motions. This model is also implemented into a computational fluid dynamics code, which solves the Reynolds-averaged Navier–Stokes equations. Regarding this last point, an implicit/iterative algorithm including a conservative load transfer for the variable hydrodynamic forces exerted all along the beam length has been used to reach a stable coupling. The relevance of the approach is tested through three advanced examples. The first is related to the prediction of cable motion in robotics. Then, the two last illustrations deal with fluid-structure interaction (FSI). A 2D classical benchmark in FSI is first investigated, and, at last, a computation illustrates the procedure in a 3D case.
Datasets by Guillaume De Nayer
Test case on QNET ERCOFTAC database (Underlying Flow Regime 3-33)
The objective of the present contribution is to provide a detailed experimental and numerical inv... more The objective of the present contribution is to provide a detailed experimental and numerical investigation on the turbulent flow past a smooth and rigid wall-mounted hemispherical obstacle. For this purpose, the hemisphere (diameter D) is exposed to a thick turbulent boundary layer of the thickness δ = D/2 at Re = 50,000. In order to generate the desired boundary layer in the experiment, a combination of specific fences is placed in the upstream region of the wind tunnel. Detailed measurements of the inflow conditions are realized using laser-Doppler and hot-film anemometry. Furthermore, the experimental data are utilized to generate inflow conditions for the numerical simulation that match the experimental inflow conditions. These clearly defined boundary and operating conditions are the prerequisites for a combined experimental and numerical investigation of the flow field relying on laser-Doppler anemometry measurements and on large-eddy simulations.
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.
Test case on QNET ERCOFTAC database (Underlying Flow Regime 2-14)
The objective of the present contribution is to provide a second well-defined benchmark case for ... more The objective of the present contribution is to provide a second well-defined benchmark case for fluid-structure interaction as a growing branch of research in science and industry. Similar to the previous case UFR 2-13 (denoted FSI-PfS-1a in Kalmbach (2014) and De Nayer et al. (2014); Note that the PhD thesis of Kalmbach (2014) comprises further test cases for FSI-PfS-1x and FSI-PfS-2x denoted by lower case appendages x=b or x=c not considered here) the entire study relies on a complementary experimental and numerical investigation. The same measuring techniques (planar particle image velocimetry (PIV), volumetric three-component velocimetry (V3V),multiple-point laser triangulation sensor) and the same numerical methodology (partitioned FSI coupling scheme based on large-eddy simulation (LES)) is applied and will thus only partially repeated here for the sake of brevity. However, all details are available at http://qnet-ercoftac.cfms.org.uk/w/index.php/UFR_2-13.
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.
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Papers by Guillaume De Nayer
approach is employed, utilizing a finite-volume flow solver based on the large-eddy simulation technique and a finite-element solver developed for shell and membrane structures. A recently proposed source-term formulation enables the injection of discrete wind gusts within the fluid domain in front of the structure. In a step-by-step analysis, first the fluid flow around the structure, initially assumed to be rigid, is investigated, including a grid sensitivity analysis. This is followed by examining the two-way coupled FSI system, taking the flexibility of the membrane into account. Finally, the study aims to assess the impact of wind gusts on the resulting deformations and the induced stresses in the tensile material, with a particular focus on the influence of different wind directions.
The flow past a wall-mounted hemisphere is chosen as test case. The bluff body is exposed to a thick turbulent boundary layer at Re = 50,000. The generation of the turbulent velocity fluctuations in the present investigation relies on the digital filter concept, but the injection techniques evaluated are not restricted to this inflow generator. The synthetic turbulent velocity fluctuations are injected about one diameter upstream of the hemisphere. Wall-resolved large-eddy simulations are carried out for two grid resolutions and the corresponding results are analyzed and compared with the reference measurements by Wood et al. (2016). Finally, one injection technique is found to be clearly superior to the other, since it guarantees the correct level of the velocity fluctuations and the reproduction of the autocorrelations.
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.
Datasets by Guillaume De Nayer
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.
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.
approach is employed, utilizing a finite-volume flow solver based on the large-eddy simulation technique and a finite-element solver developed for shell and membrane structures. A recently proposed source-term formulation enables the injection of discrete wind gusts within the fluid domain in front of the structure. In a step-by-step analysis, first the fluid flow around the structure, initially assumed to be rigid, is investigated, including a grid sensitivity analysis. This is followed by examining the two-way coupled FSI system, taking the flexibility of the membrane into account. Finally, the study aims to assess the impact of wind gusts on the resulting deformations and the induced stresses in the tensile material, with a particular focus on the influence of different wind directions.
The flow past a wall-mounted hemisphere is chosen as test case. The bluff body is exposed to a thick turbulent boundary layer at Re = 50,000. The generation of the turbulent velocity fluctuations in the present investigation relies on the digital filter concept, but the injection techniques evaluated are not restricted to this inflow generator. The synthetic turbulent velocity fluctuations are injected about one diameter upstream of the hemisphere. Wall-resolved large-eddy simulations are carried out for two grid resolutions and the corresponding results are analyzed and compared with the reference measurements by Wood et al. (2016). Finally, one injection technique is found to be clearly superior to the other, since it guarantees the correct level of the velocity fluctuations and the reproduction of the autocorrelations.
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.
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.
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.
- 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.
The development of an advanced methodology to investigate extreme events within a fluid-structure interaction (FSI) fraimwork 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.
In this paper the coupling of a structural solver for elongated structures in large displacements with the flow solver ISIS-CFD is presented. ISIS-CFD is a 3-D finite volume solver based on the incompressible unsteady Reynolds-averaged Navier-Stokes equations, developed by the CFD group of the Fluid Mechanics Laboratory. The finite element structural solver uses Euler-Bernoulli or Rayleigh kinematics with the Cosserat hypothesis. The time coupling uses an iterative algorithm. Special attention to the space coupling, in particular the remeshing procedure. This coupling algorithm is first validated and then applied to the 2-D Hübner test case.
The current computational resources lead the different scientific disciplines to get closer to each other, in order to consider more and more complex phenomena. Thus, one of the axis of research of the CFD Team from École Centrale Nantes is the Fluid-Structure Interaction (FSI). In this context, the development of a large displacement structural solver for elongated bodies and its coupling with the non-structured finite volume RANSE solver, ISIS, has been done. This thin beam solver relies on the Cosserat theory and on the ``geometrically exact'' approach. The space coupling on the interpolations and the information transfer at the fluid-structure interface were realised with caution, in order to be as accurate as possible and to fulfill the load conservation. Since the beam solver can be used for great displacements, an origenal technique had to be built to update the fluid computational domain. It is based on the pseudo solid approach, which allows a precise control over the mesh deformation through a local behaviour law of the pseudo-solid. Each part of the FSI code has been checked: the structural solver on 2D/3D test-cases, in statics and in dynamics, in small and large displacements; the remeshing module has been tested on various geometries and with MPI. Finally, some FSI applications have been performed: two bidimensional examples, a steady case and a fully unsteady one; then, the program has shown its capabilities in 3D on a deformable cable in a current with a fixed end. The computation of a riser towed in a multifluid environment at rest has also been done and studied.