Fluid-structure Interaction Code Research Articles

A numerical study on the flow-induced vibrations of flexible cylinders attached with fixed short fairings

Fairings have been used to suppress the vortex-induced vibrations(VIV) of deep water cylinders in the offshore and marine engineering applications. When properly functioning, they can reduce both the cylinder oscillation amplitude in the cross-flow direction and the fluid drag force in the in-line direction. Plenty research reports can be found on the rigid cylinder VIV suppression performances of fairings while very few research work discuss their VIV suppression performances on flexible cylinders. In the present work, a self-developed high-resolution fluid–structure interaction code based on the computational fluid dynamics is used to simulate the flexible cylinder responses attached with differently covered fixed short fairings. The Reynolds number based on the incoming velocity and the bluff body diameter is 1.68×104. The short fairings are found not to induce strong galloping oscillations in the flexible cylinders while the galloping is prominent in the rigid cylinder cases at large reduced velocities. The different short fairing setups are found to influence the local fluid–structure energy transport distributions and the travelling wave directions in the flexible cylinder responses, which eventually influences the flexible cylinder oscillation amplitude distributions, response frequencies and mode orders.

On the viscoelastic dynamic beam modelling

There is a renewed interest in the development of underwater robotic fish due to the advancement in control systems. The cross section of real fish broadly consists of solid bone at the core followed by viscoelastic matrix (tissue mass). The precise material development for such systems is still a challenge, as the desired motion requires viscoelastic dynamic response with probably varying axial stiffness. It is thus important to develop physically accurate material model of fish that can be employed in fluid–structure interaction codes. The primary objective of the present work is to develop viscoelastic dynamic material model, where the viscous response is obtained by Kelvin–Voigt model. The detailed formulation of the model is presented followed by its numerical implementation by finite element (space discretization) and Newmark (time discretization) methods. The correctness of the developed MATLAB code is successfully demonstrated by solving several problems having known analytical solutions. The entire code is made available in the supplementary material file, which can be readily deployed in the design and development of robotic fish systems.

Verification and convergence study of a spectral-element numerical methodology for fluid-structure interaction

A high-order in space spectral-element methodology for the solution of a strongly coupled fluid-structure interaction (FSI) problem is developed. A methodology is based on a partitioned solution of incompressible fluid equations on body-fitted grids, and nonlinearly-elastic solid deformation equations coupled via a fixed-point iteration approach with Aitken relaxation. A comprehensive verification strategy of the developed methodology is presented, including h-, p- and temporal refinement studies. An expected order of convergence is demonstrated first separately for the corresponding fluid and solid solvers, followed by a self-convergence study on a coupled FSI problem (self-convergence refers to a convergence to a reference solution obtained with the same solver at higher resolution). To this end, a new three-dimensional fluid-structure interaction benchmark is proposed for a verification of the FSI codes, which consists of a fluid flow in a channel with one rigid and one flexible wall. It is shown that, due to a consistent problem formulation, including initial and boundary conditions, a high-order spatial convergence on a fully coupled FSI problem can be demonstrated. Finally, a developed framework is applied successfully to a Direct Numerical Simulation of a turbulent flow in a channel interacting with a compliant wall, where the fluid-structure interface is fully resolved.

Underwater explosive bubble interaction with an adjacent submerged structure

An experimental and computational investigation was conducted to study the interaction between bubbles generated by an underwater explosive (UNDEX) and a nearby steel plate structure. The experiments were performed for different standoff distances to investigate the interaction between the gas bubble and the rigid structure. High-speed photography was utilized to capture the underwater explosive gas bubble’s behavior, and a series of pressure transducers were used to record the emitted pressure histories. The numerical simulations were performed with the Dynamic System Mechanics Advanced Simulation software, which is a full y coupled Eulerian–Lagrangian fluid–structure interaction code. The numerical simulations were validated with the experiments in terms of the detonation pressure, structural surface pressures, and UNDEX gas bubble growth and collapse. Results show that the UNDEX standoff distance greatly influences the gas bubble’s shape, migration speed, bubble jetting behavior, and loading into its adjacent structure. Moreover, the impulse generated by the shock wave on the plate surface proved to be small in comparison to the impulse generated by the collapse of the UNDEX gas bubble. The magnitude of impulse depends on standoff distance, collapse symmetry, and relative collapse location.

A numerical study on the vortex-induced vibration of flexible cylinders covered with differently placed buoyancy modules

Buoyancy modules are essential parts in the offshore oil and gas drilling and production practices. They provide weight compensation for the riser and help avoid excessive top tension exerted by the floating platform. However, the deployment of buoyancy modules with larger diameter than the bare riser may alter the geometric shape of the riser system and greatly influence the hydrodynamic performance. Engineering designs with buoyancy modules are mostly based on experience and rigid cylinder forced vibration experimental data. Since it is difficult if not impossible to perform real sea experiments, numerical simulation becomes important in aiding the buoyancy module design and setup. In the present work, a fluid–structure interaction code based on the computational fluid dynamic model is firstly validated with the riser experimental data from MARINTEK (the Norwegian Marine Technology Research Institute) and then used to simulate the riser responses with different continuous buoyancy module setups. The Reynolds number based on the incoming velocity and the bluff body diameter ranges from 4×103 to 1.68×104. The simulation results suggest that the shape alteration from the buoyancy deployment will change the vortex-induced vibration(VIV) response mode along with the vibration frequency. The cylinder fatigue damage, travelling wave response and the fluid–structure energy transfer characters are discussed under different buoyancy module setups.

Aeroelastic vibration analysis of a 1.5 stage compressor

Aeroelastic vibration problems are commonly found in modern compressors operating in off-design conditions. Large amplitude vibration could lead to high cycle fatigue (HCF) of blade and usually occurs in front stages of axial compressors. In this study, the influence of tip gap size on aeroelastic stability is analyzed in a 1.5 stage compressor with an in-house fluid-structure interaction code. A three-dimensional unstructured finite-volume compressible flow solver is applied in the fluid domain and a structure dynamic solver with the modal superimposition method for blade motion is used in the structure domain. Rotor tip clearances of 1%, 2% and 3% of tip axial chord at maximum rotor loading conditions at off-design speeds are analyzed for aeroelastic stability. The tip leakage flow and vortex structure can be seen near the blade tip region at a larger tip gap size. The aeroelastic stability of rotor blade at different tip gap sizes is mainly influenced by the 1st torsion mode, and the variation of aerodynamic damping is not monotonous. The intensity of the tip vortex and shock wave are the key factors affecting the aeroelastic stability of rotor when tip gap size increases.

High fidelity simulation and validation of crossflow through a tube bundle and the onset of vibration

Flow induced vibrations can be detrimental to all manner of engineering applications with fluid flow. Heat exchangers with bundles of thin tubes experiencing a crossflow are especially susceptible to the unexpected onset of vibration. Heat-exchanger design necessarily involves extensive modeling and testing to ensure that significant vibrations cannot develop for any expected flow conditions. A properly validated numerical simulation can work in conjunction with physical experiments to identify problematic vibrations and allow for rapid iteration of design at relatively low expense. A high-fidelity fluid–structure interaction code has been developed by fully coupling CFD LES/DNS code Nek5000 and CSM code Diablo. The coupled code is used to simulate crossflow through a tube bundle in a geometry recreated after a physical experiment. Validation against this experiment involves comparing the amplitude and frequency spectrum for three different flow velocities. The velocities compared straddle the onset of large magnitude vibrations. The simulation accurately captures the onset of the vibrations with a marginally lower predicted frequency of vibration. When the onset velocity is compensated for the difference in natural frequency, the simulation results closely match the experiment. The continuous nature of the simulation measurements helps illustrate the fluid mechanics behind the pin motion and reveals the front to back propagation of the vibrations as the fluid velocity increases.

The response of an elastic splitter plate attached to a cylinder to laminar pulsatile flow

The flow-induced deformation of a thin, elastic splitter plate attached to the rear of a circular cylinder and subjected to laminar pulsatile inflow is investigated. The cylinder and elastic splitter plate are contained within a narrow channel and the Reynolds number is mostly restricted to Re=100, primarily covering the two-dimensional flow regime. An in-house Fluid-Structure Interaction code is employed for simulations, which couples a sharp-interface immersed boundary method for the fluid dynamics with a finite-element method to treat the structural dynamics. The structural solver is implicitly (two-way) coupled with the flow solver using a partitioned approach. This implicit coupling ensures numerical stability at low structure-fluid density ratios. A power spectrum analysis of the time-varying plate displacement shows that the plate oscillates at more than a single frequency for pulsatile inflow, compared to a single frequency observed for steady inflow. The multiple frequencies obtained for the former case can be explained by beating between the applied and plate oscillatory signals. The plate attains a self-sustained time-periodic oscillation with a plateau amplitude in the case of steady flow, while the superimposition of pulsatile inflow with induced plate oscillation affects the plateau amplitude. Lock-in of the plate oscillation with the pulsatile inflow occurs at a forcing frequency that is twice of the plate natural frequency in a particular mode and this mode depends on the plate length. The plate displacement as well as pressure drag increases at the lock-in condition. The percentage change in the maximum plate displacement, and skin-friction and pressure drag coefficients on the plate, due to pulsatile inflow is quantified. The non-linear dynamics of the plate and its coupling with the pulsatile flow are briefly discussed.

An embedded strategy for the analysis of fluid structure interaction problems

A Navier–Stokes solver based on Cartesian structured finite volume discretization with embedded bodies is presented. Fluid structure interaction with solid bodies is performed with an explicit partitioned strategy. The Navier–Stokes equations are solved in the whole domain via a Semi-Implicit Method for Pressure Linked Equations (SIMPLE) using a colocated finite volume scheme, stabilized via the Rhie–Chow discretization. As uniform Cartesian grids are used, the solid interface usually do not coincide with the mesh, and then a second order Immersed Boundary Method is proposed, in order to avoid the loss of precision due to the staircase representation of the surface. This fact also affects the computation of fluid forces on the solid wall and, accordingly, the results in the fluid–structure analysis. In the present work, first and second order approximations for computing the fluid forces at the interface are studied and compared. The solver is specially oriented to General Purpose Graphic Processing Units (GPGPU) hardware and the efficiency is discussed. Moreover, a novel submerged buoy experiment is also reported. The experiment consists of a sphere with positive buoyancy fully submerged in a cubic tank, subject to harmonic displacements imposed by a shake table. The sphere is attached to the bottom of the tank with a string. The position of the buoy is determined from video records with a Motion Capture algorithm. The obtained amplitude and phase curves allow a precise determination of the added mass and drag forces. Due to this aspect the experimental data can be of interest for comparison with other fluid–structure interaction codes. Finally, the numerical results are compared with the experiments, and allow the confirmation of the numerically predicted drag and added mass of the body.

English

Before using any simulation method for load simulations, it must be verified and validated. However, it is not enough only to find a discrepancy between the model and the reference data, but it is also required to identify the section within the code responsible for any bugs. Therefore, the aim of this paper is to present a set of basic validation cases to be used in fluid‐structure interaction (FSI) simulations. By splitting the FSI method into its key functionalities of forces, torques, translations, rotations, etc., each part of the coupling can be validated by itself with a specific case. Each of the cases consists of a free decay pendulum in water. To gain experimental reference data, the cases are set up in a water basin and the motion over time is recorded with an optical measurement system. In a second step, the validation cases are set up in a fluid‐multibody FSI code and simulated. The motions from numerical simulation and experiment are compared and used to draw a conclusion on the integrity of the experimental data presented and the validity of the tested FSI code.

Some manufactured solutions for verification of fluid-structure interaction codes

This paper presents manufactured solutions (MS) for verification of a fluid-structure interactions code. MS provide benchmark solutions for direct evaluation of the solution errors. The method of manufactured solutions (MMS) is a straight forward and general procedure for generating exact analytical solutions with a sufficiently rich structure to ensure that all terms of the differential equations are exercised in the simulations. When used with systematic grid refinement studies, the MMS provides strong code verification with a theorem-like quality. Manufactured solutions for fluid-structure interaction (FSI) problems with large displacements are presented with sample results from grid convergence studies.

Highly efficient optimization mesh movement method based on proper orthogonal decomposition

AbstractIn this paper, we implement the method of proper orthogonal decomposition (POD) to generate a reduced order model (ROM) of an optimization‐based mesh movement scheme. In this study it is shown that POD can be used effectively to generate an ROM, that accurately reproduces the full order mesh movement algorithm, with a decrease in computational time of over 99%. We further introduce a novel training procedure whereby the POD models are generated in a fully automated fashion. The technology is applicable to any mesh movement method and enables potential reductions of up to four orders of magnitude in mesh movement related costs. The proposed model can be implemented without having to pre‐train the POD model, to any fluid–structure interaction code with an existing mesh movement scheme. Copyright © 2010 John Wiley & Sons, Ltd.

A Numerical Study of Labyrinth Seal Flutter

A numerical study of a labyrinth-type turbine seal flutter in a large turbofan engine is described. The flutter analysis was conducted using a coupled fluid-structure interaction code, which was originally developed for turbomachinery blade applications. The flow model is based on an unstructured, implicit Reynolds-averaged Navier–Stokes solver. The solver is coupled to a modal model for the structure obtained from a standard structural finite element code. During the aeroelasticity computations, the aerodynamic grid is moved at each time step to follow the structural motion, which is due to unsteady aerodynamic forces applied onto the structure by the fluid. Such an integrated time-domain approach allows the direct computation of aeroelastic time histories from which the aerodynamic damping, and hence, the flutter stability, can be determined. Two different configurations of a large-diameter aeroengine labyrinth seal were studied. The first configuration is the initial design with four fins, which exhibited flutter instability during testing. The second configuration is a modified design with three fins and a stiffened ring. The steady-state flow was computed for both configurations, and good agreement was reached with available reference data. An aeroelasticity analysis was conducted next for both configurations, and the model was able to predict the observed flutter behavior in both cases. A flutter mechanism is proposed, based on the matching of the structural frequencies to the frequencies of waves traveling in the fluid, in the interfin cavities and in the high- and low-pressure cavities.

A two-system, single-analysis, fluid—structure interaction technique for modelling abdominal aortic aneurysms

This work reports on the implementation and validation of a two-system, single-analysis, fluid-structure interaction (FSI) technique that uses the finite volume (FV) method for performing simulations on abdominal aortic aneurysm (AAA) geometries. This FSI technique, which was implemented in OpenFOAM, included fluid and solid mesh motion and incorporated a non-linear material model to represent AAA tissue. Fully implicit coupling was implemented, ensuring that both the fluid and solid domains reached convergence within each time step. The fluid and solid parts of the FSI code were validated independently through comparison with experimental data, before performing a complete FSI simulation on an idealized AAA geometry. Results from the FSI simulation showed that a vortex formed at the proximal end of the aneurysm during systolic acceleration, and moved towards the distal end of the aneurysm during diastole. Wall shear stress (WSS) values were found to peak at both the proximal and distal ends of the aneurysm and remain low along the centre of the aneurysm. The maximum von Mises stress in the aneurysm wall was found to be 408kPa, and this occurred at the proximal end of the aneurysm, while the maximum displacement of 2.31 mm occurred in the centre of the aneurysm. These results were found to be consistent with results from other FSI studies in the literature.

PIV Validation of Blood-heart Valve Leaflet Interaction Modelling

The aim of this study was to validate the 2D computational fluid dynamics (CFD) results of a moving heart valve based on a fluid-structure interaction (FSI) algorithm with experimental measurements. Firstly, a pulsatile laminar flow through a monoleaflet valve model with a stiff leaflet was visualized by means of Particle Image Velocimetry (PIV). The inflow data sets were applied to a CFD simulation including blood-leaflet interaction. The measurement section with a fixed leaflet was enclosed into a standard mock loop in series with a Harvard Apparatus Pulsatile Blood Pump, a compliance chamber and a reservoir. Standard 2D PIV measurements were made at a frequency of 60 bpm. Average velocity magnitude results of 36 phase-locked measurements were evaluated at every 10 degrees of the pump cycle. For the CFD flow simulation, a commercially available package from Fluent Inc. was used in combination with inhouse developed FSI code based on the Arbitrary Lagrangian-Eulerian (ALE) method. Then the CFD code was applied to the leaflet to quantify the shear stress on it. Generally, the CFD results are in agreement with the PIV evaluated data in major flow regions, thereby validating the FSI simulation of a monoleaflet valve with a flexible leaflet. The applicability of the new CFD code for quantifying the shear stress on a flexible leaflet is thus demonstrated.

A Newton method using exact jacobians for solving fluid–structure coupling

This paper aims at introducing a partitioned Newton based method for solving nonlinear coupled systems arising in the numerical approximation of fluid–structure interaction problems. We provide a method which characteristic lies in the use of exact cross jacobians evaluation involving the shape derivative of the fluid state with respect to solid motion perturbations. Numerical tests based on an implementation inside a 3D fluid–structure interaction code show how the exactness of the cross jacobians computation guarantee the overall convergence of the Newton’s loop.

Containment of blast phenomena in underground electrical power plants

Gas explosion accidents in electrical power plants can be originated by electrical faults in components containing oil for insulation, such as transformers, junction boxes, etc. A ground discharge can cause the pyrolysis of part of the oil and the production of a gaseous mixture, which can generate fires or deflagrations in contact with air. A deflagration gives rise to a pressure shock wave, which can propagate throughout the underground plant if not suitably confined. Fluid-dynamic, structural and, in some cases, fluid-structure interaction analyses are needed to evaluate the overpressures occurring during the blast phenomena and the consequent damages on structural elements. In the present paper, a numerical approach to this kind of problem is proposed and applied to a case study dealing with a typical transformer cell in an underground power plant. The fluid-structure interaction code PLEXIS-3C is used to perform both fluid and structural fast dynamic computation.

Response of Water-Filled Thin-Walled Pipes to Pressure Pulses: Experiments and Analysis

Experiments are performed to verify modeling techniques used in fluid-structure interaction codes that predict the response of liquid-filled piping systems to strong pressure pulses. Pressure pulses having a 150-μs rise time, a 2000-psi (13.8 MPa) magnitude, and a 3-ms duration are propagated into straight, water-filled Ni 200 pipes (3-in. (7.6-cm) O.D. 0.065-in. (0.165-cm) wall). Attenuation of the pressure pulse and the strain and deformation along the pipes are measured. The experiments are modeled in WHAM, a two-dimensional, finite-element, compressible fluid-structure interaction code. The experimental and analytical results are discussed in detail and are found to compare favorably.

Experiment and analysis on pressure pulse propagation in a plastically deforming pipe

This paper presents an experimental and analytical study on pressure pulse propagation phenomena in a plastically deforming pipe filled with water. The simple and well-defined experiments have been performed in order to provide the fundamental data to verify a two-dimensional fluid-structure interaction code. The experimental facility consists of a test pipe and a pulse gun filled with water at room temperature. The pressure pulse was generated in the pulse gun by low velocity SK explosive, high velocity pentolite or a low energy detonator. Experiments have been performed on 3 m long, 16 cm OD and 0.287 cm thick stainless steel-316 pipe both with and without elbow section. Transient pressure, transient strain and plastic deformation are measured when the pressure pulse propagates along the test pipe which is jointed rigidly to the pulse gun. On entering the test pipe, the pulse attenuates rapidly to the yield pressure of the pipe (14 MPa) in about 40 cm. Large plastic deformation occurs over wider range in low velocity explosive experiments than high velocity explosive experiments. Peak pressure reduces in the elbow section more in the high velocity explosive experiments than the low velocity explosive experiments. The experimental results have been analyzed by using a two-dimensional code. The comparison between experiment and prediction for the pressure pulse propagation behavior shows fairly good results. However, the strains are 30–100% over-estimated by the code. This discrepancy is attributable to the influence of the strain rate on stress and the limitation of the calculational model for simulating the detailed structure of the test facility.