Transient Fluid-structure Interaction Research Articles

The simulation of deformation and vibration characteristics of a flexible hydrofoil based on static and transient FSI

Flexible hydrofoils can be regarded as the simplified lifting bodies of composite propellers, and these hydrofoils are suitable for use in the study of the fluid-structure interaction (FSI) computational method. The hydrodynamic simulation of a rigid cantilevered rectangular hydrofoil is performed first, then the boundary rotational motion and remeshing approach are used for the numerical calculation of the hydrofoil's pitching hydrodynamic performance, and finally the tunnel boundary effect is perfectly simulated via the steps of structured meshing, rotational parameter transforming, and remeshing. The mesh number, boundary layer, orthogonality angle, and turbulent intensity parameters are obtained through comparisons of various models; thus, the Laminar Separation Bubble (LSB) and turbulent transition are captured. Then the static FSI simulation of the flexible hydrofoil is conducted, showing that the lift of the flexible hydrofoil is low and the drag is high for the two-way FSI compared to the one-way FSI. The center of pressure, the maximum deformation, and the stress move toward the leading edge, showing the necessity of the two-way FSI calculation. The transient FSI simulation of the flexible hydrofoil is then studied using the Large Eddy Simulation (LES) turbulence model to calculate the pressure load, which can simulate the turbulence fluctuation as the exciting source of the structural frequency-domain load. The calculated first peak frequencies are 96 Hz and 78 Hz at 4° and 8°, respectively. The modal vibration shapes of the POM hydrofoil are calculated showing the first mode of bending, the second mode of twisting and the higher mode of bend-twist coupling. The calculated wet natural frequencies of the POM hydrofoil and the steel hydrofoil show good agreements with experimental values. As the modal rank increases, the POM hydrofoil's wet modal frequency decrease varies from 53.8% to 29.2%, whereas the steel hydrofoil's wet modal frequency decrease varies from 36.9% to 14.2%.

Dynamic load and stress analysis of a large horizontal axis wind turbine using full scale fluid-structure interaction simulation

A dynamic load and stress analysis of a wind turbine is carried out using transient fluid-structure interaction simulations. On the structural side, the three 50 m long commercial glass-fiber epoxy blades are modelled using shell elements, accurately including the properties of the composite materials. On the fluid side, a hexahedral mesh is obtained for every blade and for the hub of the machine. These meshes are then overlaid to a structured background mesh through an overset technique. The displacements prescribed by the structural solver are imposed on top of the rigid rotation of the turbine. The atmospheric boundary layer (ABL) is included using the k-epsilon turbulence model. The computational fluid dynamics (CFD) and computational solid mechanics (CSM) solvers are strongly coupled using an in-house code. The transient evolution of loads, stresses and displacements on each blade is monitored throughout the simulated time. The ABL induces oscillating axial displacements in the outboard region of the blade. Furthermore, the influence of gravity on the structure is accounted for and investigated, showing that it largely affects the tangential displacement of the blade. The oscillating deformations lead to sensible differences in the torque provided by each blade during its rotation.

Preliminary Study on Mesh Stiffness Models for Fluid-structure Interaction Problems

One of the challenges in modern computational engineering is the simulation of fluid-structure interaction (FSI) phenomena where one of the crucial issues in the multi-physics simulation is the choice of stiffness model for mesh deformation. This paper focuses on the application of iteratively implicit coupling procedure on two transient FSI cases of vortex induced-vibration (VIV) that manifest oscillating flexible structures. The aim is to study various mesh stiffness models in the Laplace equation of diffusion employed within the arbitrary Lagrangian-Eulerian (ALE) methodology to handle the moving mesh. In the first case where a laminar flow interacted with a flexible splitter, it was demonstrated that a near FSI boundaries increased-stiffness model prevails to manage a large deformation of the moving structure as compared to a near volume increased-stiffness model. However, the potential technique could not be exploited to the second FSI configuration, where the effect of the turbulence of flow was included. It was found that the mesh topology near the FSI interface was collapsed. Instead of utilizing the same approach, a mesh stiffness based on a wall distance was found to be auspicious. Thus, the mesh stiffness model in the FSI simulation is case-dependent.

A new approach for computing the steady state fluid–structure interaction response of periodic problems

A special type of fluid–structure interaction (FSI) problems are problems with periodic boundary conditions like in turbomachinery. The steady state FSI response of these problems is usually calculated with similar techniques as used for transient FSI analyses. This means that, when the fluid and structure problem are not simultaneously solved with a monolithic approach, the problem is partitioned into a fluid and structural part and that each time step coupling iterations are performed to account for strong interactions between the two sub-domains. This paper shows that a time-partitioned FSI computation can be very inefficient to compute the steady state FSI response of periodic problems. A new approach is introduced in which coupling iterations are performed on periodic level instead of per time step. The convergence behaviour can be significantly improved by implementing existing partitioned solution methods as used for time step coupling (TSC) algorithms in the time periodic coupling (TPC) framework. The new algorithm has been evaluated by comparing the convergence behaviour to TSC algorithms. It is shown that the number of fluid–structure evaluations can be considerably reduced when a TPC algorithm is applied instead of a TSC. One of the most appealing advantages of the TPC approach is that the structural problem can be solved in the frequency domain resulting in a very efficient algorithm for computing steady state FSI responses.

Numerical evaluation of systematic errors of a non-invasive intracranial pressure measurement

Intracranial pressure (ICP) monitoring procedure can be applied to aid in secondary brain damage prevention. A high invasiveness of commonly used ICP measuring methods poses a risk of complications, and therefore new non-invasive methods are currently being developed. A promising non-invasive ICP measurement method is based on the existence of pressure balance state, which is driven by the unique morphological property of ophthalmic artery (OA). The value of ICP can be obtained by evaluating blood flow or artery characteristics in different OA segments, intracranial OA segment (IOA) and extracranial OA segment (EOA). In order to increase measurement accuracy, the systematic errors must be evaluated, which requires an implementation of a numerical model encompassing various physical phenomena. In this paper, a developed numerical model is presented, which was used to solve a transient fluid–structure interaction (FSI) problem of the pulsatile blood flow in a straight, physically meaningful anisotropic, hyperelastic OA, with ICP and external pressure (Pe) loads. It was found that the systematic error based on mean cross-sectional area difference between IOA and EOA segments was {–1.48, –1.37, –1.17} mmHg with ICP = {10, 20, 30} mmHg, respectively. The systematic error based on mean blood flow velocity difference between IOA and EOA segments was {–1.84, –1.76, –1.625} mmHg with ICP = {10, 20, 30} mmHg, respectively. The presented numerical model examined the worst-case scenario in terms of boundary conditions, which were immovable, while lengths of OA segments were physiologically relevant statistical means; however, the obtained systematic errors still met the clinical standards of ANSI/AAMI, where it is stated that the error should not exceed the ± 2 mmHg in the range of 0–20 mmHg of ICP. Boundary conditions and compliance affects the systematic error in both ways (reduce or increase it); this may explain the low systematic errors obtained in experimental studies by other authors.

High resolution adaptive framework for fast transient fluid-structure interaction with interfaces and structural failure – Application to failing tanks under impact

• We provide a new computational framework for transient fluid-structure interaction. • Complex phenomena such as structural failure and fluid interfaces are accounted for. • High-accuracy is achieved via a multi-purpose adaptive strategy for fluid & structure. • Advanced experiments for tanks under impact are simulated with very accurate results.

Bubble-sphere interaction beneath a free surface

The bubble-sphere interaction beneath a free surface is investigated experimentally and numerically, which is associated with underwater explosion and cavitation. We use the electric discharge technique to generate an oscillating bubble that is set above a movable sphere and below a free surface. The transient fluid-structure interaction (FSI) is recorded by high-speed imaging. We identify three distinctive bubble jet impact patterns, i.e., (i) jet impacts on the sphere top with water layer between them; (ii) the bubble partly envelops the sphere at first and then the jet impacts on the sphere top without water layer; (iii) the jet impacts on an annular ring, splitting the bubble into a toroidal bubble and a new singly-connected bubble. Such transient FSI events are modeled by the boundary integral method and the foregoing bubble splitting is modeled by an improved vortex ring model. The bubble, free surface, and sphere dynamics in experiments are found to be captured extremely well by numerical simulations. The bubble dynamic behaviors are associated with three dimensionless parameters and a systematic study of these parameters are provided. Lastly, the two types of cavitation phenomena on sphere surface are discussed and the detailed physical mechanisms are analyzed.

Numerical studies on the instantaneous fluid–structure interaction of an air-inflated flexible membrane in turbulent flow

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.

Numerical Simulation of Fluid–Structure Interaction Problem Associated with Vertical Launching System

We develop a combined Lagrangian–Eulerian method for transient fluid–structure interaction problems. Based on the ghost fluid framework for improving interface tracking accuracy between a fluid (hot rocket exhaust plume) and a high strain rate deforming solid (rear cover of a vertical launch system), the numerical coupling between the two media ensures an accurate description of the flexible structure. A nine-node quadrilateral element based on total Lagrangian formulation is used, while the hydrodynamic finite difference method is used for the supersonic exhaust plume that forms a complex flow within the plenum. The Lagrangian, Eulerian, and fluid–structure interaction coupling methods are verified by ANSYS results and related theories. A two-dimensional simulation of the full vertical launch system operation mode is conducted. This requires an accurate reproduction of the complex flowfield generated by the rapid rear cover opening under a high-pressure plume during rocket launch. This fluid–structure interaction problem solution may be used for future design upgrades when a vertical launch system is exposed to unusually harsh interactive gas and structure conditions.

Numerical investigation on global responses of surface ship subjected to underwater explosion in waves

Transient fluid-structure interaction (FSI) has always been a challenging problem in ship mechanics. Underwater explosion and its highly nonlinear impact on nearby structures are more complicated. In this paper, the global responses of a surface ship subjected to an underwater explosion in waves are numerically investigated considering the nonlinear coupling between the fluid and structure solvers. The fluid field is modeled with the boundary element method (BEM), while the potential decomposition theory is adopted to take the surface waves into account. The global responses of the ship structure are modeled with the mode superposition method. Then a FSI model is established using the mode decomposition method of the acceleration potential to consider the interaction. An underwater explosion experiment is carried out to validate the numerical model, which shows good agreement between the numerical and experimental results. With the presented model, the interactions between a ship and an underwater explosion bubble both in still water and waves are simulated. The dynamics of the ship global responses and the influence of the waves are analyzed which shows noteworthy nonlinearity.

Modelling of delamination due to hydraulic shock when piercing anisotropic carbon-fiber laminates using an abrasive waterjet

Abstract Abrasive waterjet (AWJ) piercing of composite laminates typically generates interlaminar delamination due to the hydraulic shock (‘water hammer’) associated with liquid jet impact. This paper extends previous liquid jet impact literature by examining the effects of hydraulic shock on an anisotropic carbon-fiber/epoxy laminate using finite element analysis and experiments. A transient fluid-structure interaction model was developed to simulate the AWJ impact process, thereby providing a detailed explanation of the underlying mechanisms leading to substrate damage in an anisotropic material. The fluid domain of the numerical model examined the effects of varying pressure and nozzle size on the magnitude of the hydraulic shock. The forces generated in the fluid domain acted as the loads in the transient model which utilized cohesive zone modelling to predict the initiation of delamination cracks in the carbon-fiber/epoxy substrate. The numerical results showed that increased pressure and nozzle size resulted in increased hydraulic shock loading, and increased ply debonding. These results were verified experimentally on AWJ pierced specimens using 3D x-ray micro-tomography. The numerical and experimental results showed that the composite anisotropy produced an asymmetric shock loading along the liquid-solid interface, which contributed to the asymmetric delamination in the upper ply of the composite.

A partitioned approach for the coupling of SPH and FE methods for transient nonlinear FSI problems with incompatible time‐steps

SummaryWe propose a method to couple smoothed particle hydrodynamics and finite elements methods for nonlinear transient fluid–structure interaction simulations by adopting different time‐steps depending on the fluid or solid sub‐domains. These developments were motivated by the need to simulate highly non‐linear and sudden phenomena requiring the use of explicit time integrators on both sub‐domains (explicit Newmark for the solid and Runge–Kutta 2 for the fluid). However, due to critical time‐step required for the stability of the explicit time integrators in, it becomes important to be able to integrate each sub‐domain with a different time‐step while respecting the features that a previously developed mono time‐step coupling algorithm offered. For this matter, a dual‐Schur decomposition method originally proposed for structural dynamics was considered, allowing to couple time integrators of the Newmark family with different time‐steps with the use of Lagrange multipliers. Copyright © 2016 John Wiley & Sons, Ltd.

Torque Analysis of IPMC Actuated Fin of a Micro Fish like Device Using Two-Way Fluid Structure Interaction Approach

In this paper, a numerical simulation of three dimensional model of IPMC actuated fin of a fish like micro device is presented using two-way fluid structure interaction approach. The device is towed by the surface vessel through a tow cable. Fin is acting as dorsal fin of the fish to control depth of the device and also acts as a stabiliser against its roll motion. Fin's displacement disturbs water flow streamlines around it, as a result velocity and pressure profile of fluid's domain changes around the actuated fin. As fin's position continuously changes throughout its actuation cycle, this makes it transient structural problem coupled with a fluid domain. Fin's displacement is received by the fluid and resulting fluid forces are received by the fin making it a two-way fluid structure interaction (FSI) problem. Such problems are solved by multi field numerical simulation approach. This multifield numerical simulation is performed in ANSYS WORKBENCH by coupling transient structural and Fluid Flow (CFX) analysis systems. It is desirous to determine the torque acting on the fin due to fluid forces through its actuation cycle by IPMC actuators. The objective of this study is to develop the methodology (two-way fluid structural interaction (FSI)) used to simulate the transient FSI response of the IPMC actuated fin, subjected to large displacement against different flow speeds. Efficacy of fin as depressor and riser is also required to be judged by monitoring the forces acting on wing in response to its displacement under IPMC actuation. Same approach is also applicable to the self-propelled systems.

Active Transient Sound Radiation Control from a Smart Piezocomposite Hollow Cylinder

Abstract The linear 3D piezoelasticity theory along with active damping control (ADC) strategy are applied for non-stationary vibroacoustic response suppression of a doubly fluid-loaded functionally graded piezolaminated (FGPM) composite hollow cylinder of infinite length under general time-varying excitations. The control gain parameters are identified and tuned using Genetic Algorithm (GA) with a multi-objective performance index that constrains the key elasto-acoustic system parameters and control voltage. The uncontrolled and controlled time response histories due to a pair of equal and opposite impulsive external point loads are calculated by means of Durbin’s numerical inverse Laplace transform algorithm. Numerical simulations demonstrate the superior (good) performance of the GA-optimized distributed active damping control system in effective attenuation of sound pressure transients radiated into the internal (external) acoustic space for two basic control configurations. Also, some interesting features of the transient fluid-structure interaction control problem are illustrated via proper 2D time domain images and animations of the 3D sound field. Limiting cases are considered and accuracy of the formulation is established with the aid of a commercial finite element package as well as comparisons with the current literature.

A non-intrusive partitioned approach to couple smoothed particle hydrodynamics and finite element methods for transient fluid-structure interaction problems with large interface motion

We propose a non-intrusive numerical coupling method for transient fluid-structure interaction (FSI) problems simulated by means of different discretization methods: smoothed particle hydrodynamics (SPH) and finite element (FE) methods for the fluid and the solid sub-domains, respectively. As a partitioned coupling method, the present algorithm can ensure a zero interface energy during the whole period of numerical simulation, even in the presence of large interface motion. In other words, the time integrations of the two sub-domains (second order Runge---Kutta scheme for fluid and Newmark integrator for solid) are synchronized. Thanks to this energy-conserving feature, one can preserve the minimal order of accuracy in time and the numerical stability of the FSI simulations, which are validated with a 1D and a 2D trivial numerical test cases. Additionally, some other 2D FSI simulations involving large interface motion have also been carried out with the proposed SPH---FE coupling method. Finally, an example of aquaplaning problem is given in order to show the feasibility of such coupling method in multi-dimensional applications with complicated structural geometries.

Hydro-elastic analysis of marine propellers based on a BEM-FEM coupled FSI algorithm

ABSTRACT A reliable steady/transient hydro-elastic analysis is developed for flexible (composite) marine propeller blade design which deforms according to its environmental load (ship speed, revolution speed, wake distribution, etc.) Hydro-elastic analysis based on CFD and FEM has been widely used in the engineering field because of its accurate results however it takes large computation time to apply early propeller design stage. Therefore the analysis based on a boundary element method-Finite Element Method (BEM-FEM) Fluid-Structure Interaction (FSI) is introduced for computational efficiency and accuracy. The steady FSI analysis, and its application to reverse engineering, is designed for use regarding optimum geometry and ply stack design. A time domain two-way coupled transient FSI analysis is developed by considering the hydrodynamic damping ffects of added mass due to fluid around the propeller blade. The analysis makes possible to evaluate blade strength and also enable to do risk assessment by estimating the change in performance and the deformation depending on blade position in the ship's wake. To validate this hydro-elastic analysis methodology, published model test results of P5479 and P5475 are applied to verify the steady and the transient FSI analysis, respectively. As the results, the proposed steady and unsteady analysis methodology gives sufficient accuracy to apply flexible marine propeller design.

The influence of computational assumptions on analysing abdominal aortic aneurysm haemodynamics.

The variation in computational assumptions for analysing abdominal aortic aneurysm haemodynamics can influence the desired output results and computational cost. Such assumptions for abdominal aortic aneurysm modelling include static/transient pressures, steady/transient flows and rigid/compliant walls. Six computational methods and these various assumptions were simulated and compared within a realistic abdominal aortic aneurysm model with and without intraluminal thrombus. A full transient fluid-structure interaction was required to analyse the flow patterns within the compliant abdominal aortic aneurysms models. Rigid wall computational fluid dynamics overestimates the velocity magnitude by as much as 40%-65% and the wall shear stress by 30%-50%. These differences were attributed to the deforming walls which reduced the outlet volumetric flow rate for the transient fluid-structure interaction during the majority of the systolic phase. Static finite element analysis accurately approximates the deformations and von Mises stresses when compared with transient fluid-structure interaction. Simplifying the modelling complexity reduces the computational cost significantly. In conclusion, the deformation and von Mises stress can be approximately found by static finite element analysis, while for compliant models a full transient fluid-structure interaction analysis is required for acquiring the fluid flow phenomenon.

Coupling of SPH-ALE method and finite element method for transient fluid–structure interaction

This article presents an interface-energy-conserving coupling strategy for transient fluid–structure interaction. The solid sub-domain is discretized by finite element method with Newmark time integrator, whereas for the fluid sub-domain we use the mesh-less method SPH-ALE with 2nd order Runge–Kutta scheme. This paper proposes a method to impose a mean interface normal velocity continuity in such a way that the algorithmic interface energy is zero during the whole period of numerical simulation. This coupling method thus ensures that the coupled problem shall be stable in time. Secondly it will converge in time with the rate of convergence of the worst time integrator chosen for each problem. The proposed method is first applied to a mono-dimensional problem by which we investigate the phenomena of propagation of shock waves across the fluid–structure interface. A good agreement is observed between the numerical result and the analytical solution in the 1D shock wave propagation test cases. Finally, a multi-dimensional example is presented and compared to another coupling approach.

Systolic fluid–structure interaction model of the congenitally bicuspid aortic valve: assessment of modelling requirements

A transient fluid–structure interaction (FSI) model of a congenitally bicuspid aortic valve has been developed which allows simultaneous calculation of fluid flow and structural deformation. The valve is modelled during the systolic phase (the stage when blood pressure is elevated within the heart to pump blood to the body). The geometry was simplified to represent the bicuspid aortic valve in two dimensions. A congenital bicuspid valve is compared within the aortic root only and within the aortic arch. Symmetric and asymmetric cusps were simulated, along with differences in mechanical properties. A moving arbitrary Lagrange–Euler mesh was used to allow FSI. The FSI model requires blood flow to induce valve opening and induced strains in the region of 10%. It was determined that bicuspid aortic valve simulations required the inclusion of the ascending aorta and aortic arch. The flow patterns developed were sensitive to cusp asymmetry and differences in mechanical properties. Stiffening of the valve amplified peak velocities, and recirculation which developed in the ascending aorta. Model predictions demonstrate the need to take into account the category, including any existing cusp asymmetry, of a congenital bicuspid aortic valve when simulating its fluid flow and mechanics.

Effect of the transient fluid-structure interaction on sound radiation from a partially submerged cylindrical pile

A time-domain structural acoustics model of a partially submerged cylindrical pile excited by an impact force has been developed to predict underwater sound radiation for arbitrary field conditions. Radial boundary conditions are formulated using the velocity potential so pressure waveforms near the pile wall can be estimated; however, the rapid decay observed in measured waveforms is not easily predicted because radiation loading on the structure is also transient. The sudden acceleration of the wall initially results in a net flow of energy from the structure into surrounding fluids. Subsequently part of this energy radiates to the far field as a compressional wave; however, as the forced wall motion decreases and the spatial pressure gradient at the wall reverses direction, fluid accelerates back to the pile and re-excites the wall. A model for this transient fluid-structure interaction was formulated and integrated into the time-domain structural acoustics numerical model. By accounting for energy flow back into the structure, predicted pressure waveforms are in good agreement with field data. Results indicate that a significant portion of the kinetic energy remains in the near field and does not radiate sound, and that temporal characteristics of the impact force may influence far field sound radiation.