Fluid-structure Interaction Interface Research Articles

Composed Fluid–Structure Interaction Interface for Horizontal Axis Wind Turbine Rotor

In this paper, we propose a technique for high-fidelity fluid–structure interaction (FSI) spatial interface reconstruction of a horizontal axis wind turbine (HAWT) rotor model composed of an elastic blade mounted on a rigid hub. The technique is aimed at enabling re-usage of existing blade finite element method (FEM) models, now with high-fidelity fluid subdomain methods relying on boundary-fitted mesh. The technique is based on the partition of unity (PU) method and it enables fluid subdomain FSI interface mesh of different components to be smoothly connected. In this paper, we use it to connect a beam FEM model to a rigid body, but the proposed technique is by no means restricted to any specific choice of numerical models for the structure components or methods of their surface recoveries. To stress-test robustness of the connection technique, we recover elastic blade surface from collinear mesh and remark on repercussions of such a choice. For the HAWT blade recovery method itself, we use generalized Hermite radial basis function interpolation (GHRBFI) which utilizes the interpolation of small rotations in addition to displacement data. Finally, for the composed structure we discuss consistent and conservative approaches to FSI spatial interface formulations.

A Temporal Consistent Monolithic Approach to Fluid-Structure Interaction Enabling Single Field Predictors

We present a monolithic approach to large-deformation fluid-structure interaction (FSI) problems that allows for choosing fully implicit, single-step and single-stage time integration schemes in the structure and fluid field independently, and hence is tailored to the needs of the individual field. The independent choice of time integration schemes is achieved by temporal consistent interpolation of the interface traction. To reduce computational costs, we introduce the possibility of field specific predictors in both structure and fluid field. These predictors act on the single fields only. Possible violations of the interface coupling conditions during the predictor step are dealt with within the monolithic solution procedure. We present full detail of such a generalized monolithic solution procedure, which is fully consistent in its non-conforming temporal and spatial discretization. The incorporated mortar approach allows for non-matching spatial discretizations of the fluid and solid domain at the FSI interface and is fully integrated in the resulting monolithic system of equations. The method is applied to a variety of numerical examples. Thereby, temporal convergence rates, the special role of essential boundary conditions at the fluid-structure interface, and the positive effect of predictors are demonstrated and discussed. Emphasis is put on the comparison of different time integration schemes in fluid and structure field, for what the achieved freedom of choice of time integrators is fully exploited.

Space-mapping in fluid–structure interaction problems

Transient response due to gust loads can lead to structural failure despite the fact that the aero-elastic system is asymptotically stable. Unsteady aeroelastic analysis should therefore be included in the load calculation cycle of the aircraft design process. Especially in the transonic regime, partitioned strong coupling algorithms perform better than loose coupling algorithms and allow larger time-steps in the unsteady simulation. However, the simulation of high fidelity unsteady fluid–structure interaction using strong coupling algorithms is currently too expensive in order to be useful in industry.To accelerate high fidelity partitioned fluid–structure interaction simulations we apply space-mapping, which is a technique that originates from the field of multi-fidelity optimization. Without loss of generality we assume the availability of a cheap low fidelity fluid solver and an expensive high fidelity fluid solver. The space-mapping approach is used to accelerate the high fidelity computation using black-box input/output information of both the high fidelity fluid solver and low fidelity fluid solver. In order to achieve this, a space-mapping function is defined on the fluid–structure interface which keeps track of the differences between the high fidelity model and the low fidelity model during the coupling iterations. Reformulating the root-finding problem on the fluid–structure interaction interface using the space-mapping function results in the Aggressive Space-Mapping algorithm.The Aggressive Space-Mapping algorithm is applied to 1-D and 2-D test cases in order to assess the speedup with respect to the Quasi-Newton Inverse Least Squares algorithm. The latter is considered to be the current state of the art in partitioned strong coupling algorithms. The observed speedup depends mainly on the type of FSI problem and the time step size. The maximum observed speedup is about 1.5.The application of the space-mapping technique to partitioned fluid–structure interaction problems is found to be a promising approach. The framework is non-intrusive and allows the reuse of existing solvers which is especially useful in an industrial environment. It is expected that the space-mapping technique can be combined with higher order time integration schemes that maintain accuracy over a large range of time step sizes.

Passive movement of human soft palate during respiration: A simulation of 3D fluid/structure interaction

This study reconstructed a three dimensional fluid/structure interaction (FSI) model to investigate the compliance of human soft palate during calm respiration. Magnetic resonance imaging scans of a healthy male subject were obtained for model reconstruction of the upper airway and the soft palate. The fluid domain consists of nasal cavity, nasopharynx and oropharynx. The airflow in upper airway was assumed as laminar and incompressible. The soft palate was assumed as linear elastic. The interface between airway and soft palate was the FSI interface. Sinusoidal variation of velocity magnitude was applied at the oropharynx corresponding to ventilation rate of 7.5L/min. Simulations of fluid model in upper airway, FSI models with palatal Young's modulus of 7539Pa and 3000Pa were carried out for two cycles of respiration. The results showed that the integrated shear forces over the FSI interface were much smaller than integrated pressure forces in all the three directions (axial, coronal and sagittal). The total integrated force in sagittal direction was much smaller than that of coronal and axial directions. The soft palate was almost static during inspiration but moved towards the posterior pharyngeal wall during expiration. In conclusion, the displacement of human soft palate during respiration was mainly driven by air pressure around the surface of the soft palate with minimal contribution of shear stress of the upper airway flow. Despite inspirational negative pressure, expiratory posterior movement of soft palate could be another factor for the induction of airway collapse.

A Porous Elastic Model for Bacterial Biofilms: Application to the Simulation of Deformation of Bacterial Biofilms Under Microfluidic Jet Impingement

The presence of bacterial biofilms is detrimental in a wide range of healthcare situations especially wound healing. Physical debridement of biofilms is a method widely used to remove them. This study evaluates the use of microfluidic jet impingement to debride biofilms. In this case, a biofilm is treated as a saturated porous medium also having linear elastic properties. A numerical modeling approach is used to calculate the von Mises stress distribution within a porous medium under fluid-structure interaction (FSI) loading to determine the initial rupture of the biofilm structure. The segregated model first simulates the flow field to obtain the FSI interface loading along the fluid-solid interface and body force loading within the porous medium. A stress-strain model is consequently used to calculate the von Mises stress distribution to obtain the biofilm deformation. Under a vertical jet, 60% of the deformation of the porous medium can be accounted for by treating the medium as if it was an impermeable solid. However, the maximum deformation in the porous medium corresponds to the point of maximum shear stress which is a different position in the porous medium than that of the maximum normal stress in an impermeable solid. The study shows that a jet nozzle of 500 μm internal diameter (ID) with flow of Reynolds number (Re) of 200 can remove the majority of biofilm species.

Variational principles of nonlinear dynamical fluid–solid interaction systems

Based on the fundamental equations of continuum mechanics, the concept of Hamilton's principle and the adoption of Eulerian and Lagrangian descriptions of fluid and solid, respectively,variational principles admitting variable boundary conditions are developed to model mathematically the nonlinear dynamical behaviour of the responses and interactions between fluid and solid. The nonlinearity of the fluid is introduced through nonlinear field equations and nonlinear boundary conditions on the free surface and fluid–solid interaction interface. The structure is treated as a nonlinear elastic body. This model assumes the fluid inviscid, incompressible or compressible and the fluid motion irrotational or rotational but isentropic along the flow path of each fluid particle. The stationary conditions of the variational principles include the governing equations of nonlinear elastic dynamics, fluid dynamics and those relating to the fluid-structure interaction interface as well as the imposed boundary conditions. A family of variational principles are obtained depending on the assumptions introduced into the mathematical model (i.e. fluid incompressible, motion irrotational, etc.) and these provide a foundation to construct numerical schemes of study to assess the dynamical behaviour of nonlinear fluid–solid interaction systems. Two simple illustrative examples are presented demonstrating the applicability of the proposed theoretical approach.