Session A03: FSI: General

Topology Optimization of Fluid-structure Interaction Problems: the TOBS Method Approach

This work presents a coupled topology optimization methodology for minimizing the compliance energy of a structure composed of a metal or polymer strip located in a fluid flow channel. The methodology proposed here aims to apply the Topology Optimization of Binary Structure (TOBS) method to seek for the optimum layout of the structure. This is achieved using COMSOL Multiphysics Live link with MATLAB. The steady state fluid-structure and sensitivity analyses are carried out in COMSOL. The TOBS method is implemented in MATLAB to obtain the optimized topology configuration of the structure. A number of numerical examples is presented to validate the optimization procedure and to demonstrate the effectiveness of the implemented algorithm. Also, key parameters, e.g. fluid velocity at the inlet, Young's modulus of the solid material and structure aspect ratio are investigated.   

An Eulerian method for mixed soft and rigid body interactions in fluids

Fluid-structure interaction problems are encountered in many engineering and biological applications. In this work, we presented a fully Eulerian approach for fluid-structure interactions that is simple to implement and capable of simulating multi-body interactions. When the solid is rigid, a projection step is formulated as a composite linear system that simultaneously enforces the rigidity and incompressibility constraints. When the solid is soft, a reference map technique is applied to characterize the body deformation in an Eulerian framework. Several examples including single soft and rigid flags, multiple rigid bodies, and soft-rigid combinations will be presented, with potential applications to biological systems.   

A Soft Material Anemometer

Micro air vehicles (MAVs) often operate in wind speeds of the same magnitude as their own velocity. In order to navigate and control them efficiently, there is a need to estimate the wind speed. Because of their low weight, around 100 g, a wind speed sensor must be small and lightweight. The sensor must also be able to withstand impulses from impacts or wind gusts. Most conventional sensor concepts today are unable to fulfill these requirements. We suggest a new flow sensor concept, based on electrically conductive soft materials, denoted organic elastic filament velocimetry (OEFV). This technique estimates flow velocity by measuring the strain a polymer ribbon in the flow experiences. The polymer ribbon can withstand strains of the order of $\epsilon \sim 1$, making it extremely durable, in contrast to most conventional materials. The ribbon is manufactured in polydimethylsiloxane (PDMS) and made piezoresistive by adding a thin layer of silver nanowires, so that the resistance of the ribbon can be related to the flow velocity. The large aspect ratio of the ribbon (length to thickness) simplifies the description of the flow around it and amplifies the sensor output. A fairly simple model of the sensor behavior is constructed and compared to experimental data.   

A feasibility study on the potential for employing piezoelectric cantilever beams as vortex flow sensors

The Train of Frozen Boxcars (TFB) model was previously developed to study the effective one-way coupling of the force due to the advection of a vortex or train of vortices over a stiff piezoelectric cantilever beam. The TFB approach involves the advection of several boxcars of different amplitudes, widths and separations as a model for the fluidic force acting on the beam. In this talk, we explore utilizing the TFB model as a way to deduce the force that acts on the piezoelectric beam due to the vortex and study its potential for predicting properties associated with vortex flow such as vortex circulation, diameter and separation between vortices. Preliminary results indicate that while the original TFB model can predict the properties of an individual vortex rather well, the model requires tweaks in order to better predict vortex properties when a train of vortices are involved.   

Pendulum in a Flow: Case of a Balanced Pendulum

Fluid-structure interactions are the basics of the complexity of Aerodynamics, enhancing resonance in structures and turbulence in flows. Even simple systems like a pendulum can become more complex, as a hysteretic bistability shows up for a range of flow velocities when the pendulum confronts a flow. This is predicted by a simple balance of weight and aerodynamical forces, but non stationary response can be seen through spontaneous transitions between both stable positions. This dynamic can also be observed when substracting the weight of the pendulum. By analyzing trajectories in different phase spaces, we recover a stochastic measurement of the drag and lift coefficients. Moreover, the pendulum oscillates around the horizontal at a frequency that is linked to the evolution of the normal drag coefficient with the angular position of the pendulum. The instantaneous lift and drag coefficients inferred from the dynamical behavior of the pendulum seems to be governed by the dynamical vortex shedding phenomena, which we currently investigate experimentally.   

Wind tunnel testing for hydrodynamic load characterization of icosahedron-shaped coral reef arks

Coral reefs play an important role in the balance of the marine ecosystem. They provide shelters to marine species, protect coastlines from the damaging effects of waves and tropical storms, serve as a source of nitrogen and other nutrients for marine food chains. An artificial structure named coral reef arks with the shape of icosahedron is being proposed. Their diameter are 3 meters and need to withstand ocean currents ranging from 0.5 to 2.0 m/s. Wind tunnel force measurements for one solid and one hollow icosahedron models are conducted at three different free stream tunnel speeds to investigate the hydrodynamic characteristics of the structures. Based on the model diameter of 0.152m, the tunnel speeds give rise to Reynolds numbers of 0.26, 0.37 and 0.45 million, which correspond to ocean current speeds of 0.10, 0.14 and 0.17 m/s. Results show that the drag force coefficient is reduced from 0.46 to 0.37 when the test model is changing from solid to hollow icosahedron. The dominant frequencies at Strouhal numbers of 0.24 and 0.50 for the solid icosahedron model are reduced to Strouhal numbers of 0.16 and 0.19 for the hollow model. The ``pinging test'' clarifies that these dominant Strouhal numbers are induced by the flow rather than the natural frequencies of the structure.   

Aerodynamically-Adaptive Wings using Flow-Interactive Control

Controlled interactions between a wing surfaces and the embedding flow are explored to effect tunable structural (e.g., stiffness, damping) and aeroelastic (bending and twist) properties through the aerodynamic load distributions. These interactions and therefore the aerodynamic loads, are regulated using actively-distributed air bleed that is driven through the wing’s surfaces by the flow-induced pressure differences and is regulated by integrated louvers. Wind tunnel investigations using a modular 3-D half-span flexible wing model have explored quasi-static and time-dependent, transitory coupling between bleed-induced aerodynamic loads and wing’s aeroelastic properties. The flow structure is investigated using particle image velociometry (PIV) and the structural response of the wing model is assessed over a broad range of angle of attack using a motion analysis system and an array of accelerometers. It is shown that bleed actuation which leads to significant modifications of the aerodynamic loads and of the near-wake flow field and can be exploited for temporal control of its dynamical bending characteristics.