Session C48: Fluid-Structure Interactions (FSI) II

Vortex-induced vibration of a flexibly-mounted cylinder placed in the proximity of a stationary parallel cylinder

Some computational results on the vortex-induced vibrations (VIV) of a flexibly-mounted cylinder placed in the proximity of a stationary parallel cylinder will be presented. Simulations are performed at moderate Reynolds numbers (100<Re<1000), where VIV features of the non-stationary cylinder are examined for various gap sizes between the two cylinders and for different diameters of the stationary cylinder. An in-house fluid-structure interactions solver based on HORUS platform -- an open-source LES/DNS flow solver -- is used for simulations with two- and three-dimensional (2- and 3-D) cylinders. The results for 2-D cylinders of the same diameter at Re=150, the reduced velocity of 5 and for the centre-to-centre distance of 3.5D* (D* being the cylinder diameter) show a broken-symmetry vortex-shedding pattern behind the oscillating cylinder. This is caused by interactions between vortices shedding from the oscillating and stationary cylinders, as well as formation of a gap flow between them. Previous computational studies considered only 2-D cylinders and employed RANS flow solvers coupled with turbulence models.   

Modal Analysis of Strongly Coupled Fluid-Structure Interaction Problems

With recent trends in many energy markets, the use of hydroelectric turbines is shifting from base power delivery to grid regulators. The emergence of intermittent renewable energy sources is increasing the need for the use of turbines in off-peak conditions and frequent starts and stops. These cause transient phenomena that can be damageable to the fatigue life of the main turbine components. To tackle this issue, knowledge of the turbines vibration modes frequency and damping coefficient is essential. In this context, we developed a new modal analysis method for strongly coupled fluid-structure interaction (FSI) problems. The proposed technique couples linear elastic equations and linearized Navier-Stokes equations in a finite element formulation. This allows for the evaluation of the frequency and damping coefficient of the coupled modes of complex structures subject to important flows without the need for computationally heavy fully coupled non-linear time-integration FSI simulations. For now, a proof of concept has been elaborated using the 2D flag flutter problem and it was validated using 3D hydrofoil experiments.   

Modal analysis of a spinning disk in a dense fluid as a model for high head hydraulic turbines

In high head Francis turbines and pump-turbines in particular, RSI is an unavoidable source of excitation that needs to be predicted accurately. Precise knowledge of turbine dynamic characteristics, notably the variation of the rotor natural frequencies with rotation speed and added mass of the surrounding water, is essential to assess potential resonance and resulting amplification of vibrations. In these machines, the rotating disk-like structures of the runner crown and band as well as the head cover and bottom ring give rise to the emergence of diametrical modes and a mode split phenomenon for which no efficient prediction method exists to date. Fully coupled FSI methods are too computationally expensive; hence, a simplified method based on the modal force approach would be a powerful tool for the design and expected life prediction of these turbines. This work presents the development of an analytical model for a rotating disk in a dense fluid, which accurately predicts the natural frequency split as well as the natural frequency drift that are observed in experiments. Additionally, the analytical model gives an explanation on the physical origin of the mode split phenomenon.   

Force and wake observations for a circular cylinder undergoing forced 2-DOF motion in a free stream

A series of forced motion experiments were performed for a circular cylinder undergoing combined in-line and cross-flow motion in a free stream. Variation of the in-line amplitude, cross-flow amplitude, reduced velocity, and phase between in-line and cross-flow motions were made for a Reynolds number of 7620, with a total of 9555 experiments. Forces were measured for all experiments and PIV visualization of the wake was made for select experiments. Specific phases between in-line and cross-flow motion are shown to have multiple possible wakes depending on the length of the experiment, with specific motion combinations being particularly sensitive to the conditions of the experiment. Although all forced motions were symmetric, asymmetry was observed in the wake for specific combinations of motion parameters, leading to asymmetric forcing and large mean lift. The measured forces and resulting wake are particularly sensitive to the phasing between in-line and cross-flow motion, indicating that if using forces derived through forced motion experiments for the prediction of 2-DOF vortex-induced vibrations, one should have sufficient resolution in the phase parameter in order to properly capture the variation in forces.
  

Insights on the pressure distributions of controlled ice-induced vibration experiments

Increased need for energy pushed offshore wind, oil, and gas companies into deeper waters in the Arctic and Subarctic regions, bringing extra challenges with it. It is observed that dynamic interaction between the offshore structure and floating ice sheet might result in ice-induced vibrations, which can contribute to fatigue damage. To better understand this complex phenomenon, forced ice-induced vibration experiments are carried out in the ice tank at the Hamburg Ship Model Basin (HSVA), Hamburg, Germany. In the tests, different rigid structures are forced through the still ice by systematically changing the ice speed. Pressure distributions in the ice-structure interface and resulting structural motions are analyzed using two multivariate analysis methods: 1) proper orthogonal decomposition and 2) smooth orthogonal decomposition. Results indicate that both methods capture the dominant pressure variations successfully and result responses that are physically interpretable. For example, first pressure mode illustrates the ductile pressure variation on the structure, which varies in the ice drift direction, and some combinations of second and third pressure modes represent the oscillations in the cross-flow direction, which illustrate the complex nature of ice-induced vibrations.   

Burst-and-coast swimming in zebrafish

Swimming kinematics of small fish such as zebrafish are characterized by intermittent sequences consisting in an active swimming phase directly followed by a passive coast phase. These specific sequences are based on a coupling between sensing and decision: fish use the passive time to sense their environment and prepare their next move. Fish essentially use vision and the lateral line system to see and sense their surrounding environment. The mechanisms that govern this “sensing” to “decision-making” (S2D) process are still to be understood and detailed. This work is an attempt to characterize these sequences using several archetypal model experiments and models gathering hydrodynamics, statistics and behavioral sciences. We will focus on new results obtained from real fish experiments in free swimming and forced gait configurations (using a controlled swimming channel). We will show how the statistics of these S2D sequences evolve with the conditions of the experiment; the important parameters being here the external flow conditions and the size of the habitat, but also the species and the maturity state of the fish (larva, juvenile or adult). We believe that these results will have direct implications on the design and implementation of biomimetic robotic systems.   

Swimming via size-change: High efficiency propulsion using resonant fluid-structure interactions

Cephalopods use large-scale structural deformation to propel themselves underwater, changing their internal volume by 20-50%. In this work, the hydroelastic response of a swimmer comprised of a fluid-filled elastic-membrane is studied via analytic, numerical, and experimental methods. The self-propelled soft-body fluid and solid dynamics are shown to benefit greatly from the irreversible jet flow, the reversible internal added-mass variation, and the pulsation in tune with the swimmer’s immersed fundamental frequency. It is shown that even a simplistic size-changing structure can utilize these physical mechanisms to achieve quasi-propulsive power ratios of greater than 100%, i.e. self-propulsion for these swimmers requires less energy than towing at the same speed.   

Breakdown of continuum models of fluid structure interaction in a nanoporous biological material

Determining how porous materials interact with permeating fluids is important for understanding their mechanical properties. Existing approaches often treat the permeating fluid as a continuous medium, but for many materials the pore dimension can be on the order of a nanometer, close to the size of a single water molecule. It is not clear how the discreteness of the permeating fluid affects the macroscopic mechanical properties of these materials. Here we show the bacterial spore, a dormant biological nanoporous structure, exhibits mechanical properties that challenge theoretical models based on continuum treatment of water. We found a statistical mechanical treatment of the confined water correctly predicts a range of equilibrium and dynamic properties of the spore, including an extreme slowdown of relaxation kinetics and a highly nonlinear mechanical response. Because the underlying assumptions of this approach are based on the geometry and not specific to the spore chemistry, these findings could also be applicable to other nanoporous materials.   

Enhancing and controlling parametric instabilities in structures: Application to an electromagnetic pendulum.

Parametric instabilities are dynamical instabilities arising when the mechanical state of a structure is properly modulated in time. It is sometimes seen as a phenomenon to avoid for sailing ships (parametric rolling) or landing helicopters (ground resonance) but it has also been exploited in vibrating fluids (Faraday waves) or NanoElectroMechanical Systems (parametric amplification). One well-known limitation in fully exploiting classic parametric instabilities based on small periodic modulation of a mechanical state is that inherent friction forces rapidly cancel sub-harmonic parametric resonances. To overcome this drawback, we suggest to formerly modify the state of a mechanical system close to its diverging instability, so that it becomes possible to periodically modulate a system between a naturally oscillating and diverging state. This original way of enhancing and controlling parametric instabilities is illustrated here through the numerical and experimental implementation of an electromagnetic pendulum. Not only we find it is possible to greatly enhance the number of subharmonic instability regions, but it is feasible to control the width of those regions, opening a promising way for frequency filtering in NanoElectroMechanical Systems.