Simulations of propelling and energy harvesting articulated bodies via vortex particle-mesh methods

The emergence and understanding of new design paradigms that exploit flow induced mechanical instabilities for propulsion or energy harvesting demands robust and accurate flow structure interaction numerical models. In this context, we develop a novel two dimensional algorithm that combines a Vortex Particle-Mesh (VPM) method and a Multi-Body System (MBS) solver for the simulation of passive and actuated structures in fluids. The hydrodynamic forces and torques are recovered through an innovative approach which crucially complements and extends the projection and penalization approach of Coquerelle et al. and Gazzola et al. The resulting method avoids time consuming computation of the stresses at the wall to recover the force distribution on the surface of complex deforming shapes. This feature distinguishes the proposed approach from other VPM formulations. The methodology was verified against a number of benchmark results ranging from the sedimentation of a 2D cylinder to a passive three segmented structure in the wake of a cylinder. We then showcase the capabilities of this method through the study of an energy harvesting structure where the stocking process is modeled by the use of damping elements

Hydroaeroelasticity

The textbook is devoted to investigate the stability problems for deformable systems streamlined by fluid or gas flow. Special attention is paid to the study of hydrodynamic forces acting on deformable surfaces. The textbook will be intended for engineering students and postgraduate students of higher educational institutions

Pragudega elastsete astmeliste talade stabiilsus

Väitekirja elektrooniline versioon ei sisalda publikatsiooneKäesolevas väitekirjas vaadeldakse elastsete astmeliste talade stabiilsust. Töö aluseks on autori kuus teaduslikku publikatsiooni, millest kolm on avaldatud viimase kolme aasta jooksul. Väitekiri koosneb neljast osast: kokkuvõtvast osast ehk kokkuvõtteartiklist, publikatsioonide koopiatest, kirjanduse ülevaatest ja autori elulookirjeldusest. Antud töös uuritakse elastseid talasid, millele mõjub teljesuunaline koormus. Talad on astmelised ning astme kohtades asuvad defektid ehk praod, mis antud uurimuses on stabiilsed. Pragude sügavus ja asukoht mõjutab talade stabiilsust ning stabiilsuse tundlikkust antud parameetrite suhtes on analüüsitud kombineerides elastsusteooria ja lineaarse purunemismehaanika meetodeid. Esimeses peatükis tuuakse ajalooline ülevaade kirjandusest. Teises peatükis esitatakse uurimuse põhialused prao mõju analüüsiks. Praoga tala uurimiseks kasutatakse nn. kaalutu väändevedru mudelit. Selle mudeli kohaselt asendatakse praoga tala konstruktsiooniga, mis koosneb kahest tala tükist (elemendist). Need elemendid on omavahel ühendatud kaalutu väändevedruga, mille jäikus on võrdeline pinge intensiivsuse koefitsendiga prao tipu juures. Järgnevas neljas peatükis uuritakse kriitilise koormuse sõltuvust prao parameetritest erinevate talade ja kinnitustingimuste korral. Esimesel juhul on vaatluse all konsooltala, teisel juhul on vabale otsale lisatud elastne kinnitus. Kolmandaks uuritakse konsooltala, mis asub elastsel alusel ning lõpetuseks tala, mis on seest õõnes (nelinurkne toru).In the present thesis critical buckling loads of stepped beams are studied and the sensitivity of the critical load on the parameters of stable cracks as location and depth is analysed. Combining the methods of the elastic beam theory and of the linear elastic fracture mechanics an approximate method for the stability analysis of beams and columns subjected to the axial pressure is developed. Introducing the additional compliance matrix the flexibility of the beam in the vicinity of a crack is prescribed by means of the compliance of the structure. This, in turn, is coupled with the stress intensity factor which can be calculated by methods of the linear elastic fracture mechanics. Critical buckling loads of stepped columns subjected to the axial pressure and weakened with cracks emanating from re-entrant corners of steps are established. Numerical results are presented for uniform and hollow beams with single step of the cross section, also for two-stepped beams. The beams under consideration are simply supported or clamped at the ends, also cantilevers, elastically fixed. The case of beams resting on elastic foundation is studied separately. The dissertation is based on the six papers of the author (two of these are published during the last two years). The dissertation consists of the review of the obtained results, the copies of the papers, the list of literature and CV of the author. The dissertation is organized as follows. Section 1 contains historic background of the stability analysis, the aim and the structure of the dissertation. In section 2 the concept of local flexibility is described in detail. In sections 3, 4, 5 and 6 the method is applied to partcular cases of beams. The first case concerns elastic beams that are clamped at one end and free at another end. Secondly elastically fixed beams are studied in greater detail. In section 5 beams resting on elastic foundation are considered. Finally, in section 6 beams with hollow cross sections are studied. The influence of crack length and step location on the stability of the beams has been analyzed

Vector offset operators for deformable organic objects.

Many natural materials and most of living tissues exhibit complex deformable behaviours that may be characteriseda s organic. In computer animation, deformable organic material behaviour is needed for the development of characters and scenes based on living creatures and natural phenomena. This study addresses the problem of deformable organic material behaviour in computer animated objects. The focus of this study is concentrated on problems inherent in geometry based deformation techniques, such as non-intuitive interaction and difficulty in achieving realism. Further, the focus is concentrated on problems inherent in physically based deformation techniques, such as inefficiency and difficulty in enforcing spatial and temporal constraints. The main objective in this study is to find a general and efficient solution to interaction and animation of deformable 3D objects with natural organic material properties and constrainable behaviour. The solution must provide an interaction and animation framework suitable for the creation of animated deformable characters. An implementation of physical organic material properties such as plasticity, elasticity and iscoelasticity can provide the basis for an organic deformation model. An efficient approach to stress and strain control is introduced with a deformation tool named Vector Offset Operator. Stress / strain graphs control the elastoplastic behaviour of the model. Strain creep, stress relaxation and hysteresis graphs control the viscoelastic behaviour of the model. External forces may be applied using motion paths equipped with momentum / time graphs. Finally, spatial and temporal constraints are applied directly on vector operators. The suggested generic deformation tool introduces an intermediate layer between user interaction, deformation, elastoplastic and viscoelastic material behaviour and spatial and temporal constraints. This results in an efficient approach to deformation, frees object representation from deformation, facilitates the application of constraints and enables further development

Quasi-Monolithic Graph Neural Network for Fluid-Structure Interaction

Using convolutional neural networks, deep learning-based reduced-order models have demonstrated great potential in accelerating the simulations of coupled fluid-structure systems for downstream optimization and control tasks. However, these networks have to operate on a uniform Cartesian grid due to the inherent restriction of convolutions, leading to difficulties in extracting fine physical details along a fluid-structure interface without excessive computational burden. In this work, we present a quasi-monolithic graph neural network framework for the reduced-order modelling of fluid-structure interaction systems. With the aid of an arbitrary Lagrangian-Eulerian formulation, the mesh and fluid states are evolved temporally with two sub-networks. The movement of the mesh is reduced to the evolution of several coefficients via proper orthogonal decomposition, and these coefficients are propagated through time via a multi-layer perceptron. A graph neural network is employed to predict the evolution of the fluid state based on the state of the whole system. The structural state is implicitly modelled by the movement of the mesh on the fluid-structure boundary; hence it makes the proposed data-driven methodology quasi-monolithic. The effectiveness of the proposed quasi-monolithic graph neural network architecture is assessed on a prototypical fluid-structure system of the flow around an elastically-mounted cylinder. We use the full-order flow snapshots and displacements as target physical data to learn and infer coupled fluid-structure dynamics. The proposed framework tracks the interface description and provides the state predictions during roll-out with acceptable accuracy. We also directly extract the lift and drag forces from the predicted fluid and mesh states, in contrast to existing convolution-based architectures