OpenFOAM Finite Volume Solver for Fluid-Solid Interaction

This paper describes a self-contained parallel fluid-structure interaction solver based on a finite volume discretisation, where a strongly coupled partitioned solution procedure is employed. The incompressible fluid flow is described by the Navier-Stokes equations in the arbitrary Lagrangian-Eulerian form, and the solid deformation is described by the Saint Venant-Kirchhoff hyperelastic model in the total Lagrangian form. Both the fluid and the solid are discretised in space using the second-order accurate cell-centred finite volume method, and temporal discretisation is performed using the second-order accurate implicit scheme. The method, implemented in open-source software OpenFOAM, is parallelised using the domain decomposition approach and the exchange of information at the fluid-solid interface is handled using global face zones. The performance of the solver is evaluated in standard two- and threedimensional cases and excellent agreement with the available numerical results is obtained

Reinforcement Learning Experiments and Benchmark for Solving Robotic Reaching Tasks

Reinforcement learning has shown great promise in robotics thanks to its ability to develop efficient robotic control procedures through self-training. In particular, reinforcement learning has been successfully applied to solving the reaching task with robotic arms. In this paper, we define a robust, reproducible and systematic experimental procedure to compare the performance of various model-free algorithms at solving this task. The policies are trained in simulation and are then transferred to a physical robotic manipulator. It is shown that augmenting the reward signal with the Hindsight Experience Replay exploration technique increases the average return of off-policy agents between 7 and 9 folds when the target position is initialised randomly at the beginning of each episode

rl_reach: Reproducible reinforcement learning experiments for robotic reaching tasks

rl_reach is publicly available at this URL: https://github.com/PierreExeter/rl_reach[EN] Training reinforcement learning agents at solving a given task is highly dependent on identifying optimal sets of hyperparameters and selecting suitable environment input/output configurations. This tedious process could be eased with a straightforward toolbox allowing its user to quickly compare different training parameter sets. We present rl_reach, a self-contained, open-source and easy-to-use software package designed to run reproducible reinforcement learning experiments for customisable robotic reaching tasks. rl_reach packs together training environments, agents, hyperparameter optimisation tools and policy evaluation scripts, allowing its users to quickly investigate and identify optimal training configurations. rl_reach is publicly available at this URL: https://github.com/PierreExeter/rl_reachSIEuropean Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 713654

FLUID-STRUCTURE INTERACTION ON A FIXED FAN BLADE

High global electricity demand is pushing engineers towards providing hydropower electromagnetic generators with more resistant rotary equipment against well-known problems such as fatigue and vibrational cracking. The aim is to make power plants immune against high time and cost consuming refurbishments. In these systems, one of the rotating parts that is most susceptible to such failures is the ventilation fan. It is often an axial fan with blades distributed at one or two ends of the machine. The blades are attached to, and rotating with, the same shaft as the rotor, pushing the air through the rotor and stator towards the cooler. The blades are often manufactured by simple bent plates that are welded to the rotor, to keep the cost at minimum. They operate in an air flow that is highly restricted to the space that is available when the electromagnetic parts of the machine have been designed, causing temporally and spatially varying and non-ideal flow angles. For such conditions it is vital to study fluid-structure interaction on the blades to be able to avoid fan blade failures. Broken fan blades may cause severe damages to other parts of the machine, at enormous costs of repair and down-time. The present work provides a numerical study of the aeroelastic behaviour of a fixed blade resembling a blade of a double-sided axial fan of a hydropower generator. The focus is on flow-induced fluttering and resonance due to vortex shedding. The fluid-structure interaction is captured using the solids4Foam toolbox, which is an open source module for OpenFOAM, including specific solvers for solid and fluid mechanics and fluid-structure interaction. The turbulence is modelled using the scale-adaptive SAS model, which is able to capture vortex shedding with a combination of moderate computational costs and acceptable accuracy.Figure 1 shows the blade geometry, which has an extruded circular arc cross-section that is connected to a base plate at the lower side and has a thin clearance to a cover at the upper side. Iso-surfaces of the Q-criterion in the left picture, show the vortical structures at the tip and trailing edge. In the right picture, an instantaneous exaggerated deformed shape of the blade is shown under the working condition.AcknowledgementsThe research presented was carried out as a part of the Swedish Hydropower Centre (SVC). SVC is established by the Swedish Energy Agency, EnergiForsk and Svenska Kraftn\ue4t together with Lule\ue5 University of Technology, The Royal Institute of Technology, Chalmers University of Technology and Uppsala University, www.svc.nu. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and C3SE, partially funded by the Swedish Research Council through grant agreement no. 2018-05973.References[1]\ua0\ua0\ua0 OpenCFD, OpenFOAM: The Open Source CFD Toolbox. User Guide Version 1.4, OpenCFD Limited. Reading UK, Apr. 2007. [2]\ua0\ua0\ua0 C. W. Bergan, B. W.\ua0 Solemslie, P. 8stby, & O. G. Dahlhaug, “Hydrodynamic damping of a fluttering hydrofoil in high-speed flows”,\ua0International Journal of Fluid Machinery and Systems, vol.\ua011, No. 2, pp. 146-153, April-June 2018, ISSN: 18829554. DOI: http://dx.doi.org/10.5293/IJFMS.2018.11.2.146

On the major role played by the curvature of intracranial aneurysms walls in determining their mechanical response, local hemodynamics, and rupture likelihood

The properties of intracranial aneurysms (IAs) walls are known to be driven by the underlying hemodynamics adjacent to the IA sac. Different pathways exist explaining the connections between hemodynamics and local tissue properties. The emergence of such theories is essential if one wishes to compute the mechanical response of a patient-specific IA wall and predict its rupture. Apart from the hemodynamics and tissue properties, one could assume that the mechanical response also depends on the local morphology, more specifically, the wall curvature, with larger values at highly-curved wall portions. Nonetheless, this contradicts observations of IA rupture sites more often found at the dome, where the curvature is lower. This seeming contradiction indicates a complex interaction between local hemodynamics, wall morphology, and mechanical response, which warrants further investigation. This was the main goal of this work. We accomplished this by analysing the stress and stretch fields in different regions of the wall for a sample of IAs, which have been classified based on particular local hemodynamics and local curvature. Pulsatile numerical simulations were performed using the one-way fluid-solid interaction strategy implemented in OpenFOAM (solids4foam toolbox). We found that the variable best correlated with regions of high stress and stretch was the wall curvature. Additionally, our data suggest a connection between the local curvature and local hemodynamics, indicating that the curvature is a property that could be used to assess both mechanical response and hemodynamic conditions, and, moreover, to suggest new metrics based on the curvature to predict the likelihood of rupture.Comment: Preprint submitted to Acta Biomaterialia, with 27 pages and 11 figure

Fluid-structure interaction of a large ice sheet in waves

With global warming, the ice-covered areas in the Arctic are being transformed into open water. This provides increased impetus for extensive maritime activities and attracts research interests in sea ice modelling. In the polar region, ice sheets can be several kilometres long and subjected to the effects of ocean waves. As its thickness to length ratio is very small, the wave response of such a large ice sheet, known as its hydroelastic response, is dominated by an elastic deformation rather than rigid body motions. In the past 25 years, sea ice hydroelasticity has been widely studied by theoretical models; however, recent experiments indicate that the ideal assumptions used for these theoretical models can cause considerable inaccuracies. This work proposes a numerical approach based on OpenFOAM to simulate the hydroelastic wave-ice interaction, with the Navier-Stokes equations describing the fluid domain, the St. Venant Kirchhoff solid model governing the ice deformation and a coupling scheme to achieve the fluid-structure interaction. Following validation against experiments, the proposed model has been shown capable of capturing phenomena that have not been included in current theoretical models. In particular, the developed model shows the capability to predict overwash, which is a ubiquitous polar phenomenon reported to be a key gap. The present model has the potential to be used to study wave-ice behaviours and the coupled wave-ice effect on marine structures.Comment: 23 pages 9 figures, submitted journal pape

Assessing the compressive and impact behavior of plastic safety toe caps through computational modelling

Toe caps are one of the most important components in safety footwear, but have a significant contribution to the weight of the shoe. Efforts have been made to replace steel toe caps by polymeric ones, since they are lighter, insulated and insensitive to magnetic fields. Nevertheless, polymeric solutions require larger volumes, which has a negative impact on the shoe’s aesthetics. Therefore, safety footwear manufacturers are pursuing the development of an easy, low-cost and reliable solution to optimize this component. In this work, a solid mechanics toolbox built in the open-source computational library, OpenFOAM®, was used to simulate two laboratory standard tests (15 kN compression and 200 J impact tests). To model the polymeric material behavior, a neo-Hookean hyper-elasto-plastic material law with J2 plastic criteria was employed. A commercially available plastic toe cap was characterized, and the collected data was used for assessment purposes. Close agreements, between experimental and simulated values, were achieved for both tests, with an approximate error of 5.4% and 6.8% for the displacement value in compression and impact test simulations, respectively. The results clearly demonstrate that the employed open-source finite volume computational models offer reliable results and can support the design of toe caps for the R&D footwear industry.This work was funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT-Portuguese Foundation for Science and Technology under the projects UIDB/05256/2020; UIDP/05256/2020 and FAMEST-Footwear, Advanced Materials, Equipment’s and Software Technologies (POCI-01-0247-FEDER-024529)

Ship resistance when operating in floating ice floes: a combined CFD&DEM approach

Whilst climate change is transforming the Arctic into a navigable ocean where small ice floes are floating on the sea surface, the effect of such ice conditions on ship performance has yet to be understood. The present work combines a set of numerical methods to simulate the ship-wave-ice interaction in such ice conditions. Particularly, Computational Fluid Dynamics is applied to provide fluid solutions for the floes and it is incorporated with the Discrete Element Method to govern ice motions and account for ship-ice/ice-ice collisions, by which, the proposed approach innovatively includes wave effects in the interaction. In addition, this work introduces two algorithms that can implement computational models with natural ice-floe fields, which takes randomness into consideration thus achieving high-fidelity modelling of the problem. Following validation against experiments, the model is shown accurate in predicting the ice-floe resistance of a ship, and then a series of simulations are performed to investigate how the resistance is influenced by ship speed, ice concentration, ice thickness and floe diameter. This paper presents a useful approach that can provide power estimates for Arctic shipping and has the potential to facilitate other polar engineering purposes.Comment: 26 pages 18 figures, submitted journal pape