1,721,012 research outputs found
Wet and dry transom stern treatment for unsteady and nonlinear potential flow model for naval hydrodynamics simulations
No full textWe present a model for the fast evaluation of the total drag of ship hulls operating in both wet and dry transom stern conditions, in calm or wavy water, based on the combination of an unsteady semi-Lagrangian potential flow formulation with fully nonlinear free-surface treatment, experimental correlations, and simplified viscous drag modeling. The implementation is entirely based on open source libraries. The spatial discretization is solved using a streamline upwind Petrov Galerkin stabilization of an iso-parametric, collocation based, boundary element method, implemented using the open source library deal.11. The resulting nonlinear differential-algebraic system is integrated in time using implicit backward differentiation formulas, implemented in the open source library SUNDIALS. The Open CASCADE library is used to interface the model directly with computer-aided design data structures. The model accounts automatically for hulls with a transom stern, both in wet and dry regimes, by using a specific treatment of the free-surface nodes on the stern edge that automatically detects when the hull advances at low speeds. In this case, the transom stern is partially immersed, and a pressure patch is applied on the water surface detaching from the transom stern, to recover the gravity effect of the recirculating water on the underlying irrotational flow domain. The parameters of the model used to impose the pressure patch are approximated from experimental relations found in the literature. The test cases considered are those of the U.S. Navy Combatant DTMB-5415 and the National Physical Laboratory hull. Comparisons with experimental data on quasi-steady test cases for both water elevation and total hull drag are presented and discussed. The quality of the results obtained on quasi-steady simulations suggests that this model can represent a promising alternative to current unsteady solvers for simulations with Froude numbers below 0.35
Low-Frequency Variations of Force Coefficients on Square Cylinders with Sharp and Rounded Corners
Full text linkSquare lighting poles with sharp or round corners have been observed to exhibit large amplitude oscillations with a frequency that is near that of their first mode. This frequency is one order of magnitude lower than that of the vortex shedding frequency for the observed wind speeds. As such, lock-in resonance between the vortex shedding and the poles motions has been ruled out as the cause of these oscillations. In this work, numerical simulations of flows over square cylinders with different aspect ratios and cross-sectional shapes have been carried out to explore and quantify low-frequency variations of the aerodynamic forces and their sources. The results show that low-frequency components are a basic aspect of the flow over finite length cylinders and are associated with the three-dimensional flow characteristics near the free end of the cylinder. This aspect is related to the loss of synchronization of the vortex shedding which, in turn, results in variations of the mean drag coefficient along the cylinder. This ability to numerically simulate the flow over a lighting pole and detailed results, as presented here, could be used to overcome wind tunnel test limitations imposed by the size of the test section and blockage ratio, and of meeting geometric similarity requirements for high aspect ratio configurations. Consequently, results from such simulations could play an important role in improving code specifications which, to date, have been based on wind tunnel simulations
Simulation of the dynamics of an olympic rowing boat
No full textEditors: P. Wesseling, E. Oñate, J. Périau
A hybrid reduced-order model for segregated fluid-structure interaction solvers in an ALE approach at high Reynolds number
No full textThis study introduces a first step for constructing a hybrid reduced-order models (ROMs) for segregated fluid-structure interaction in an Arbitrary Lagrangian-Eulerian (ALE) approach at a high Reynolds number using the Finite Volume Method (FVM). The ROM is driven by proper orthogonal decomposition (POD) with hybrid techniques that combines the classical Galerkin projection and two data-driven methods (radial basis networks, and neural networks/ long short term memory). Results demonstrate the ROM's ability to accurately capture the physics of fluid-structure interaction phenomena. This approach is validated through a case study focusing on flow-induced vibration (FIV) of a pitch-plunge airfoil at a high Reynolds number (Re=107)
A stable semi-lagrangian potential method for the simulation of ship interaction with unsteady and nonlinear waves
No full textIn this contribution, we present preliminary results of a model for the simulation of three dimensional unsteady nonlinear water waves generated by a ship hull advancing in calm water, under development at the MathLab laboratory of the International School for Advanced Studies of Trieste, Italy. In the framework of potential flow theory, the governing equation is the Laplace's equation, complemented by fully nonlinear free surface boundary conditions. To prevent downstream transport of the mesh nodes and avoid remeshing, such boundary conditions are written in semi-Lagrangian formulation. The resulting boundary value problem is treated as a system of nonlinear differential-algebraic equations, in which the unknowns are the positions of the nodes of the computational grid, along with the corresponding potential and potential normal derivative values. Among these, only vertical positions and potential values associated with free surface nodes, are differential components, as their time evolution is prescribed by ODEs derived from the semi-Lagrangian free surface boundary conditions. All other unknowns are instead algebraic components, their values satisfying algebraic equations resulting from the BEM discretization of the Laplace's equation. The time advancing of nonlinear differential-algebraic system is performed by means of a Backward Differentiation Formula implicit method with variable step size and variable order, implemented in the framework of the open source library Sundials. The collocated and iso-parametric BEM discretization of the Laplace's equation has been implemented employing the open source library deal.II. The semi-Lagrangian free surface boundary conditions are stabilized by means of a Streamwise Upwind Petrov--Galerkin (SUPG) method. The unstructured quadrilateral grids needed for the simulations, are automatically generated on arbitrary CAD hull geometries. The test cases considered are that of a Wigley hull and the US Navy Combatant, DTMB 5415, advancing in calm water. The simulations results obtained are compared with experimental results
Hybrid Data-Driven Closure Strategies for Reduced Order Modeling
Full text linkIn this paper, we propose hybrid data-driven ROM closures for fluid flows.
These new ROM closures combine two fundamentally different strategies: (i)
purely data-driven ROM closures, both for the velocity and the pressure; and
(ii) physically based, eddy viscosity data-driven closures, which model the
energy transfer in the system. The first strategy consists in the addition of
closure/correction terms to the governing equations, which are built from the
available data. The second strategy includes turbulence modeling by adding eddy
viscosity terms, which are determined by using machine learning techniques. The
two strategies are combined for the first time in this paper to investigate a
two-dimensional flow past a circular cylinder at Re=50000. Our numerical
results show that the hybrid data-driven ROM is more accurate than both the
purely data-driven ROM and the eddy viscosity ROM.Comment: arXiv admin note: text overlap with arXiv:2205.1511
Propagating geometry information to finite element computations
Full text linkThe traditional workflow in continuum mechanics simulations is that a geometry description -for example obtained using Constructive Solid Geometry (CSG) or Computer Aided Design (CAD) tools-forms the input for a mesh generator. The mesh is then used as the sole input for the finite element, finite volume, and finite difference solver, which at this point no longer has access to the original, "underlying" geometry. However, many modern techniques-for example, adaptive mesh refinement and the use of higher order geometry approximation methods really do need information about the underlying geometry to realize their full potential. We have undertaken an exhaustive study of where typical finite element codes use geometry information, with the goal of determining what information geometry tools would have to provide. Our study shows that nearly all geometry-related needs inside the simulators can be satisfied by just two "primitives": elementary queries posed by the simulation software to the geometry description. We then show that it is possible to provide these primitives in all of the frequently used ways in which geometries are described in common industrial workflows, and illustrate our solutions using a number of examples
On the thermal welding of paper-based polylaminate packages: Modelling, numerical implementation and sensitivity analysis
No full textThis work presents a numerical model for the simulation of pack- age sealing in industrial machines for beverage packaging. The simulations are aimed at the prediction of the temperature field in all the layers of the polylaminate material composing the package. The package sealing is in fact carried out by means of thermal welding. Thus, accurate predictions of the temperatures following the package heating via hot air jet and right before the folding flaps are pressed together is paramount to in turn predict sealing success. The heat equation is solved in the package volume by means of a plate FEM formulation in which arbitrary order Lagrangian shape function are used for both the longitudinal and the normal discretization. The resulting semi-discretized equations are time advanced by means of an Implicit Euler scheme with constant time step. The solution of the system is complemented by forward sensitivity computation to obtain, at each time step, quantitative assessment of the effect of process parameters variations on the temperature output. The numerical results are compared to experimental measurements so as to validate the developed simulation tool. The results obtained suggest that the solver is able to reproduce with satisfactory accuracy the experimen- tal temperature field evolution in the portion of the package interested by the thermal welding
Curvature-adapted Remeshing of CAD Surfaces
Full text linkAbstractA common representation of surfaces with complicated topology and geometry is through composite parametric surfaces as is the case for most CAD modelers. A challenging problem is how to generate a mesh of such a surface that well approximates the geometry of the surface, preserves its topology and important geometric features, and contains nicely shaped elements. In this work, we present an optimization-based surface remeshing method that is able to satisfy many of these requirements simultaneously. This method is inspired by the recent work of Lévy and Bonneel (Proc. 21th International Meshing Roundtable, October 2012), which embeds a smooth surface into a high-dimensional space and remesh it uniformly in that embedding space. Our method works directly in the 3d spaces and uses an embedding space in R6 to evaluate mesh size and mesh quality. It generates a curvature-adapted anisotropic surface mesh that well represents the geometry of the surface with a low number of elements. We illustrate our approach through various examples
pi-BEM: A flexible parallel implementation for adaptive, geometry aware, and high order boundary element methods
No full textMany physical phenomena can be modelled using boundary integral equations, and discretised using the
boundary element method (BEM). Such models only require the discretisation of the boundary of the domain,
making the setup of the simulation straightforward and lowering the number of degrees of freedom. However,
while many parallel efficient libraries are available for the Finite Element Method (FEM), the implementation of
scalable BEM solvers still poses many challenges. We present the open source framework π-BEM (where π stands
for parallel): a novel boundary element method solver, combining distributed and shared memory paradigms to
achieve high scalability. π-BEM exploits high performance libraries and graph partitioning tools to deliver a
parallel solver employing automatic domain decomposition, high order elements, local refinement capabilities,
and exact geometry-adaptivity (using CAD files). A preliminary fast multipole accelerator is included in the
implementation. Every aspect of the library is modular and easily extendible by the community. We discuss the
internal structure of the code, and present some examples to demonstrate the reliability and scalability of our
implementation
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