1,721,107 research outputs found
Numerical simulations of optimal control problems for the Reynolds averaged Navier-Stokes system closed with a two-equation turbulence model
No full textOptimal control theory in fluid dynamics has become popular in the last several years because of its use in the design of new engineering devices and optimization of existing ones. In recent years the optimal control of turbulent flows has gained attention but turbulence modelling and its control is still an open problem. In this work we study a distributed optimal control problem for a flow modelled by the Reynolds averaged Navier–Stokes system closed by a two-equation k−ω turbulence model which takes into account the limits imposed on turbulent viscosity. The complete adjoint system is derived with a Lagrangian multiplier approach allowing to write a cost functional directly based on the turbulence kinetic energy. We assess a cost functional composed of two terms, the first for a velocity matching profile problem and the second for turbulence enhancement or reduction. The optimality system is solved numerically with a finite element code parallelized with standard message passing interface libraries. Numerical results in two and three-dimensional spaces are reported to show the validity of this numerical approach
An optimal control approach to a fluid-structure interaction parameter estimation problem with inequality constraints
No full textIn this work, we present a new optimal control approach to fluid-structure interaction parameter estimation problems. The goal is to obtain the desired deformation by controlling the solid material properties, such as the Young modulus. We consider a stationary monolithic FSI problem where solid and liquid forces at the interface are automatically balanced. We consider inequality constraints in order to bound the Young modulus control admissible set. For the optimization, we adopt the Lagrange multiplier method with adjoint variables and obtain the optimality system which minimizes the augmented Lagrangian functional. We implement a projected gradient-based algorithm in a multigrid finite element code suitable for the study of large solid displacements. In order to support the proposed approach, we perform numerical tests with different objectives and control constraints
Optimal control problems for the Navier-Stokes system coupled with the k-ω turbulence model
No full textOptimal control of fluid-dynamics systems has gained attention in the last several years from the scientific community because of its potential use in design of new engineering devices and optimization of existing ones. Many research works have extensively studied the optimal control for the system of Navier-Stokes but the problem of turbulence in these works is usually not taken into account because of the many difficulties arising from the numerical implementation and solution of the optimality system. In this work turbulence is considered by coupling the k ω two-equation turbulence model with the averaged Navier-Stokes system. The complete optimality system is derived and the existence of a weak solution proven. Some numerical examples are reported
Four Parameter Heat Transfer Turbulence Models for Heavy Liquid Metals
No full textIn advanced Gen IV nuclear reactors heavy liquid metals are considered as coolant for their high conductivity and specific neutronic properties. These fluids have a very low Prandtl number and show a peculiar heat transfer where conduction can be the dominant mechanism at very high Reynolds numbers. In ordinary fluids various turbulence models are available to match the experimental data: Similarity between velocity and thermal turbulent fields is assumed in almost all commercial Computational Fluid Dynamics codes and the simple eddy diffusivity model with constant turbulent Prandtl number is implemented. In low Prandtl number fluids this model fails to reproduce standard correlations build from experimental data. Therefore it is important to develop new heat transfer turbulence models that are able to reproduce numerically the physical behavior. In this work we present different turbulence models to study the heat transfer in heavy liquid metal turbulent flows. Results obtained with the simple eddy diffusivity model are reported. More complex four parameter turbulence models are also presented and numerical results in simple geometries are reported. For a large range of forced flows with no similarity between velocity and thermal fields a four parameter turbulence model is a powerful tool for predicting the heat transfer
CFD simulations in heavy liquid metal flows for square lattice bare rod bundle geometries with a four parameter heat transfer turbulence model
No full textThe study of heat transfer in heavy liquid metals has gained more attention in the last several years due to their applications in new advanced nuclear reactors. These fluids are characterized by low Prandtl numbers and a peculiar heat transfer that cannot be accurately reproduced with standard turbulence approximations, such as the Simple Eddy Diffusivity model (SED), commonly used in commercial codes. In this paper we report the results obtained for the SED and a more advanced k–e–kt –et four parameter turbulence model for simulations in square lattice bare rod bundle geometries with different pitch-to-diameter ratios. We compare these numerical results with the available experimental data and correlations for the prediction of the Nusselt number
An improved monolithic multigrid Fluid-Structure Interaction solver with a new moving mesh technique
No full textFluid-Structure Interaction simulations have gained popularity in the research community because of their applications in several industrial and biological fields. In such problems mesh movement is necessary in order to clearly evaluate the deformed solid state and the stresses. In many cases, especially when large displacement occurs, the movement of the mesh nodes can reduce accuracy and convergence properties of the solver. In this paper we present an improved fluid structure interaction solver with a new moving mesh algorithm based on a multilevel Arbitrary Lagrangian Eulerian method to be used in the computation of the arbitrary fluid displacement field. This algorithm is used together with a multigrid, monolithic, fluid structure interaction solver for large displacement problem in which the mesh overlapping is more likely to happen. Numerical simulations in two and three-dimension for both hexahedral and tetrahedral meshes are reported in order to better investigate the capabilities of this solver
Boundary Control Problems in Convective Heat Transfer with Lifting Function Approach and Multigrid Vanka-Type Solvers
No full textThis paper deals with boundary optimal control problems for the heat and Navier-Stokes equations and addresses the issue of defining controls in function spaces which are naturally associated to the volume variables by trace restriction. For this reason we reformulate the boundary optimal control problem into a distributed problem through a lifting function approach. The stronger regularity requirements which are imposed by standard boundary control approaches can then be avoided. Furthermore, we propose a new numerical strategy that allows to solve the coupled optimality system in a robust way for a large number of unknowns. The optimality system resulting from a finite element discretization is solved by a local multigrid algorithm with domain decomposition Vanka-type smoothers. The purpose of these smoothers is to solve the optimality system implicitly over subdomains with a small number of degrees of freedom, in order to achieve robustness with respect to the regularization parameters in the cost functional. We present the results of some test cases where temperature is the observed quantity and the control quantity corresponds to the boundary values of the fluid temperature in a portion of the boundary. The control region for the observed quantity is a part of the domain where it is interesting to match a desired temperature value
Numerical comparison of different solution methods for optimal boundary control problems in thermal fluid dynamics
No full textIn this paper we propose and compare different methods for the solution of the control-adjoint-state optimality system which minimizes an objective functional in temperature. The minimization is constrained by the energy convection-diffusion equation with velocity field defined by the incompressible Navier- Stokes system. Three methods, based on different solution spaces, for solving the adjoint-state optimality system are compared. In the first one, as in the standard approach, the controlled temperature field is assumed to belong to a regular class of solutions with smooth derivatives and the resulting control-adjoint-state optimality system is solved in a segregated way. In the second one we introduce a fully coupled solution approach, where, in order to obtain a more robust numerical algorithm, the boundary control is extended to the interior and Dirichlet conditions are implicitly enforced through a volumetric force term. In the last approach we introduce Discontinuous Galerkin formulation for the energy equation in order to seek discontinuous solutions. Numerical two and three-dimensional test cases arenreported in order to show the validity of the proposed approaches. The results are compared in term of solution smoothness and achievement of low values of the objective functional
A preliminary investigation of the growth of an aneurysm with a multiscale monolithic Fluid-Structure interaction solver
Full text linkpeer reviewedIn this work we investigate the potentialities of multi-scale engineering techniques to approach complex problems related to biomedical and biological fields. In particular we study the interaction between blood and blood vessel focusing on the presence of an aneurysm. The study of each component of the cardiovascular system is very difficult due to the fact that the movement of the fluid and solid is determined by the rest of system through dynamical boundary conditions. The use of multi-scale techniques allows us to investigate the effect of the whole loop on the aneurysm dynamic. A three-dimensional fluid-structure interaction model for the aneurysm is developed and coupled to a mono-dimensional one for the remaining part of the cardiovascular system, where a point zero-dimensional model for the heart is provided. In this manner it is possible to achieve rigorous and quantitative investigations of the cardiovascular disease without loosing the system dynamic. In order to study this biomedical problem we use a monolithic fluid-structure interaction (FSI) model where the fluid and solid equations are solved together. The use of a monolithic solver allows us to handle the convergence issues caused by large deformations. By using this monolithic approach different solid and fluid regions are treated as a single continuum and the interface conditions are automatically taken into account. In this way the iterative process characteristic of the commonly used segregated approach, it is not needed any more
Marker reconstruction by using best-fit quadric approximation
No full textIn this work, an interface advection problem has been considered, by using markers approximation. The functions described with this new algorithm have been implemented in a C++ library able to initialize, advect and rebuild a cloud of markers, as well as compute a quadric best fit for the marker cloud in every cell. We test the developed functions by using an analytical velocity field on a two bubble advection problem
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