6 research outputs found
OpenFOAM for Francis turbine transients
The flexibility and fast responsiveness of hydropower systems make them a reliable solution to overcome the intermittency of renewable energy resources and balance the electrical grid. Therefore, investigating the complex flow fields during such operation is essential to increase the reliability and lifetime of future hydropower systems. The current article concerns the utilization of OpenFOAM for the numerical study of Francis turbines during transient load change operations. The details of employed models and numerical schemes are thoroughly explained. The Laplacian smoothing algorithm is applied for the deformation of the guide vane domain. The impact of different mesh diffusivity parameters on both load rejection and acceptance operations is studied. It is shown that general slip boundary conditions cannot be used for slipping points on the guide vane upper and lower surfaces. Instead, different alternatives are introduced and compared. The developed framework is tested on a high-head Francis turbine. Different transient operations are simulated and results are compared to the experimental data. It is shown that OpenFOAM can be employed as a trustworthy CFD solver for numerical investigation of Francis turbines transient operations
A comparison of interpolation techniques for non-conformal high-order discontinuous Galerkin methods
The capability to incorporate moving geometric features within models for
complex simulations is a common requirement in many fields. Fluid mechanics
within aeronautical applications, for example, routinely feature rotating (e.g.
turbines, wheels and fan blades) or sliding components (e.g. in compressor or
turbine cascade simulations). With an increasing trend towards the
high-fidelity modelling of these cases, in particular combined with the use of
high-order discontinuous Galerkin methods, there is therefore a requirement to
understand how different numerical treatments of the interfaces between the
static mesh and the sliding/rotating part impact on overall solution quality.
In this article, we compare two different approaches to handle this
non-conformal interface. The first is the so-called mortar approach, where flux
integrals along edges are split according to the positioning of the
non-conformal grid. The second is a less-documented point-to-point
interpolation method, where the interior and exterior quantities for flux
evaluations are interpolated from elements lying on the opposing side of the
interface. Although the mortar approach has significant advantages in terms of
its numerical properties, in that it preserves the local conservation
properties of DG methods, in the context of complex 3D meshes it poses notable
implementation difficulties which the point-to-point method handles more
readily. In this paper we examine the numerical properties of each method,
focusing not only on observing convergence orders for smooth solutions, but
also how each method performs in under-resolved simulations of linear and
nonlinear hyperbolic problems, to inform the use of these methods in implicit
large-eddy simulations.Comment: 37 pages, 15 figures, 5 tables, submitted to Computer Methods in
Applied Mechanics and Engineering, revision
Flow Characteristics of Preliminary Shutdown and Startup Sequences for a Model Counter-Rotating Pump-Turbine
Pumped Hydropower Storage (PHS) is the maturest and most economically viable technology for storing energy and regulating the electrical grid on a large scale. Due to the growing amount of intermittent renewable energy sources, the necessity of maintaining grid stability increases. Most PHS facilities today require a geographical topology with large differences in elevation. The ALPHEUS H2020 EU project has the aim to develop PHS for flat geographical topologies. The present study was concerned with the initial design of a low-head model counter-rotating pump-turbine. The machine was numerically analysed during the shutdown and startup sequences using computational fluid dynamics. The rotational speed of the individual runners was decreased from the design point to stand-still and increased back to the design point, in both pump and turbine modes. As the rotational speeds were close to zero, the flow field was chaotic, and a large flow separation occurred by the blades of the runners. Rapid load variations on the runner blades and reverse flow were encountered in pump mode as the machine lost the ability to produce head. The loads were less severe in the turbine mode sequence. Frequency analyses revealed that the blade passing frequencies and their linear combinations yielded the strongest pulsations in the system
An in-depth numerical analysis of transient flow field in a Francis turbine during shutdown
Power production from intermittent renewable energy resources, such as solar and wind, has increased in the past few decades, leading researchers and engineers to establish techniques to preserve a stable electrical grid. Consequently, hydraulic turbines are being used more frequently in transient operating modes to regulate the grid. The present work provides a comprehensive numerical study on the transient flow field of a high-head Francis turbine model throughout the shutdown sequence. The computations were performed using OpenFOAM, utilizing the SST-SAS turbulence model. A Laplacian smoothing scheme is employed to conduct the mesh deformation of the guide vane domain. The time-averaged draft tube velocity field at the steady Best Efficiency Point (BEP) is validated against experimental data. Then different aspects of the transient flow field in the shutdown sequence are carefully assessed and explained for the first time. Short-Time Fourier Transform (STFT) analysis is carried out on the fluctuating part of the static pressure and force signals. High-amplitude low-frequency oscillations, due to the formation of a Rotating Vortex Rope (RVR) were observed during a specific period of the shutdown sequence. Thereafter, at deep part load conditions, the RVR vanishes and, a wide range of stochastic frequencies are identified at minimum load. A signal coherence analysis was accomplished to distinguish the deterministic and stochastic frequencies. The variation of the velocity field in the draft tube is described in detail with the help of velocity triangles. An in-depth explanation of the formation and variation of vortical structures during the whole sequence is presented. The physical mechanism of formation and destruction of the RVR is thoroughly explained
Conservative handling of arbitrary non-conformal interfaces using an efficient supermesh
This work presents a new and efficient strategy to handle non-conformal interfaces with the aim of assuring the conservation of fluxes in Finite Volume problems. A conservative interpolation is developed for general transport equations. Due to the arbitrary connectivity between the interfaces, the interpolations require flux-based weights defining a complex numerical stencil. In this context, a new method is proposed to simplify the coupling at the interface based on the construction of a simplified supermesh. Here, a supermesh is not completely defined, instead, the interface faces are logically duplicated (or multiplied) to generate a one-to-one connectivity between them. The simplified supermesh named pseudo-supermesh eliminates the interpolations and assures the conservation of fluxes based on the trivial connectivity. The area and the geometrical centroid of the new faces are redefined according to the overlapped sector of the original faces using the local supermeshing approach. Since the arbitrary polygons resulting from the face intersections are not generated and introduced into the mesh, computational cost and implementation efforts are saved. The proposed method is tested focusing on the conservation of fluxes and on the accuracy, showing conservation to machine precision and second order convergence as expected. In order to be able to solve large problems, the methodology is designed and implemented to be run in parallel architectures showing an excellent efficiency.Fil: Aguerre, Horacio Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; Argentina. Universidad Tecnológica Nacional. Facultad Regional Concepción del Uruguay; ArgentinaFil: Marquez Damian, Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; Argentina. Universidad Tecnológica Nacional. Facultad Regional Santa Fe; ArgentinaFil: Gimenez, Juan Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería y Ciencias Hídricas; ArgentinaFil: Nigro, Norberto Marcelo. Universidad Nacional del Litoral. Facultad de Ingeniería y Ciencias Hídricas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; Argentin