Riemann solution for hyperbolic equations with discontinuous coefficients

summary:This paper deals with a Riemann solution for scalar hyperbolic equations with discontinuous coefficients. In many numerical schemes of Godunov type in fluid dynamics, electromagnetic and so on, usually hyperbolic problems are solved to estimate fluxes. The exact solution is generally difficult to obtain, but good approximations are provided in many situations like Roe and HLLC Riemann solvers in fluid. However all these solvers assumes that the acoustic waves speeds are continuous which is not true as we will show in this paper. A new Riemann solver is then proposed based on previous work of the author and an application to a gas-particle model for a 90 degree curved bend is performed

Spray drag effect of fluidized sand for a supersonic vehicle

This paper deals with fluidized sand simulation in order to estimate the impact of sand particle motion on the BLOODHOUND SuperSonic Car (SSC) drag forces, such phenomenon is known as a spray drag effect. A gas-particle model is used to simulate the sand particles that rise from the ground because of the strong shockwave-desert surface interaction. A finite volume scheme is used to discretise the continuous model with a special treatment of the solid phase equations. An indefinitely differentiable and anisotropic limiter to reinforce the method stability and reduce any excessive smearing is applied. To estimate the area where sand particles are detached from the ground, a criterion based on pressure change is proposed. The model is first validated on a curved 90 bend test case with comparison to experimental results and then applied to the supersonic car

New Simplified Algorithm for the Multiple Rotating Frame Approach in Computational Fluid Dynamics

This paper deals with rotating effects simulation of steady flows in turbomachinery. To take into account the rotating nature of the flow, the frozen rotor approach is one of the widely used approaches. This technique, known in a more general context as a multiple rotating frame (MRF), consists on building axisymmetric interfaces around the rotating parts and solves for the flow in different frames (static and rotating). This paper aimed to revisit this technique and propose a new algorithm referred to it by a virtual multiple rotating frame (VMRF). The goal is to replace the geometrical interfaces (part of the computer-aided design (CAD)) that separate the rotating parts replaced by the virtual ones created at the solver level by a simple user input of few point locations and/or parameters of basic shapes. The new algorithm renders the MRF method easy to implement, especially for edge-based numerical schemes, and very simple to use. Moreover, it allows avoiding any remeshing (required by the MRF approach) when one needs to change the interface position, shape, or simply remove or add a new one, which frequently happened in practice. Consequently, the new algorithm sensibly reduces the overall computations cost of a simulation. This work is an extension of a first version published in an ASME conference, and the main new contributions are the detailed description of the new algorithm in the context of cell-vertex finite volume method and the validation of the method for viscous flows and the three-dimensional (3D) case which is of significant importance to the method to be attractive for real and industrial applications.BCAM-BALTOGAR CFD Platform for Turbomachinery Simulation and Design (BFA/DFB - 6/12/TK/2012/00020

Enhanced and restored signals as a generalized solution for shock filter models. Part II—Numerical study,” J.Math

Abstract In Part I of this paper, we proposed a well-posed generalized model for signal enhancement and restoration based on shock filters. A theoretical study of the Cauchy problem in the framework of generalized functions algebra was developed in detail. In Part II, we investigate the numerical aspects of the model. We derive an efficient, explicit numerical scheme in both one and two dimensions, and investigate the schemes' stability and convergence. Through experimental tests, we demonstrate the effectiveness of the numerical schemes when restoring and enhancing signals in various situations with a limited number of iterations. Moreover, we show the impact of the coefficients introduced in the model on the procedure's processing time

Efficient Rotating Frame Simulation in Turbomachinery

This paper deals with the simulation of steady flows in tur- bomachinery. Two approaches are proposed, the first one is the classical multiple-rotating frame method (MRF) by multi- zone approach where the different zones are separated by non- overlapping interfaces and solved independently. Since each zone is loaded separately, a transferring system should be prop- erly implemented at the interface boundaries. Two techniques are considered, in the first one the conservative variables are in- terpolated between zones while in the second one the fluxes are transferred through the interfaces. The other proposed approach is a new version of the MRF using a virtual interface (VMRF). This is a simplified of the pre- vious one where the interfaces are created virtually at the solver level, rendering the method easy to implement especially for edge-based numerical schemes, and avoiding any re-meshing in case one needs to change interface position, shape or simply re- move or add new one. Finally, numerical tests are performed to demonstrate the efficiency of the proposed methods by compari- son with commercial codes (ANSYS FLUENT)

BBIPED: BCAM-Baltogar Industrial Platform for Engineering Design

Currently, commercial software for computational fluid dynamics offers a good set of features to deal with traditional designs. Within a competitive market industrial innovation is a key factor that must be faced by companies. However, the design of solutions to deal with industrial challenges cannot be done within commercial software due to the lack of flexibility. Open source initiatives are a good option but the learning curve is high, specially for industrial engineers profiles. In this paper, we present the BBIPED platform which has been designed to deal with turbomachinery applications in a simple and friendly way. The main goal is to keep the platform as simpler as possible providing the enough flexibility to include out-of-the-box solutions to cope with industrial challenges. BBIPED platform provides links with currently existing remarkable open source initiatives altogether with our own developments. Particularly, it is remarkable a first approach for automatic mesh generation based on geometry parametrization solution, and the provision of novel techniques to deal with multiple rotating frame (MRF): Multizone MRF an Virtual MRF. Case tests were designed to test the solutions and to assess and validate the results against commercial suites with promising results

Identification of defects originated during the filling of cast pieces through particles modelling

In casting processes, strong recirculation zones may trap air, gases and strip particles of sand off the mold affecting seriously the quality of the cast pieces. Especially during the filling of molten pieces with large surfaces, several faults were detected which are responsible of considerable economic losses in such casting processes. The aim of this work is focused on the correct identification of these physical phenomena, through Computational Fluid Dynamics (CFD). A fully experimental work correlating a saline solution with similar properties than the liquid alloy was carried out in order to guess the flow behavior inside the mold. Different parameters such as filling time, temperature and velocity of the liquid alloy together with the geometrical design of the mold were taken into account. The simulated evolution profiles and propagation speeds were compared with the laboratory experiments showing good agreement, validating thus the numerical model. Through an advanced particles modeling feature from a commercial package, the potential to predict and later correct some casting defects was demonstrated. As a conclusion, the mold together with supply channels need to be carefully optimized in order to control the correct direction of solidification avoiding the appearance of oxides but also to prevent stripping the sand off the walls

Aerodynamic study of a tricycle wheel subsystem for drag reduction

This paper deals with a computational fluid dynamics (CFD) and experimental drag analysis on an isolated rotating wheel subsystem (including its accessories: tire, suspension, A-arms, and fender) of a motor tricycle vehicle with two wheels in front. The main goal of the present work is to study the effect of the fender on the wheel subsystem drag and its optimization. The Star CCM+ commercial code was used for the numerical simulations. Different flow conditions were simulated and some results were validated by comparison to wind tunnel experimental results. To perform drag optimization, several aerodynamic fender shapes were designed and simulated as part of the subsystem. A drastic drag reduction up to 30.6% compared to the original wheel subsystem was achieved through numerical simulations

Lagged and instantaneous dynamical influences related to brain structural connectivity

Contemporary neuroimaging methods can shed light on the basis of human neural and cognitive specializations, with important implications for neuroscience and medicine. Different MRI acquisitions provide different brain networks at the macroscale; whilst diffusion-weighted MRI (dMRI) provides a structural connectivity (SC) coincident with the bundles of parallel fibers between brain areas, functional MRI (fMRI) accounts for the variations in the blood-oxygenation-level-dependent T2* signal, providing functional connectivity (FC).Understanding the precise relation between FC and SC, that is, between brain dynamics and structure, is still a challenge for neuroscience. To investigate this problem, we acquired data at rest and built the corresponding SC (with matrix elements corresponding to the fiber number between brain areas) to be compared with FC connectivity matrices obtained by 3 different methods: directed dependencies by an exploratory version of structural equation modeling (eSEM), linear correlations (C) and partial correlations (PC). We also considered the possibility of using lagged correlations in time series; so, we compared a lagged version of eSEM and Granger causality (GC). Our results were two-fold: firstly, eSEM performance in correlating with SC was comparable to those obtained from C and PC, but eSEM (not C nor PC) provides information about directionality of the functional interactions. Second, interactions on a time scale much smaller than the sampling time, captured by instantaneous connectivity methods, are much more related to SC than slow directed influences captured by the lagged analysis. Indeed the performance in correlating with SC was much worse for GC and for the lagged version of eSEM. We expect these results to supply further insights to the interplay between SC and functional patterns, an important issue in the study of brain physiology and function.Comment: Accepted and published in Frontiers in Psychology in its current form. 27 pages, 1 table, 5 figures, 2 suppl. figure

Numerical investigation of the aerodynamic performance for a wells-type turbine in a wave energy converter

Ocean waves constitute an extensive energy resource, whose extraction has been the subject of intense research activity in the last three decades. Among the different variants of Wave Energy Converters, the principle of the Oscillating Water Column (OWC) is one of the most promising ones. An OWC comprises two key elements: A collector chamber, which transfers the wave oscillations' energy to the air within the chamber by back and forth displacement, and a power take off system, which converts the pneumatic power into electricity or some other usable form. The Wells turbine is a self-rectifying air turbine, a suitable solution for energy extraction from reciprocating air flow in an OWC. In the present work, the steady state, inviscid flow in the Wells turbine is investigated by numerical simulations. The relatively novel Virtual Multiple Reference Frame (VMRF) technique is used to account for the rotary motion of the turbine, and the overall performance is compared with results in the literature