Simulating engineering flows through complex porous media via the lattice Boltzmann method

In this paper, recent achievements in the application of the lattice Boltzmann method (LBM) to complex fluid flows are reported. More specifically, we focus on flows through reactive porous media, such as the flow through the substrate of a selective catalytic reactor (SCR) for the reduction of gaseous pollutants in the automotive field; pulsed-flow analysis through heterogeneous catalyst architectures; and transport and electro-chemical phenomena in microbial fuel cells (MFC) for novel waste-to-energy applications. To the authors’ knowledge, this is the first known application of LBM modeling to the study of MFCs, which represents by itself a highly innovative and challenging research area. The results discussed here essentially confirm the capabilities of the LBM approach as a flexible and accurate computational tool for the simulation of complex multi-physics phenomena of scientific and technological interest, across physical scales

On the steady and unsteady turbulence modeling in ground vehicle aerodynamic design and optimization

Computational Fluid Dynamics is nowadays largely employed as an effective optimization tool in the automotive industry, especially for what concerns aerodynamic design driven by critical factors such as the engine cooling system optimization and the reduction of drag forces, both limited by continuously changing stylistic constraints. The Ahmed reference model is a generic car-type bluff body with a slant back, which is frequently used as a benchmark test case by industrial as well as academic researchers, in order to investigate the performances of different turbulence modeling approaches. In spite of its relatively simple geometry, the Ahmed model possesses many of the typical aerodynamic features of a modern passenger car - a bluff body with separated boundary layers, recirculating flows and complex three-dimensional wake structures. Several experimental works have pointed out that the flow region which presents the major contribution to the overall aerodynamic drag is the wake flow behind the vehicle model: therefore, a more exact simulation of the wake and separation process seems to be essential for the accuracy of numerical drag predictions. As a consequence, a significant effort has been put in many computational studies carried out on the Ahmed model in the last two decades, in order to fix benefits and deficiencies of various turbulence modeling practices, from the steady-state RANS approach to the fully unsteady LES approach. Though now there are some generally accepted remarks, such as the difficulties encountered by some classical steady-state RANS models in giving accurate results for some critical flow regimes, it is authors' opinion that there are still some issues that need to be addressed, particularly for what concerns the differences and the possible improvements related to the passage from steady to unsteady approaches. In this paper a numerical investigation of turbulent flow around the Ahmed model, performed with the open-source CFD toolbox OpenFOAM®, is presented. Several URANS turbulence models, as well as different wall treatments, have been extensively tested on the notoriously critical 25° rear slant angle configuration of the Ahmed body. Simulations with the same models, but run in steady-state RANS mode, have been also provided in order to evaluate which kind of approach could be the best compromise as a sufficiently accurate and time-saving optimization tool for ground vehicle aerodynamic design. Drag predictions and other flow features, especially in terms of velocity profiles visualization in the rear region, have been critically compared with the experimental data available in the literature and with some prior numerical studies

High reynolds number hybrid RANS/LES modeling with turbulent time scale bounding

An existing two-equation eddy viscosity turbulence model was modified by the authors by application of the normal Reynolds stresses "realizability" constraint to the turbulent time scale τ=k/ε. The model was then sensitized to local grid spacing through relatively straightforward modifications in the k-equation destruction term, obtaining a scale-resolving hybrid RANS/LES formulation. The resulting hybrid form was implemented into an open source finite volume CFD code and evaluated for the resolution of high Reynolds number essentially incompressible flows. The cases studied here include aerodynamic and aeroacoustic predictions for a standard simplified car mirror shape

On the rans modeling of turbulent airflow over a simplified car model

The need for reliable CFD simulation tools is a key factor for today’s automotive industry, especially for what concerns aerodynamic design driven by critical factors such as the engine cooling system optimization and the reduction of drag forces, both limited by continuously changing stylistic constraints. The Ahmed body [1] is a simplified car model nowadays largely accepted as a test-case prototype of a modern passenger car because in its aerodynamic behavior is possible to recognize many of the typical features of a light duty vehicle. Several previous works have pointed out that the flow region which presents the major contribution to the overall aerodynamic drag, and which presents severe problems to numerical predictions and experimental studies as well, is the wake flow behind the vehicle model. In particular, a more exact simulation of the wake and separation process seems to be essential for the accuracy of drag predictions. In this paper a numerical investigation of flow around the Ahmed body, performed with the open-source CFD toolbox OpenFOAM®, is presented. Two different slant rear angle configurations have been considered and several RANS turbulence models, as well as different wall treatments, have been implemented on a hybrid unstructured computational grid. Pressure drag predictions and other flow features, especially in terms of flow structures and velocity field in the wake region, have been critically compared with the experimental data available in the literature and with some prior RANS-based numerical studies.</jats:p

On the detailed multidimensional modeling of HT PEM fuell cells and stacks

A detailed multidimensional CFD-electrochemical model of a single HT PEM fuel cell has been firstly developed, in order to assess its reliability as an engineering simulation tool for HT PEM based energy systems. The model performances have been validated against ad hoc experimental measurements made on a single HT PEM cell, including different anode gas compositions (either pure H-2 or Syngas).In a second stage, a reduced single cell model has been derived from the fully detailed configuration, with the aim of evaluating its feasibility in HT PEM stack performance predictions. The reduced model was then replicated for the reproduction of a mini-stack configuration, carrying out some preliminary simulations on the latter

Numerical and experimental study of asymmetric water impacts of wedge-shaped sections

In this paper we investigate numerically and experimentally the hydrodynamics related to the asymmetric water impact of a two dimensional wedge shape. Most of the currently available theoretical, numerical and experimental studies on water impacts neglect the effects of geometrical and kinematic asymmetries. Nonetheless, impact asymmetry might be of great relevance under some particular sailing circumstances, such as high speed planing hulls or generic vessels moving with multiple degrees of freedom into steep waves.In the numerical part of our study, we use the Volume Of Fluid (VOF) method to analyze the combined effects of geometrical and kinematic asymmetry, by systematically varying the wedge roll angle and the direction of the impact velocity. The experimental apparatus includes high speed image recording, pressure sensors and accelerometers and is in turn designed to allow simultaneous changes in the roll angle and velocity direction. Numerical results and experimental measurements are compared in terms of impact related forces and fluid flow characteristics, evidencing similarities and discrepancies among the two datasets

Evaluation of a scale-resolving methodology for the multidimensional simulation of GDI sprays

The introduction of new emissions tests in real driving conditions (Real Driving Emissions&mdash;RDE) as well as of improved harmonized laboratory tests (World Harmonised Light Vehicle Test Procedure&mdash;WLTP) is going to dramatically cut down NOx and particulate matter emissions for new car models that are intended to be fully Euro 6d compliant from 2020 onwards. Due to the technical challenges related to exhaust gases&rsquo; aftertreatment in small-size diesel engines, the current powertrain development trend for light passenger cars is shifted towards the application of different degrees of electrification to highly optimized gasoline direct injection (GDI) engines. As such, the importance of reliable multidimensional computational tools for GDI engine optimization is rapidly increasing. In the present paper, we assess a hybrid scale-resolving turbulence modeling technique for GDI fuel spray simulation, based on the Engine Combustion Network &ldquo;Spray G&rdquo; standard test case. Aspects such as the comparison with Reynolds-averaged methods and the sensitivity to the spray model parameters are discussed, and strengths and uncertainties of the analyzed hybrid approach are pointed out. The outcomes of this study serve as a basis for the evaluation of scale-resolving turbulence modeling options for the development of next-generation directly injected thermal engines