1,721,027 research outputs found
A 3D-CFD methodology to investigate boundary layers and assess the applicability of wall functions in actual industrial problems: A focus on in-cylinder simulations
No full textIn the industrial practice, 3D-CFD in-cylinder simulations still largely rely on RANS turbulence models and high-Reynolds wall treatments, i.e. based on wall functions. However, the use of the latter represents a potential source of error, leading to poor estimations of shear stress and heat flux at the wall. In fact, universal laws of the wall can be claimed only under very restricted conditions, which are hardly (to say never) met in industrial applications. As a result, typical dimensionless profiles of velocity and temperature on the combustion chamber walls are far from standard wall functions. In the present paper, a methodology to investigate the presence of dimensionless profiles comparable to universal wall laws in boundary layers of actual industrial problems is presented. In particular, attention is focused on 3D-CFD in-cylinder simulations. While the existing literature deals with DNS or hybrid URANS/LES approaches applied to simplified geometries and low revving speed conditions (for computational cost reasons), in the present paper a RANS k-ε turbulence model with a low-Reynolds wall treatment is adopted. In addition, an alternative strategy to extract velocity and temperature dimensionless profiles from the computed fields is proposed. The methodology is preliminary tested on a 2D plane channel (where the existence of wall functions is a priori acknowledged), at both quasi-isothermal and highly non-isothermal conditions. Afterwards, it is applied to the well-known “GM Pancake” engine test case, showing that both u+ and T+ calculated on the combustion chamber walls remarkably differ from analytical standard wall functions. Finally, in order to demonstrate the importance of dimensionless profiles to properly predict heat transfer, two different high-Reynolds simulations of the “GM Pancake” engine are proposed, one with standard wall functions and one with u+ and T+ profiles provided by the low-Reynolds analysis. While the former underestimates heat fluxes, the latter provides results in good agreement with the experiments
Validation of a zonal hybrid URANS/LES turbulence modeling method for multi-cycle engine flow simulation
No full textA zonal hybridization of the RNG (Formula presented.) - (Formula presented.) URANS model is proposed for the simulation of turbulent flows in internal combustion engines. The hybrid formulation is able to act as URANS, DES or LES in different zones of the computational domain, which are explicitly set by the user. The resulting model has been implemented in a commercial computational fluid dynamics code and the LES branch of the modified RNG (Formula presented.) - (Formula presented.) closure has been initially calibrated on a standard homogeneous turbulence box case. Subsequently, the full zonal formulation has been tested on a fixed intake valve geometry, including comparisons with third-party experimental data. The core of the work is represented by a multi-cycle analysis of the TCC-III experimental engine configuration, which has been compared with the experiments and with prior full-LES computational studies. The applicability of the hybrid turbulence model to internal combustion engine flows is demonstrated, and PIV-like flow statistics quantitatively validate the model performance. This study shows a pioneering application of zonal hybrid models in engine-relevant simulation campaigns, emphasizing the relevance of hybrid models for turbulent engine flows
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
An integrated 2D/3D numerical methodology to predict the thermal field of electric motors
Full text linkThe present work aims at providing a predictive numerical methodology for the thermal characterization of electric motors. The methodology relies on a 2D -FE simulation for the estimation of the electromagnetic (iron and joule) losses. The latter are then exploited in a 3D-CFD Conjugate Heat Transfer analysis for the evaluation of the thermal field. The CFD model includes both the solid components and the fluid domains. The main novelty of the paper is represented by the copper coil modelling. In fact, copper, air, epoxy resin and enamel are synthetized in a single homogeneous body able to reproduce the thermal behaviour without including the single components, to reduce the computational cost. The methodology is validated against experimental data on a three-phase squirrel-cage induction motor. As for the experimental data (available at three different operating conditions), temperature distributions are measured by thermocouples at the test bench for the validation of the 3D-CFD CHT model. In addition, experimental estimations of the losses are available for the validation of the 2D electromagnetic simulations. The numerical results in terms of motor performance, electromagnetic losses and thermal field are discussed and are proved to be close to the experimental counterparts, for all the investigated conditions
Towards grid-independent 3D-CFD wall-function-based heat transfer models for complex industrial flows with focus on in-cylinder simulations
No full textConvective heat transfer heavily affects both efficiency and reliability in many industrial problems. For this reason, its proper estimation is mandatory since the early design stage. 3D-CFD simulations represent a powerful tool for the prediction of the heat fluxes. This is even more true considering that typical operating conditions of many applications prevent experimental characterization. As for 3D-CFD computations, the combination of Reynolds Averaged Navier Stokes (RANS) turbulence modeling and high-Reynolds wall treatment is still widely diffused in the industrial practice, to save both computational cost and time. The adoption of a high-Reynolds wall treatment based on wall functions, which permits the use of relatively coarse near-wall grids, implies specific restrictions for the height of the near-wall cell layer. In particular, the first cell-centroid must be placed in the fully turbulent (log-) region of the boundary layer. The main drawback of a cell-centroid falling into the viscous sub-layer consists in a huge overestimation of both wall shear stress and wall heat transfer. The lower the y+ is (i.e. the lower the wall distance is), the higher the predicted values are. As for many other industrial applications, Internal Combustion Engine (ICE) in-cylinder simulations remarkably suffer from the presence of low y+ values in the computational domain, mostly at part-loads and low-revving speeds. At specific operating points, such as idle conditions, it is nearly impossible to maintain y+ in the log-region, even during the compression stroke, when the velocity field should allow the dimensionless distance to reach the highest values in the engine cycle. To avoid such undesired overestimations of shear stress and heat transfer, a modified formulation of the thermal law of the wall (T+) to be used in the viscous sub-layer is proposed in the present paper. To further reduce the grid-dependency of the high-Reynolds wall treatment, a similar modification is applied to the velocity wall function (u+). Resulting wall heat flux and wall shear stress are shown to be grid-independent, at least for y+>3. The proposed alternative modeling for u+ inside the viscous sub-layer is validated in terms of flow field against experimental Laser-Doppler Anemometry (LDA) data and Direct Numerical Simulation (DNS) results. Despite the present analysis focuses on in-cylinder simulations, the alternative u+ and T+ formulations can be applied to any complex flow. Furthermore, the proposed modified laws of the wall can be adopted in conjunction with any wall-function-based heat transfer model
CFD Simulations and Potential of Nanofluids for PEM Fuel Cells Cooling
No full textPolymer Electrolyte Membrane Fuel Cells (PEMFCs) are undergoing a rapid development, due to the ever-growing interest towards their use to decarbonize power generation applications. In the transportation sector, a key technological challenge is their thermal management, i.e. the ability to preserve the membrane at the optimal thermal state to maximize the generated power. This corresponds to a narrow temperature range of 75-80°C, possibly uniformly distributed over the entire active surface. The achievement of such a requirement is complicated by the generation of thermal power, the limited exchange area for radiators, and the poor heat transfer performance of conventional coolants (e.g., ethylene glycol). The interconnection of thermal/fluid/electrochemical processes in PEMFCs renders heat rejection as a potential performance limiter, suggesting its maximization for power density increase. To this aim, suspensions of coolants and nanoparticles (nanofluids) have been proposed for PEMFCs cooling, although their characterization has often been limited to the superior thermal conductivity, overlooking a comprehensive understanding, and leaving a relevant research gap. In this paper, nanofluids cooling is simulated using 3D-CFD in a small laboratory scale (25 cm2) model of a hydrogen-air PEMFC with a liquid cooling circuit. The variation of the coolant fluid is studied considering flow uniformity, heat rejection, pressure losses, and power generation, ultimately leading to a high-level analysis on the trade-off between heat transfer/storage, relevant for coolant channels in PEMFCs. The study elucidates the membrane conditions and the compositional requirements for ethylene glycol and water based nanofluids to lead to a net gain in the generated power density, modelled in the range of +5/10% for high particle loading (10%) and envisaged to reach +15% for hypothesized ideal compositions. The study clarifies the role of nanofluids for PEMFC cooling and redefines their enabler contribution in the development of high power density PEMFCs, indicating guidelines for their application-designed formulation
Optimization via genetic algorithm of a variable-valve-actuation spark-ignition engine based on the integration between 1D/3D simulation codes and optimizer
No full textIn this work, a turbocharged spark ignition engine equipped with a variable valve actuation device is investigated to numerically optimize the Brake specific fuel consumption (BSFC) at different loads and speeds by employing a genetic algorithm. The engine is preliminary analyzed at the test bench under both part and full load operations and different valve strategies. A system schematization is realized in a 1D code. The developed model is integrated with user-defined sub-models for the description of the in-cylinder processes, and then is validated over the measurements. A 3D CFD model of a single cylinder is developed in a commercial code and validated against experimental mean in-cylinder pressure and combustion indicators. The validated 1D engine model is coupled to an external optimizer, to identify the optimal calibration, performing multi-variable and multi-objective optimizations with the adoption of the MOGA genetic algorithm. The latter aims at minimizing the BSFC in a BMEP sweep, at fixed speed, while controlling the load through the Inlet Valve Closure (IVC) at fully opened throttle valve. The optimization results show that an advanced control of the intake valve strategy allows a maximum BSFC advantage of 26% at medium/high loads and medium speeds, if compared to the manufacturer-advised engine calibration. The outcomes of the optimization process are also confirmed by the 3D CFD tool. The latter not only contributes to the tuning of the 1D model, but it also provides an in-depth on detailed 3D aspects, such as turbulence and knock, that could not be assessed via a simplified 1D approach. The presented methodology represents a valuable tool to refine the virtual calibration of VVA engines and to support the design phase, thus remarkably reducing the experimental efforts. Moreover, it is a promising example of integration between 1D and 3D CFD tools
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