1,721,142 research outputs found
Effects of the Domain Zonal Decomposition on the Hybrid URANS/LES Modeling of the TCC-III Motored Engine Flow
No full textHybrid URANS/LES turbulence modeling is rapidly emerging as a valuable complement to standard LES for full-engine multi-cycle simulation. Among the available approaches, zonal hybrids are potentially attractive due to the possibility of clearly identify URANS and LES zones, eventually introducing further zone types with dynamically switching behavior. The present work aims at evaluating the impact of different zonal configurations on the simulated flow statistics using the well-assessed TCC-III experimental engine setup. More specifically, different methods (URANS, LES or seamless DES) are applied inside the cylinder volume, as well as into the intake/exhaust ports and plenums. For each of the five tested configurations, in-cylinder flow features are compared against the reference TCC-III experimental measurements, in terms of velocity field statistics and quality indices. In addition, a detailed analysis using Proper Orthogonal Decomposition (POD) is carried out to quantitatively compare the results from experiments and simulation sets. The study outcomes are used as a starting point for discussing the applicability of zonal hybrid turbulence modeling to realistic engine geometries, critically analyze the model assumptions (e.g. the domain zonal decomposition) and provide guidelines for general application of such method
Evaluation of a scale-resolving methodology for the multidimensional simulation of GDI sprays
No full textThe introduction of new emissions tests in real driving conditions (Real Driving Emissions—RDE) as well as of improved harmonized laboratory tests (World Harmonised Light Vehicle Test Procedure—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’ 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 “Spray G” 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
Hybrid URANS/LES Turbulence Modeling for Spray Simulation: A Computational Study
Get PDFTurbulence modeling for fuel spray simulation plays a prominent role in the understanding of the flow behavior in Internal Combustion Engines (ICEs). Currently, a lot of research work is actively spent on Large Eddy Simulation (LES) turbulence modeling as a replacement option of standard Reynolds averaged approaches in the Eulerian-Lagrangian spray modeling framework, due to its capability to accurately describe flow-induced spray variability and to the lower dependence of the results on the specific turbulence model and/or modeling coefficients. The introduction of LES poses, however, additional questions related to the implementation/adaptation of spray-related turbulence sources and to the rise of conflicting numerics and grid requirements between the Lagrangian and Eulerian parts of the simulated flow. About the latter, an efficient alternative might be found in hybrid URANS/LES formulations, which are still relatively unexplored for spray modeling applications and for ICE modeling in general. In this work, we conduct a systematic analysis aimed to assess the effects of several URANS, LES and hybrid turbulence modeling formulations on the spray dynamics. The hybrid form is based on a purposely developed version of the k-g URANS closure, and the simulation campaign is focused on a standard n-dodecane evaporating spray case in a constant volume vessel configuration. The spray is modeled within the Eulerian-Lagrangian framework, with primary and secondary breakup taken into account by means of the Kelvin-Helmholtz-Rayleigh-Taylor (KHRT) model. Further, we investigate on the effects due to the Stochastic Turbulence Dispersion (STD) of parcels. Numerical experiments are carried out via the open-source CFD code OpenFOAM. The results are validated against the baseline experimental data for evaporating ECN Spray A and with previous computational findings available in literature
Feasibility of passive solar tracking through the thermal expansion of a PCM medium in a residential TES application: a numerical analysis
No full textPhase Changing Materials (PCMs) are widely adopted and studied for Thermal Energy Storage (TES) applications, due to their inherent capability of storing and releasing high amounts of thermal energy in a narrow temperature range. At the same time, some of the materials commonly implemented in PCM-based TES devices (e.g. paraffin waxes or other organic materials) are known to experience a significant volumetric expansion (up to 20% or more) during their solid-to-liquid phase transition. Such expansion is generally considered a side effect, which should be accounted for to avoid damaging the PCM containment structure in the TES device. Recently, the thermally driven expansion of PCMs has been considered as a driving force for passive solar tracking, showing promising technical developments for dedicated solar tracking devices. In the present paper, we evaluate the feasibility of using the volumetric expansion cycles in a PCM-based TES device for PV solar tracking purposes, thus assuming an innovative and efficient integration between thermal and PV solar installations. To this aim, the temporal evolution of the temperature and density fields inside the PCM are modeled through a finite-difference/finite-volume numerical implementation. Accurate charge and discharge profiles of the TES device are implemented, assuming data from a previously investigated solar-assisted heating/cooling plant for a typical residential application in southern Italy. Outcomes from this numerical analysis allow to perform a parametric study in terms of specific tracking capability of the chosen PCM (paraffin wax) vs. the installed PV modules surface
Effects of the LES-Mode SGS Viscosity Formulation on the Hybrid URANS/LES Modeling of Turbulent Fuel Sprays
No full textThe LES hybridization of standard two-equation turbulence closures is often achieved leaving formally unchanged the turbulent viscosity expression in the URANS and LES modes of operation. Although generally convenient in terms of ease of implementation, this choice leaves some theoretical consistency questions unanswered, the most obvious being the actual meaning of the two transported turbulent scalars and their exact role in the modeled viscosity build-up. A possible remedy to this is represented by the simultaneous modification of one or both the turbulent transport equations and of the turbulent viscosity formula, for which a standard LES behavior is enforced whenever needed. The present work compares a conventional DES-based hybrid model with a consistency-enforcing modified variant for turbulent fuel spray simulation. In our case, LES-mode consistency is accomplished by excluding the second turbulent scalar quantity from the viscosity calculation. In this way, the turbulent kinetic energy acts as SGS kinetic energy and the SGS viscosity is evaluated according to the well known one-equation LES implementation. The conventional and modified hybrid models are applied to the simulation of the ECN non-reacting spray-A case, which is representative of a typical high-speed turbulent fuel spray injection. Moreover, the analysis is extended to study the effect of different fuel injection pressures and ambient conditions. The spray is modeled using Eulerian-Lagrangian approach, with primary and secondary breakup taken into account by means of the Kelvin-Helmholtz-Rayleigh-Taylor model. Simulations are carried out using the open source code OpenFOAM. As a result, the modified hybrid model is found to be suitable to properly capture the relevant physics of the spray. In addition, different behaviors are observed for the two hybridization strategies: while the conventional model operates basically as a pure-LES, the consistency-enforcing modified variant applies a more intense switching between URANS and LES modes of operation. This leads to some benefit, in terms of accuracy, with respect to the pure-LES approach when a possibly not optimized mesh is used
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
Get PDFThe 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
Design and optimization of an experimental test bench for the study of impulsive fluid-structure interactions
No full textIn this work, the design and the optimization of an experimental test bench for the experimental characterization of impulsive water-entry problems are presented. Currently, the majority of the experimental apparatus allow impact test only in specific conditions. Our test bench allows for testing of rigid and compliant bodies and allows performing experiments on floating or sinking structures, in free-fall, or under dynamic motion control. The experimental apparatus is characterized by the adoption of accelerometers, encoders, position sensors and, above all, FBG (fiber Bragg grating) sensors that, together with a high speed camera, provide accurate and fast data acquisitions for the dissection of structural deformations and hydrodynamic loadings under a broad set of experimental conditions
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