Federico Brusiani

Multi-dimensional modeling of the air/fuel mixture formation process in a PFI engine for motorcycle applications

The preparation of the air-fuel mixture represents one of the most critical tasks in the definition of a clean and efficient SI engine. Therefore it becomes necessary to consolidate the numerical methods which are able to describe such a complex physical process. Within this context, the authors developed a CFD methodology into an open-source code to investigate the air-fuel mixture formation process in PFI engines. Attention is focused on moving mesh algorithms, Lagrangian spray modeling and spray-wall interaction modeling. Since moving grids are involved and the mesh quality during motion strongly influences the computed in-cylinder flow-field, a FEM-based automatic mesh motion solver combined with topological changes was adopted to preserve the grid quality in presence of high boundary deformations like the interaction between the piston bowl and the valves during the valve-overlap period. The fuel spray was modeled by using the Lagrangian approach, and the spray sub-models (atomization and breakup) were tuned according to experimental validations carried out in previous works. Specific submodels were implemented to describe the impingement of fuel spray with the engine walls. The evolution of the resulting liquid film was also taken into account by solving the mass and momentum equations with the Finite-Area discretization method. The proposed methodology was applied to simulate a single-cylinder SI engine for motor-scooter applications at a low load operating condition. This operating point was chosen since these engines often run very close to idle conditions when they are used in the urban areas.

Advanced Modelling of a New Diesel Fast Solenoid Injector and Comparison with Experiments

Upcoming Euro 4 and Euro 5 emission standards are increasing efforts on injection system developments in order to improve mixture quality and combustion efficiency. The target features of advanced injection systems are related to their capability of operating multiple injection with a precise control of the amount of injected fuel, low cycle-by-cycle variability and life drift, within flexible strategies. In order to accomplish this task, injector performance must be optimised by acting on: optimisation of electronic, driving circuit, detailed investigation of different nozzle hole diameter configurations, assessment of the influence of manufacturing errors on hole diameter and inlet rounding on injector performance.

Implementation of a Finite-Element Based Mesh Motion Technique in an Open Source CFD Code

Nowadays, Computational Fluid Dynamic (CFD) codes are widely used in different industrial fields. Although hardware and numerical model improvements, the mesh generation remains one of the key points for a successful CFD simulation. Mesh quality is influenced by the adopted mesh generator tool and, after all, by the designer’s experience and it becomes very important when moving meshes are required. In fact, mesh skewness, aspect ratio, and non-orthogonality have to be controlled during the deforming process since their wrong evolution could produce an unphysical behavior of the computed flow field. Mesh motion could be performed by different strategies: dynamic smoothing operation and dynamic re-meshing operation, are, today, two of the mainly used approaches. All of them can be combined to guarantee the correct reproduction of motion profile and a good mesh quality level. In this context, the authors have implemented a moving mesh methodology in the Open Source CFD code OpenFOAM®. A multiple number of meshes is used to cover the whole simulation period, and the grid point motion is accommodated by an automatic mesh motion techinque with polyhedral cell support. The Laplace equation is chosen to govern mesh motion. This guarantees that an initial valid mesh remains valid for arbitrary boundary motion. Mesh to mesh interpolation is performed by using a cell based, distance weighted interpolation technique. The proposed approach was tested on a real IC-engine geometry. In particular, the mesh quality evolution during motion, the numerical results and the computational costs were evaluated.Copyright

The Role of Simulation in the Development of a Fast-Actuation Solenoid C.R. Injection System

Upcoming Euro 4 and Euro 5 emission standards are increasing efforts on injection system developments in order to improve mixture quality and combustion efficiency. The target features of advanced injection system are related to their capability of operating multiple injection with a precise control of amount of fuel injected, low cycle-by-cycle variability and life drift, within flexible strategies. In order to accomplish this task, performance must be optimised since injection system concept development by acting on. The extensive use of numerical approach has been identified as a necessary integration to experiments in order to put on the market high quality injection system accomplishing strict engine control strategies. The modelling approach allows focusing the experimental campaign only on critical issues saving time and costs, furthermore it is possible to deeply understand inner phenomena that cannot be measured. The lump/ID model of the whole system built into the AMESim® code was presented in previous works: particular attention was devoted in the simulation of the electromagnetic circuits, actual fluid-dynamic forces acting on needle surfaces and discharge coefficients, evaluated by means 3D-CFD simulations. In order to assess new injection system dynamic response under multiple injection strategies reproducing actual engine operating conditions it is necessary to find to proper model settings. In this work the integration between the injector and the system model, which comprehends the pump, the pressure regulator, the rail and the connecting-pipes, will be presented. For reproducing the dynamic response of he whole system will be followed a step-by-step approach in order to prevent modelling inaccuracies. Firstly will be presented the linear analysis results performed in order to find injection system own natural frequencies. Secondly based on linear analysis results will be found proper injection system model settings for predicting dynamic response to external excitations, such as pump perturbations, pressure regulator dynamics and injection pulses. Thirdly experimental results in terms of instantaneous flow rate and integrated injected volume for different operating conditions will be presented in order to highlight the capability of the modelling methodology in addressing the new injection system design.Copyright

Basic Numerical Assessments to Perform a Quasi-Complete LES Toward IC-Engine Applications

Still today, the numerical representation of a fully 3D turbulent flow remains one of the most challenging task. In Computational Fluid Dynamics (CFD), turbulent flows can be numerically solved with different levels of accuracy bounded between Reynolds Averaged Navier Stokes (RANS) and Direct Numerical Simulation (DNS) methods. Today, the RANS is the standard approach to perform turbulent flow simulations. It allows a good reproduction of the mean flow conditions guaranteeing, at the same time, an acceptable computational cost for practical engineering applications. Unlike the RANS, DNS is the most complete approach that can be used to numerically solve a turbulent flow because, in this case, all the turbulent scales are directly solved. However, today the DNS approach remains inapplicable in industrial field because of the prohibitive computational power required. Between RANS and DNS, a third method can be considered for the solution of high Reynolds turbulent flows: Large Eddy Simulation (LES). LES approach allows the direct solution of the largest turbulent scales (anisotropy turbulence) while the smallest scales (isotropy turbulence) are numerically modelled by a sub-grid scale model. For this reason, with respect to RANS, LES is expected to give an improvement about turbulent flow numerical solution when the physical behaviour of the considered fluid domain is dominated by the large scales of motion. At the same time, the computational cost of a LES simulation is quite lower than for DNS simulation. Even if during the last years LES has helped to improve the comprehension of complex turbulent fluid dynamic systems, for its applications in industrial fields further insights are needed. In particular, one of the main problem linked to the LES method regards the definition of a simulation methodology by which to obtain an high solution level (i.e. high level of energy directly solved) at the lowest computational cost. To fulfill this requirement, the authors defined a new LES simulation methodology based on Adaptive Mesh Refinement (AMR). The first results obtained by its application on a backward facing step test case are here presented and discussed in detail.Copyright

Application of Adaptive Large Eddy Simulation Methodology in IC-Engine Related Problems

Today, Reynolds Averaged Navier Stokes (RANS) simulation approach remains the most widely used method in computational fluid dynamic studies of IC-Engines because it allows a good prediction of the mean flow properties at an affordable computational cost. The main limit of the RANS approach resides in the method used to predict turbulence that fails in the reproduction of anisotropic turbulence conditions. It can result in a lack of accuracy in reproducing the main physical processes, as spray evolution (mixture formation), heat transfer, and combustion, governing the IC-Engine physics. To fix this problem, the large Eddy Simulation (LES) approach can be considered.In LES the governing equations are filtered in space, rather than time-averaged as in RANS. It allows the direct solution of all the turbulent scales up to a cut-off length defined by the filter dimension. Therefore, in LES a more accurate description of the turbulence and of all the physical processes correlated to it has to be expected. However, even if the LES method allows an irrefutable improvement in turbulent flow solution accuracy, today its application to industrial IC-Engine design is still rare because of its high computational cost.During the last few years, significant advances in numerical methods, sub-grid scale models, and hardware performance have supported LES applications in many industrial fields. This paper is intended to work in the same direction by presenting a new LES methodology based on the coupling between LES and an adaptive mesh refinement (AMR) procedure. The main goal of this procedure is to guarantee a good resolution of the turbulent flow field adapting the filter size to the local turbulence length scale. The developed procedure allows a significant reduction of the total mesh size and, therefore, of the computational cost. The LES-AMR method was tested on an IC-Engine geometry for which experimental results were available.Copyright

Definition of a LES Numerical Methodology for the Simulation of Engine Flows on Fixed Grid

To improve the overall engine performance, it is necessary to clearly understand the main unsteady phenomena that occur inside an IC engine. Since experimental technique can provide only lump parameters, the CFD numerical approach has been identified as a valid alternative tool to perform detailed investigations on the fluid dynamics behaviours. The numerical analysis of engine flows is commonly performed by using RANS approach. Adopting a RANS methodology only the mean flow variable distributions could be obtained because the time average of the generic flow variable fluctuation is zero by definition. To perform an effective analysis about the unsteady characteristic of a generic flow and, in particular, of an engine flow it is necessary to improve the numerical solution level adopting the LES (Large Eddy Simulation) approach. LES solves directly the large scales of motion (responsible for the main energy transport inside the flow) while only the small scales are modelled using a Sub-Grid Scale model. Moreover, the LES approach could also be used as test bench case to properly define and understand how it is possible to improve the solution accuracy of RANS simulation. This paper regards the LES analysis of a steady non-reactive wall-bounded flow over a test bench engine geometry. In particular, two LES models, i.e., the Wall Adaptive Local Eddy-Viscosity (WALE) [25] model and the one-equation Dynamic Model by Kim and Menon [23, 24, 29] have been tested. The numerical set-up has been defined performing a preliminary parametric CFD simulations on a basic flow over a backward facing step case. In particular, a bounded second order central differencing scheme was adopted and a discussion of the kinetic energy conservation attitude of such a scheme is performed. LES results have been compared to available experimental LDA measurements of mean and rms fluctuations of both axial and tangential velocity components and with numerical predictions obtained by an optimized RANS simulation of the same case. This paper shows the advantages and the limits of the LES simulation approach applied to IC engine flows.Copyright