Simulation of In-Cylinder Flow Phenomena with ANSYS Piston Grid An Improved Meshing and Simulation Approach Dipl.-Ing. (FH) Günther Lang, CFDnetwork Engineering Dipl.-Ing. Burkhard Lewerich, CFDnetwork Engineering Dr.-Ing. Joachim Ebner, Powertrain CAE Combustion, BMW Group, Munich SYNOPSIS ANSYS Piston Grid (PG) is a software-package that extents the functionality of ANSYS CFX. It enables fully automated simulation of in-cylinder flow phenomena in internal combustion engines (ICE). Central part of PG is a main program that runs during the ICE simulation and controls the Meshing-Tool (ICEM) and the CFD-Solver (CFX). Both the meshing and the simulation are totally script controlled, no user input at runtime is required. PG has been introduced in the ICE development process at BMW three years ago. It became apparent, that the quality and reliability of the automatic meshing is of key-importance for the successful implementation of PG in the engineering environment. During the last three years, the PG process has been optimized, leading to a sophisticated meshing and simulation strategy which are presented in this paper. This improved meshing enables PG to obtain considerable higher performances in ICE simulations than conventional methods. 1. INTRODUCTION The efficiency of today s internal combustion engines has already reached a considerable high level. To achieve further optimization, a more detailed analysis of the involved processes is required. Thereby the degree of detail of each single process has to be increased as well as the understanding of the interaction of these processes. Because of this complexity simulation plays an important role in today s engine development. The main challenges simulation is faced to in this field are short development times, complex geometries and difficult physical phenomena. A large number of geometries and operating points have to be investigated. For that purpose, a high level of meshing automation, solver stability and capability to describe all physical phenomena in modern turbocharged and direct injected engines are essential requirements in order to predict the internal process. ANSYS provides Piston Grid (PG) as special tool to satisfy the aforementioned demands for ICE simulations. PG acts as framework for ICE simulations by providing the main engine data and simulation settings. During the simulation, PG controls the Meshing-Tool (ICEMCFD) and the CFD-Solver (CFX). Both the meshing and the simulation are totally script based and run without user input at runtime. To enable this, the user-defined meshing script must be able to generate a high quality
mesh for all engine configurations and for all valve and piston positions. The script for the CFX- Solver settings has to contain all required data for the simulation. The introduction of PG in BMW engine development showed that the meshing strategy is of high importance for the successful implementation of PG in the engineering process. With the experience to apply PG for simulations of various different engines it was found, that a high flexibility and reliability of the meshing strategy is required. This experience yielded in over three years of development an improved meshing and simulation strategy. By now, this customized PG Process is the standard tool for nearly all kind of ICE simulations at BMW. The following section introduces the implementation of PG within the ANSYS 11 software package. The functionality and the key features of PG and the resulting simulation process are described in detail. Subsequent, the essential parts for running a PG simulation are presented under the term The Standard PG Process. Section 3 illustrates the key role of the meshing strategy within PG simulations. The demands of modern ICEs and their impacts on the meshing strategy as well as the need to improve the Standard PG Process are discussed briefly. The consideration of the key features of the Improved PG Process closes this section. The following section illustrates the meshing strategy used within the Improved PG Process. All features of the improved meshing process are presented and the reasons for implementing the respective meshing features are shown. Section 5 gives an overview over typical simulation cases the Improved PG Process is designed for. A showcase project schedule illustrates the main working steps and shows the expected turn-around times. These illustrations are based on the experience of CFDnetwork with customer-related ICE simulations. The closure of the paper gives a short summary and points out, that CFDnetwork provides professional services for ICE simulations with ANSYS PG. 2. Implementation of Piston Grid in ANSYS 11 PG is a script based command line tool within the ANSYS 11 software package. There is no GUI support for PG now. The complete setup of a PG simulation consists of several scripts, which contain control parameters for the involved applications PG, ICEM and CFX. The essential files for running a PG simulation are the main PG control-file containing basic simulation characteristics, the meshing script (replay-file) and the file introducing the numerical set-up for the simulation (ccl-file). 2.1. Basic structure of the Piston Grid Simulation Process As already mentioned, the main role of PG is to connect the meshing tool (ICEM) and the CFX- Solver. From this the schematic representation of a typical PG simulation shown in Figure 2.1 can be derived: CFX start CFX-Solver start CFX-Pre stop CFX-Solver start ICEM ICEM Figure 2.1: Basic loop of the Piston Grid simulation process
PG stops CFX at a certain point of the simulation when bad mesh quality or other user-defined parameters forces it. These parameters are the so called remeshing criteria, e.g. when the mesh quality falls below a user-specified limit (e.g. due to valve or piston motion) or when a change in the mesh topology is required. After the simulation reaches a remeshing criterion, PG stops the simulation and provides ICEM with the current geometry (valve and piston positions). A new mesh is generated by the meshing script and PG starts the CFX-Solver to continue the simulation. This procedure runs automatically until the end of the entire simulation without any further user input. 2.2. The General Piston Grid Process - Setup and Workflow To start the simulation process, the user has to prepare different files and scripts. The essential scripts involved in the General PG Setup are as follows: 1. Engine and Simulation Data (PG control-file) 2. Geometry preparation (ICEM geometry file = tetin-file) 3. ICEM meshing strategy (replay-file) 4. CFX-Solver settings (ccl-file) Engine and simulation data include the engine speed, valve and piston motion definitions and the subdivision of the combustion cycle in its single phases. These phases are called sequences in PG. Each sequence represents a certain engine configuration. The whole combustion cycle for a four-stroke engine with valve overlap can be divided into four characteristic configurations: only exhaust valves open, exhaust and intake valves open, only intake valves open and both valves closed. So, four sequences have to be defined in the control-file. For every sequence, the user has to provide the remaining parts 2 to 4 of the General PG Setup. The second part of the General PG Setup is the geometry preparation in ICEM. For every specified sequence, the user has to provide a geometry file. Besides necessary geometry simplifications, the user has to follow the PG specific naming conventions. Figure 2.2: The Standard Piston Grid Workflow
The third part, the replay-file, defines the meshing strategy for the particular sequence. Independent of the valve lift or piston position, the meshing script has to generate automatically a mesh of sufficient high quality. The meshing script presented here is completely user-defined. The scope of functionalities applied varies between very simple to very complex problems. It will be demonstrated later that the applied meshing strategy plays a decisive role in a PG simulation. Before starting the simulation, the user has to define all necessary numerical settings and save them in a separate CFX setup-file (ccl-file). The ccl-file contains the initial and boundary conditions as well as basic settings like the global convergence criteria and the turbulence model for the simulation. It has to be pointed out here, that once a good meshing strategy is defined and working solver settings are found, these scripts can be used for any further ICE simulation. Thus for each new engine, the geometry preparation becomes the unique timeconsuming work step. After completing the four parts of the General Setup, the simulation can be started. If no convergence problems occur, e.g. at very small valve lifts where strong pressure gradients and high velocities (Ma > 2) appear, the simulation runs to the specified end time without any further user input. The post-processing of the simulation results can be performed with CFX-Post in the batch mode. Figure 2.2 shows the above described aspects of a PG simulation and the applications involved which are Piston Grid, ICEM and CFX and CFX-Post. Because the given simplified scheme only contains essential parts for a PG run, the workflow is called the Standard Workflow. As it will be shown in the next section, the workflow is influenced by the implemented meshing strategy presented in this paper. The above presented case assumes a simple meshing script that does not affect the workflow. 3. The Improved PG Process The Improved PG Process is in a wide range a result of the further improved meshing script. Implementing all desired meshing features requires a more complex and more time consuming PG Setup, here referred to as The Extended PG Setup. Also additional workflow steps compared to the Standard Workflow are needed. Subsequent a brief overview of the reasons for improving the Standard PG Process, this section demonstrates the differences between the Standard and the Improved Process. 3.1. Reasons for Improving the Standard PG Process On the one hand, the meshing script has to be flexible enough to come along with all demands imposed by modern engines. These engines have very complex geometries and special features like a variable valve control or direct injection system. On the other hand, the meshing has to be totally stable (stability of the meshing process directly influences the turn-around times in a strong way). In order to achieve reduced turn-around times and a higher level of accuracy compared to conventional methods, the meshing has to meet to following requirements: High stability and reliability High flexibility due to complex engine geometries and due to modern engine applications Operating points with low maximum valve lifts Masking and piston shape High accuracy Minimization of numerical errors Continuous boundary layers Local and simulation-time dependent mesh-refinement
User independent meshing As mentioned already, the Improved Meshing Strategy affects the PG Setup respectively additional preparation steps in the workflow are required. The main aspects of the Extended PG Setup used in the BMW environment and the associated Improved Workflow are illustrated in the following sub-sections. 3.2. The Extended PG Setup Because of the improved meshing strategy, the Extended PG Setup contains additional issues in all four parts compared to the Standard PG Setup. Within the engine and simulation data one has to specify characteristic engine dimensions which are necessary for adapting the meshing to a particular engine geometry. The ICEM geometry preparation gets more complex because several features of the improved meshing demand for special preparation steps. Additionally, an algorithm for defining various remeshing criteria supplements the CFX-Solver settings. The additional parts of the Extended PG Setup are summarized in Figure 3.1. Figure 3.1: Additional parts of the Extended PG Setup compared to the Standard Setup 3.3. The Improved PG Workflow After completing the Extended PG Setup, the user has to follow strictly the defined Workflow. The additional step of the Improved PG Workflow compared to the Standard one is an additional Pre-Meshing step. Figure 3.2 illustrates the schematic sketch of the Improved PG Workflow. In particular, the Pre-Meshing step includes the pre-meshing of non-moving geometry parts and the automatic generation of meshes in artificial extreme positions (worst-case meshing). The non-moving geometry parts like the intake and exhaust ports or the region near the spark plug are meshed only once before running the simulation. These meshes are used in each meshing during the simulation. The worst-case meshing allows to check the mesh quality in artificial extreme positions, e.g. both valves in their minimal lift position and the piston in the upper dead center respectively both valves and the piston in their maximum lift position (these geometrical engine configurations are artificial, because they do not appear in the real combustion cycle).
This approach gives the user detaild information of the mesh quality and main mesh characteristics which will be generated during the simulation. This Pre-Meshing step also guarantees the stability of the meshing. Figure 3.2: Characteristics of the Improved PG Workflow Beside the already mentioned Pre-Meshing step the main difference from the Improved Workflow to the Standard one is the communication between the involved applications. Mainly data exchange between ICEM and the CFX-Solver is required, e.g. the information of a change in the mesh topology in ICEM has to be transferred to CFX due to the setup of boundary conditions. The most simple mesh topology is a consistent mesh containing only tetraeder cells with continuous prism elements to resolve the boundary layer. Depending on the piston and valve lifts, the mesh may contain an extruded region in the cylinder or a hexaeder-mesh for small valve gaps. These special mesh topologies demand appropriate adaption of the CFX- Solver settings like sub-domain or interface definitions. Therefore the meshing script adapts a template containing the CFX-Solver settings for the simplest mesh topology automatically. On the other hand, ICEM depends on information from CFX like the aforementioned remeshing criteria. For example a switch is introduced which defines a certain piston lift for change the mesh topology between meshes containing mesh-extrusion in the cylinder region and meshes without that feature. According to this information, ICEM is able to apply the desired meshing features and to generate the required mesh topology. The plotted communication arrows indicate that nearly all applications depend on each other and require information from each other. It is not essential, that every communication path occurs during the simulation. 3.4. The Meshing Strategy - Key Feature of the Improved PG Process Finally, the central role of the meshing strategy has to be pointed out. A robust and flexible meshing is absolutely necessary to assess ICE simulations with PG successfully. For this reason, the meshing strategy used in the BMW engine development was improved successively over the last three years. For covering a wide range of ICE applications and geometries, the meshing strategy is quite complex. This assumes a high amount of expertise in applying the Improved PG Process correctly or in extending it with additional features. In the following section the meshing strategy implemented in the Improved PG Process is presented in detail.
4. Meshing Strategy of the Improved Piston Grid Process The meshing strategy developed for the Improved PG Process includes several special meshing features from which the most important are presented below. At first, a detailed description of the Hexa-Mesh feature is given to provide an insight into the integration of a single meshing feature into the workflow. All the other meshing features are implemented in a similar way. Hexa-Mesh in valve gaps This meshing feature enables creating grids down to a minimal valve lift of 0.05 mm. The need for simulations down to valve lifts of 0.05 mm evolves from the full variable valve train. This system may realize a maximum valve lift of 1 mm at certain operating points. In such a case, it would be insufficient to simulate only down to a valve lift of 0.3 mm with reasonable mesh sizes and sufficient mesh quality. A pure continuous tetraeder-prism mesh turned out to be feasible solely to a minimal valve lift of about 0.3 mm. There are two main reasons for this: in small valve gaps, the required tetraeder-sizes produce too much mesh nodes and in association with this high spatial resolution, the meshing algorithm requires an extreme high geometry quality which is mostly not available. The solution for this problem is to generate a Hexa-Mesh within the valve gap region. Figure 4.1 shows the valve gap region at three different valve positions resolved with the Hexa-Mesh. CA 330 Valve lift: 0.05 mm CA 350 Valve lift: 0.15 mm CA 365 Valve lift: 1.00 mm Figure 4.1: Hexa-Mesh in the valve gap regions at three different valve positions To apply the Hexa-Mesh feature, two preparation steps have to be performed. The first one is to specify the desired valve lift range in which the Hexa-Mesh feature is used, e.g. from 0.05 mm to 1 mm. For every valve a different range can be specified. The second and work-intensive step includes additional geometry preparation. Special curves and surfaces with well defined name patterns have to be provided. Figure 4.2 shows these special geometry components. Hex_Curve_Seat_1 n Hex_surface_Seat_1 m Hex_Curve_Valve_1 n Hex_Surface_Valve_1 l Figure 4.2: Geometry preparation needed for the Hexa-Mesh feature
After these preparation steps the meshing runs fully automated. Before each meshing process, the meshing script compares the actual valve lift and the user-defined lift range for applying the Hexa-Mesh feature. According to the current valve position, the required Hexa-Mesh is generated. Thereby, the sizes of the hexaeder cells are user-specified. The connection to the surrounding tetraeder-prism mesh occurs by grid interfaces. Special care was put on the connection between the hexaeder and the prism cells in the boundary layer region. The Hexa- Mesh algorithm calculates the distribution of the wall-nearest hexaeder cells according to the identical method used for defining the prism-heights. By this a nearly one-to-one passage could be realized within the boundary layer region. The resulting mesh contains Hexa-Meshes and hence differs in its topology from the standard tetraeder-prism mesh. For a correct simulation setup, the interfaces and sub-domains have to be defined. This is done automatically by a separate adaption step of the ccl-template by the ICEM meshing script. The implementation of the Hexa-Mesh feature demonstrates how close all scripts and applications of the Improved PG Process are linked together. All meshing features presented in the following are implemented in a similar way. Dynamic geometry construction The meshing script constructs surfaces which depend on the piston or valve motion for each new meshing automatically. Surfaces which are affected by this are the cylinder liner and the valve rods. The aim is to avoid any overlapping surface elements which might cause problems for the ICEM meshing algorithm. By this, the dynamic geometry construction supports the meshing robustness. Dynamic control of meshing parameters The dynamic control of the meshing parameters provides the adaptation of the cell sizes to the current engine configuration and to the current piston and valve positions. Piston Grid innately provides structures which enable this mechanism. Therefore, the user specifies the meshing parameters which have to be changed during the simulation process within the PG Setup. Such parameters are for example the surface mesh sizes, the prism heights or the tetraeder sizes within densities. Additionally, the user has to provide all dependencies of these variable parameters. The Improved PG Process contains a complete logic for the variable meshing parameters. The dynamic control of meshing parameters helps to reduce the number of mesh nodes and accordingly the simulation time required. Also the quality of the results rises with adequate prism-layer thickness and higher spatial discretization in critical regions. Pre-Meshing of non-moving geometry parts and of special surfaces The idea behind this meshing feature is that geometry parts which are stationary during the entire simulation need only to be meshed once. This is for example the case for the exhaust and intake ports. Additionally, the pre-meshing provides almost all surface meshes of static and moved walls These surface meshes are used for all meshes during the simulation By this, the meshing of the boundary layer is kept nearly constant. The advantage of the pre-meshed parts is to deliver very similar meshes for minimizing interpolation errors. An improved meshing robustness and a reduced meshing effort for each single meshing process are the side benefits. Local remeshing The worst cells of a distorted mesh are typically near the region of the moving parts. Especially the cells in valve gaps are exposed to high shear rates by the valve motion. The cells quickly get worse in these local defined regions and undershoot the minimal mesh quality, whereas the bulk
of grid elements are not distorted at all. The Standard PG Process would start the meshing for the whole geometry and create a complete new grid. The mapping of the old solution onto the new mesh obviously causes interpolation errors. The local remeshing feature loads the distorted grid, deletes only the cells of bad quality with additional two surrounding cell layers and generates new mesh elements only within the emerging holes. Typically only a few percentage of the mesh nodes are affected. The local remeshing feature ensures a minimal meshing effort, a maximum robustness and minimization of the interpolation errors at the same time. Mesh-Extrusion in the Cylinder-Region If the piston lift exceeds an user-defined limit, the ICEM meshing script inserts a pre-meshed triangulated extrusion plane in the combustion chamber. Below this plane, the mesh generation is performed by extrusion of the triangles. The resulting meshes are very similar within the extruded parts. Due to the piston motion, the mesh nodes of two different extrusion meshes vary only in their z-coordinates while the x and y values remain constant. By this, the interpolation of the previous simulation results reduces to a linear problem. This produces more accurate results than a full 3-dimensional interpolation. 5. Typical Engineering Tasks of the Improved PG Application and Showcase Project Schedule This section gives an overview of typical engineering tasks for the Improved PG Application. Thereby, the base case is the simulation of the entire combustion cycle with injection and mixture formation for a full model. The effort for handling, simulation and post-processing times for the base case and for different variations are discussed briefly. Figure 5.1 sketches the typical schedule for a customer project according to the experience of CFDnetwork. Figure 5.1: Project schedule for a full model simulation with fuel injection and mixture preparation
As it can be taken from Figure 5.1, the Extended PG Setup for the base (first) case within an engine project consumes up to two weeks. Within the Extended Setup everything from the CAD data transfer to the implementation of the initial and boundary conditions is included. It is important to point out that the base case is special and requires greatest care (compared to following variations). The introduced geometry simplification has to be discussed with the customer. In order to adapt the meshing to the particular engine and to meet the customer requirements concerning mesh characteristics and mesh quality several feedback loops of the pre-meshing step (meshes in artificial extreme positions) are needed. Every customer has its own guidelines concerning cell sizes, expansion factors and prism-heights. Also the implementation of the correct initial and boundary conditions has to be proved carefully. These most critical working steps are marked red in Figure 5.1. The great benefit of PG now is that once the Setup for the base case has been completed, different variations of the base case can be realized with nearly negligible effort. Examples for this variations are shape analyses, valve lift variations or changes of the injection settings. The most time consuming project part is the simulation itself. A mixture formation simulation of a full model over the entire combustion cycle typically needs between two and four weeks. It is obvious, that the simulation time varies depending on the level of parallelization, single processor speed and the used numerical settings and models (e.g. standard is the use of a twoequation turbulence model). The meshing is the only single processor operation within the PG simulation. Typically about one day is required for the overall mesh generation. For postprocessing the same aspects as for the Extended Setup plays a role. Once the automated process delivers all required quantities and pictures, the handling time for following simulations reduces to a minimum. The following list contains typical applications for the Improved PG Process: 1. Simulation of gas exchange (with/without injection and mixture formation, half or full model) 2. Analysis of different engine operating points (spread variations, injection timing) 3. Optimization of intake and exhaust port geometry The first two cases cover the whole combustion cycle, whereas the third case may only consider the flow induced by the intake or the exhaust port. The stated time data refer to the aforementioned base case of the engine project. 6. Summary and Outlook The Improved PG Process is a reliable meshing and simulation approach for ICE engines. Reduced and predictable turn-around times and increased accuracy can be achieved compared to the Standard PG Process or conventional methods. The Improved PG Process results from three years of experience with PG at BMW. By now, Improved PG is the standard application for ICE simulations at BMW and the introduced strategy has proven itself in numerous different engine designs. Functional demonstration using the example of a mixture formation simulation for a DI gasoline engine with complex geometry also has been performed by CFDnetwork Engineering. As part of the BMW methodology development, CFDnetwork engineers possess deep insight in the Improved PG Process. This expertise enables us to offer comprehensive services with ANSYS Piston Grid. There is a strong interest to implement the features of the Improved PG Process as a standard into next ANSYS Piston Grid version. Then, special customer requirements or new meshing features can be implemented. The Improved PG Process provides the framework for advancing in spray and combustion calculations. CFDnetwork Engineering helps you to establish Piston Grid in your simulation process in order to improve the quality of the simulation results and at the same time to reduce turn-around times.