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Accelerating vehicle concept modeling
LMS Virtual.Lab Vehicle Concept Modeling is a dedicated concept analysis tool that builds upon the LMS Virtual.Lab Structures solutions for component, subsystem and full vehicle modeling, analysis and post-processing. Using this tool, concept models can be easily built and used to understand the influence of modifications of beam-like sections and joints in vehicles and therefore tune the global vehicle dynamics, i.e. the global modes.
Introduction
 Today’s customers expect a broad range of diverse vehicle variants with high quality at a reasonable price. In order to meet this demand, the time-to-market and development cost must be reduced. It is thus important to even more base design decisions on virtual prototypes and to optimize the whole vehicle development process. A major challenge and ongoing revolution in digital product development consists of achieving an "Analysis leads Design" process, in which an upfront engineering analysis phase essentially precedes the detailed geometrical design and in which CAE supports concept analysis to define the design requirements in order to meet the functional performance targets. Performing upfront engineering basically allows solving a certain percentage of problems early and hence to obtain an initial CAD model with higher quality.  A key enabler of the "Analysis leads Design" process is the morphing technology, available in LMS Virtual.Lab Structures. Using morphing, it is possible to restyle a predecessor finite element model towards new styling information and obtain very early in the concept phase a simulation model to perform functional performance simulations. However, using the full-scale finite element model to perform modification predictions and tune the vehicle towards targets set on the frequency of the global modes for example can be quite tedious and time consuming. LMS Virtual.Lab Vehicle Concept Modeling offers a tool to create a reduced concept model of an existing finite element model.  The concept model is created by dividing the vehicle structure into functional components (beams, surfaces and assembly joints) and by modelling those in a simple way; the structural members with length significantly greater than the cross sectional area (pillars, cross members, floor pan…) are modelled with beam elements, the joints in between those with static superelements, and the remainder of the vehicle can be reduced down using a modal model. This concept model can then be used to identify quickly which beams and sections influence a lot the global modes, and of course then also in optimization processes where targets are set on the frequency of the global modes and the design parameters are the joint and beam modifications. Theoretical formulation of the Vehicle Concept Model  In order to create the vehicle concept model, the user will first need to decide which primary beam-like members and which joints will be taken into account in global vehicle dynamics modification and optimization studies. Once this is decided, the reduced beam and joint concept model can be created. As a second step, the original finite element model will be reduced by means of a modal reduction and only a limited set of nodes is kept after reduction: the layout nodes and some wire-frame nodes for post-processing of global modes. As a final step, the reduced model is used in the modification analyses. Concept modifications are considered through FE beam elements added between the layout nodes and stiffness matrices for the joints between the beam sections. The beam elements have 4 stiffness properties (Area, 1st and 2nd bending and Torsion); they can be considered all at a time or independently. Since the reduced model runs very fast, numerous modification as well as optimization analyses can be performed in a limited time.
1. Reduced Beam Model
For each primary beam member cross-section, the equivalent beam properties are computed as follows:
- A Select a cut node where an intersection plane must be applied.
B An axis system that defines the approximate beam direction and intersection plane. C Cut the primary member’s shell elements along the intersection plane. Locate a beam center node in the geometrical center of the intersection plane. D Connect the beam center node with the surrounding mesh. More specifically, the nodes nearest to the intersection plane are located, and interpolation equations are used to connect these nodes with the beam center node. E Compute the equivalent beam properties: area of the intersection plane and moments of inertia with regards to the beam center node. F Reduce the original model by modal reduction
 Typically, along each primary beam member a number of intersection planes are defined, for which equivalent beam properties are computed. The average beam properties are then taken as equivalent representation of the primary beam member. A scaled version of this equivalent beam can then be added to the original structure by defining beam elements between successive beam center nodes. By adding scaled beams to a reduced modal model, one obtains a much smaller model with an easy parameterization (one only needs to modify the properties of the added beams).
2. Reduced Joint Model
 After computing equivalent beams for the primary beam members, one must separate the joint from the original structure. This is a very straightforward process in LMS Virtual.Lab; the user has only to pick a random element of the joint and the surrounding beams. This yields a typical layout as the model below: a joint model that includes the interpolation relations to the beam center nodes.
Guyan reduction is then used to compute a static superelement that contains stiffness relations between the end points of the joint (i.e. the beam center nodes). The Guyan reduction analysis case set-up and solver driving is also done automatically within Virtual.Lab. The static superelement consists of system matrices, which can be scaled and used in a modification and optimization framework.
An application example
As an example, vehicle concept modeling is applied here to a racing car chassis. The FE model (4391 nodes, 6012 elements) is shown in fig 1. Modal analysis of the nominal model shows that the first six natural modes are fundamental modes (i.e. without area deformations in the cross-sections of beam members) that lie in the range [0-117] Hz.
| Objective | Nominal | | f1 | 54.599 | | f2 | 55.116 | | f3 | 87.480 | | f4 | 98.445 | | f5 | 102.10 | | f6 | 116.65 | | fsum | 514.39 |
Based on engineering judgement, the beams and joint layout has been defined and the remainder of the chassis has been reduced by modal reduction towards the visualization wireframe shown, see fig 3 and 4.  To show the prediction quality of this concept model, all beams have been modified with 10%, corresponding to 10% increase of the shell thickness of the original FE model. As can be seen from figure 5, the concept modelling technique is valid in the range that is governed by the fundamental modes (up to 120Hz). The modes above 120 Hz involve area deformations in the beam members’ cross-sectional area; when this occurs, the beam modification approach is no longer valid. The goal is now to optimize the global dynamics of the racing car, e.g. by changing the frequency of the first mode or to maximize Fsum, the sum of the first six natural frequencies. This can be by using the embedded designof- experiments and optimization technology inside LMS Virtual.Lab. As an example, figure 6 and 7 show the effect on the frequency for the first global mode (i.e. the sensitivity), when the cross section or one of the moments of inertia is changed.  
Conclusions
It is clear that the concept modeling technology as presented here is a very valuable tool in the concept phase, as it enables to quickly and accurately obtain concept design directions for the primary structural beam-like members and joints, when compared to the conventional modifications on full finite element models. The user can create these concept models with a minimum of interactive effort inside LMS Virtual.Lab Vehicle Concept Modeling, and can analyze and optimize the concepts also inside LMS Virtual.Lab.
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