Engine Dynamic Analysis Using Flexible Bodies

It’s time to shorten engine dynamic analyses, thanks to a tighter integration between Altair OptiStruct® and AVL EXCITE. Finite element models are extensively used in research and development of automotive engines. Modal condensation techniques are used to reduce large Degrees of Freedom (DOF) models to Flexible Bodies, in order to be used in dynamic analysis.

There are two types of Flexible Bodies used in Multi-Body Dynamic (MBD) analysis: fixed (such as an engine block) and moving (such as a crankshaft or connecting rod). The Flexible Bodies are created using the Component Mode Synthesis (CMS) with the Craig-Bampton reduction method. The number of DOF can be reduced from the number of Finite Element DOF to just a few hundred Craig-Bampton modes. In addition to the reduced mass, stiffness, and damping matrices, the inertial invariants need to be calculated for the moving Flexible Bodies. Also, some of the model information can be stored along with interior point modal vectors so that the MBD software can show movement of the Flexible Body in its post-processor. The movement is calculated by multiplying the calculated modal participation coefficients by the modal vector for each point for each time or frequency step.


After the MBD analysis is performed by Software such as AVL EXCITE and SIMPACK, the calculated modal participation coefficients for the Flexible Body can used to calculate the elemental stresses and strains in either the time or frequency domain. This is done by multiplying the modal participation coefficients by the modal stresses or strains for each time or frequency step. The modal stresses and strains are calculated during the Flexible Body reduction step and stored for later use in the stress or strain recovery step. The stresses and strains can then be passed to Fatigue Software using the .op2 or .h3d files.

Crankshaft meshing for AVL EXCITE.

Crankshaft meshing for AVL EXCITE.

With most Finite Element Software, the reduced matrices, model information, interior point modal vectors, and the full mass matrix are written to a number of different files. Special DMAP, macro’s, or scripts are needed to tell the Finite Element software to create these files After the files are generated, a translator is used to calculate the inertial invariants for moving parts and then generate the input data file for the MDB software. This calculation and translation can take hours for some models. The intermediate files can be many Gigabytes of data. With Altair OptiStruct, this step is not required. OptiStruct automatically calculates the inertial invariants and directly writes the MBD input data file directly. These are the .fbi file for SIMPACK and the .exb file for AVL EXCITE. In addition, no DMAP, macro, or scripts are required. The only input data needed is PARAM, SIMPACK for SIMPACK or PARAM, EXCEXB for ALV EXCITE.

The modal condensation time for OptiStruct can be reduced from many hours, or even days, with Lanczos to just 1 to 2 hours using the AMSES solver that is built into OptiStruct. The AMSES solver uses a multi-level sub-structuring technique to dramatically reduce this calculation time. AMSES is provided at no additional cost in OptiStruct and runs on both Linux and Windows.

The solver is so fast that these reductions can be done on a local workstation, or even laptop, rather than on an expensive server. Many Flexible bodies can be made up of more than one component, for example a crankshaft and flywheel. OptiStruct offers tied contact to connect these two components even if the meshes do not match at the interface.

Comparison between full and partial model surface results.

Comparison between full and partial model surface results.

For the second step of stress or strain recovery, most Finite Element software needs to recreate the modal stresses and strains, which can take hours. With OptiStruct, the modal stresses and strains are stored in the .h3d file during the Flexible Body creation run. Using these stored modal stresses and the modal participation coefficients from the MBD analysis, the element stresses and strains can be calculated in just a few seconds or minutes, as it is just a matrix-matrix multiply.

The integration of OptiStruct can reduce the time required for the entire engine dynamic analyses process from days to just a few hours by utilizing AMESE with fast Flexible Bodies generation, reduced matrices and inertial invariants with MBD input data file, and fast fatigue analysis calculation of element stresses or strains.

Additional Resources:

On-Demand Webinar: Powertrain NVH and Durability Analysis with HyperWorks.

Harold Thomas
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Harold Thomas

About Harold Thomas

Dr. Thomas is resposible for the business development direction of Altair’s solver product, OptiStruct. He has been working in the areas of Structural Optimization, Finite Element Analysis, and Computational Fluid Dynamics for more than 30 years. He studied Mechanical Engineering at the Columbia University in New York City and received his Ph.D. in Aeronautical Engineering from the University of California in Los Angeles (UCLA).