CHAIR OF

MULTIBODY

DYNAMICS

The chair deals with the holistic fluid-structure-interaction of dynamic systems, whereby the resulting vibration phenomena and excitation mechanisms are the focus of the analyses. For that purpose, a suitable description of the structure is necessary, whereby the usual methods of FEM or MBS can be used. The subsequent coupling with the fluid generally implies a description, wherein profound numerical and informational knowledge are necessary. Due to the diversity of technical applications, there are no generally valid interfaces and a problem-adapted description has to be implemented.

Dynamic FEM-simulation

The description of elastic structures is advantageously done using finite element simulations. Problem-adapted models can be used to investigate the static or dynamic system behaviour. In the context of the dynamic assessment of these structures, modal analyses can be performed, which describe the eigen behaviour of the structure and represent a fundamental step for further investigation of the system. Using suitable measuring technology, a comparison can be made between the model used and the real structure. In case of known external excitation, the vibration behaviour can also be determined by transient simulations, which do not only indicate critical frequencies but also allow the calculation of resulting vibration amplitudes. An essential aspect of these investigations is the description of non-linear properties (material behaviour, contact, bearing stiffness etc.), which must be integrated in an appropriate way. Due to the description form, large translational or rotational movements or the implementation of numerically stiff force elements lead to high computation times. This behaviour can be taken into account by using suitable reduction methods, which can significantly decrease the numerical effort. For the FEM simulations both commercial programs as well as in-house developments (especially under the focus of advanced reduction methods and the integration of elastic structures in MBS) are used. Contact

Eigen forms of a turbocharger housing				Vibrations of an oil pan

Multi-body simulation with rigid and elastic components

MBS algorithms are predestined for the description of systems, which perform large translations and/or rotations during operation. If, in addition to the pure rigid body properties, elastic deformations are also taken into account, a description on basis of FE-formulations is suitable, whereby a reduction of the degree of freedom is inevitable. As a result, the dynamic system behaviour can be analysed under the effect of any nonlinear force elements. Contact

MBS model of the e-mobility vehicle "Editha" of the OvGU				Deformation of an exhaust turbocharger during operation (scaling factor: 500)

Integration of non-linear bearing properties (journal bearings, roller bearings, air bearings etc.)

In addition to the mass properties, the system stiffness and damping define the dynamic behaviour. The bearing properties (journal bearings, roller bearings, air bearings) are of particular importance, as they are usually highly nonlinear. In general, the description is due to differential relations, which depend on the current kinematic state of the system. Based on this, specialised force elements can be created for MBS applications, which consider the nonlinear characteristics by solution of the differential equations and implement the resulting forces in the calculation. Corresponding algorithms have been developed in the context of various dissertations as well as funded research projects and are also part of the in-house developed MBS software EMD (homepage). Contact

Pressure development in the floating ring bearing of an exhaust turbocharger				Bearing element with contact force distribution				Crank drive with elastic con-rod

Holistic analysis of structure and support

The holistic description of the interaction of structure and support, in addition to the formal representation of the bearings as a non-linear spring-damper elements, also enables the inclusion of elastic deformations (global and local) in the system properties. Furthermore, the information on surface speeds can also be used to investigate acoustic issues, which make a significant contribution to sound reduction and thus to customer satisfaction. Contact

Approach for a holistic description of the acoustic behaviour of a crank drive (Source: Duvigneau et al. A holistic approach for the vibration and acoustic analysis of combustion engines including hydrodynamic interactions. Archive of Applied Mechanics, 2016. DOI 10.1007/s00419-016-1153-5)

Modelling of temperature development in support and structure

In addition to the influence of elastic deformation on the bearing properties and thus the vibration behaviour, further field problems and their interaction have to be taken into account. This initially refers to the temperature field, which causes a change in the dynamic viscosity and the clearance in the journal bearing. This leads to a change in the pressure build-up, which in turn influences the vibration behaviour and also the temperature development. The modelling of these interactions can only be done holistically and by taking into account the transient behaviour, which means that the aspects described above must be fully considered. In the context of rotor dynamics of turbochargers, this issue is the subject of the AiF-funded project Thermally extended rotor dynamics of turbochargers, IGF-No .: 18760. Contact

Holistic modelling approach

Interaction between airflow and structural vibrations

In addition to the consideration of non-linear bearing properties, further couplings between structural vibrations and the surrounding fluid can also be observed. In this context, the flow of impeller-wheel was investigated and a procedure was developed to determine the excitation of the structural vibration based on the pressure field. Finally, a one-sided CFD-FEM coupling was used, as the influence of the structural vibrations on the air flow could be neglected. In contrast, this reaction is not negligible for the investigation of self-balancing units. The behaviour of the liquid, which is used in fluid balancers or realizes the positioning of the balancing weights in ball-balancers, is dependent on the rotor dynamics, but also has a direct effect on the system dynamics. For the holistic analysis a direct coupling is therefore inevitable. Contact

Impeller-wheel PIV measurement				Impeller-wheel CFD-simulation				Impeller-wheel coupled structure simulation