The dynamic behaviour of rotor-bearing systems represents an ongoing field of research.

By increasing the rotational speed of the rotor an instability can be detected which is often referred to as "oil-whirl" or "half-frequency-whirling" in literature. As the frequency of this "whirling" instability meets an eigenfrequency of the associated elastic rotor, its oscillation amplitudes increase tremendously which is also known as "oil-whip". These "oil-whirl" and "oil-whip" effects can be rated as rather critical and should be avoided during the operation of the rotor-bearing system.
Various modifications (compared to the "classical" cylindrical bearing) have been proposed in literature in order to suppress or at least to decrease these unwanted effects. By modifying the shape of the bearing sleeves improvements of the rotor's dynamic behaviour are expected. Starting from an initially circular shape, the bearing sleeve is elastically deformed by piezoelectric actuators which leads to a complex fluid-solid-interaction.

The effects of the piezo actuated shaping on the rotor-bearing system are studied by means of systematic stability and bifurcation analysis, focusing on time-efficient modelling methods.

Contact: Prof. W. Seemann, A. Bitner

Systems with friction are widespread in all kinds of applications. Unfortunatelly the precise simulative prediction of the resulting friction force in frictional contacts is still an unsolved task which has led to the developement of many empirical friction laws. Despite their great value for many practical and theoretical applications, a deeper insight into the actual contact situation and the corresponding friction force with its dependencies is necessary to improve technical systems e.g. with regard to energy efficiency and wear.

Early works on this topic by Greenwood & Williamson and Archard for the purely elastic and Bowden & Tabor for the purely plastic deformation case have at least led to a justification for Coulombs friction law. Their research efforts indicate that the real contact area is almost proportional to the normal contact force whereat this relation can be attributed to surface roughness in both load cases. By the additional assumption that the friction force is proportional to the real contact area Coulombs friction law can be justified.

Further investigations on the dry contact of two sliding metallic bodies depending on various physical parameters and sliding speed are performed. For this purpose, a thermomechanical model is developed and evaluated for different contact configurations considering the surface roughness of both contact bodies in particular. Subsequent investigations on the consequences of the calculated friction coefficient in the context of friction-induced vibrations are carried out.

Contact: Prof. C. Proppe, L. Oestringer

Radial compressor in turbochargers is often considered in theory as periodic system, but in fact it features inevitable small imperfections caused by material defects and manufacturing tolerances which break the cyclic periodicity. This is called mistuning. The loss of periodicity changes drastically the dynamic behavior of the compressor. Typically the forced response level of the mistuned bladed disk is larger than the tuned design. Because of the random nature of mistuning, the determination of the largest resonant response at any frequency has to be considered as a stochastic problem.

Mistuning receives significant attention from the research community since the late 1960s. Models using coupled lumped mass oscillators have allowed the fundamental phenomena of mistuning to be understood. More recently finite element model are used to explore with a better precision the behavior of mistuned compressor. To minimize the computational costs the finite element model has to be reduced first before performing a Monte Carlo Analysis. In the last decade several model reduction methods were developed. The way in which mistuning is implemented depends on the used reduction method and the vast part of them accounts only for a frequency mistuning model or a proportional mistuning model, in which the ideal tuned configuration is not modified.

Today a key topic is to combine this reduction methods with a more realistic introduction of mistuning, such as geometric mistuning. In other words, it will be tried to take directly random geometry modifications into consideration in a stochastic problem.

Contact: Prof. C. Proppe, M. Koebele

Surrogate modeling is a method that can be applied if quantities of interest cannot be easily directly measured or simulated, e.g. if a simulation run or experiment is very expensive. In this case, a surrogate model for the outcome is obtained and used instead. Due to increasing complexity of models, surrogate modeling is playing an increasing role in various engineering, but also other scientific disciplines. In this work, a meteorological problem is analyzed in collaboration with the Institute of Meteorology and Climate Research - Department Troposphere Research at KIT.

The West African monsoon is a major wind system that affects regions between latitudes 9° and 20° N. The monsoon is the result of the seasonal shifts of the ITCZ (Intertropical Convergence Zone) and seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean. The forecast of this monsoon has shown to suffer from remarkable uncertainties in several quantities. Some of these quantities are the local rainfall and the north-south shift which have a great impact on the inhabitants, particularly on the agriculture. In order to quantify the uncertainty in the forecast of the West African monsoon, a sensitivity analysis for a range of uncertain parameters in the weather model is conducted.

For this study the ICON model which is operationally used by the Deutscher Wetterdienst (DWD) is applied to carry out weather simulations. Since for the analysis a steady state for the monsoon quantities over simulated days is intended, the simulated time and thus the computational cost for one simulation run is very high. Therefore, surrogate models seem to be a promising opportunity. In this work, Gaussian Process Surrogates are used to achieve a relation between input parameters and monsoon quantities. The surrogate model is then used to carry out a global sensitivity analysis given defined ranges and probability density functions for all parameters. The results can offer an indication which parameter definitions should be specified more detailed by conducting further studies in order to reduce the uncertainty in forecasted monsoon quantities. Furthermore, the surrogate model can serve as a basis for parameter identification studies.

Contact: Prof. C. Proppe, M. Fischer