Determining the Influence of Casing Vibrational Behavior on Rotordynamics

Abstract

Casings of machinery and support structures have an influence on the rotordynamic behaviour which is commonly considered by simplified models (e.g. one degree of freedom models). These are in many cases insufficient. Hence, more accurate modelling approaches are required which can be used in the design process or the rotordynamic calculation to achieve a better representation of the overall vibrational behaviour. To study the effects of casing and supporting structures on rotordynamics, the casing modal parameters of an axial compressor are determined by an experimental modal analysis. In parallel, a numerical model is established. As experimental data are rarely found in the literature, this work focuses on the parameter identification of the casing structure. The results are subsequently incorporated into a model updating strategy, in order to tune and improve the numerical model. Experimental and numerical data are compared to assess the quality of the data and the results gained. The ultimate objective is a reduced order model, which can be integrated in existing rotordynamic tools via an interface while keeping the calculation time low.

Operational Modal Analysis of an Axial Compressor Rotor and Casing System for the Online Identification of a Digital Twin

“…According to the bearing manufacturer's calculation, the damping values in the bearing decrease with increasing speed. The damping values, which are close to zero for the operation case, are supported by the results from the Campbell diagram studied in [20]. Mode shapes seem to be more suitable considering the ambient influence, but their interpretation is much more time consuming and complex and thus requires further research, particularly when looking at rotating machinery.…”

“…The identified modal parameters from EMA are displayed in Table 1. More details can be found in [20]. The lower modes seem not to be strongly influenced by the rotor as the mass ratio between rotor and casing is approximately 1:17.…”

“…An example of a real mechanical engineering application is given in the following section using the axial compressor test rig at the Institute of Turbomachinery and Fluid-Dynamics (TFD) of Leibniz Universität Hannover. An analysis of the supposed structure was already performed by the authors of this work using experimental modal analysis (EMA) and standstill conditions to determine the casing influence on rotordynamics [20]. Since accessibility is not necessarily guaranteed on a big structure such as the axial compressor, the use and further qualification of OMA as an SHM tool is of great relevance.…”