The main objective of the presented study is an evaluation of the effectiveness of various methods for estimating statistics of rotor-shaft vibration responses. The computational effectiveness as well as the accuracy of statistical moment estimation are essential for efficient robust design optimization of the rotor-shaft systems. The compared methods include sampling techniques, the perturbation approach, the dimension reduction method and the polynomial chaos expansion method. For comparison, two problems of the rotor-shaft vibration analysis are considered: a typical single-span rotor-shaft of the eight-stage centrifugal compressor driven by the electric motor and a large multi-bearing rotor-shaft system of the steam turbo-generator. The most important reason for the observed scatter of the rotor-shaft vibration responses is the inherently random nature of residual unbalances as well as stiffness and damping properties of the journal bearings. A proper representation of these uncertain parameters leads to multidimensional stochastic models. It was found that methods that provide a satisfactory balance between the estimation accuracy and computational effectiveness are sampling techniques. On the other hand, methods based on Taylor series expansion in most of the analyzed cases fail to approximate the rotor-shaft response statistics.


Keywords: stochastic moment estimation, Latin hypercube sampling, polynomial chaos expansion, rotor- shaft system, lateral vibration analysis.

In this paper, we present finite element formulations for general three-dimensional convex polyhedra for use in a common finite element framework that are well suited, e.g., for modeling complex granular materials and for mesh refinements. Based on an universally applicable interpolant for any convex polyhedron, different interpolation schemes are investigated in the context of nonlinear elastostatics. The modeling benefits and the numerical performance regarding the mechanical response and the computational cost are analyzed by several examples.


Keywords: nonlinear finite elements, polyhedral elements, 3D interpolation, finite elasticity.

This paper proposes the wave based method for the steady-state dynamic analysis of the in-plane behaviour of 2D structural solids. This novel prediction technique relaxes the frequency limitations of the commonly used finite element method through an improved computational efficiency. This efficiency is obtained by selecting basis functions which satisfy the governing equations a priori, in accordance with the indirect Trefftz approach. Special attention is paid to problems in which singularities appear in the problem solution. For these problems, the conventional set of basis functions is extended with functions which can represent the singularity accurately. The capabilities of this novel method for mid-frequency applications, as compared to the standard finite element method, are demonstrated by means of two numerical examples.


Keywords: structural dynamics, wave based method, indirect Trefftz method, plate membrane, stress singularities, corner functions.

This work deals with the construction of a mixed and extensible domain decomposition method for incompressible flows. In the scheme proposed here, the solution is sought at the intersection of two spaces, one containing the solution of the Navier-Stokes equations considered separately in each subdomain, and the other one containing the solutions of the compatibility equations on the interfaces. A solution verifying all the equations of the two spaces is achieved iteratively. One difficulty is that the interface problem is large and dense. In order to reduce its cost while maintaining the extensibility of the method, we defined an interface macroproblem in terms of a few interface macro unknowns. The best option takes advantage of the incompressibility condition to prescribe an interface macroproblem which propagates the information to the whole domain by making use of only two unknowns per interface. Several examples are used to illustrate the main properties of the method.


Keywords: Navier-Stokes, Domain Decomposition Method, Multiscale Method.