Emre Dikmen

Influence of Multiphysical Effects on the Dynamics of High Speed Minirotors—Part I: Theory

In Part I of this work, a theoretical analysis showed that the surrounding air in the closed confinement between rotor and casing has a significant effect on the dynamic behavior of high speed minirotors. In order to validate the developed theoretical model, an experimental setup is designed and the dynamic behavior of the rotor with medium gap confinement is studied. The experimental setup has flexible supports, which consist of beams with adjustable length. The support stiffness is changed by altering the beam length. Modal analysis of the rotor is done in free-free conditions in order to test the capability of the rotordynamic model without the supports and multiphysical effects. The experimental and simulation results agree well with a difference of 1%. Then modal analysis of the whole structure is done at standstill and during operation in the absence of the casing. In this way, multiphysical effects are eliminated and only support effects on the dynamics of the structure are observed. The supports appear to have significant effect on the natural frequencies of the flexural modes of the system. Different support modeling techniques are studied and adequate equivalent models are obtained. These models are then implemented into the structural model of the rotor. Finally, multiphysical effects are tested at different speeds with different support stiffnesses. Experiments are performed with and without the casing for determining the change in the natural frequencies and onset of instability. The surrounding fluid has a significant effect on the stability of the system while the natural frequencies do not change significantly. The experimental and theoretical results are in fair agreement for predicting the natural frequencies and the onset of instability.

THERMAL MODELING OF A MINI ROTOR-STATOR SYSTEM

In this study the temperature increase and heat dissipation in the air gap of a cylindrical mini rotor stator system has been analyzed. A simple thermal model based on lumped parameter thermal networks has been developed. With this model the temperature dependent air properties for the fluid-rotor interaction models have been calculated. Next the complete system has also been modeled by using computational fluid dynamics (CFD) with Ansys-CFX and Ansys. The results have been compared and the capability of the thermal networks method to calculate the temperature of the air between the rotor and stator of a high speed micro rotor has been discussed.

Design of an Experimental Setup for Testing Multiphysical Effects on High Speed Mini Rotors

Recently, there have been numerous research projects on the development of minirotating machines. These machines mostly operate at speeds above the first critical speed and have special levitation systems. Besides, the multiphysical effects become significant in small scale. Therefore, advanced modeling approaches should be developed and innovative experimental rigs with the foregoing requirements should be constructed in order to test the developed techniques. In the current study, the design of an experimental setup for testing the multiphysical effects has been outlined. First, the previously developed multiphysical models (Dikmen, E., van der Hoogt, P., de Boer, A., and Aarts, R., 2010, “Influence of Multiphysical Effects on the Dynamics of High Speed Minirotors—Part I: Theory,” J. Vibr. Acoust., 132, p. 031010; Dikmen, E., van der Hoogt, P., de Boer, A., and Aarts, R., 2010, “Influence of Multiphysical Effects on the Dynamics of High Speed Minirotors—Part II: Results,” J. Vibr. Acoust., 132, p. 031011) for the analysis of small scale rotors are described briefly for background information. Second, an analysis of the effect of the rotor parameters (diameter, length, rotation speed, etc.) on the dynamics of the rotor under multiphysical effects is presented. Afterward the design process which includes the design decisions based on these results, the availability, simplicity, and applicability of each component is presented in detail. Finally, the experimental results have been presented and the efficiency of the design has been evaluated. In summary, the design requirements for an experimental setup for testing multiphysical effects on minirotors have been analyzed. The design procedure and evaluation of the design have been presented.

Flow Induced Instability on High Speed Mini Rotors in Laminar Flow

In this study, a modeling approach is developed to examine laminar flow effects on the rotordynamic behavior of high-speed mini rotating machinery with a moderate flow confinement. The existing research work mostly focuses on the flow-induced forces in small gap systems, such as bearings and seals, in which the flow is mostly laminar and inertia effects are ignored. In other studies, medium gap systems are analyzed, taking the inertia effects into consideration, but the surrounding flow is considered as turbulent. However, in high speed mini rotating machinery, the large clearances and the high speeds make the inertia effects significant, even in the laminar flow regime. In the current study, the flow-induced forces resulting from the surrounding fluid are analyzed and these models are combined with the structural finite element (FE) models for determining the rotordynamic behavior. The structure is analyzed with finite elements based on Timoshenko beam theory. Flow-induced forces, which include inertia effects, are implemented into the structure as added mass-stiffness-damping at each node in the fluid confinement. The shear stress is modeled with empirical and analytical friction coefficients, and the stability, critical speeds, and vibration response of the rotor is investigated for different friction models. In order to validate the developed modeling approach, experiments were conducted on a specially designed setup at different support properties. By comparing the experiments with the theoretical models, the applicability of the different friction models are examined. It was found that the dynamic behavior is estimated better with empirical friction models compared to using the analytical friction models.

A flexible rotor on flexible supports: modeling & experiments

In this study, a flexible rotor with variable support stiffness has been analyzed. Simple support models consisting of mass, spring systems are extracted from modal analysis of the isolated support and by applying static loads to the finite element model of the supports. The derived equivalent models of the supports are then implemented in the finite element based structural model which predicts the dynamic behavior of the rotor. Finally experimental modal analysis of the rotor is performed with different support stiffnesses. The experimental and theoretical results have been compared and different support modeling approaches have been examined.