Peter van der Hoogt

Power harvesting in a helicopter rotor using a piezo stack in the lag damper

A piezoelectrically augmented helicopter lag damper has been simulated for the purpose of harvesting electrical energy within the rotor of the aircraft. This energy can then be consumed locally for sensing, processing, and transmission of data to the cockpit. An 8.15-m radius rotor is considered, and in-plane rigid lagging motion forms the prime excitation of the damper. The piezoelectric stack is installed within the rod of the damper in such a manner that the stack is submitted to all damper loads. MATLAB and Simulink are used to simulate a simplified blade model. A number of electrical harvesting circuits are investigated, and the piezo stack is optimized for each circuit. Also the effect of nonlinear capacitance of the piezo material is investigated revealing a profound effect. The important design parameters are identified and optimized resulting in a power output of 5.1 W for a steady 130-knot forward flight profile.

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.

Life assessment by fracture mechanics analysis and damage monitoring technique on combustion liners

A methodology has been developed and tested including a multi-disciplinary framework towards integrated analysis of gas turbine combustors. The sub-elements consist of combustion dynamics, stress and modal analysis, fracture mechanics and structural health monitoring have been interlinked indicating the damage evaluation to life assessment. The interaction between the interrelated combustion driven flame dynamics, acoustic pressure fluctuations and liner wall vibration has been investigated in the laboratory combustor test system. During the operation, the combustion, acoustics and wall vibrations have been coupled together. The dynamic combustion process generates high amplitude pressure oscillations resulting in vibration of the liner structure at about constant elevated temperature in base load operation. The thermo-acoustic instabilities have a significant destructive impact on the life of the liner material due to high cyclic vibration levels at high temperature. A structural health monitoring (SHM) method has been established to identify the damage, detect the flaw existence and determine the location, severity and progress of the damage for the combustion liners. Vibration-based and acoustic emission (AE) techniques have been applied in the test system to assess the structural behavior. The applicability of the technique has been tested by examining the dynamic modal parameters of the structure. The method enables a reliable assessment on the liner specimen at elevated temperatures by means of non-destructive evaluation under continuous operation of the combustor. The combustion liner specimen material has been assessed by calculating the near-tip fields at the crack tip by finite element based stress and fracture mechanics analysis. An algorithm based on J-Integral has been utilized to analyze the crack growth behavior under various loading conditions considering both linear and non-linear elastic fracture mechanics concepts. The location and the direction of the cracking on the liner specimen have been predicted. The presented work interrelates the different mechanisms in gas turbine combustors and the applicability of the concepts has been verified and validated in the test systems.

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.

Sensitivity of Combustion Driven Structural Dynamics and Damage to Thermo-Acoustic Instability: Combustion-Acoustics-Vibration

The dynamic combustion process generates high amplitude pressure oscillations due to the thermo-acoustic instabilities, which are excited within the gas turbine. The combustion instabilities have a significant destructive impact on the life of the liner material due to the high cyclic vibration amplitudes at elevated temperatures. This paper presents a methodology developed for mechanical integrity analysis relevant to gas turbine combustors and the results of an investigation of combustion-acoustics-vibration interaction by means of structural dynamics. In this investigation, the combustion dynamics was found to be very sensitive to the thermal power of the system and the air-fuel ratio of the mixture that feed into the combustor. The unstable combustion caused a dominant pressure peak at a characteristic frequency, which is the first acoustic eigenfrequency of the system. Besides, the higher-harmonics of this peak were generated over a wide frequency-band. The frequencies of the higher-harmonics were observed to be close to the structural eigenfrequencies of the system. The structural integrity of both the intact and damaged test specimens mounted to the combustor were monitored by vibration-based and thermal-based techniques during the combustion operation. The flexibility method was found to be accurate to detect, localize and identify the damage. Furthermore, a temperature increase was observed around the damage due to the hot gas leakage from the combustor that can induce detrimental thermal stresses to consume the lifetime.

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.