Structural Dynamics

Researcher:

Dr Jie Yuan

Workpackage 3 (WP3) of SYSDYMAT involves modelling and design of dry film lubricated dovetail fan blade roots. Fan blade systems in turbofans provide the main thrust of modern jet engines. Dry film lubricant is widely used in dovetail joints to mitigate fretting fatigue problems and potentially to produce more frictional damping. This project aims to explore an improved fan blade root design to obtain dynamic performance gains through novel dovetail interface geometries with a lubricated coating.

The specific objectives are:

1. To develop FE modelling of dry film lubricant coated blade roots.
2. To validate the FE model using the DOGBONE testing rig.
3. To perform simplified model-based design optimization for an improved design.
4. To verify the optimal design both numerically and experimentally.

Recent highlights include

1. The effects of lubricated coatings on contact parameters have been investigated using imperial 1D frictional rig. The results show that the dry film lubricant can significantly reduce the friction coefficient. The roughness of surfaces plays a key role in sustaining the coating on the interface.
2. A highly efficient FE model has been developed for dynamic analysis of fan blade systems with dovetail joints, thanks to recently proposed reduced order modelling techniques. The method can improve the computational speed by more 100 times when compared to classical computational techniques.

Postdoc:

Fabian Hualca 

The aim of the ARIAS project is to advance the current understanding of the physics in seal aeroelasticity. Experimental validation of analytical methods will support future engine certification and enable the design of more efficient and quieter engines.

The specific objectives are:

1. To design and construct a unique static seal flutter rig facility.
2. To conduct a series of static rig tests which will quantify and rank the effects of design parameters on flutter stability and understand their interdependencies.
3. To develop methods to predict seal flutter occurrence and to completely model the required multi-physics effects that play a part in seal flutter.

To perform validation of the approach in a full rotating rig (at CTA).

Recent Highlights:

Designing a novel and unique seal flutter rig comes with its own challenges. For example; it must operate inside an acoustic chamber, be able to accommodate different seal geometries and cavity volumes, have a reversible flow configuration, allow for variable seal tip gaps, and should be able to accommodate all the necessary instrumentation. All the above challenges are bounded by the size of the acoustic chamber, flow conditions required, and budget. The main features of the seal flutter rig include; two inlets for reversibility of flow direction, variable cavity volume, a conical test section to vary the seal tip gap, and different seal geometries.

Researcher:

Giuliano Tuzzi

Modern jet engine designs are more susceptible to shaft-disc-blade dynamic interaction, since the component frequency ranges are closer. This project aims to understand, model and validate experimentally shaft-disc-blade mode-coupling in aero engines.

The specific objectives are:

To understand all the physical phenomena involved in shaft-disk-blade mode-coupling.

To model and predict mode shapes and response of rotating assemblies with mode-coupling.

To investigate causes and features of a particular shaft bending to disc umbrella coupled-mode observed during an engine development test.

To validate experimentally the numerical results using the ARES rig.

Recent Highlights:

1. A previously unknown coupling behaviour of flexible shafts with flexible discs has been identified.  In this case, asymmetry in the supporting structure leads to a coupling between axial and bending modes of the shaft, which in turn couple with zero nodal diameters (0ND) and 1ND modes of the disc. The resulting modes are a superposition of several shaft and disc modes. This approach has then been extended by considering the gyroscopic effect, multiple discs and tuned and mistuned bladed discs, highlighting a highly complicated and fully-coupled response of the system.
2. In parallel, a real asymmetrical supporting structure has been designed and manufactured in order to validate experimentally these results on the ARES rig.
3. Further developments of this project include experimental testing of different configurations with flexible discs/bliscs in order to validate the theoretical models.

Postdoc:

Michal Szydlowski  

The aim of Work Package 3 (WP3) of the Cornerstone project to increase confidence in rotor condition monitoring, so that data acquired form working engines and outcome of its analysis can influence future design choices.  In addition a working system could support a pilot’s decisions during flight.

The specific objectives are:

1. Installing and instrumenting a representative engine geometry with a dense sensor and exciter network
2. Developing data analysis techniques in order to reduce and analyse the data acquired and to highlight significant events.
3. Develop methods to interpret highly reduced data sets from a representative engine setup in order to provide improved engine condition monitoring.

Recent Highlights:

A concept of a test rig (HERMES), based on a GNOME engine, has been developed and is currently at the detail design stage. To overcome the problems of large-scale data acquisition, a hardware-software signal acquisition architecture, based on a network of specialized data capture and processing devices, has been developed. The system is designed to meet the requirements of vibration testing in a research environment, with a specific focus on reasonable cost, flexibility and scalability. Rather than using a single, powerful and expensive system that ‘does it all’, the system was designed so the data capture and processing loads are distributed between several nodes of a network, where each node consists of a low-cost data acquisition system (DAQ). Compression, intelligent data reduction and analysis methods are being developed and will be implemented on the test rig as part of the acquisition and processing network.

Researcher:

Luke Muscutt

Workpackage 3 of the multi-university Cornerstone project aims to investigate complex vibrational phenomena of a real engine using a comprehensive sensor array.  This data will be used to determine which sensors are most informative on a real engine.

Specific objectives are:

• To build an experimental test rig consisting of a Gnome helicopter engine housed in a vacuum chamber to reduce aerodynamics effects. The whole setup will be mounted on a seismic block to provide vibrational isolation from the surroundings.
• To heavily-instrument the engine with various sensor types to provide significant amounts of data from the different parts of the engine, including the casing, bearings, blades, and rotor.
• To create an advanced data acquisition and control system to run the rig.
• To use advanced processing tools to identify vibrational events.

 Recent highlights:

1. The design process for the new test facility is nearly completed, and large components of the rig are currently being manufactured.  The engine will be housed inside a large low pressure chamber, which will eliminate aero-elastic interaction, and allow the focus to be purely on the structural dynamic response.
2. One of the two available Gnome engines has been stripped to bearing level, and after small design modifications it will be reassembled with a low level of instrumentation to ensure safe initial operation.

Researcher:

Ali Tatar

The main motivation of this PhD research project is to investigate the impact of including a planetary gearbox on the global dynamics of rotating systems.

 The specific objectives are:

1. To understand the coupled dynamic behaviour of a rotating system with a planetary gearbox.
2. To investigate how planetary gearbox parameters influence the global dynamics of a rotating system.
3. To develop a rotor dynamics software package (GEAROT) for predicting the dynamic response and analysing the dynamic behaviour of the planetary-geared rotor system.

Recent Highlights:

1. Development of a six degree of freedom dynamic model of a planetary geared rotor system.
2. Identifying the vibration modes of planetary geared rotor systems with and without gyroscopic effects and analysing the impact of the planetary gearbox on the vibration modes of the global rotor system using the modal energy analysis.
3. Investigating the effects of the gearbox parameters on the modal behaviour of planetary geared rotor systems and determining the influential gearbox parameters on the global dynamics of the rotor system.
4. Designing a geared rotor test rig by coupling two shafts with a planetary gearbox to carry out the experimental modal analysis.
5. Validation of the numerical computations of the developed hybrid dynamic model against the experimental modal analysis results.

Post-doc:

Dr Aykut Tamer

Reliability of the engine auxiliary pipe system is critical to reducing maintenance costs and engine downtime. A major threat to pipe system reliability is vibrations, which should be kept at mild levels. Effective damping of pipe vibrations is therefore essential in order to support increasing jet-engine performance.

The specific objectives are:

• To develop numerical methods for damping assessment for complex pipe systems.
• To validate the methods through experiments.
• To evaluate of novel pipe support designs using the validated method.

 Recent highlights:

1. A numerical method based on strain energy has been developed.
2. The method has been applied to straight and curved pipes resting on rubber supports.
3. Experiments have shown satisfactory correlation between measurements and simulations.

Demonstration of the method on a multi-pipe network is in progress.

Researcher:

Daniel Heller

Current Blade Tip-Timing (BTT) seems to have reached its limits and a novel approach to measuring blade vibration is required. The target of this work is to introduce a BTT application and analysis methodology which has not previously been pursued.

The specific objectives are:

• To undertake a literature review of the state-of-the-art for BTT.
• To determine multi-harmonic and multi-mode vibrations occurring simultaneously.
• To detect non-linear vibration responses arising from excitation and aerodynamic instabilities.
• To ascertain axial, radial, and tangential blade positions during operation.

 Recent highlights:

Blade Tip-Timing (BTT) and clearance sensor waveform analysis (BLASMA) is a recently patented methodology for extracting blade vibration parameters from sensor waveforms. At first, waveforms are generated when rotating and vibrating blades pass beneath a sensor that is mounted on the casing. The dynamic pattern of vibratory blade motion leads to a modulation of the waveform that is different from the imaginary and constant waveform modulation due to purely rotating rigid blades. Such waveforms can serve for vibration analysis with the following inexhaustive list of methods:

1. BLASMA-SCP uses signal characteristics
2. BLASMA-IWE is based on the inverse of the waveform equation
3. BLASMA-OPT employs an optimisation routine.

Researcher:

Dr Ye Yuan

Frictional joints, such as Under Platform Dampers in aero-engines, show very poor repeatability during reassembly, leading to large uncertainty in their dynamic predictions. In this project a robust design strategy will be developed to ensure more repeatable joint performance.

Specific objectives are:

• To develop a methodology, which for the first time employs a surrogate model to optimise the geometric configuration of UPDs for robust damping performance.
• To identify novel UPD designs that provide good, and repeatable damping performance.
• To validate the designs against experimental data from the UPD test rig at Imperial.

 Recent highlights:

1. The objective of this project is to develop a methodology, which for the first time, employs a surrogate model to optimise the geometric configuration of UPD for robust damping performance. The objective function takes into account the variations of geometric configuration due to manufacturing tolerance, which could significantly alter the dynamic behaviour of the blade.
2. A surrogate model has the advantage of yielding a satisfactory solution with relative few function counts, since it is a computationally cheap approximation of the expensive objective functions evaluated at sample points. This surrogate approach is used to guide the search for improved solutions at untried points/configurations.
3. The expected end result is a damper design that may not provide as much damping as current highly optimised designs, but it will provide damping consistently, leading to an more robust overall damping performance.

Researcher:  

Alfredo Fantetti

Friction is a major source of uncertainty for the correct prediction of the dynamic response of complex jointed structures such as aeroengines. The aim of the project is to improve the understanding of friction and understand how it affects the global dynamics of structures.

The specific objectives are:

1. To understand the physics of contact interactions by measuring friction hysteresis loops with a 1D friction rig.
2. To measure the main contact parameters needed as input for nonlinear dynamics simulations with friction.
3. To understand how friction at contacting interfaces affects the global dynamics of the system.

Recent highlights:

1. A matrix of hysteresis loops has been measured for varying normal loads, sliding distances, excitation frequencies and nominal areas of contact on the two friction rigs built at Imperial College London and Politecnico di Torino.
2. Ultrasound measurements were also performed in combination with hysteresis loop measurements in order to gain insights from both measuring techniques.
3. Dynamic substructuring has been performed on the friction rig model to generate insights on the relationship between friction and dynamics.
4. Numerical Modelling has been performed using a number of techniques including finite element, boundary element and harmonic balance solvers.
5. The Imperial College friction rig has been upgraded to measure the normal contact stiffness.

Researcher:

Alex Haslam

This project focuses on how vibrations are transmitted between the different rotors within modern jet engines. The rolling-element bearings which support the rotors are complex to model, making this phenomenon difficult to predict in simulation.

The specific objectives are:

1. To develop a lightweight bearing model which can accurately predict the vibration transmissibility.
2. To use this bearing model to predict the response of rotor-bearing systems in an efficient manner.
3. To investigate experimentally the vibration transmission on a customised rotor-bearing test rig.
4. To validate the modelling approach by comparing the experimental results with simulation.

Recent highlights:

1. In order to first investigate the relevant physical mechanisms, an in-depth parameter study has been carried out, focussing on the response of a simplified rotor-bearing system. A framework for efficiently simulating the response of such systems was developed in the process.
2. Two rigs have been used to experimentally validate the modelling approach. Initially, a static loading test rig was designed and built, in order to measure the stiffness of a bearing. These results were found to match the predictions from the model very well. Additionally, a more complex rotor-bearing test rig, consisting of a flexible shaft with two balancing discs supported by two ball bearings, was assembled and commissioned. Piezo-electric actuators have been connected to the front bearing, which can replicate the excitation from a second rotor.