Aditya Khanna

Preventing Offshore Wind-Turbine Dynamic Failures

Vortices shed in the wake of a subsea power cable, apply fluctuating hydrodynamic forces, causing the cable to vibrate. If the vortex shedding frequency approaches the natural frequency of the cable, high amplitude vibration known as ‘Vortex-Induced Vibration’ or VIV occurs. VIV induces bending of the cable and sliding between the layers of the cable structure, often resulting in fatigue failure. While the research body on the modelling of the dynamic response and fatigue behaviour of power cables is mature, simultaneous modelling of these phenomena is still in its infancy.

The project aims to develop an analytical, simulation and experimental framework that can model the multibody dynamic response and resulting fatigue damage accumulation in a unified manner. The analytical approach supported by more complex simulation and experimentation is preferred for the modelling of complex nonlinear phenomena.

A multibody simulation model will be developed using FE and multibody simulation models in conjunction with a widely used global hydrodynamic model, Orcaflex. The analytical reduced order model, known as the wake oscillator model, will be utilised to predict VIV conditions efficiently in a wider range investigation. In this modelling approach, the fluctuating forces generated by vortex shedding are idealised by a nonlinear oscillator with a limit cycle. The structural motion interacts with the wake oscillator through a forcing term, forming a coupled system. The team at UQ have used a similar approach for prediction of Aeolian vibration in power lines, wind turbine flutter, brake squeal and railway wheel squeal. The developed modelling approach will advance previous studies by considering the nonlinear bending response of the helically wound power cable armour and conductors, which may improve the accuracy of fatigue damage calculations. A range of cable configurations will be considered, aimed at developing preventative guidelines against premature fatigue failures.

Investigation of Railway Studs and Squats

The research seeks to develop a generalised validated mathematical model for rail studs, and in particular to examine how this mechanism differs from that for rail squats. Railway studs and squats are track defects that grow via dynamic loading over successive train wheel passages. The model would be used to predict growth of studs and to evaluate and determine optimum railway vehicle and track conditions to mitigate this rail defect. An extensive experimental and field study would be used to validate the results.

Passive control of wind turbine tower vibration

Wind turbines, though designed to harvest wind energy, are also subjected to complex aerodynamic loads during operation. Studying the fluid-structure coupling, especially dynamic instabilities, remains one of the most important structural engineering issues for the wind energy industry. With an exponential growth in wind energy production, it is critical to continue improving the safety and availability of wind turbines, while avoiding unnecessary conservatism in their design.

Passive vibration control techniques, such as Tuned Mass Dampers (TMDs), are extensively utilised for controlling wind-induced vibration (and the resulting cyclic stresses) in tall structures. Distributed TMDs are a promising candidate for the suppression of multi-modal and multi-directional wind excitation within the tight space constraints of the wind turbine structure. This PhD project will develop theoretical and computational models of wind turbines with distributed TMDs as the means for passive vibration control. Methods for the efficient prediction of wind turbine tower aeroelastic excitations will be developed.

The project will perform the fundamental task of quantifying second-order aerodynamic effects that are currently ignored in design codes, while also developing a predictive modelling technique that is computationally efficient. Aerodynamic loads resulting from blade rotation, crosswinds, and, vortex shedding, are not considered in most dynamic models of wind turbines. In this PhD project, these complex aerodynamic loads will be quantified (experimentally and numerically) and coupled with lumped-parameter and finite-element models of the turbine.