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A time-varying parameter model of a body oscillating in pitch |
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| Authors: | AP McCabe GA Aggidis TJ Stallard |
| Institution: | aRenewable Energy Research Group, Engineering Department, Lancaster University, Lancaster, LA1 4YR, UK bJoule Centre, School of Mechanical, Aerospace and Civil Engineering, University of Manchester, M60 1QD, UK |
| Abstract: | Among the assumptions upon which linear time-invariant models of floating bodies are based is that the body motions are so small that any change in the body’s angular position can be disregarded. However, it is often a major design requirement of a wave energy conversion device that the response amplitude is large, thereby invalidating one of the assumptions of the linear model. In particular, the immersed geometry of a body undergoes considerable variation when it is moved in pitch. With regard to this we investigate the difference in performance between a quasi-linear model in which the change of immersed surface is modelled by time-varying parameters and a basic linear model in which the immersed surface is time-invariant. The time-varying parameter model is realized by interpolation between the appropriate parameter values of a set of linear time-invariant (LTI) models derived for the different immersed surfaces that occur at discrete body displacements. It is shown that the responses predicted using the time-varying parameter model are closer to those measured experimentally than those of a standard frequency-domain model. Particular improvement occurs when the responses are large, such as at or near the resonance frequency. A problem which may limit the general use of the model is also discussed. |
| Keywords: | Time-varying parameter model Time-domain model Potential-flow hydrodynamics |
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