Jump bifurcation phenomenon during varying wind speeds in floating offshore wind turbines

Floating Offshore Wind Turbines (FOWTs) are susceptible to an instability which has come to be called negative damping. Conventional land based wind turbine controllers when used with FOWTs may cause large amplitude platform pitch oscillations. Most controllers have since been improved to reduce motions due to this phenomenon. In this paper, the motions induced using one of the original controllers is studied. The current study is performed using the coupled time domain program FAST-SIMDYN that was developed in Marine Dynamics Laboratory (MDL) at Texas A&M University. It is capable of studying large amplitude motions of Floating Offshore Wind Turbines. FOWTs use various controller algorithms of operation based on the available wind speed depending on various power output objectives i.e., to either maximize or level out power absorption. It is observed that the transition region for controllers is often chaotic. So most studies focus on operations away from the transition region below and above the transition wind speeds. Here we study the transition region using the theoretical insight of non-linear motion response of structures. This study reveals the presence of a very interesting and potentially hazardous nonlinear phenomenon, bifurcation. This finding could help explain the chaotic motion response that is observed in the transition region of controllers. Understanding the nature and cause of bifurcation could prove very useful for future design of FOWT controllers.

A Novel Method to Choose the Experimental Parameters in Large Amplitude Oscillatory Shear Rheology

Large Amplitude Oscillatory Shear (LAOS) rheology is a technique to analyze materials that are viscoelastic in nature. The raw values of stress and strain that were taken out from rheometer during the large amplitude oscillatory shear test are used in the constitutive models. The model parameters from the constitutive model are then analyzed on the materials being tested. Various test protocols and geometries will be used to analyze the materials of interest during LAOS rheological examination. The selection of test protocols and usage of geometry are less studied in testing various kinds of materials using LAOS. Cone and plate and parallel plate geometries are generally used for LAOS. The test protocols would be varying amplitude and varying frequencies. In the present work, quantification of relative performance of test protocols and geometry that have been used during the analysis of cross-linked poly vinyl alcohol hyaluronic acid (PVAHA) gels as material systems are studied using data envelopment analysis (DEA). The methodological approach using output oriented constant return to scale (CRS) and output oriented variable return to scale (VRS) are tested with the decision making units (DMU) as the geometry and test protocols used. These results are then combined with the Shannon's entropy to rank the efficient DMUs. Using Shannon's entropy combined with CRS and VRS, it is suggested that the use of parallel plate geometry with the test protocol of 0.5 rad/s and 50 frequency is best suitable for the cross-linked hyaluronic acid and poly vinyl alcohol gels examined.

Coloured-noise-induced transport in a model of the thermochemical reactor

In this paper, effects of coloured noise on the stochastic excitement in a model of the thermochemical flow reactor are studied. Transport phenomena associated with noise-induced generation of large-amplitude oscillations are investigated depending on the correlation time of coloured noise. We study how probability of the noise-induced excitement is related to the stochastic sensitivity of the system to coloured noise with certain correlation characteristics. Parameter zones of the high stochastic sensitivity are found and discussed in connection with occurrence of resonance. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’.