Rotational sampling in wind turbines: Difference between revisions

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The loads on both [[horizontal-axis wind turbine]]s (HAWTs) and [[vertical-axis wind turbine]]s (VAWTs) are cyclic; that is, the [[thrust]] and [[torque]] acting on the blades is dependentdepend on where the blade is. In a horizontal axis wind turbine, both the apparent wind speed seen by the blade and the [[angle of attack]] dependdepends on the blade's position of the blade. This phenomenon is described as [[Rotational sampling in wind turbines|rotational sampling]]. This article will provide an insight into the cyclic nature of the loads that arise because of rotational sampling for a horizontal axis wind turbine.

Rotational sampling can be divided into two parts: [[Deterministic system|deterministic]] and [[Stochastic process|stochastic]]. Deterministic processes present themselves as spikes on a [[Power spectral density|power spectrum]], whereas stochastic processes are broader, i.e., spread over a wider frequency range.

{{Further|Wind turbine design}}

==Background==

[[Analysis]] of the loads on a [[wind turbine]] can be carried out through the use of power spectra. A power spectrum is defined as the power [[spectral density]] function of a signal plotted against frequency. The power spectral density function of a plot is defined as the [[Fourier transform]] of the [[covariance function]].<ref>Remote sensing: models and methods for image processing, R. a. Schowengerdt</ref><ref>Remote Sensing: Models and Methods for Image Processing, Robert A. Schowengerd</ref> Regarding the analysis of loads, the analysisit involves [[time series]], in which case the covariance function becomes the [[autocovariance]] function. In the signal processing sense, the autocovariance can be related to the [[autocorrelation]] function.

==Deterministic processes==

===Sources of deterministic processes===

Upon completing a single revolution, a blade has produced an ever-changing [[torque]], and so power. Some of these changes are due to deterministic processes, i.e., processes that can be determined and do not require statistical methods. Examples of deterministic processes are listed below:

* Gravitational loading