Propeller and vortex ring state for floating wind turbines during surge.
Lead Research Organisation:
Heriot-Watt University
Department Name: Sch of Engineering and Physical Science
Abstract
This is a PhD research project in mechanical engineering, more specifically in floating offshore wind turbine aerodynamics. The impact on the aerodynamic performance of the rotor as the platform moves in the wind direction will be investigated using computational fluid dynamics. The scenarios considered will be those with platform motion high enough to enter the turbine into propeller state and vortex ring state, two events that can lead to a significant reduction in the turbine's performance as a result of the turbine interacting with its own wake.
Organisations
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509474/1 | 01/10/2016 | 30/09/2021 | |||
| 1813026 | Studentship | EP/N509474/1 | 01/10/2016 | 31/03/2020 | Ryan Kyle |
| Description | The aim of this work was to investigate in more detail 'propeller state' for a floating offshore wind turbine when it moves with the waves in the wind direction. Propeller state is when the rotor of the wind turbine acts more like a propeller than a turbine and stops extracting energy from the wind; an occurrence which is of course detrimental to the performance of the turbine. The results have shown that these propeller-like conditions are the result of a reduction in the wind speed at the turbine as it moves with the wind, but whilst it rotates at the same speed, so that the air passes over the wind turbine blades at an angle (angle of attack) so low that it causes a negative force. This is opposite to normal operating conditions and the rotor will be forced to slow down its rotation and thus power output will be reduced. This will happen at blade locations closest to centre of the turbine, resulting in the most significant impact in this area. As the relative wind speed at the turbine drops further due to the movement of the turbine in the wind direction, progressive blade locations going out towards the tip will then also produce a negative force related to propeller state. This was shown quite clearly using computational fluid dynamics. A second aim of the work was to identify the cause of 'vortex ring state' (VRS), a phenomenon often connected to propeller state and first seen with helicopters in vertical descent. This is when the turbulent tip flow cannot be moved downwind like normal and then come into contact with the next passing blade, breaking down the aerodynamics of that blade and at the region of contact. In the work here, it was shown that VRS can happen when the duration the turbine spends moving quickly with the wind direction is similar to, or greater than, the time it takes for one blade to pass (120 degrees if you have three blades). If the wind speed at the rotor is zero or below, the vortex at the tip will not move downwind and the following blade does indeed strike that vortex as was again shown with computational fluid dynamics. As there is also a root vortex being formed at the centre of the rotor that cannot be moved downwind, it was also revealed that the vortex here builds up and engulfs the hub of the turbine. Although the chances of propeller state and VRS happening are only about 3% in UK waters (based on wave archive data), it is clear that when it does happen the rotor will be negatively affected and so these phenomena should be considered at the design phase. At the very least, operators should be aware of the consequences of each and be able to respond appropriately if it happens in practice to avoid any damage to the turbine. |
| Exploitation Route | The findings here can be used by those in the area of floating offshore wind energy systems to develop a potential remedy and thus allow for a further improvement in the performance of floating offshore wind turbines. For industrial applications, the results could highlight to operators the likelihood of this happening and then the appropriate mitigation strategy can be adopted when it becomes visible in the field, be it a reduction or full cessation of power output to prevent component damage. |
| Sectors | Energy |