Aerodynamic analysis of TLP offshore wind turbine under wind and waves

Based on the real-scale NREL 5MW wind turbine as the analysis object, combined with the TLP floating platform, using the CFD software fluent and the Volume of Fluid (VOF) multiphase flow model, this paper simulates and analyzes the aerodynamic characteristics of the TLP floating offshore wind turbine under the wind and wave conditions by applying surge motion through UDF, and carries out an in-depth analysis of its power, axial thrust change, and blade pressure distribution. The power and axial thrust will fluctuate and change more violently as the amplitude and frequency of surge movement increase corresponding to waves. Low wave height, low amplitude, and low frequency are more obvious for local disturbance of power and axial thrust. After the surge motion is applied, the blade pressure at different times changes significantly. With the increase of amplitude, the pressure difference at the same time increases at the entire blade root, blade center, and blade tip, which focus mainly on the leading edge of the blade suction surface, and is most obvious at the blade root and least obvious at the blade tip. When the surge motion is not applied, with the increase of wave height, the front edge of the blade suction will obtain a greater negative pressure, and the pressure difference of each part will increase. The research results provide a technical reference and theoretical basis for the optimization design of the aerodynamic performance.


Introduction
The floating offshore wind turbine (FOWT) was first proposed by Professor Heronemus [1] of the Massachusetts Institute of Technology in 1972.He installed many small wind turbines on a large floating platform, which can be used as the prototype of the FOWT.Since the new century, the FOWT has gradually become a research hotspot in the field of wind power and marine engineering, with the United States, Europe, and Japan taking the lead in this field.Many research institutes, universities, and enterprises have successively invested in the research and development of FOWTs, and have achieved fruitful results.
Zuo [2,3] used ABAQUS finite element analysis software to conduct a numerical simulation of the NREL 5MW wind turbine under wind and wave loads, and analyzed its dynamic response.Oguz [4] conducted a free oscillation test, regular and irregular wave test and wind condition simulation for a 5 MW TLP FOWT at a water depth of 70 m.Tran and Kim [5] apply the unsteady BEM and use dynamic grid technology to conduct unsteady computational fluid dynamics simulation to analyze the pitch motion of FOWT.Wang [6] et al. used the free eddy current method to analyze the influence of surge frequency and amplitude.The results show that the average output power decreases first and then increases with the increase in frequency.
Based on the real scale NREL 5MW wind turbine as the analysis object, combined with the TLP floating platform, using the CFD software fluent, using the Volume of Fluid (VOF) multiphase flow model, and applying surge motion through UDF, this paper simulated and analyzed the aerodynamic characteristics of the TLP FOWT, studied the power and axial thrust changes of the TLP FOWT, and compared and analyzed the pressure distribution of the wind turbine blades.

Wind Turbine Model
Based on the 5MW wind turbine, the TLP NREL 5MW offshore wind turbine is established, and the Solidworks software is used to conduct full-size modeling of the complete wind turbine model including the platform and mooring line, as shown in Figure 1.

Flow Field and Boundary Conditions
Figure 2 shows the flow field calculation area and calculation boundary conditions.The sea area is a rectangle with a length of 10R and a width of 6.3R.The distance from the inlet of seawater and air velocity to the rotating region is 3R, and the distance from the pressure outlet to the rotating region is 7R.The area where the air domain and the rotation domain contact each other is set as the interface surface.The inlet of the seawater area is used to define seawater velocity inlet, the inlet of the air domain is set as air velocity inlet, and the outlet of the seawater domain and air domain is set as pressure outlet.In addition, the surrounding boundaries of the blade surface, air domain and seawater domain are used to define non-slip wall surfaces.

Computational Mesh
The seawater domain, air domain and internal rotation domain are meshed by using Fluent Meshing software.Figure 3 shows the grid distribution of the entire computing domain.Apply the boi function in Fluent Meshing software to refine the wave area.The surface grid of the model is divided by a tetrahedron grid, and the internal body grid is a hexahedron grid with higher calculation accuracy.The structural model of the TLP offshore floating wind turbine is very complex, and the blade tip and mooring line positions are too sharp.Appropriate grid sizes are selected and set for the platform, mooring line, tower and engine room wind turbine of the whole TLP sea water floating wind turbine, further refining the surface grid of the TLP FOWT.The whole flow field has a total of 658.6w grids, including 190.6w grids in the inner rotating region and 468w grids in the air and sea regions.characteristics under the combined action of wind and wave, VOF multiphase flow model is applied to the simulation calculation and the surge motion is added to simulate the more real working condition of TLP floating seawater wind turbine.The SST k-w turbulence model is used to solve the unsteady RANS equation.Considering the rotation of the wind wheel, the sliding grid method is used for simulation.In this paper, the 20s is calculated under the rated wind speed and small amplitude wave conditions, and then the UDF function of fluent is used.The DEFINE_PFORILE macro applies the method of additional velocity to convert the surge motion condition into the change of the wind speed inlet in the air domain, and the wave condition in the sea domain remains unchanged.Set each transient time step as 0.02s, the number of sub-iteration steps as 40 steps, and the simulation time after applying the surge motion as the 60s for the simulation solution.The main research content of this chapter is the aerodynamic analysis of the surge motion of the TLP FOWT under the action of wind and waves.The parameters of wave conditions and motion conditions are shown in Table 1

Numerical Model Verification
Set the rated wind speed at the speed inlet to 11.4m/s.The speed of the TLP FOWT is 12.1 r/min.The rotation time for four periods under unsteady conditions is calculated.The torque of the wind turbine is monitored.Figure 4 shows the overall torque of the TLP floating offshore wind turbine.When calculating 20 s, the torque of the TLP FOWT is stable at 4 ×10 6 N, due to the interference of the tower drum, when the wind turbine blade rotates 120° azimuth to the front of the tower drum, the power decreases significantly, and three fluctuations occur in a cycle.This phenomenon is consistent with the research conclusion of Wen [7] , which verifies the accuracy of the model.3 Aerodynamic analysis of TLP offshore wind turbine under wind and waves

Analysis of power and axial thrust affected by surge motion
When the wave height is H=2m and H=4m, the corresponding amplitude is A=2m, A=4m, and the frequency is f=0.1Hz,f=0.2Hz, respectively, the surge motion of TLP floating offshore wind turbine.The wave height, amplitude and frequency select the parameter values that are easy to appear under normal wave conditions.At the wave height of H=2m and H=4m, respectively, the surge motion with amplitude of A=2m, A=4m and frequency of f=0.1Hz and f=0.2Hz is applied, and the numerical simulation time of 40~60s is selected for power and axial thrust impact analysis.
It can be seen from Figure 5 that the local disturbance of low wave height, low amplitude and low frequency on the power and axial thrust of the TLP FOWT is more obvious when the wave generates a surge motion.In addition, from the overall curve, the power and axial thrust of the TLP floating offshore wind turbine will fluctuate more sharply as the amplitude and frequency of surge movement increase corresponding to the wave.This is because the corresponding induced speed will change more significantly as the amplitude and frequency of surge movement increase.Under the condition of high amplitude and high-frequency surge motion corresponding to wave height, the power change of TLP floating offshore wind turbine is more significant.No matter how the wave height, amplitude and frequency change, their average power values remain the same.Under the same frequency condition of a single degree of freedom surge motion, the high amplitude has a greater influence on power, and its sensitivity increases with the increase of frequency.Similar to the power change, the axial thrust change of the TLP FOWT is more significant under the condition of high amplitude and high-frequency surge motion corresponding to the wave height.At the same time, no matter how the wave height, amplitude and frequency change, their average axial thrust values are basically the same, indicating that the single degree of freedom surge motion with different amplitude and frequency has no effect on the average axial thrust.In addition, under the condition of the same frequency of the single-degree-of-freedom surge motion, the influence of high amplitude on the minimum value of axial thrust is greater than that of the maximum value, and its sensitivity increases with the increase of frequency.

Blade pressure distribution
Figure 6 shows the pressure distribution at the root (0.35), middle (0.67), and tip (0.95) of the blade at the time of T/2 and T under the conditions of wave height H=2m, H=4m and F=0.1s, H=2m, A=2m, H=4m and A=4m.It can be found that after the surge motion is applied, the pressure difference at different times changes significantly, and the pressure difference at T/2 is greater than that at T time.
With the increase of amplitude, the pressure difference at the same time increases at the entire blade root (0.35), blade center (0.67), and blade tip (0.95), mainly at the leading edge of the blade suction surface, and is most obvious at the blade root, and least obvious at the blade tip.At the same time, when the platform surge motion is not applied, with the increase of wave height, the front edge of the blade suction will obtain a greater negative pressure, and the pressure difference of each part will increase.On the whole, the pressure changes at the blade root (0.35), the blade center (0.67), and the blade tip (0.95) are not obvious.In addition, the pressure difference at the root (0.35), middle (0.67) and tip (0.95) of the blade with platform surge motion applied are greater than that of the blade without platform surge motion applied at the time of T/2; At moment T, the pressure difference at the blade root (0.35), blade center (0.67) and blade tip (0.95) with platform surge motion applied is less than that without platform surge motion applied.This is because T/2 and T are in the balance position of backward surge motion and forward surge motion, respectively.When the wind turbine tends backward surge motion, the blade will obtain greater pressure difference, and then the maximum power and maximum axial thrust will be obtained.When there is a tendency of forward surge motion, the wind turbine will obtain the minimum pressure difference, and then the minimum power and minimum axial thrust will be obtained.

Conclusions
The main conclusions are as follows: (1) The power and axial thrust of the TLP FOWT will fluctuate and change more sharply as the amplitude and frequency of surge movement increase corresponding to the wave.When waves generate a surge motion, the local disturbance of low wave height, low amplitude and low frequency on the power and axial thrust of the TLP FOWT are more obvious.At the same frequency of surge motion, the effect of high amplitude on the minimum value of axial thrust is greater than the maximum value, and its sensitivity increases with the increase of frequency.
(2) After the platform surge motion is applied, the blade pressure changes significantly at different times.With the increase of amplitude, the pressure difference increases at the same time at the entire blade root, blade center and blade tip, which focus mainly on the leading edge of the blade, and is most obvious at the blade root and least obvious at the blade tip.

Figure 1
Figure 1 Overall schematic diagram of TLP NREL 5MW offshore wind turbine

Figure 2
Figure 2 Schematic diagrams of the calculation model

Figure 3
Figure 3 Computational mesh

Figure 4
Figure 4 Schematic diagrams for torque monitoring of TLP floating offshore wind turbine under rated conditions

Figure 5
Figure 5 Power and axial thrust changes of TLP floating offshore wind turbine under different conditions

Figure 6
Figure 6 Pressure distribution of TLP floating offshore wind turbine blades at different sections T/2 and T time