1. Introduction
Tidal current power can be described as a highly dense, reliable, and predictable renewable energy source. It has been reported that an available tidal power of 95 and 61.3 TWH/year are available in the UK and China, respectively [
1,
2]. To make full use of the high flow velocity of the water’s surface in actual horizontal axis tide power stations, horizontal axis turbine is generally supported by floating type or fixed carriers from the bottom of the sea toward near the free surface [
3]. The turbine faces the effects of water cavitation, free surface, and velocity gradient caused by waves. The impact not only makes turbine power characteristics worse but also causes accumulation of interference load, fatigue, and turbine fracture or support structure, which can lead to major accidents.
When the horizontal axis tidal current energy turbine runs near the free-surface, the movement of the turbine causes the deformation of the wave surface and vice versa [
4,
5,
6,
7,
8]. The interacting of the turbine and the wave involves the unsteady and nonlinear interaction between the vortex, turbulence, and wave surface, which complicates the flow field around the turbine. Therefore, it is important to accurately predict the hydrodynamic performance of a horizontal axis turbine under wave conditions. Recently, great interest has developed for turbine dynamic loading as a result of wave-current interactions. Many experimental and numerical studies have investigated the dynamic behavior of scale model horizontal axis tidal current turbine performance immersed in towing and flume tanks, which highlights the strong influence of surface waves [
9,
10,
11,
12,
13,
14,
15,
16]. It was reported that the mean values of both power and thrust coefficients in the presence of waves were identical to cases of current conditions.
In 2007, Barltrop et al. [
9] studied the influence of waves on the tidal current turbines. In the towing pool, the 3-blade horizontal axis turbine was tested. The turbine radius was 0.16 m, airfoil S814, and maximum chord length was 66.5 mm. The average value of torque and thrust within a range of the turbine’s regular wave and Reynolds number range were measured, where the relative velocity on the turbine radius of 0.7 was tested. The results show that there was no difference between each parameter’s average value and the wave-free condition, yet the instantaneous value of thrust and torque obviously changed. In 2010, Gallway et al. [
10] also carried out similar model tests on the three-blade horizontal axis turbine in the towing basin. The turbine diameter was 0.5 m and the regular deep-water wave (wavelength/depth) was 0.4 m. These results are similar to Barltrop’s, who found that the instantaneous change of thrust is about 37% and torque 35%. In 2013, Benoit et al. [
11] tested a three-blade horizontal axis turbine with a diameter of 0.8 m in an experimental towing basin. The test results show that interaction between wave and current increases the load fluctuation amplitude, which must be considered in fatigue analysis. In 2014, Henriques et al. [
12] carried out a surface wave model test on a three-blade horizontal axis turbine in a high-speed circulating tank at the University of Liverpool, measuring the thrust and power of the turbine under two different regular wave conditions and comparing the measurement results with those under uniform flow (no wave) conditions. A similar conclusion was obtained: the average value of thrust and power coefficient of the turbine under regular wave condition was basically the same as the measured value of uniform flow, yet the instantaneous value had obvious periodic change, and the change frequency was consistent with the wave frequency. In recent years, similar hydrodynamic tests of wave turbines were carried out by Ethan [
13], Pascal [
14], and Mustafa [
15]. In 2019, Zhang Jisheng [
16] carried out a physical model test to explore the velocity change and turbulent characteristics of the wake field of a horizontal axis tidal current turbine under the interaction of waves and flows. Compared with the condition without waves, the existence of waves was conducive to the recovery of the water flow behind the supporting structure, yet it caused a bigger loss of near wake velocity behind the water area where the turbine blades rotated. The turbulence intensity decreased when the wave period increased and increased when the wave height increased. In 2013, Harbin Engineering University (HEU) successfully developed and installed the "Haineng II" 200 kW floating horizontal axis tidal current power station, whose operation on the sea shows that waves have a significant impact on turbine power characteristics and floating carrier motion.
Most previous numerical studies that used the blade-element momentum (BEM) model to evaluate the dynamic loading behavior of the horizontal axis tidal current turbine due to surface wave’s effects utilized linear wave theory to estimate the horizontal and vertical wave particle velocities [
14,
17,
18]. The main problem of linear wave theory is that the current effects on waves are not accounted for; the current velocity is simply superimposed to the horizontal wave particle velocities. On the other hand, the BEM model based on the lift and drag obtained from two-dimensional airfoil cannot be used to analyze complex three-dimensional flow effects, nor can it obtain detailed flow field information.
It can be seen from the above research that the hydrodynamic problem of horizontal axis turbine under wave-flow condition was mainly studied by a model test. Then, the variation trend of the hydrodynamic load of the turbine under wave-flow condition was obtained, but the specific rules about the influence of wave parameters on the load are not given. Therefore, in this article, based on the computational fluid dynamics (CFD) method, the hydrodynamic load was calculated within the horizontal axis turbine under wave-current conditions. The hydrodynamic loads decomposed, and the influence rules of wave parameters and the blade tip immersion depth on those loads were obtained. Further, we summarized the approximate fast prediction method about hydrodynamic load under wave-current condition. Lastly, we provided, the reference of hydrodynamic load forecast for the horizontal axis of the turbine under complex condition.
4. Load Decomposition Analysis
According to the time-history curve under the wave condition, the axial load coefficients and energy utilization ratios fluctuated in multiple frequencies based on the rotation frequency of the turbine and the wave frequency when the flow velocities were zero. Therefore, the axial load coefficient and energy utilization can be written as the following series:
Based on the principle of the least square method, according to Equations (7) and (8), the correlation coefficients were obtained by fitting the time-history curves of the axial load coefficients and energy utilization ratios.
To verify Equations (7) and (8),
Figure 9 shows the comparison of CFD calculation value and fitting curve. FIT-1 represents the fitting result when
. FIT-2 gives the fitting results when
and
. It can be seen that the fitting values of FIT-1 was basically the same as calculated values. Those of FIT-2 reflect the characteristics of the turbine loads, which showed that the coupling influence of the rotational and wave frequencies was small. The small difference appeared only near the peak point, so that the follow-up analysis was based on the FIT-2 fitting formula.
The corresponding expression of the FIT-2 curve in
Figure 9 is shown. Based on the characteristics of Equations (9) and (10), the rotation and wave frequencies are separated to obtain their respective influences on hydrodynamic loads, which can be convenient for the prediction of hydrodynamic loads.
4.1. Load Analysis Under the Different Blade Tip Immersion Depths
According to Equations (9) and (10), the correlation coefficients were obtained by fitting the time-history curves of the axial load coefficients and energy utilization ratios. When i equaled 2, the coefficients of axial load coefficients and energy utilization ratios series expanded under different blade tip immersion depths, as shown in
Table 2 and
Table 3.
In
Table 2 and
Table 3, with the increase of the blade tip immersion depth,
and
gradually increased, and the average value of axial load coefficient and energy utilization ratio gradually increased. The first-order item coefficient (
,
,
,
) was obviously greater than the second-order item coefficient (
,
,
,
) and the influence of the quadratic term can be ignored. The fluctuation amplitude (
,
) based on the wave frequency was obviously greater than the turbine rotation frequency fluctuation (
,
), and both decreased gradually with the increase of the blade tip immersion depth. However, the amplitude attenuation based on the rotation frequency fluctuation became more rapid. Therefore, under the wave condition, the axial load coefficient and energy utilization ratios of the turbine fluctuated over time based on rotation and wave frequencies. The fluctuation amplitude based on the wave frequency was significantly greater than the rotation frequency. With the increase of immersion depth, the fluctuation amplitude decreased gradually, but the time mean values of axial load coefficients and energy utilization ratios increased gradually.
4.2. Load Analysis under the Different Wave Periods
The coefficients of load coefficient and energy utilization ratio series expansion under different wave periods are shown in
Table 4 and
Table 5, respectively.
According to
Table 4 and
Table 5, when the wave period increased, the values of
and
hardly changed, and the average values of axial load coefficients and energy utilization ratios also hardly changed. The influence of the second-order term coefficient (
,
,
,
) was neglected. The fluctuation amplitudes (
,
) based on the rotation frequency of the turbine hardly changed with the increase of the wave period, while the fluctuation amplitude (
,
) based on the wave frequency increased with the increase of the wave period. The increase was obvious when the period was smaller because the wave frequency changes were obvious in the small wave period.
Figure 10 shows the change curve of
and
with wave frequency. As can be seen,
and
decreased linearly when wave frequency increased.
4.3. Load Analysis Under the Different Wave Heights
Table 6 and
Table 7 show the coefficients and load coefficient for the energy utilization ratio series under different wave heights.
When wave height increased, the average values of axial load coefficients and energy utilization ratios hardly changed. The influence of the second-order term coefficient (
,
,
,
) was neglected. The fluctuation amplitudes (
and
) based on the rotation frequency of the turbine hardly changed when wave height increased, while the fluctuation amplitudes (
and
) based on the wave frequency increased when wave height increased.
Figure 11 shows the variation curve of
and
with wave height.
and
increased linearly with wave heights.
5. Conclusions
In this paper, the hydrodynamic loads of a horizontal turbine under wave-flow condition were numerically calculated based on the CFD method. The research conclusions can be summarized as follows:
(1) Under the condition of wave flow, the instantaneous values of axial load coefficients and energy utilization ratios of the turbine produced multi-frequency fluctuations based on wave and rotation frequencies. The fluctuation amplitude based on wave frequency was obviously greater than those based on rotation frequency.
(2) With the increase of the blade tip immersion depth, the time-mean values of the axial load coefficients and energy utilization ratios gradually increased, but the increasing rates decreased. The fluctuation amplitude based on the wave and rotation frequencies both decreased rapidly.
(3) When the wave period increased, the time-mean values of axial load coefficients and energy utilization ratios hardly changed. The fluctuation amplitudes based on the rotation frequency hardly changed when the wave period increased, while the fluctuation amplitudes based on the wave frequency basically increased linearly.
(4) When wave height increased, the time-mean values of axial load coefficients, energy utilization ratios, and the fluctuation amplitudes based on rotation frequency, hardly changed and no obvious rules were found. The fluctuation amplitudes based on the wave frequency increased linearly when wave height increased.