CFD Simulation Of Flow Over Complex Terrain Under The Effect Of Tropical Cyclone

The southeast coastal area is abundant in wind resources with an annual average wind power density above200W/m2and a utilization hour of more than6000h. However, it is often subject to tropical cyclones which can seriously influence the wind farm construction and operation. In fact, the cyclones may be generally classified into benefit and destructive cases, respectively. The beneficial cyclone can bring high profit to the offshore wind farm while the destructive cyclone may result in a great loss to the wind developers.On the other hand, when air flows over mountains which are common in southeast coastal area, the complex terrain features and physiognomy will observably change wind speed and turbulence characteristic of boundary-layer flow which will have a powerful effect on micro-sitting and the load of wind turbine. Due to lack of well-documented field test data, little work has been done in the past. The design principle of wind turbines, IEC standard, recommends the S level in the tropical cyclone affected areas, design parameters specified by the designer. It is haphazard and hard to operate. In order to ensure the healthy development of wind power in China, it is very necessary to study wind field characteristics of complex terrain under the effect of tropical cyclone. This will have important theoretical and practical significance for wind resource assessment and micro-siting of wind turbine.First, based on the research work of meteorologists and the mean and fluctuating field test data of tropical cyclones supplied by China Meterological Administration in recent ten years, a mathematical model that can represent the flow characteristics in mainstream direction of the tropical cyclone is built. This paper carried out the numerical simulation of complex terrain under the effect of tropical cyclone and wind tunnel experiment for verification.A modified k-ε model is applied in the numerical simulation and some improvements are made to the parameters of the model. A user-defined wall function for the near-wall treatment was used. The most important point is to ensure that the key physical quantities such as velocity, turbulent kinetic energy and turbulent intensity, do not change when the fluid flowing through the computational domain.The steady equilibrium tropical cyclone atmospheric boundary layer is achieved by exerting wall shear stress. The wind tunnel experiment of typical complex terrain in the tropical cyclone boundary layer was carried out to verify the applicability of numerical models. The spire-roughness element method was used to simulate tropical cyclone boundary layer. In order to realize the simulation of high turbulence, we replaced the regular roughness element with inside hexagonal bolt creatively. Through comparison with the experiment data, the accuracy of the numerical model is proved.We focused on the two-dimensional isolated hill, two-dimensional continuous double hills and three-dimensional isolated hill to analyze the spatial distribution characteristics of the average velocity magnitude, speed-up ratio, turbulent kinetic energy and turbulent intensity.The main conclusions are as follows:For the two-dimensional steep hill under the effect of tropical cyclone, the mean velocity and turbulent intensity became the strongest around the hilltop and the downwind hillfoot, with the maximum speed-up ratio and turbulence intensity of1.4and0.25, respectively. Wind turbines should not be installed behind the downwind hillfoot in the distance of4.5times of the hill height. It shows that slope plays a crucial role in the flow separation by comparing flow field of different slope hills.For the two-dimensional continuous double hills under the effect of tropical cyclone, the choicest spot for wind turbines is the hilltop of the higher mountain. The maximum speed-up ratio of the two-dimensional continuous double hills got some reduction compared with the two dimensional isolated hill. The flow over lower hill was much affected by the nearby higher hill. The IEC recommended design principle underestimated the wind turbulence intensity in southeast coastal wind farms and we suggest that larger turbulence intensity should be adopted.For the three-dimensional isolated hill, the distribution of mean velocity and fluctuation is almost symmetrical in the spanwise direction. On both sides of the hill, the velocity distribution is better and the turbulence is small. The maximum wind speed value and the separation zone of the three-dimensional isolated hill are smaller than the two-dimensional isolated hill.