导流型垂直轴风力机内部流场数值模拟中若干问题的探讨

Some Aspects on Flow Field Simulation for Guiding VAWT

导流型垂直轴风力机是以在传统垂直轴风轮外增加一圈导流叶片而得名,其气动性能往往高于常规垂直轴风力机。导流型垂直轴风力机的绕流场是典型的内外流结合的多尺度、非定常、不稳定的三维漩涡流场。对这种复杂流场进行全三维、真正意义的非定常数值模拟,所需的计算量远远超出微机的现有能力,且其所需的计算时间也往往超出企业产品的设计周期。通过讨论在对该种风力机流场进行数值模拟时的两个问题,旨在寻求一种计算量与计算时间适中,并具有一定计算精度的计算模型:其一,在处理旋转风轮与静止导风轮之间的非定常计算信息传递时,采用计算量较小的多重参考坐标系法代替计算量极大的滑移网格法的可行性;其二,在数值模拟中,采用二维简化模型代替三维实际模型的可行性。研究结果表明:滑移网格法与多重参考坐标系法对导流型垂直轴风力机外特性的计算结果具有相当的一致性,然而对流场内特性的计算结果存在一定的差异,这种差异在风轮迎风面中表现地较明显;二维简化模型与三维实际模型的数值计算均能获得相似的外特性曲线与相同的最佳尖速比,然而其计算的外特性参数在数值上普遍低于对应的三维模型计算结果。就导流型垂直轴风力机流场的数值模拟而言,采用二维简化模型并配合多重参考坐标系法,不仅可以获得一定精度的计算结果,而且可大幅缩短计算量与计算时间,符合工业设计的要求,可适合于对导流型垂直轴风力机性能的初步评估。

Guiding vertical axis wind turbine（Guiding VAWT） is named after a guiding impeller surrounding a vertical axis wind impeller,which has higher aerodynamic performance than the traditional VAWTs.The flow field inside the Guiding VAWT is a multi-scale,unsteady,three-dimensional vortex one with the combined characteristics of both the internal flow and external flow.The unsteady,three-dimensional simulation of this complex flow field needs large amount of calculation not only far beyond the ability of current personal computer,but also hardly acceptable for the design period of enterprise＇s product.By discussing two aspects of the numerical simulation for the flow field inside the Guiding VAWT,efforts are put on finding a simplified model for the numerical simulation of the Guiding VAWT,which needs only moderate computing capability and time but can obtain enough precision.Two aspects are discussed in detail： One is the feasibility of the Multiple Reference Frame Approach needing less computing capability in place of the Sliding Mesh Approach needing larger computing capability during the delivery of the unsteady calculation information between the rotating wind impeller and the stationary guiding impeller;the other is the feasibility of the simplified two-dimensional model in place of the actual three-dimensional model.Results indicate that the total performance of Guiding VAWT simulated respectively by the Multiple Reference Frame Approach and Sliding Mesh Approach are almost identical but the flow field details are somewhat different,which appears obvious in the windward area of the wind impeller.Obtained respectively by the simplified two-dimensional model and the actual three-dimensional model,the total performance curves of the Guiding VAWT are similar with the same optimal tip speed ratio.But the total performance obtained by two-dimensional model is always lower in quantity than that obtained by three-dimensional model.As far as the tentative simulation of the flow field inside the Guiding VAWT is concerned,the simplified two-dimensional model with Multiple Reference Frame Approach can be adopted,which needs moderate calculating capability and time to obtain computation results with due precision to satisfy the requirement of industrial design.