利用顶部柔膜颤振抑制三维方形棱柱气动力

通过风洞实验研究了高宽比H/d=5的正方形棱柱顶部柔性薄膜颤振对柱体气动力特性的影响规律.模型宽度为40 mm,来流风速4～20 m/s,对应Reynolds数为10960～54800.柔性薄膜为厚度0.04 mm的高压聚乙烯膜,长度l=0.5～4 cm不等.实验发现,柔性薄膜的振动状态对三维方柱气动力特性有显著影响....     

表面粗糙度对双曲冷却塔风压分布的影响

对不同粗糙条参数组合下冷却塔风压分布进行测试,分析粗糙度对平均风压和脉动风压的影响.研究结果表明:粗糙条宽度b对风压分布影响较小,而粗糙条高度k和数量n影响较大;粗糙度系数k/s能较准确描述粗糙度大小,当k/s一致时,不同粗糙条数量n与高度k组合下的平均风压基本一致,但高度大的脉动风压小;粗糙度对正压分布和背压大小影响较小,而最小负压系数幅值、脉动风压系数峰值随粗糙度的增大而减小,背压稳定区宽度则略有扩大;尽管通过增大模型表面粗糙度能有效地在较低雷诺数条件下实现高雷诺数下的平均风压分布,但脉动风压模拟的准确性需要通过大型冷却塔风压实测进行验证.     

湍流两相流的脉动速度联合PDF输运方程

概率密度函数（PDF）的方法是构造两相湍流模型的一种重要的方法.构建气体-颗粒速度联合PDF输运方程的关键是颗粒所见气体微团速度的Langevin方程.首先由N-S方程出发,精确推导出颗粒所见气体微团脉动速度的Langevin方程,进而通过理论分析表明,对比通常采用的颗粒所见气体微团瞬时速度的Langevin方程而言,采用前者能有效地减少关联量的统计偏差.最后,给出了颗粒-气体脉动速度的联合PDF输运方程.     

基于PSO-BP神经网络的隧道内气动压力幅值预测

将BP神经网络技术用于隧道内气动压力变化幅值的预测,使用粒子群算法(particle swarm optimization,PSO)对其进行优化,构建PSO-BP神经网络模型。为了验证模型的准确性和可靠性,利用收集到的数据样本对模型进行训练测试,并引入交叉验证法评估2种模型的性能。研究结果表明：PSO-BP神经网络能够准确预测不同情况下的气动压力幅值,而且在平均相对误差、平均绝对误差、均方根误差以及样本的决定系数等方面均比未优化的BP神经网络的好,具有更高的预测精度。通过建立的PSO-BP压力幅值预测模型,得到了压力幅值在不同条件下的变化规律。     

Scalar Statistics along Inertial Particle Trajectory in Isotropic Turbulence

The statistics of a passive scalar along inertial particle trajectory in homogeneous isotropic turbulence with a mean scalar gradient is investigated by using direct numerical simulation. We are interested in the influence of particle inertia on such statistics, which is crucial for further understanding and development of models in non-isothermal gas-particle flows. The results show that the scalar variance along particle trajectory decreases with the increasing particle inertia firstly; when the particle＇s Stokes number St is less than 1.0, it reaches the minimal value when St is around 1.0, then it increases if St increases further. However, the scalar dissipation rate along the particle trajectory shows completely contrasting behavior in comparison with the scalar variance. The mechanical-to-thermal time scale ratios averaged along particle, （r）p, are approximately two times smaller than that computed in the Eulerian frame r, and stay at nearly 1.77 with a weak dependence on particle inertia. In addition, the correlations between scalar dissipation and flow structure characteristics along particle trajectories, such as strain and vorticity, are also computed, and they reach their maximum and minimum, 0.31 and 0.25, respectively, when St is around 1.0.     

有限长正方形棱柱绕流的双稳态现象

运用大涡模拟(large eddy simulation, LES) 方法对有限长正方形棱柱绕流进行了研究. 棱柱高宽比为5,一端固定于平板上,另一端为自由端. 平板表面边界层厚度可忽略不计. 基于自由流速度和棱柱宽度的雷诺数为3 900. 由计算结果发现,有限长棱柱的尾流在自由端后下扫流的作用下具有显著的三维性. 尾流中会交替出现两种典型的流动状态:一种是柱体两侧所脱落的涡在尾流中形成类似卡门涡街的交错排列状态;另一种是柱体尾流中涡团对称分布且无明显的周期性. 两种典型流动状态的交替出现,对有限长棱柱气动力特性有直接的影响.