Numerical investigations on dynamic stall of low Reynolds number flow around oscillating airfoils

This paper presents a 2D computational investigation on the dynamic stall phenomenon associated with unsteady flow around the NACA0012 airfoil at low Reynolds number (Re_c ≈ 10⁵). Two sets of oscillating patterns with different frequencies, mean oscillating angles and amplitudes are numerically simulated using Computational Fluid Dynamics (CFD), and the results obtained are validated against the corresponding published experimental data. It is concluded that the CFD prediction captures well the vortex-shedding predominated flow structure which is experimentally obtained and the results quantitatively agree well with the experimental data, except when the blade is at a very high angle of attack.     

Hybrid LES/RANS modelling of free surface flow through vegetation

Vegetated channels are environmentally friendly and frequently used to convey water for drainage and recreational purposes. The design and assessment of these channels often requires the use of numerical models which are based on the Reynolds Averaged Navier-Stokes (RANS) approach or Large Eddy Simulations (LES). It is well accepted that both approaches have their advantages and disadvantages. To overcome these disadvantages a hybrid model combining the RANS and LES methodologies is proposed in this work. The major task for the model development is to couple the RANS and the LES models effectively. Various methods have been investigated and the results are as follows. At the inflow boundary of the computational domain, a semi-analytical velocity profile for submerged vegetation is used as the RANS inflow condition to shorten the unrealistic flow transition region. At the interface of the upstream RANS region and the downstream LES region, turbulence fluctuations are artificially generated using a spectral line processor, with the mean velocity determined by using the frozen cloud assumption. At the interface of the upstream LES region and the downstream RANS region, a virtual momentum sink is imposed to dissipate the sub-grid scale fluctuations and to shorten the transition region. The final model has been verified against experiments of flow through submerged and emergent vegetation, as well as a partly vegetated channel.     

基于矩形NAM和偏微分方程的灰度图像压缩方法

借助矩形NAM图像表示方法和偏微分方程技术,提出一种新的灰度图像压缩方法.该方法在编码端把原始图像逆布局为若干矩形子块,并采用坐标压缩方法存储每个子块的位置、大小.在解码端,采用偏微分方程进行图像修复,有效消除解码图像中的方块效应.该方法的时间复杂度为 ,其中n为灰度图像像素数.实验表明:在保持图像质量的前提下,该方法具有更高的压缩比和更少的块数,具有较高实用价值.     

灰色理论在古陶瓷断代中的应用

利用模式识别方法可以推测古陶瓷出产年代。将古陶瓷中各化学成分的含量组成一个向量,并与该向量的灰色模型GM(1, 1)参数构成新向量,用于描述古陶瓷样品。以灰色关联度为距离定义,使用近邻法对古陶瓷样品年代进行预测,其留一法预测精度达79.55%。实验结果验证了灰色理论可以用于古陶瓷断代。     

基于听觉感知模型的多通道语音增强系统

提出以模拟听觉感知模型的非均匀滤波器组为基础的多通道语音增强系统,与基于均匀滤波器组的语音增强系统相比,该方法达到相同频率分辨率所需的通道数较少。采用Itakura-Saito距离对系统进行客观评价,仿真结果表明,该系统增强后的语音比均匀多通道系统增强后的语音具有更好的改善效果。     

基于核密度估计的活动轮廓模型

基于核密度估计的活动轮廓模型如果没有适当的扰动机制,往往不能在弧度突变的边缘上获得较好的收敛结果,且在大噪声环境下鲁棒性较差。针对该问题,提出一个新的代价函数。该函数通过融合边缘映射的曲率信息,改善原算法在突变边缘的收敛效果,降低算法对初始轮廓的依赖。     

一种医学图像的轮廓提取方法

针对医学图像的模糊性和灰度不均导致目标轮廓难以准确提取的问题,提出使用改进的遗传算法控制主动轮廓模型完成边界提取的方法。采用保优算子保留遗传性状,选择适当的交叉算子,在进化后期可实现由整体寻优到局部寻优的转变。实验结果证明,该方法在提取目标轮廓时抗模糊能力强、鲁棒性好。     

无线传感器网络S-MAC协议性能研究

提出一种二维Markov链模型用于S-MAC协议的性能分析与评价。该模型利用信号流图和母函数的方法分析S-MAC协议的介质访问控制(MAC)服务时延,结合M/G/1/K排队模型分析S-MAC协议在非饱和负载下的性能。仿真实验和数值分析结果证明该模型可以较好地描述S-MAC的接入方式。     

一种改进的Chord网络模型

经典的Chord模型中为维护Chord环路由信息而周期性执行的Stabilize操作产生了大量的消息转发。提出一种改进的Chord模型,通过使用优化的双向Finger表,使得只在节点加入或离开时才进行路由更新,降低了消息转发的开销,同时优化了路由定位算法。     

带移动节点的无线传感器网络时间同步方法

为提高无线传感器网络中节点间时间同步的精度并且降低节点能量消耗,提出了一种基于移动节点与静态节点混合网络的时间同步方法.先建立一个带移动的节点的分簇网络结构,再结合发送者一接受者与改进的发送者一发送者两种同步模型从多个移动节点同时发起同步.通过数学分析和仿真得出该同步方法在同步时间精度和节点节能方面具有良好的性能.