Flow structure of the wake behind an elliptic cylinder close to a free surface

The flow fields around an elliptic cylinder of axis ratio AR=2 adjacent to a free surface were investigated experimentally using a water channel. The main objective is to understand the effect of the free surface on the flow structure in the near-wake. The flow fields were measured by varying the depth of cylinder submergence, for each experimental condition, 350 velocity fields were measured using a single-frame PIV system and ensemble-averaged to obtain the spatial distribution of turbulent statistics. For small submergence depths a large-scale eddy structure was observed in the near-wake, causing a reverse How near the free surface, downstream of the cylinder. As the depth of cylinder submergence was increased, the flow speed in the gap region between the upper surface of the cylinder and the free surface increased and formed a substantial jet flow. The general flow structure of the elliptic cylinder is similar to previous results for a circular cylinder submerged near to a free surface. However, the width of the wake and the angle of downward deflection of the shear layer developed from the lower surface of the elliptic cylinder are smaller than those for a circular cylinder.     

自由表面对小攻角翼型流动分离特性的影响

该文介绍了在实验段为1.7m ×0.4m×0.4m的小型风浪槽中进行的自由表面附近,小攻角翼型流场分离特性的PIV实验结果.实验测量了NACA0012翼型在10°攻角下与自由表面多种不同间距时,翼面附近速度场,而后将各间距下连续采集的约500副瞬态流场结果进行统计分析,获得了翼面时均速度场等信息,讨论了时均流场结构和速度分布随翼型与水面间距的变化规律及水面对翼面流场分离特性的影响.实验的雷诺数Re=5.7′104.实验结果表明,随着翼型与自由表面间距的不同,翼型表面流动结构和气流在翼型表面的分离结构发生明显的变化,其明显的变化特征为,随着翼型从无水面干扰的工况向水面接近的过程中,上表面的分离先逐渐增强,而后被抑制,该流动演化特性进而可引起作用于翼型的气动特性的变化.     

PIV MEASUREMENTS OF THE NEAR-WAKE FLOW OF AN AIRFOIL ABOVE A FREE SURFACE

The near-wake flow of a NACA0012 airfoils mounted above a water surface were experimentally studied in a wind/wave tunnel. The main objective of this study is to investigate the influence of the free surface on the structure of the airfoil trailing wake. The flow structure was measured with different ride heights between the airfoil and free surface using a Particle Image Velocimetry (PIV) system. The Reynolds number based on the chord length of the airfoil was about 3.5×103. For each experimental condition, large amount of instantaneous velocity fields were captured and ensemble-averaged to get the spatial distributions of mean velocity and mean vorticity, as well as turbulence statistics. The results show that the flow structures of the airfoil wake varies remarkably with the change in the ride height.     

Experimental analysis of flow fields inside intake heads of a vacuum cleaner

The flow structure inside the intake head greatly affects the working efficiency of a vacuum cleaner such as suction power and aero-acoustic noise In this study, the flow inside intake heads of a vacuum cleaner was investigated using qualitative flow visualization and quantitative PIV (Particle Image Velocimetry) techniques The aerodynamic power, suction efficiency and noise level of the intake heads were also measured In order to improve the performance of the vacuum cleaner, inner structure of the flow paths of the intake head, such as trench height and shape of connection chamber were modified. The flow structures of modified intake heads were compared with that of the original intake head The aero-acoustic noise caused by flow separation was reduced and the suction efficiency was also changed due to flow path modification of intake head In this paper, the variations of flow fields for different intake heads are presented and discussed together with results of aerodynamic power, suction efficiency and noise level     

近自由表面翼型尾流速度场的PIV测量及POD分析

翼型的流场结构对地效飞行器气动特性有着重要影响,针对这一问题,该文介绍了在风浪槽中进行的近自由表面翼型的流场PIV试验测量及其试验结果.实验分别测量了翼型与自由表面不同间距的多种工况下,零攻角NACA0012翼型尾流速度场随时间的演化过程,并将同一间隙条件下连续采集的瞬态速度场结果进行时均统计分析,获得了尾流的时均速度场等信息,用于对比分析翼型与自由表面不同间隙时的流场结构特征.实验结果表明,随着翼型与自由表面间距的不同,翼型尾流场的流动结构和气流在翼型表面的分离特性发生明显的变化,进而可能引起作用于翼型的气动力的变化.而后将本征正交分解POD (Proper Orthogonal Decomposition) 方法应用于PIV瞬态速度场的后处理,研究分析了翼型尾流各本征模态下的湍流结构.结果显示低阶本征模态中存在明显的相干结构,且大尺度的流动结构与流场中绝大部分的能量分布直接相关,而高阶本征模态代表了流动的小尺度结构.     

Evaluation of recursive PIV algorithm with correlation based correction method using various flow images

The hierarchical recursive local-correlation PIV algorithm with CBC (correlation based correction) method was employed to increase the spatial resolution of PIV results and to reduce error vectors. The performance of this new PIV algorithm was tested using synthetic images, PIV standard images of Visualization Society of Japan, real flows including ventilation flow inside a vehicle passenger compartment and wake behind a circular cylinder with riblet surface. As a result, most spurious vectors were suppressed by employing the CBC method, the hierarchical recursive correlation algorithm improved the sub-pixel accuracy of PIV results by decreasing the interrogation window size and increased spatial resolution significantly. However, with recursively decreasing of interrogation window size, the SNR (signal-to-noise ratio) in the correlation plane was decreased and number of spurious vectors was increased. Therefore, compromised determination of optimal interrogation window size is required for given flow images, the performance of recursive algorithm is also discussed from a viewpoint of recovery ratio and error ratio in the paper.     

EXPERIMENTAL STUDY OF FLOW STRUCTURE BEHIND A CIRCULAR CYLINDER WITH WAVY SURFACE BY PIV

1. INTRODUCTIONThe flow around a s mooth circular cylinderhas been extensively studied by many researchers ,as can be seen fromthe comprehensive reviews ofWilliamson[1]and Rockwell[2]. Wang et al .[3]in-vestigated the 3D flow structure behind a circularcy…     

近自由表面对称翼型气动特性的实验研究

本实验的目的主要以地效飞行器为背景,研究其中的空气动力学地面效应原理,是对地面效应的探索性基础实验研究。以NACA0012翼型为研究对象,在风水槽中对其进行了近水面的气动特性随攻角和间隙比变化的实验。分析了翼型表面压力系数随高度和随攻角变化时的分布规律。结果发现,随着高度的降低前驻点会沿着下翼面向下游移动;随着高度的降低和攻角的增加源于地面效应的下翼面高压区域向后缘扩展;在3°～5°的攻角范围内得到了有明显提高的升阻比,但是在间隙比h/c=0.15附近时会出现一次明显的升阻比减低现象。实验数据较真实的反应出近自由表面对称翼型的气动特性,可以很好的解释及印证地面效应的基本原理。