Volumetric measurements of an adverse-pressure-gradient turbulent boundary layer using single-camera light-field PIV
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摘要: 作为一种新兴的体三维粒子图像测速技术，光场单相机三维粒子图像测速技术（Single-Camera Light-Field Particle Image Velocimetry，LF-PIV）能够仅用单个相机获得三维速度场，其结果已在许多复杂三维流动测量中得到验证。LF-PIV的优势主要在于其紧凑简便的硬件设备以及对光学窗口较宽松的要求。应用LF-PIV技术对一个自相似的逆压湍流边界层（Adverse Pressure Gradient Turbulent Boundary Layer，APG-TBL）进行测量，该实验在澳大利亚莫纳什大学（Monash University）航空航天与燃烧湍流研究实验室（Laboratory for Turbulence Research in Aerospace and Combustion，LTRAC）水洞中完成。实验对远、近壁面测量所得到的各600组瞬态三维流场数据进行分析验证，并与相同工况下的2D-PIV实验结果对比，证明基于DRT-MART重构技术的LF-PIV能够进行基本的湍流边界层测量。Abstract: As a novel volumetric particle image velocimetry technique, Single-Camera Light-Field PIV (LF-PIV) is able to reconstruct three-dimensional flow fields using a single camera. The merits of LF-PIV lie in its concise hardware setup and minimum optical access requirement. Its capability has been proved in many experimental investigations. In this study, LF-PIV is used to measure a self-similar Adverse Pressure Gradient Turbulent Boundary Layer (APG-TBL). Experiments are performed in a large water tunnel at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Monash University. Twenty independent batches of light-field PIV images are captured for both inner and outer flow, each consisting of 250 instantaneous image pairs. Instantaneous 3D velocity fields are reconstructed with the GPU accelerated DRT-MART and 3D cross-correlation methods and compared with two-dimensional PIV (2D-PIV) results. Preliminary results show that though limited by the experiment conditions and PIV algorithms developed in 2016, similar accuracies to 2D-PIV are achieved near and above the boundary layer. With the volumetric calibration method that compensates optical distortions caused by lens defect and misalignment between the Microlens Array (MLA) and image sensor, the resolution of LF-PIV is greatly improved.
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图 2 LTRAC实验水洞及LF-PIV对上下两层边界层测量实验示意图
Figure 2. Schematic of the LTRAC water tunnel and the LF-PIV experimental setup of the inner and outer layer measurements
图 3 (a) 激光光路实物图; (b)相机设置实物图; (c)粒子图像局部放大图。其中左上角红色六边形框表示微透镜的排布方式，框内为单个微透镜所成的像
Figure 3. (a) experimental setup of the laser path; (b) experimental setup of the LF-PIV system; (c) partially magnified particle image. The red hexagonal frame in the upper left corner indicates the arrangement of the micro lenses array, and the inside of the frame is the subimage formed by a single microlens
图 4 LF-PIV对湍流边界层测量的实验结果，图示结果为时均流场，将远、近壁面两组测量结果进行拼合后的展示
Figure 4. Overview of averaged volumetric velocity field of APG-TBL measured by LF-PIV and assembled with inner and outer layer data. The outer flow is in the positive x-direction, and the wall lies here on the right-hand side
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