Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction
Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction
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DOI：
10.1007/bf03181763
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发表时间：
2006-04
影响因子：
1.7
通讯作者：
C. Nguyen;J. Wells
中科院分区：
文献类型：
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作者：
C. Nguyen;J. Wells
在粒子图像测速（PIV）中，通常通过对测量的速度场进行微分来估计速度梯度，这放大了测量的速度中的噪声。如果在边界附近寻找梯度，由于示踪剂位移小、速度矢量有效位置的不确定性、示踪剂图案的强烈变形和激光反射，这种噪声通常大于体相流体。本文考虑一种改进的粒子图像畸变（PID）方法，直接计算固定壁面处的速度梯度，称为“PIV/IG”（“界面测量”）。从合成的二维PIV图像的结果表明，我们的方法实现了更高的信噪比和精度比速度微分。此外，我们已经开发了一个程序来重建三维速度梯度在一个固定的墙壁上的两个非零组件从PIV/IG数据中获得的立体视图;这些方程大大简化由于无滑移条件。明渠湍流底壁的实验数据似乎受到某种形式的像素锁定的影响。与标准PIV一样，这强调了图像中足够的示踪剂直径、足够的接种密度和相机传感器的动态范围的重要性。
Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field, which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectlycalculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic 2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor.