Abstract
In this paper, we propose a supervised learning strategy for the fluid motion estimation problem (i.e., extracting the velocity fields from particle images). The purpose of this work is to design a convolutional neural network (CNN) for estimating dense motion field for particle image velocimetry (PIV), which allows to improve the computational efficiency without reducing the accuracy. First, the network model is developed based on FlowNetS, which is recently proposed for end-to-end optical flow estimation in the computer vision community. The input of the network is a particle image pair and the output is a velocity field with displacement vectors at every pixel. Second, a synthetic dataset of fluid flow images is generated to train the CNN model. To our knowledge, this is the first time a CNN has been used as a global motion estimator for particle image velocimetry. Experimental evaluations indicate that the trained CNN model can provide satisfactory results in both artificial and laboratory PIV images. The proposed estimator is also applied to the experiment of turbulent boundary layer. In addition, the computational efficiency of the CNN estimator is much superior to those of the traditional cross-correction and optical flow methods.
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The original FlowNetS receives RGB images as input, hence six channels in all are required. However, PIV images are typically created using monochrome cameras. Therefore, two-channel input for this CNN is also sufficient.
Available online: https://github.com/mrahrauld/PIVLab.
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Acknowledgements
This work was supported in part by the National Natural Science Foundation of China under Grant 61473253, the Foundation for Innovative Research Groups of the National Natural Science Foundation of China under Grant 61621002 and the Fundamental Research Funds for the Central Universities.
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A data sets of particle images for training
A data sets of particle images for training
As introduced in Sect. 3.2, in order to train the neural network, we generate a PIV dataset with more than 13 thousand items (where each item consists of an image pair and a ground truth motion field). The dataset includes various flow motion structures generated by computational fluid dynamics (CFD). Specifically, cases such as Uniform, Back-step, Cylinder are simulated by our own. DNS turbulence and surface quasi-geostrophic (SQG) cases are provided in Carlier (2005) and Resseguier et al. (2017), respectively. Finally, the Johns Hopkins Turbulence Databases can be found in Li et al. (2008), Perlman et al. (2007). Details of the dataset are given in Table 4. The data as well as the trained PIV-NetS model can be requested from the authors and will be accessed online soon.
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Cai, S., Zhou, S., Xu, C. et al. Dense motion estimation of particle images via a convolutional neural network. Exp Fluids 60, 73 (2019). https://doi.org/10.1007/s00348-019-2717-2
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DOI: https://doi.org/10.1007/s00348-019-2717-2