A combined method is proposed to determine the water entry acceleration at a low impact velocity through image processing. The procedure includes: (1) a sequence of images for water impact are recorded by a high speed camera, (2) the sub-pixel image processing method is employed to calculate the displacement with an accuracy on the “sub-pixel” level, (3) the acceleration of the object is acquired by differentiating the displacement twice and with results being further filtered by a carefully designed low-pass Butterworth filter. A theoretically based analysis is conducted for designing the parameters of the low-pass filters. It is shown that the water entry can be regarded as a procedure with a slowly changing velocity. The method is validated with the standard sinusoidal motion and the water entry of a sphere. This approach could be considered as an auxiliary method during the early-stage study of the water entry, and it could be further applied to some complicated circumstances, like the water entry of spinning spheres.

中文翻译：

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水冲击力测量的组合数据处理方法

提出了一种通过图像处理来确定低冲击速度下的入水加速度的组合方法。该过程包括：（1）用高速相机记录一系列水冲击图像；（2）使用子像素图像处理方法以“子像素”级别的精度计算位移； （3）通过对位移进行两次微分来获取物体的加速度，并通过精心设计的低通巴特沃斯滤波器进一步过滤结果。进行基于理论的分析以设计低通滤波器的参数。结果表明，入水可以看作是速度缓慢变化的过程。该方法已通过标准正弦运动和球体的进水验证。