因为平均流速不是由同一时间各测点流速取平均值得出的，而是由一段时间内各点的流速取平均值得到的。
The average flow velocity can not get from the average flow velocity of every monitoring point in the same time, but by a period of time.
因为平均流速不是由同一时间各测点流速取平均值得出的，而是由一段时间内各点的流速取平均值得到的。
The average flow velocity can not get from the average flow velocity of every monitoring point in the same time, but by a period of time.
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