But, if a field is fairly simple, you should be able to get some general feeling for whether your answer should be positive, negative or zero just by thinking about which way is my flow going.

但如果流速场很简单的话，大家理应有一种直觉，可以感受出来，结果应该为正为负，还是为零。

youdao

So zero is supposed to be neither positive nor negative, and here I am just simplifying and saying it's actually going to be negative.

所以0不是一个正数，也不是负数，在这里我只是简单地，说明它实际上会是负数。

youdao