伯努利原理

It is a fact that all things obey the conservation of energy, and fluids are no exception to that rule. For any fluid body, its component has 3 main parts that can be easily changed -- kinetic energy $\left(\frac{1}{2}mv^2\right)$, gravitational potential energy $\left(mgh\right)$ and potential energy stored as pressure $\left(PV\right)$.

Bernoulli's principle is a statement of conservation of energy for any non-compressible fluid. It says that an increase in the flow speed of the fluid on a levelled surface would lead to a drop in pressure. This can be easily seen via the conservation of energy.

Since the fluid is flowing on a levelled plane, we know that its gravitational potential energy stays constant. Since the sum of kinetic energy and potential energy stored as pressure is constant, an increase in one would mean a decrease in another, therefore, an increase in speed would lead to a direct decrease in pressure.