There are 3 main laws that describe the flowing behavior of the fluid: the law of conservation of mass, the law of momentum, and the law of conservation of energy. This article only involves fundamental introduction to these 3 laws and will not involve too many math.

To begin with, let’s look at the conservation of mass. The conservation of mass states that the mass of the fluid will not change spontaneously during the flowing process. This is easy to understand, because the fluid molecules cannot be generated or be eliminated without any external factors. Therefore, the mass of fluid during the flowing process is constant.

Let’s look at an example. The figure below demonstrates an shrinking tube that has different cross-sectional areas on two ends.

According to the law of conservation of mass, the mass of fluid that flows into the tube in every second should equal to the mass of fluid that flows out of the tube in every second. Therefore, when fluid flows into this shrinking channel, it should accelerate. This can be seen from the result of simulation based on CFD.

Next, let’s look at the law of momentum. Momentum is the product of mass and velocity. The law of momentum states that the force leads to the change of the momentum of objects. If there is an external force acting on the fluid, the momentum of fluid will change; if the momentum of fluid change, then there must be a pair of interactive force acting on both fluid and the object that exert influence on fluid. In short, the change in the momentum of the fluid implies the generation of various forces acting on the object that is interacting with fluid. These forces include lift and drag.

One example that can illustrate the law of momentum is the interaction between jet and objects like ping pong ball where the jet hit on. Let’s examine one scenario. There is a jet, and there is a ping pong ball above the jet. The jet will hit the ball. Since jet cannot penetrate the ball, the fluid in the jet will decel when it hits the ball. This tells us that a force between the jet and the ball will be generated, and the ping pong ball can actually levitate in the air! Here is a video that can demonstrate this phenomenon.

Finally, let’s look at the conservation of energy in the flowing process.