1. Fluid dynamics is the study of fluids in motion. It is one of the most complex branches of mechanics, as illustrated by such familiar examples of fluid flow as a river in flood or a swirling cloud of cigarette smoke. While each drop of water or each smoke particle is governed by Newton's laws of motion, the resulting equations can be exceedingly complex. So, many situations of practical importance can be represented by idealized models that are simple enough to permit detailed analysis.
When fluid is in motion, its flow can be characterized as being one of two main types. The flow is said to be steady, or laminar, if each particle of the fluid follows a smooth path, such that paths of different particles never cross each other. In steady flow, the velocity of fluid particles passing any point remains constant in time.
Because the motion of real fluids is very complex and not fully understood, we make some simplifying assumptions in our approach. In our model of ideal fluid flow, we make the following assumptions:
1. The fluid is non-viscous. Internal friction is neglected.
2. The flow is steady. The velocity at each point remains constant.
3. The fluid is incompressible. The density of the fluid is constant.  2.

3. Originally Posted by abdelwahab
Fluid dynamics is the study of fluids in motion. It is one of the most complex branches of mechanics, as illustrated by such familiar examples of fluid flow as a river in flood or a swirling cloud of cigarette smoke. While each drop of water or each smoke particle is governed by Newton's laws of motion, the resulting equations can be exceedingly complex. So, many situations of practical importance can be represented by idealized models that are simple enough to permit detailed analysis.
When fluid is in motion, its flow can be characterized as being one of two main types. The flow is said to be steady, or laminar, if each particle of the fluid follows a smooth path, such that paths of different particles never cross each other. In steady flow, the velocity of fluid particles passing any point remains constant in time.
Because the motion of real fluids is very complex and not fully understood, we make some simplifying assumptions in our approach. In our model of ideal fluid flow, we make the following assumptions:
1. The fluid is non-viscous. Internal friction is neglected.
2. The flow is steady. The velocity at each point remains constant.
3. The fluid is incompressible. The density of the fluid is constant.
I don't know who you think that "we" is, but the list of assumptions leads to the most simple models of fluid mechanics, models useful in some contexts but totally useless in others. For instance, in inviscid flow there can be no drag, and the flow is also irrotational, hence no eddies and no turbulennce.

Incompressible fluid flow is sometimes a usefule approximation, but in some applications, for instance rocketry, it is inappropriate.

BTW, do you have a point ?  Bookmarks
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