As I said at the beggining of this thread, my intention was, and is, giving all interested members of this comunity a brief view around general hydrodynamics. A brief, but hope, useful view, able to be understood by everybody.
Many Open Rovers may have a deep knowledge about the subject, my intention is making the explanation for those having a little or no previous knowledge. Of course, and has far as my knlowledge could reach, im for entering any deeper question that could be of the interest of any of you.
NOTE: Many conceptual simplifications will be done and rough examples used. Dont hersitate asking for clarification or more accurate explanations.
OK. Lets go for the basics.
Whats a fluid ? Best way for starting a mental model is the Newton's concept. Yes, the same Sir Isaac Newton we all know.
Lets imagine matter builded from tiny "bricks" that are linked together. Yes, molecules or atoms. Their relative positions and distances are fixed. No "brick" is able to move with respect to the sorrounding ones. That's why its a SOLID. All constructions elements are fixed at their positions, a solid can only be moved as a whole.
What happens if that solid is broken in, lets say, two pieces ? We've got two solids, having each one the same properties than the original one.
From a mechanical point of view, that force, that holds all bricks together, may be said to have two main properties; strenght and range.
Suppose the case of glass, and suppose a glass bar subjected to a traction force. It will not elongate even a bit, but once a given force is achieved, it suddenly brakes with no prior elongation or distortion.
Lets now think on an steel bar. Once the stress starts to work, the bar starts to elongate. Elasticity comes into play. Steel will not get suddenly broken as glass did, but will get more and more distorted, until achieving the stress limit.
In the case of glass, the "bricks" linking force is very strong, but its range is very short. Just by taking the bricks a minimum distance appart, the force is exceeded.
In the case of steel, the required force may be lower, but .... the bar can be a lot elongated before braking. Its elasticity can be interpreted as if the "bricks" where linked by forces that have a very long range, which are able to keep "things" joined even through certain distances.
What about liquids ? The same bricks, joined by the same forces ...... but. Now, those building bricks mentioned at the begining, are linked in such a way, that are allowed to freely move, but always keeping their relative distances.
If all components can freely move, a liquid does not have to keep a given shape. In fact a liquid has no shape. It's shape will adapt to that of the container where it is kept.
But, as the distance between "bricks" must be always the same, the volume of the liquid will also be constant.
Think on real bricks. 50 bricks, put togheder in an orderly way, will have the same volume, no mind the resulting shape of the joint.
What if neither the "bricks" distances nor the location are holded ? Then, the resulting matter will be free to addopt any shape, like liquids, but also to addopt any volume. That's a gass.
Solid, liquid or gass, are nothing more than an expression of the degree of freedom of the matter components.
When the mechanical behaviour of a solid is studied from the engineer point of view, it is seen as a whole. Most calculations made around solids do not mind on the shape, but in general concepts like Center of Mass, material properties that are supposed to be the same through the whole body, moments and forces, punctual vectors ...... and all related most times to a given material quantity.
Solids most times, can be studied as units, as bodies that have a given mass, volume and shape. For example, only four numbers can define a solid motion, XYZ velocity vector and time stamp.
In the case of fluids, things become a lot more abstract, and hence, related maths a lot more complex. But ..... lets go slowly, math makes no sence if there is no concept to explain.
For an enclosed fluid, obliged to addapt to a container shape, things are easier, but ... what about open free fluids ?
When studying a fluid behaviour, no borders can be used most times. A moving fluid, the wind, the ocean, has no shape, no starting or ending point. Where are the borders of the water that sorrounds a sailing ship ? There is no defined mass, and hence, none of the properties that classic mechanics apply to solids.
Then, how to approach the fluids study ? It must be assumed that a fluid is a continuum, meaning that there are no defined limits, and hence, no defined dimensions, no begining and no ends.
The way to approach a fluid is the counter than approaching a solid.
While the solid is studied as whole, and how it is composed is a secondary question that can be deduced from the whole, fluids are studied from the components (the bricks) and the whole is deduced from the parts.
Studying how each "brick" behaves, and how each one, is influenced by all the others, allows for knowing how the whole fluid will behave.
NEXT: Free flow, internal forces.