BASIC ESSENTIALS ON MECHANICS OF THE FLUIDS (1 Part)

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The Mechanics of Fluids is the branch of the science that studies the balance and the movement of the fluids, this is, liquids and gases. In the fluids, there can take place a relative movement of the molecules or atoms that is a part of the internal structure so much in movement as at rest, situation that never takes place in the solid ones.

The mechanics of fluids can split into two differentiated parts. The first one of them is the one that studies, basically, the movement of fluids that circulates along a concrete trajectory, in that the typical phenomenon is his transport. In this type of circulation of fluids, these circulate canalized along the interior of conductions or river beds, and by it it is named an internal flow. It is a basic science in all the engineerings. When the fluid object of study is the water, the part of the mechanics of fluids that studies his movement is the Hydraulics.

The second part into which the mechanics of fluids splits is when these circulate, instead of for the interior of conductions, to turn in a set of solid particles, being named an external flow, since instead of circulating the fluid for the interior of the solid one (a conduction), it is the fluid the one that wraps the whole exterior surface of the solid ones.

following lover of the knowledge commented on Them, In chemical technology, the knowledge of the external flow of fluids is necessary thinking that applies himself in multitude of basic operations typical of the chemical industry, as sedimentation, filtration. Also, it turns out to be basic in the treatment of all the operations involve transmission of heat and transference of matter, that is to say, in all the so-called basic operations of the Chemical Engineering. The external flow of fluids has been developed historically by the Chemical Engineering, from what it is a branch of the mechanics of fluids of special importance.

basic Aspects.
Tackling the definition we have that, In flow of fluids, it performs vital importance to know the pressure since with his knowledge it can be controlled and measure itself the flow. Since the pressure, according to his definition, is the normal force exercised on a surface, to measure the pressure it will be necessary to insert a probe in the point where the pressure wants to be known, it sounds that it consists of exhibiting a section. They can be considered to be three definitions of pressure as the way of measuring the same one, that is to say, since the measuring probe of the pressure is placed:

• static Pressure: It is the pressure exercised by the fluid on a plane parallel to the direction of the current, due to the shocks of the molecules as consequence of a chance movement (p). For a fluid in movement the static pressure must measure oneself to the section of the probe parallel to the movement of the fluid. In case of flown at rest, there is no difference in how the probe of pressure is placed.

• Pressure of impact or of shock or of stagnation: It is the pressure exercised by the fluid on a plane perpendicular to the direction of the current, due to the shocks of the molecules for the chance movement and the movement of the fluid (p 1/2pv ²), being p the thickness of the fluid and v the module of the punctual speed of the fluid). Therefore, the probe will have to have the section perpendicular and faced up the direction of the current.

• kinetic, dynamic Pressure or of speed: It is the difference between the pressures of impact and statics (1/2pv ²), which will be void in case of flown at rest.

In this image we have the static pressure and of impact in a fluid in movement; also it is possible to tackle pressure according to other different causes like the probe of measurement is inserted, as absolute pressure, hydrostatics, manometric (Image prepared for @centaurox)

The pressure hydrostatics to the pressure that exercises the gravitational weight of the portion of liquid placed over the measuring probe, being really a difference of pressure between two points. Nevertheless, this pressure hydrostatics is despicable in case of gases. Also other types of pressure can be defined according to the team to which it measures itself, that will be seen later in the section dedicated to the teams of measurement.

Base in the units of pressure.
The unit of pressure is the Pascal (Pa Ξ 1 N/m2), it is more frequent to express the pressure in kPa or MPa. Also the use of bar is very frequent, atm and kg/cm2, units similar and practically equivalent all of them (1 bar Ξ 105 Pa, 1 atm Ξ 1.013 bar, 1 bar Ξ 1.02 kg/cm2), and does not forget the unit of pressure of the English system psi (pound per square inch, 1bar = 14.50 psi).

Dynamics of pressures of service
The static pressures of the fluids during his flow change very much according to the circumstances. The pressure of the water in the networks of supply of the cities ranges between 2 bar and 7 bar, while the pressure of aspiration of the bombs is usually next to the atmospheric one, that of discharge can overcome 100 bar. The gaseous flows usually qualify like of fall, average or high pressure, according to which the pressure of the circulating gas is lower than 1.2 bar, between 1.2 and 3.5 bar or between 3.5 and 100 bar, respectively. All these pressures are attainable for the compressors of several stages.

Taking in account in such a way that pressures superior to the stated ones are applied, one speaks about technology of high pressures, being able to include in same industrial processes as important as the syntheses of the ammonia and of the methanol and many polymerizations, hidrogenaciones.

Instruments of measurement of pressures.
According to the team that is used for the measurement of the pressure in a point, it is necessary to differ between absolute pressure (when the team measures the entire pressure) or pressure sobreatmosférica or manometric. (It differs from pressure with regard to the atmospheric one). In the ambience of the earth's crust, the air is exercising a pressure continues, from what often it is considered to be a positive pressure to pressures superior to the atmospheric one, and gap or negative pressure to the low ones. Nevertheless, speaking in terms of absolute pressure, impossible termodinámicamente is the existence of negative pressures.

The teams that measure the atmospheric pressure are named barometers, and of them, there are barometers that measure the atmospheric absolute pressure (columns of liquid, like that of Torricelli), and barometers that measure the atmospheric pressure with regard to other one of reference with the one that was calibrated (measured to column of fluid). Actually, the absolute pressure only can measure oneself to column barometers of fluid. All the rest barometers and other meters of absolute pressure measure pressures with regard to a reference, which are the calibrated one in case of barometers, or the atmospheric pressure in case of other meters of pressure. With regard to these meters of pressure sobreatmosférica that measure with regard to the atmospheric pressure, it is necessary to have present that this one changes sensitively throughout the days. The teams that measure pressures of mechanical form are named manometers, which can measure pressures sobreatmosféricas (when they measure the pressure in a point with regard to the ambience) or of distinguishing pressures when they measure differences of pressure between two points.

interesting Other that my name is her to tension under the scheme of this science, it is the manometric pipe in U, the manometric fluid is at rest (it does not circulate), from what the pressure in both branches is the same to equality of height if it is occupied by the same manometric fluid. This way, to measure the pressure by means of a pipe in U, it breaks of that maximum height in which manometric fluid exists in both branches (points of identical pressure). From this height, the different contributions of pressure are taken into account in every branch, which are the same on the height in which it starts having flown manometrically in both branches.

Analyze in a deposit of compressed air it has a tap, which connects to a branch of a manometer in U that contains mercury, and other one is to the air.

The points 1 and 2 of the manometer are to the same pressure, since there is to the same height the same manometric fluid that is at rest (P1 = P2). Despising the pressure hydrostatics exercised by the gases where it is equivalent to despise his potential energy, on the point 1 only there exercises pressure the air compressed into the tank with his pressure PA. Likewise, on the point 2, there exercises pressure so much a column of mercury of height hm and, over this one, the atmospheric pressure, since this branch is opened, therefore we have: PA = pmghm Patm. (Image prepared for @centaurox)

It is important to have these knowledge like in the engineering, as in the industries inside his structures of plant and team one handles grace to this knowledge, in system of cooling the mechanics of fluids is even applied, in a next publication we will keep on tackling this topic greetings and successes for all.

Bibliography.
Mechanics of the fluids and hydraulic for Jaime Ernesto Díaz Ortíz – 2006.
Applied mechanics of fluids - Page 26 for Robert L. Mott – 1996.