,
to confine -
to set in motion-
to assume -
solid boundaries - .
to tend -
to adhere - ,
to pour - ,
friction -
to exert a drag-
shear stress -
layer -
subscript -
plane -
nomenclature -
three-dimensional -
steady-state conditions-
born out - .
valid -
viscosity -
relation -
to vary with -
in order to -
stickiness - ,
Momentum interchange
product of -
to give rise -
viscous shear
molecular force field
mass exchange rate
small bore tube -
lubricant -
fuel oil -
poise [poiz] - /.
stoke - /.
conversion - ,
to facilitate -
in accordance with-
I. :
1. What example is given by the author at the beginning of the text? 2. How does the author explain the difference between absolute viscosity and kinematics' viscosity? 3. How do changes in temperature influence the viscosity of gases and liquids?
4. Is the mechanism of momentum exchange the same in liquids and in gases?
5. What units of measurement are used for viscosity?
II. :
a) (. 1-4).
b) - "one"...
c) "for + . + ". 4. "It is... which" (.4).
TEXT A
1. Consider a fluid confined between two parallel plates, the upper one being set in motion at a velocity U by a force F and the lower one being fixed. Assume that the distance "h" between the plates is sufficiently small for the fluid particles to move in parallel paths. From experience we have observed that fluid particles adjacent to solid boundaries tend to adhere to the surface (easily observed when pouring motor oil). This same property generates an internal friction by adjacent fluid particles exerting a drag on each other and producing a shear stress & yx = F/A between adjacent fluid layers. The subscripts "yx" indicate that the stress is in plane perpendicular to "y" and parallel to "x", a nomenclature which is obviously necessary in three-dimensional systems.
2. Under steady-state conditions Newton observed that the shear stress is directly proportional to the velocity gradient.
His observation, repeatedly borne out by subsequent investigators, is equally valid at any position; i.e., () where "u" is the fluid velocity in the "x" direction and ":" is the absolute viscosity. This empirical relation, known as Newton's equation of viscosity, defines absolute, or dynamic, viscosity ": ". It is sometimes more advantageous to define kinematics' viscosity.
|
|
|
3. The viscosity of fluids varies with temperature and pressure being much more sensitive to temperature than pressure. Changes in temperature cause opposite variations in the viscosity of gasses and liquids. An increase in the temperature of a liquid reduces its viscosity but increases the viscosity of a gas. This is intuitive for liquids but not apparent for gases.
4. Although values for viscosity are obtained by macroscopic measurements, let us consider a gas from a microscopic standpoint in order to understand the basic mechanism. From observations we tend to think of viscosity as a property related to "stickiness". Basically, however, it arises because of momentum interchange between molecules. Molecules are constantly in motion, the motion being more pronounced at higher temperatures and lower pressures. As the gas moves, slow-moving molecules strike faster-moving ones, slowing them down. It is this momentum (product of mass and velocity) interchange which gives rise to viscous shear, a measure of which is viscosity. The mechanism of momentum exchange in liquids is the same as in gases qualitatively, but the physical structure is much more complex since the molecules are closer and the molecular force fields have a greater effect on the momentum exchange in the collision process.
5. By analogy, suppose two trains loaded with coal are running on parallel tracks in the same direction. If workmen begin throwing coal from the slower train to the faster one, the train which "catches" the coal is slowed by the increased mass, because of the momentum component in the direction of motion of the train. Now imagine workmen on both trains, analogous to molecules in adjacent fluid layers, throwing coal back and forth from one train to the other. If the train initially has unequal velocities and the mass - exchange rate is equal for both trains, the faster train is slowed. So it is with the momentum interchange between fluid layers.
6. Viscosity is often measured by observing the time required for a given amount to fluid to flow from a short small-bore tube. Viscosities of fuel oils are measured at 77 and 122F, of lubricants at 100 and 210F. Viscosity is often given in metric units which have special names ": ": poise =1 g/cm=sec =100 centipoises, v: stoke = 100 centistokes. The following unites:: 1 (lbrsec/ft2) = 479 poises, v: l(ft2/sec) = 30.482 stokes.
7. Fluids which obey equation (2-5) are known as Newtonian fluids. All gases and most liquids of engineering importance are Newtonian. Fluids which do not behave in accordance with Eq. (2-5), no Newtonian fluids, will not be considered in this text.
A
1. , , U F, . , "h" . , , , , ( ). , & yx = F/A . "yx" , "" "", .
|
|
|
2. .
, , ; .., (), "u" "" ":" . , , , , ":". , .
3. , , . . , . , .
4. , , . , , "". , - . , . , . ( ) , . , , , .
5. , , . , , , - . , , . , . .
6. , . 77 122 F, 100 210 F. , ":": (poise) =1 g/cm=sec =100 centipoises, v: = 100 centistokes. :: 1 (lbrsec / FT2) = 479 , v: (ft2 / ) = 30.482 .
7. , (2-5), . . , . (2-5), , .
