PRINCPLE PROPERTIES OF FLUIDS

 

PRINCPLE PROPERTIES OF FLUIDS

      Fluids is the substance that deform continuously under shearing force, also these are substances   that can flow include liquid and gas.

Fluid mechanics is the study which deals with fluids, their properties and forces on them.

Fluids have three major properties which are

1. Density
2. Viscosity
3. Surface tension

These are very important properties to be discussed so as to know how fluids behave in different situations.

Density

This is the quantity of substance present in it per unit volume, density of substances can be expressed in different ways as follows

              1. Mass density

This is the mass of the substance per unit volume; mass density is expressed in kilogram per cubic meter as kg/m3.

       2.  Unit weight of specific weight

                           This is quantity of substance in Newton per unit volume; unit weight is expressed                                in Newton per cubic meter. 

       3. Relative density

Relative density is the ratio of quantity of substance per unit volume to the quantity of standard liquid of equal volume at standard temperature (standard liquid is water at 4⁰C).

Relative density has no SI Unit as it is ratio of two densities.

 

Viscosity

Viscosity is the fluid property due to cohesion and interaction between fluid molecules.

Viscosity offers resistance of fluid to flow or stir or pour, it is the measure of fluidity of the fluid.

Viscosity of fluid can be expressed in the following terms

1. Coefficient of dynamic viscosity μ

                This expresses the amount of shear stress required for producing certain amount of shear strain.

              2. Coefficient of kinematic viscosity v

                Is the ration between coefficients of dynamic viscosity to density of the fluid. 

                                                                                

Surface tension

Surface tension this is the property of fluid surface to offer resistance against tension, this property is caused by strong cohesion between liquid molecules and weak adhesion between liquid and air molecules.

 

If we would have strong adhesion between liquid and air molecules and weak cohesion between liquid molecules, then we would see that liquid molecules are dispersed into air.

 

This property bring about formation of meniscuses of liquids as they can be seen in narrow tubes

                  In first tube we have concave meniscus which is formed, because we have weak adhesion   between liquid molecules and strong adhesion between liquid and tube molecules.

In second tube we have convex meniscus which is formed, because we have strong cohesion between liquid molecules and weak adhesion between liquid and tube molecules.

In construction effect of surface tension can be seen Bulking of the sand as water film formed around sand particles, its surface tension tends to push sand particles apart.

 Capillarity

This property of fluid to rise or fall through narrow tube, this is caused by surface tension on the surface of the liquid which tends to drag liquid up or down through narrow tube following its direction.

 

In this property we have two kinds of effects capillary rise and capillary fall.

Capillary rise

This is the rise of liquid above its surface through narrow tube. This is caused by positive direction of surface tension as shown in A.

In A angle of contact between surface tension and vertical is acute so it pulls liquid upward, in the following manner.

 

Force pulling liquid = surface Tension x cos α (angle of contact).

 

 

Since the cos of acute angle (α < 90) is positive then, we have positive pulling force which pulls liquid upward.

 

Capillary fall

This is the fall of liquid below its surface through immersed narrow tube. This is caused by negative direction of surface tension as shown in B.

 

In A angle of contact between surface tension and vertical is obtuse so it pulls liquid downward, in the following manner.

 

Force pulling liquid = surface Tension x cos α (angle of contact).

 

Since the cos of obtuse angle (α > 90) is negative then, we have negative pulling force which pulls liquid downward.

 

In construction effect of capillary rise can be seen as water rise from soil below or around foundation to the foundation, wall and other structural parts.