Properties of fluids
Properties of Fluids mainly possesses the following properties :
1. Mass density
2. Specific Weight
3. Specific volume
4. Specific gravity
5. Compressibility
6. Viscosity
7. Surface tension
8. Capillarity
Mass Density
Mass density (or specific mass) of a fluid is its mass per unit volume. It is denoted by a symbol p (rho). In S.I units it is expressed in KN/m³
Mass density P = Mass of fluid Volume fund
Since a molecule has a certain mass regardless of its state, mass density is proportional to the number of molecules in a unit volume of fluid. It has a constant value at same temperature and pressure. It decreases with increasing temperature, but increases with increase in pressure.
For water maximum density occurs at the 4°C, which is 1000 kg/m³ (or) 10 KN/m³
Specific Weight
Specific weight (or weight Density) of a fluid is its weight per unit volume. It is represented by a symbol w (or) Y (gama). In S.I units it is expressed in N/m³
Specific weight decreases with increasing temperature and increases with increase in pressure.
Mass density (p) and specific weight (w) are related as W = Pg
Where g is acceleration due to gravity.
Specific weight of water at 4° C is 9810 N/m³ (or) 9.81 KN/m³
Specific Volume
the volume of the fluid per unit weight. Thus it is the reciprocal of specific weight It is denoted by u. in Sl units it is expressed in m³/N
Specific volume (v) =1/Specific weight (w)
Specific Gravity
the ratio of specific weight of the fluid to the specific weight of standard fluid. It is dimensionless and has no units.
Specific gravity = Specific weight of liquid liquid / Specific weight of water
For liquids, the standard fluid is water at 4° C. For gases, the standard fluid is either hydrogen or air free from carbon dioxide.
Knowing the specific gravity of any liquid, its specific weight may be calculated by,
w = Specific gravity of liquid x Specific weight of water at 4° C
= (Specific gravity of liquid) x 9810 N/m³.
Commonly used values of specific gravities in fluid flow calculations are 1.0 and 13.6 for water and mercury respectively.
Compressibility
All fluids can be compressed by the application of external forces and when this force is removed, fluids expand to their original volumes like elastic solids. Compressibility of a fluid is expressed as inverse of the bulk modulus of elasticity (K) of the fluid. Bulk modulus is defined as the ratio of increase in pressure (dp) to change in volume (du) per unit volume (v).
K=-(dp/dv/v)
Compressibility of liquids is negligible as compared to gases. Accordingly in the case of liquids effect of compressibility is ignored in most hydraulic problems. However in some special cases such as rapid closure of valve or where liquids subjected to instantaneous rise or decrease of pressure, effect of compressibility is considered. The variation in the volume of water, with the variation of pressure, is so small that for all practical purposes it is neglected. Hence water is considered to be an incompressible liquid
Viscosity
A thin liquid like water or alcohol spreads more quickly on a plate than a thick liquid like tar or castrol. This is because a thick liquid offers more resistance to the flow than the light liquid. This property of a fluid which offers resistance to the movement of one layer of fluid over adjacent layer is called as viscosity. As the viscous fluids flow there exists shearing stresses between fluid layers. The intensity of viscous shear stress (t) is given by the equation
t=u dv/dy
It is known as Newton’s equation of viscosity.
Coefficient of Viscosity: Coefficient of viscosity can be defined as the shear stress required to produce unit rate of angular deformation. It is also called as dynamic viscosity, In S.I units is expressed in N-S/m².
Kinematic Viscosity: the ratio between dynamic viscosity and the mass density is known as kinematic viscosity.
Surface Tension
Liquids have properties of cohesion and Adhesion. Cohesion means attraction between molecules of the same liquid. Adhesion means attraction between molecules of a liquid and the molecules of a solid boundary which is in contact with the liquid.
Surface tension occurs due to cohesion between liquid particles at the surface. The property of the liquid surface to exert a tension is called as Surface Tension. It is defined as the surface energy per unit area of surface film. It is denoted by o In S.I units it is expressed in N/m.
Capillarity
If any liquid has greater adhesion than cohesion, the liquid will wet the solid surface with which it is in contact and will rise at the point of contact. On the other hand, for any liquid if cohesion predominates then the liquid will not wet the solid surface and the liquid surface will be depressed at the point of contact. Such a phenomenon of rise or fall of liquid surface is known as capillarity. In S.I units it is expressed in m or mm of liquid. Capillarity: Is the rise (or) fall of liquid level in a small tube dipped in liquid.
DIMENSIONL FORMULA OF FLUIDS
