Properties of Fluid

Following are some of the properties of fluid 

1. Mass Density

"Mass density is the ratio of mass of fluid to the volume of the fluid", It is denoted by 'ρ'. The SI unit of mass density is kg/㎥. Mass density of water is 1000  kg/㎥ and of mercury is 13600  kg/㎥.

It can be written as:-

2. Specific Weight

"Specific weight is the ratio of weight per unit volume", It is denoted by ' W_s '. The SI unit of specific weight is N/㎥. Specific weight of water 9810 N/㎥ or 9.81 KN/㎥.

It can be written as:-

              

3. Specific Gravity

"Specific Gravity is the ratio of the specific weight of fluid to the specific weight of liquid." It is denoted by 'G'. It has no unit. Specific gravity of water is 1 and that of mercury is 13.6.

It can be written as:-

4. Viscosity

"Viscosity is the property of fluid by virtue of which it shows resistance against motion." 

It describes the internal resistance of fluid against motion. Fluids may have different viscosity and hence the speed of their motion is also different. 

For example, water flows faster than honey, it is because honey has higher viscosity than water

Consider two layers of fluid one over the other and distance 'y' from the solid boundary 'dy' apart with velocity 'u' and 'u+du' respectively. Due to difference in velocity 'du' in both layers the bottom layer

offers resistance to the top layer thus causing shear stress between the layers.

This shear stress is proportional to the velocity gradient (du/dy) of the the fluid denoted by 'τ'.

Where μ = Coefficient of viscosity

SI unit of viscosity is Ns/㎡. Unit of viscosity is CGS is poise(dyne-sec/cm² ).    

5. Surface Tension

"Surface tension is the property of fluid that tends the water surfaces to shrink to a minimum surface area." It is due to the cohesive forces acting between the water molecules. It can also be defined as force per unit length of the free surface. It is denoted by 'σ'. SI unit unit of surface tension is N/m.

One can see surface tension in action when insects float on water, even though density of insects is higher than water.

Surface Tension