Fluid Mechanics Basics

Hello friends,

today i am going to teach you some very basic concepts of fluid mechanics.
lets starts:

FLUID
we know in solids the molecules are very close to each to other, so they always show a fixed angular deformation as shown below

But In fluids this deformation which we generally describe as shear is not fixed, their we get continuous deformation beacause of the flow property. We term it as viscosity, later on i will explain the Viscosity. That's why here angular deformation keep on changing so here we get continuous deformation.

Gases and Liquids are having very important property of continous deformation which is termed as flow property.

So we put gases and liquids in different cateogry which is far away from solids and known as FLUIDS.

Fluid as Continuum:
                                  Continuum means continous mass. We have two types of studies of fluids one is microscopic and orther is macroscopic.

In Microscopic voids or the interstitial gap between the molecules can not be neglected. But in Macroscopic we can neglect the voids with respect to system dimensions.

so the fluid system can be treated as continous distribution of mass and this concept is known as continuum.

BASIC PROPERTIES OF FLUIDS

1. Density (ρ)
It is defined as the ratio of mass to the volume.

S.I units,
mass=kg
volume= m³
So density = kg/m³

2. specific weight (w)
It is deined as the ratio of weight to the volume of fluid.

3. Specific Gravity (S.G)
It is defined as the ratio of density of fluid with respect to standard fluid.

for liquids , water is taken as standard fluid and density of water is 1000 kg/m^3.
for gases , air is taken as standard and density of air is 1 kg/m³

4. Relative density:
 It is defined as the ratio of density of one fluid to the density of the other fluid.

The difference Between specific gravity and realtive density is that in specific gravity we took ratio w.r.t standrad fluid whereas in relative density no concept of standard fluid. It is a general ratio term.
If at any place like in examination if you found some term like this then examinar must clear by telling the other fluid also, if other fluid is not given then it is understood that specific gravity is given.

5. Compressibility (β)
This term basically defined as to check the change in volume of the fluid when pressure is changed.

this term is -ve because as pressure increases volume must decrease or vice versa. hence both are opposite so term is -ve, but we want to define compressibility as +ve so we multiply it with -ve to make is postive.

Now, we know mass is always conserved.
                                                                                    ρV= Constant
differnetiate this equation
                                                                                       ρdV + Vdρ = 0

put this value in abobe equation

Now, we define Incompressible Fluid
Actually density not change with respect to pressure

 These Fluids are termed as Incompressible fluid.

Liquids
 For liquids generally we take water it is compressible but very small change in volume with very high pressure increase.
so liquids can be treated as incompressible they are not actaully incompressible.
so mark this line i.e can be treated as.

Gases
From ideal gas equation we can see that pressure is directly prop to density.

so in gases density changes abruptly with change in pressure.so these are highly compressible.

6. Viscosity

"two adjacent layers of fluid resist the reltive motion of each other such a fundamental property is known as viscosity".

Basic cause:
The basic reason is cohesive forces which is acting between the molecules of the fluids.
they resist each other due to which friction develops or shear stresses develop, this is known as viscosity.

where

τ = shear stress

µ = viscosity

                       

                              Keep reading and keep exploring your ideas in the real world.