 Flow is nothing but generally refers to the movement of gas or liquid. The concept of flow describes how the fluids interact with their surroundings and their behavior. Flow can be either laminar or turbulent. Laminar flow is a steady flow of a liquid with different velocities in the horizontal surface in a layer. On the other hand, the flow of fluid with a speed more significant than its critical velocity is disordered and called turbulent flow.

Many factors affect the flow rate of a liquid—viscosity, velocity, density, pressure, etc. as an example. For instance, The viscosity of water at a temperature of 20 degrees Celsius is approximately 0.01 poise or 10-3 Pa.s (Pascal seconds). Here is a detailed explanation of the factors affecting the flow rate and properties of liquids.

## What is viscosity?

Viscosity is the measure of fluid’s resistance to flow. The term viscosity describes the internal friction of the moving liquid. Viscosity is measured in respect of a ratio of shearing stress to the speed gradient in a fluid.

η = 2ga2(Δp)/9ν

Where,

• ∆ρ is the density difference
• a is that the radius of the sphere
• g denotes acceleration due to gravity
• v define the velocity of the sphere

Dynamic and kinematic viscosities are the two different measurements of viscosity used to describe fluids,

### Dynamic viscosity

It measures the shear stress to the shear rate of a liquid. Dynamic viscosity gives you information on the force needed to make the fluid flow at a selected rate

### Kinematic velocity

It measures the ratio of the viscous force to the inertial force on the fluid. This gives how fast the liquid flows when a particular pressure is applied.

ν = μ/Ⲣ

• As the temperature increases, the viscosity of liquids decreases rapidly and the viscosity of gases increases with an increase in temperature.

## Coefficient of viscosity

The coefficient of viscosity is defined because the force of friction is required to take care of a difference of velocity of 1 cm/s between parallel layers of fluid. The unit is poise and one poise equals one dyne-second per square centimeter.

## Critical velocity

Critical velocity is that of the speed and direction at which the flow of a liquid changes from smooth to turbulent through a tube.

νc = kη/rρ

• K = Reynolds’s number,
• η = coefficient of viscosity of the liquid
• r = radius of the capillary
• ρ = density of the liquid.

## Surface tension

It is a force acting on a liquid to reduce its surface area. It depends on the forces of attraction between the particles within the given fluid and the forces of attraction of solid, liquid, or gas in contact with it. Likewise, it can be expressed as

T = F/L

• T = Surface tension of the liquid
• L = Length in which the force acts
• F = Force per unit length

Newton per Meter or N/m is the unit of surface tension.

### Examples of surface tension found in nature

1. Moving insects on the water
2. The tension is lowered when the garments are washed in water by using soaps and detergents.
3. Formation of water bubbles

#### Note

Water has high surface tension due to the positive hydrogen and negative oxygen attracting and sticking together. Hence, this leads to the bond getting strong and requires specific energy to interrupt. This is why water has high tension, and things can float within the water with less density.