Laminar and turbulent flows

Laminar Flow

Fluids at low velocities tend to flow without lateral mixing and fluid flows with adjacent layers past one another. The fluid neither creates neither eddies nor cross currents. This type of flow is termed as laminar flow.
Shear stress depends almost only on theviscosity-μ- and is independent ofdensity-ρ.

Turbulent Flow

Fluids flowing at higher velocities tend to create eddies and cross current this type of flow is known as turbulent flow.
Turbulence may also be generated in other ways than by flow through a pipe. It can result either from contact of the flowing stream with solid boundaries or contact between 2 layers of fluid moving at different velocities. First type of flow is called wall turbulence and the other is known as free turbulence.
Turbulent flow consists of mass of eddies of various sizes coexisting in flowing stream. Large eddies are continuously formed and they break down into smaller eddies which inturn will evolver even smaller ones and finally further smaller ones disappear.
Shear stress for turbulent flow is a function of thedensity-ρ.

Reynolds Number

Reynolds number is a dimensionless number which depends on the density, viscosity and velocity of the fluid. Reynolds number flow where fluids achieve critical velocity. It’s the stage where laminar flow changes into turbulent flow.

Nre = DVρ
μ

Where,
D dia of tube
V velocity of liquid
ρ density of liquid
μ Viscosity of liquid and
Nre dimensionless number.


Transition of laminar flow to turbulent flow occurs at wide range of Reynolds numbers. In a pipe flow is always laminar at Nre < 2100, but laminar flow can persists upto several thousands depends on some factors such as well rounded tube entrance and quiet liquid in tank.
Under normal conditions flow is turbulent at Reynolds number above 4000. Flow between 2100 and 4000 is known as a transition stage.
Q & A
1. What are the types of fluid flow?
A. Laminar flow and turbulent flow.

2. What is the range of Reynolds number in laminar flow?
A. Reynolds number ranges up to 2100 for laminar flow.

3. What is the range of Reynolds number in turbulent flow?
A. Reynolds number ranges between 2100 to 4000 for turbulent flow.



More pages