Laminar & Turbulent Flow

A substance that changes its physical form under the influence of an external force is defined as a fluid. The application of the force causes it to flow in that direction. The study of the motion of fluids is defined as Fluid Dynamics. 

Have you ever noticed a stream of water under a bridge? What is the first thing that comes to your mind? The most common qualities such as the speed, width and volume of the flowing stream. The velocity of the fluid flow determines its type of flow and based on these qualities the fluid flow can be determined as turbulent or laminar. 

A fluid flowing through a closed pipe is observed to have a smooth texture when the flow-rate is low, but disturbances and voids are visible after a certain threshold is breached on increasing the flow-rate. At such an instant, if a drop of ink is introduced into the stream of flow when it is smooth, the ink does not mix with all the layers of the fluid, while if it is introduced when turbulent, we observe that the ink mixes with layers. 

When a fluid flows through a closed pipe or in between two flat plates, the type of flow is dependent on the velocity, fluid viscosity & the Reynolds number that determines the size of the pipe. 

We will learn about the Streamline Flow & Turbulent Flow in this section. 

Streamline Flow

Laminar or Streamline Flow is defined as the flow in which the fluid flow happens in parallel layers that do not create any disruption or intermixing of the fluid layers. This type of flow is most commonly found in highly viscous, low velocity fluid flows. Thereby, the velocity of each fluid particle remains constant at any given point in time. At low velocities, there is no lateral mixing of the fluid particles while they also follow a disciplined order moving in straight parallel lines that are parallel to the walls of the medium they are confined in such as pipes. The layers appear to be gliding over each other. 

The path followed by the particles under steady flow conditions is defined as a streamline. These streamlines follow the equation of continuity: Av = constant where A is the cross-sectional area of the flow of the fluid & v is the velocity of the fluid at that point. This is defined as the flow-rate of a fluid or also known as the volume flux. In steady flow, the flow-rate remains constant. Hence, if the cross-section area is smaller, the velocity of the liquid is higher and vice-versa. 

Turbulent Flow

Turbulent flow of fluids is defined as the flow of motion in which fluid particles move in a random manner without following any specific path of motion. Due to this random zig-zag motion, eddies are formed across the area resulting in high energy loss. To imagine it in simple terms, it is like cars banging into each other without any sense or direction in a traffic jam. The speed of the fluid particles at any point in time changes continuously both in magnitude and direction. Its occurrence is majorly observed in large diameter pipes where the fluid flows with high velocity. It is caused by restrictions in the flow of installations such as valves, adapters, elbows in combination with a high velocity of the used fluid. 

The most common examples of turbulent flow include oil flow in large pipelines, flowing of volcanic lava, flow of water through pumps and turbines etc. 

How to know if the flow is Laminar or Turbulent?

It is a widely accepted fact that the Navier-Stokes equation and its simplified version are detrimental in understanding the flow types. The flow is determined by a non-dimensional number called the Reynolds number for pipe flow. 

Re = (VD/v)

Where,
D = Diameter of pipe
V = Mean velocity of the flow in pipe
v = Kinematic viscosity of the fluid

The flow type is determined as follows: 

Laminar Flow – If the Reynolds number is less than 2000

Turbulent Flow – If the Reynolds number is greater than 4000

It may be Laminar or Turbulent Flow if the Reynolds number is between 2000 and 4000. 

Summary: 

Laminar Flow – Laminar or Streamline flow in fluid dynamics is observed when a fluid is moving in layers over one another in a small pipe at low velocity in higher viscosity fluids. 

1. The fluid motion is in layers moving in a disciplined, non-obstructing manner in a straight line. 
2. The Reynolds number is lower than 2000 in laminar flow. 
3. The overall velocity is low. 

Turbulent Flow– Turbulent flow in fluid dynamics is observed when the fluid moves in large pipes in a random zig-zag motion at high velocity. 

1. The layers obstruct the motion of each other. 
2. The Reynolds number is greater than 4000
3. The overall velocity of the fluid is high.