Thursday, 2 August 2012

Laminar Flow

Ever seen the water flowing out of a thin pipe? It flows along a clear undisturbed stream which looks like a glass tube. That is called Laminar flow.

Fluids experience a resistance to flow (like friction). This resistance is known as viscosity. Higher the viscosity, harder it is for the liquid to flow. For instance; oil is a fluid that doesn't flow easily. Thus oil has high viscosity. The viscosity experienced by a fluid is one of the factors that determine the type of flow. There are three manners in which a fluid can flow: Laminar, Transient and Turbulent.

The three types of flow regimes can be distinguished by considering the fluid flow in pipes:

Laminar flow takes place in small pipes. The fluid flows at low velocities. The fluid in the pipe can be considered to flow as layers. These layers are in the form of cylinders having the same axis as the pipe. The layer touching the pipe flows at the lowest speed while the layer near the axis flows at higher speeds.

On the other hand turbulent flow is chaotic. Turbulent flow consists of many disturbances like swirling and spinning action of the fluid. Turbulent flow takes place at high velocities and occurs in large pipes. (The disturbances in turbulent flow are: eddies, vortexes and wakes. These have not been discussed in this article since the main focus is on Laminar flow. I suggest you look it up or else you will have to wait for an article on turbulent flow by me)

Transient flow is a mixture of Laminar and Turbulent flow. In case of fluid in a pipe, transient flow consists of Laminar flow along the walls of the pipe and turbulent flow at the centre of the pipe.

The type of flow is better defined by the dimensional number, Reynolds number (Re). The Reynolds number is the ratio of inertial forces to viscous forces of a liquid. 
The following are the ranges of Reynolds number corresponding to the flow regime:
Laminar flow - Re < 2300 where viscous forces are superior
Transient flow - 2300<Re<4000
Turbulent flow - Re>4000 where inertial forces are superior

To understand that the significance of laminar flow, see the video in the following link:
Before explaining the video there is a need to know what Couette flow is.

Consider two plates set apart from one another with some distance. Let the space between them be occupied by a viscous liquid. If there exists relative motion between the plates, laminar flow of the liquid will take place. This is Couette flow.

In the video, the instrument used is called Couette viscometer. It consists of a solid glass cylinder, which is placed in an open glass cylinder. There exists a cylindrical space between the two cylindrical surfaces. This space is filled with corn syrup which a highly viscous fluid. Then blobs of dyed corn syrup are added. When the cylinder is rotated the dye appears to diffuse through the colourless corn syrup medium. And when the motion is reversed, the dye separates and returns to its original position.

As I said, "the dye appears to diffuse", it does not actually undergo diffusion. The dye is mainly undergoes Couette flow. In this case the two plates are the two cylindrical surfaces and the liquid is corn syrup. Due to the laminar flow of corn syrup, the dyed syrup is merely dragged along the direction of rotation by viscosity. When viewed from the side it appears as if the dyes mixed with the colourless syrup. But, when viewed from top you will see the dyed syrup dragged into a spiral.

The video demonstrates that in laminar flow the fluid flows in layers slipping past each other. This is a highly desirable property when we want the liquid to flow smoothly and also direct it towards a target. This has been explained by taking the example of a fire hose.

A fire fighter extinguish the fire by directing the water flowing out of the fire hose towards the site of fire. When trying to reach a site at a significant height, a pump is used in order give the stream of water a larger range. So, efficient extinguishment requires two major factors:

  • the stream of water should have a good range without drawing much power from the pump.
  • the water lost while flowing from the exit of the hose to the fire site, should be minimum.
Both these factors can be achieved by laminar flow.  By reducing the turbulence, we are making the water flow in a well-defined smooth stream. In order to achieve laminar flow, disturbances in the water like bubbles, reverse flow have to be minimised. Passing the water through filters does this. The video in the following link demonstrates the basic manner in which Laminar flow can be attained:

Laminar flow is also used to study chemical reactions in a Laminar flow reactor (LFR). An LFR consists of a tube. The reactants enter through one end and the products are collected at the other end. Due to laminar flow of the reactants in the tube, the rate of flow reactants decreases as its distance from the axis of the tube increases. Thus the time the reactants spend in the LFR increases with its distance from the axis. This allows the researchers to easily study the states of the reaction.

Certain products like biological samples, pharmaceutical products and even electronic devices like semiconductor wafers have to be kept in a well-protected area free from contamination. A Laminar flow cabinets provide such a clean environment. The cabinet draws air from the surrounding are passes it through filters, which creates a laminar flow of the air - devoid of particles. Laminar flow cabinets include: Vertical Laminar Flow Cabinet and Horizontal Laminar Flow Cabinet.

Laminar flow has many uses. It is undisturbed flow which is actually beautiful. The beauty of laminar flow is quite evident when you look at laminar flow fountains!
By orphanjones


  1. Great explanation keep it up. Understood many things about laminar flow.Please discuss other flows also

  2. wonderful explanation n videos!

  3. Good attempt to describe the flow, but many mistakes are there in the description.