Precision Meters: Laminar Flow – Fluids

Whether one is considering air or fluid, the effects of laminar flow form an integral part of understanding the behaviour that the medium follows as a result of the surface over which it ‘flows.’

When a fluid is moving through a closed channel such as a pipe or between two flat plates, either of two types of flow may occur depending on the velocity of the fluid: laminar flow or turbulent flow.

Laminar flow is often known as a streamline flow and occurs when a fluid moves in parallel layers without any disruption between the layers.

The flow maybe across any surface that is ‘open’ such as the wing of an aircraft (or the whole fuselage) or ‘closed’ such as through a ‘conduit’ such as a pipe.

For example, consider the flow of air over an aircraft wing. The boundary layer is a very thin sheet of air lying over the surface of the wing (and all other surfaces of the aircraft). Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called laminar and the boundary layer is a laminar layer. Prandtl applied the concept of the laminar boundary layer to airfoils in 1904.

Fluids, flowing at low velocities, tend to allow adjacent layers to slide past one another without lateral mixing. i.e. there are no cross currents that are at an angle to the direction of the flow that would cause eddies or swirls of the fluid.

The fluid particles flow in a very orderly fashion in straight lines, parallel to the surface, over which the fluid is moving. In terms of fluid dynamics, laminar flow is a regime characterised by high momentum diffusion and low momentum convection.

When a fluid is flowing through a closed channel such as a pipe or between two flat plates, either of two types of flow may occur depending on the velocity of the fluid: laminar flow or turbulent flow.

Turbulent flow generally occurs at higher velocities when eddies or small packets of fluid particles form, leading to lateral mixing typically due to unevenness of the ‘conduit’ surface which in turn leads to the formation of cross currents, eddies or swirls that are at an angle to the direction of the flow.

In non-scientific terms laminar flow is “smooth”, while turbulent flow is “rough.”

The common application of laminar flow would be in the smooth flow of a viscous liquid through a tube or pipe. In that case, the velocity of flow varies from zero at the walls to a maximum along the centreline of the vessel.

The flow profile of laminar flow in a tube can be calculated by dividing the flow into thin cylindrical elements and applying the viscous force, relevant to the fluid, to them.

Water meter accuracy relies on fluid flowing ‘smoothly’ through the pipe line into which they are installed.

Water turbulence (Fig 1).

Precision Meters therefore recommends that, to ensure the optimum accuracy of a water meter, it is installed to the following minimum specifications with regard to laminar flow:

a) Ensure that there is a length of pipe, that is at least 10 times its diameter, free of any bends or fittings BEFORE the meter being installed.

B) Ensure that there is a length of pipe, that is at least 5 times its diameter AFTER the meter being installed and that any bends or fittings are installed AFTER this distance. (see Fig 2).

Pipe Length Dimensions (Fig 2).

Precision Meters: Laminar Flow – Fluids

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