Laminar flow

From Wikipedia, the free encyclopedia

Laminar flow (bottom) and turbulent flow (top) over a submarine hull.

Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. In fluid dynamics, laminar flow is a flow regime characterized by high momentum diffusion, low momentum convection, and pressure and velocity independence from time. It is the opposite of turbulent flow. In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough."

The (dimensionless) Reynolds number is an important parameter in the equations that describe whether flow conditions lead to laminar or turbulent flow. In laminar flow, the Reynolds number is less than 2100. An extreme case of laminar flow where viscous (friction) effects are much smaller than inertial forces, Inviscid flow, occurs when the Reynolds number is much less than 1.

For example, consider the flow of air over an airplane wing. The boundary layer is a very thin sheet of air lying over the surface of the wing (and all other surfaces of the airplane). 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.

As the boundary layer approaches the centre of the wing, it begins to lose speed due to skin friction and becomes thicker and turbulent. Here it is a turbulent layer. The process of a laminar boundary layer becoming turbulent is known as boundary layer transition. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag, due to skin friction, is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases the transition point also tends to move forward. One way to limit the size and effect of the turbulent region is to use swept-back "delta" wings. This is particularly important in supersonic aircraft.

[edit] Experiments

A famous experiment involving laminar flow uses two concentric glass cylinders with the gap filled with glycerin. A drop of ink placed in the fluid. When the outer cylinder is turned, the drop is drawn out into a thread that eventually becomes so thin that it disappears from view. At this point the ink molecules are said to be "enfolded" in the glycerin. If the cylinder is then turned in the opposite direction, the thread reforms and then becomes a drop. This experiment is typically used to show implicit order, but also nicely demonstrates the properties of laminar flow.