Laminar flow

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Laminar flow (over the bow) and turbulent flow (along the sides) 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, pressure and velocity independent 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. Reynolds numbers of less than 2000 are generally considered to be of a laminar type. When the Reynolds number is much less than 1, Creeping motion or Stokes flow occurs. This is an extreme case of laminar flow where viscous (friction) effects are much greater than inertial forces.

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.

Laminar and turbulent flow of cigarette smoke.

For a practical demonstration of laminar and non-laminar flow, one can observe the smoke rising off a cigarette in a place where there is no breeze. The smoke from the cigarette will rise vertically and smoothly for some distance (laminar flow) and then will start undulating into a turbulent, nonlaminar flow.