Laminar sublayer
Encyclopedia
The laminar sublayer, also called the viscous sublayer, is the region of a mainly-turbulent
flow that is near a no-slip boundary
and in which the flow is laminar. The existence of the laminar sublayer can be understood in that the flow velocity decreases towards the no-slip boundary. Because of this, the Reynolds number decreases until at some point the flow crosses the threshold from turbulent to laminar.
The laminar sublayer is important for river-bed ecology
: below the laminar-turbulent interface, the flow is stratified, but above it, it rapidly becomes well-mixed. This threshold can be important in providing homes and feeding grounds for benthic organisms.
Whether the roughness due to the bed sediment or other factors are smaller or larger than this sublayer has an important bearing in hydraulics
and sediment transport
. Flow is defined as hydraulically rough if the roughness elements are larger than the laminar sublayer (thereby perturbing the flow), and as hydraulically smooth if they are smaller than the laminar sublayer (and therefore ignorable by the main body of the flow).
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
flow that is near a no-slip boundary
No-slip condition
In fluid dynamics, the no-slip condition for viscous fluids states that at a solid boundary, the fluid will have zero velocity relative to the boundary.The fluid velocity at all fluid–solid boundaries is equal to that of the solid boundary...
and in which the flow is laminar. The existence of the laminar sublayer can be understood in that the flow velocity decreases towards the no-slip boundary. Because of this, the Reynolds number decreases until at some point the flow crosses the threshold from turbulent to laminar.
The laminar sublayer is important for river-bed ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
: below the laminar-turbulent interface, the flow is stratified, but above it, it rapidly becomes well-mixed. This threshold can be important in providing homes and feeding grounds for benthic organisms.
Whether the roughness due to the bed sediment or other factors are smaller or larger than this sublayer has an important bearing in hydraulics
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
and sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
. Flow is defined as hydraulically rough if the roughness elements are larger than the laminar sublayer (thereby perturbing the flow), and as hydraulically smooth if they are smaller than the laminar sublayer (and therefore ignorable by the main body of the flow).