# laminar flow

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## Laminar flow

A smooth, streamline type of viscous fluid motion characteristic of flow at low-to-moderate deformation rates. The name derives from the fluid's moving in orderly layers or laminae.

The chief criterion for laminar flow is a relatively small value for the Reynolds number, Re = &rgr;VL/μ, where &rgr; is fluid density, V is flow velocity, L is body size, and μ is fluid viscosity. Laminar flow may be achieved in many ways: low-density flows as in rarefied gases; low-velocity or “creeping” motions; small-size bodies such as microorganisms swimming in the ocean; or high-viscosity fluids such as lubricating oils. At higher values of the Reynolds number, the flow becomes disorderly or turbulent, with many small eddies, random fluctuations, and streamlines intertwining. See Creeping flow, Reynolds number, Turbulent flow, Viscosity

Nearly all of the many known exact solutions of the equations of motion of a viscous fluid are for the case of laminar flow. These mathematically accurate descriptions can be used to give insight into the more complex turbulent and transitional flow patterns for which no exact analyses are known. See Navier-Stokes equation

The theory of viscous lubricating fluids in bearings is a highly developed area of laminar flow analysis. Even large Reynolds number flows, such as aircraft in flight, have regions of laminar flow near their leading edges, so that laminar flow analysis can be useful in a variety of practical and scientifically relevant flows. See Boundary-layer flow, Fluid flow

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Laminar Flow

an ordered flow of a liquid or gas, in which the fluid moves in layers parallel to the direction of flow. Laminar flow is observed in very viscous fluids. It also occurs when the velocity of flow is sufficiently low and during the slow flow of a liquid around small objects. Specifically, laminar flow occurs in narrow (capillary) tubes, in the lubricant layer in bearings, and in the thin boundary layer that forms near the surface of a body over which a fluid is flowing. If the velocity of flow of a given fluid is increased, the laminar flow, at a certain instant, may undergo a transition to a disordered turbulent flow. As this occurs, the resistance to the motion of the fluid is changed drastically. The mode of flow of a fluid depends on the Reynolds number Re. If the value of Re is smaller than a certain critical value Ren, then laminar flow occurs. If Re > Recr, then the flow can become turbulent. The value of Recr depends on the type of flow being considered. For instance, for flow through circular tubes, Recr ≈ 2,200 (if it is assumed that the characteristic velocity is the velocity averaged across the cross section of the tube and that the characteristic dimension is that of the tube’s diameter). Consequently, at Recr < 2,200, the flow of the fluid in the tube will be laminar. The output of liquid from a tube during laminar flow is determined by the Poiseuille law.

### REFERENCES

Loitsianskii, L. G. Mekhanika zhidkosti i gaza, 3rd ed. Moscow, 1970.

## laminar flow

[′lam·ə·nər ′flō]
(fluid mechanics)
Streamline flow of an incompressible, viscous Newtonian fluid; all particles of the fluid move in distinct and separate lines.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## streamline flow

The flow of a liquid or gas past a solid body in a manner so that the velocity of the fluid, at every point, does not change with time.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

## laminar flow

The flow of air over a surface in smooth layers and without any turbulence or cross flow of layers of air. See laminar boundary layer.