4.8-Separation.pdf

Separation

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4.8 Separation

Flow separation occurs when the fluid pathlines adjacent to body deviate from the contour of the body and

produce a wake. This flow condition is very common. It tends to increase drag, reduce lift, and produce

unsteady forces that can lead to structural failure.

Consider the flow of a real (viscous) fluid past a cylinder as shown in Fig. 4.25. The flow pattern upstream of

the midsection is very similar to the pattern for an ideal fluid. However, in a viscous fluid the velocity at the

surface is zero (no-slip condition), whereas with the flow of an inviscid fluid the surface velocity need not be

zero. Because of viscous effects, a thin layer, called a boundary layer, forms next to the surface. The velocity

changes from zero at the surface to the free-stream velocity across the boundary layer. Over the forward section

of the cylinder, where the pressure gradient is favorable, the boundary layer is quite thin.

Figure 4.25 Flow of a real fluid past a circular cylinder.

(a) Flow pattern.

(b) Pressure distribution

Downstream of the midsection, the pressure gradient is adverse and the fluid particles in the boundary layer,

slowed by viscous effects, can only go so far and then are forced to detour away from the surface. This is called

the separation point . A recirculatory flow called a wake develops behind the cylinder. The flow in the wake

region is called separated flow . The pressure distribution on the cylinder surface in the wake region is nearly

constant, as shown in Fig. 4.25b. The reduced pressure in the wake leads to increased drag.

A photograph of an airfoil section with flow separation near the leading edge is shown in Fig. 4.26. This flow is

visualized by introducing smoke upstream of the airfoil section. Separation on an airfoil surface leads to stall

and loss of lift.

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Separation

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Figure 4.26 Smoke traces showing separation on an airfoil section at a large angle of attack.

(Courtesy of Education Development Center, Inc. ewton, MA)

Separation and the development of a wake region also occurs on blunt objects and cross sections with sharp

edges, as shown in Fig. 4.27. In these situations, the flow cannot negotiate the turn at the sharp edges and

separates from the body, generating eddies, a separated region, and wake flow. The vortices shed from the body

can produce lateral oscillatory forces than can induce vibrations and ultimately lead to structural failure, as

evidenced by the collapse of the Tacoma Narrows Bridge in 1940. The prediction and control of separation is a

continuing challenge for engineers involved with the design of fluid systems.

Figure 4.27 Flow pattern past a square rod illustrating separation at the edges.

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