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By:

Kms.Sailesh(1190910078) & N. M. Nikhil(1190910054)

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All solid objects travelling through a fluid (oralternatively a stationary object exposed to amoving fluid) acquire a boundary layer of fluidaround them where viscous forces occur in thelayer of fluid close to the solid surface. Boundary

layers can be either laminar or turbulent. Areasonable assessment of whether the boundarylayer will be laminar or turbulent can be made bycalculating the Reynolds number of the local flowconditions.

Flow separation occurs when a stream of gas orliquid flowing past a body separates from thebody's surface and forms a region of eddies

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Angle of attack. Adverse pressure gradient(dp/dx). Boundary layer formation.

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An adverse pressure gradient occurs when the staticpressure increases in the direction of the flow.Mathematically this is expressed as: dP / dx > 0. This isimportant for boundary layers, since increasing the fluidpressure is akin to increasing the potential energy of thefluid, leading to a reduced kinetic energy and a

deceleration of the fluid. Since the fluid in the inner part ofthe boundary layer is relatively slower, it is more greatlyaffected by the increasing pressure gradient. For a largeenough pressure increase, this fluid may slow to zerovelocity or even become reversed. When flow reversaloccurs, the flow is said to be separated from the surface.

This has very significant consequences in aerodynamicssince flow separation significantly modifies the pressuredistribution along the surface and hence the lift and dragcharacteristics.

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On an aircraft wing the boundary layer is the part of the flow close to the

wing. The boundary layer effect occurs at the field region in which allchanges occur in the flow pattern. The boundary layer distorts

surrounding non-viscous flow. It is a phenomenon of viscous forces. This

effect is related to the Reynolds number.

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Flow Over A Flat Plate

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The boundary layer over the front face of asphere or cylinder is laminar at lowerReynolds numbers, and turbulent at higherReynolds numbers. When it is laminar (Re 10^6),separation is delayed (to a point about 20^opast the shoulder, figure 3E) and the wake iscorrespondingly smaller.

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The difference inthe flowfieldsaround a smoothsphere and a rough,

or dimpled, spherecan be seen

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*The high-Reynolds number

case the laminar boundary layer

undergoes transition to a

turbulent boundary layer before

separating.The subsequent turbulent

mixing increases the momentum

and energy of the boundary

layer

so that it remains attached

toward the rear of the cylinder,well beyond the separation of

the laminar layer. Thus in this

case

the separation region is much

smaller

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