application of boundary layer theory

7/26/2019 Application of Boundary Layer Theory

1/4

APPLICATION OF BOUNDARY LAYER THEORY

Application in aerodynamics indstries

Figure 1 Aerofoil in Aerodynamics Industries

In the aerodynamics industries, the boundary layer is particularly important

because it is responsible for a considerable amount of drag. The transition between the

laminar and turbulent flow are called the boundary layer separation. One of the factors

which influences this separation is the pressure gradient. It seen that the shear stress

caused by viscosity has retarding effects upon the flow. This effect can however be

overcome if there is a negative pressure gradient offered to the flow. There are other

factors as well, such as the shape of the body and the drag. The shape of the body

determines the relative magnitude of the drag component. The drag is the force that

opposes motion, an aircraft must overcome the drag force upon it, in order to fly. A thin

body causes less pressure drag when compared to a thick body which is prone to

separation and produces considerable pressure drag. This is one of the reasons for the

streamlined shape of aircraft, because streamlining a body to decrease pressure drag

enables it, to avoid separation.


2/4

Figure 2 Cross section of an aerofoil

The geometry of the shape is such that it have a favourable gradient in pressure

to start with and up to a point P. The negative pressure gradient will counteract the

retarding effect of the shear stress (which is due to viscosity in the boundary layer. !or

the aerofoil, it has an adverse pressure gradient downstream of P. "ow the adverse

pressure gradient begins to retard. This effect is felt more strongly in the regions close

to the wall where the momentum is lower than in the region near the free#stream. !rom

figure, it can be seen that the velocity near the wall reduces and the boundary layer

thickens. A continuous retardation of flow brings the wall shear stress at the point $ on

the wall to %ero. !rom this point onwards, the shear stress becomes negative, the flow

reverses, and a region of recirculating flow develops. In this case, the flow no longer

follows the contour of the aerofoil, and it say that the flow has separated. The point $

where the shear stress is %ero is the point of separation. &epending on the flow

conditions, the recirculating flow terminates and the flow may become re#attached to the

body of the aerofoil. There are a variety of factors that could influence the re#

attachment. The pressure gradient may now be favourable due to the body geometry.

The other factor is that the flow which is initially laminar may undergo transition and may

become turbulent. A turbulent flow has more energy and momentum than a laminar flow,

and so to overcome the problem of boundary layer separation, the boundary layer is

deliberately tripped into turbulence at a point prior to the location of the laminar

separation, in which case, the fuller velocity profile of the turbulent boundary layers


3/4

allows it to sustain the adverse pressure gradient without separating and so the flow

may re#attach. On the aerofoil, the separation may occur near the leading edge, but the

effect is not significant, however, the dangerous situation is when it have separation

occurring more towards the trailing edge in which the flow will not re#attach, this

phenomenon, can lead to the loss of lift of the aerofoil which is termed as ''stall in

which an aircraft can suddenly drop from the sky and so pilots and engineers are

striving hard to avoid this problem. One of the approaches to this problem of separation

is to have a clear understanding of the boundary layer. This understanding has led to

several solutions, such as designing special wing sections to avoid the boundary layer

separation. In this case, the boundary layer may be sucked away through a porous

surface. )ust as flow separation can be understood in terms of the combined effects of

viscosity and adverse pressure gradients, separated flows can be reattached by the

application of a suitable modification to the boundary conditions. $uction is applied to

the leading edge of the airfoil at a sharp angle of attack, removing the early separation

%one and moving the separation point much farther downstream.


4/4

*eference

+ $aso ne% (-/. Airfoil boundary layer. !aculty of 0athematics and Physics

1niversity of 23ubl3ana2) Andre 4akker (--. 4oundary 2ayers and $eparation. *etrieved on 5 )une

-+5

application of boundary layer theory

Documents