Boundary Layer.ppt

文档介绍：Chapter 5 Boundary Layer (BL)
Introduction
* D’Alembert Paradox(佯谬,疑题): A body immersed in a frictionless fluid has zero drag.
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Inviscid
or
2
Boundary layer (边界层)
Although frictional effects in slightly viscous fluid are indeed present, they are confined to a thin layer near the surface of the body, and the rest of the flow can be considered inviscid. (1904, Prandtl)
Ludwig Prandtl(1875-1953)
第三届国际数学大会
德国海登堡
(论粘性很小的流体运动)
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Three Thicknesses of a Boundary Layer
2. Displacement Thickness (排移厚度,位移厚度)
d
U
y
u
x
1. BL Thickness d (名义厚度,厚度)
The locus of points where the velocity u parallel to the plate reaches 99 percent of the external velocity U.
4
u
dy
d*
d
y
U
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3. Momentum Thickness (动量厚度,动量损失厚度)
y
d
U
u
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Momentum Integral Relation for flat-plate BL
(平板边界层动量积分方程)
U=const p=const
Steady & pressible
RTT
CV
x
y
h
U
P
d
streamline
o
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Momentum equation:
Continuity:
CV
x
y
h
U
P
d
streamline
o
速度积分法求阻力
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1921
(1881-1963)
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y
u(y)
x
BC:
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