THE NO-SLIP CONDITION
Fluid flow is often confined by solid surfaces, and it is important to understand how the presence of solid surfaces affects fluid flow. We know that
water in a river cannot flow through large rocks, and must go around them.
That is, the water velocity normal to the rock surface must be zero, and
water approaching the surface normally comes to a complete stop at the surface. What is not as obvious is that water approaching the rock at any angle
also comes to a complete stop at the rock surface, and thus the tangential
velocity of water at the surface is also zero.
Consider the flow of a fluid in a stationary pipe or over a solid surface
that is nonporous (i.e., impermeable to the fluid). All experimental observations indicate that a fluid in motion comes to a complete stop at the surfaceand assumes a zero velocity relative to the surface. That is, a fluid in direct
contact with a solid “sticks” to the surface, and there is no slip. This is
known as the no-slip condition. The fluid property responsible for the noslip condition and the development of the boundary layer is viscosity and is
discussed in Chap. 2.
The photograph in Fig. 1–14 clearly shows the evolution of a velocity
gradient as a result of the fluid sticking to the surface of a blunt nose. The
layer that sticks to the surface slows the adjacent fluid layer because of viscous forces between the fluid layers, which slows the next layer, and so
on. A consequence of the no-slip condition is that all velocity profiles must
have zero values with respect to the surface at the points of contact between
a fluid and a solid surface (Fig. 1–15). Therefore, the no-slip condition is
responsible for the development of the velocity profile. The flow region
adjacent to the wall in which the viscous effects (and thus the velocity gradients) are significant is called the boundary layer. Another consequence
of the no-slip condition is the surface drag, or skin friction drag, which is
the force a fluid exerts on a surface in the flow direction.
When a fluid is forced to flow over a curved surface, such as the back
side of a cylinder, the boundary layer may no longer remain attached to the
sur face and separates from the surface—a process called flow separation
(Fig. 1–16). We emphasize that the no-slip condition applies everywhere
along the surface, even downstream of the separation point. Flow separation
is discussed in greater detail in Chap. 9.
A phenomenon similar to the no-slip condition occurs in heat transfer.
When two bodies at different temperatures are brought into contact, heat
transfer occurs such that both bodies assume the same temperature at the
points of contact. Therefore, a fluid and a solid surface have the same temperature at the points of contact. This is known as no-temp
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