14 - CONDITIONS IN THE INLET

Abstract

This chapter analyzes the conditions in the inlet. When a flat plate is placed in a uniform stream, whose velocity is parallel to the surface of the plate, a boundary layer is set up on the plate, its thickness increasing with distance from the leading edge. The first part of the boundary layer is laminar, and its thickness is proportional to the square root of the distance from the leading edge. When the thickness reaches a certain value, the flow in the boundary layer becomes unstable; soon after this, at a point called the “transition point,” the flow becomes turbulent. The boundary-layer thickness then increases rather more quickly; with a uniform velocity in the main flow and a smooth plate, the thickness varies as the four-fifths power of the distance from a point somewhere between the leading edge and the transition point. Laminar boundary layers in flows with decreasing pressure are much more stable than those in flows with constant pressure; and those in flows with increasing pressure are much less stable. Consequently, the transition point usually lies at the point of minimum pressure. In the inlet section of a duct the velocity of the main flow increases as the boundary layer grows.