Chapter 5 Boundary Layer (BL) 5.1 Introduction D'Alembert Paradox(佯谬,疑题): A body immersed in a frictionless fluid has zero drag au a ou au l-+1 p ax 2 Ox- OyChapter 5 Boundary Layer (BL) 5.1 Introduction * D’Alembert Paradox（佯谬，疑题）: A body immersed in a frictionless fluid has zero drag.           +   +   = −   +   2 2 2 2 1 y u x u x p X y u v x u u   