Overview

This book is about buildings, which are bluff bodies, that is to say, they have thickness. The opposite to a bluff body is a plate. There is a version of a plate which has a little thickness, this is called an aerofoil, and this behaves as a plate. It is worth spending a line or two on aerofoils.

The essential for an aerofoil is that it shall have a sharp trailing edge (rear end): it also has a rounded leading edge (front end). The requirement of the sharp trailing edge is that the flows over the top and bottom surfaces of the aerofoil meet at the trailing edge and flow off tangentially: this is called the "Kutta-Jowkoski Hypothesis". The implication of this is that, when the aerofoil is placed at a small incidence to an airstream, the flow over the lower surface would want to turn round the trailing edge to flow up the tail end of the upper surface to a separation point on the upper surface, equating the length of path on upper and lower surfaces. Because the trailing edge is sharp, a shear stress, which is caused because of the viscosity of the air, prevents this sharp turning, so the flow on the lower surface separates from the lower surface at the trailing edge and continues in the windward direction. This in turn creates a suction at the tail of the upper surface which draws the would-be separation point down to the trailing edge. The rounded...