In his article on the Lotus 79 in the July issue of Motor Sport, D.S.J. states that the suction effect produced by the venturi-like underside of the car is due to the fact that air passing through, a venturi drops in pressure as it expands. This is not correct. A venturi has the shape of an hour glass: a narrow part (the throat), separating two wide parts. As fluid, whether liquid or gas, passes through the venturi its velocity increases due to the area contraction, and then increases again. It is the velocity increase which is basically responsible for the pressure drop: the gain in kinetic energy is compensated for by a drop in potential energy (or pressure).
To say the air expands implies a density reduction, which occurs to a significant degree (say 10%) only when the local Mach number comes close to 0.5, i.e. a local air speed of about 380m.p.h. Even in the venturi throat, it seems unlikely that such velocities are reached even for a car at maximum speed.
Before coming across this conceptual error of basic aerodynamics, I had been highly impressed by D.S.J. ‘s implied ability to determine the purpose of the Brabham-Alfa’s fan by studying the angle of the blading (caption to photograph on p. 935).
Wettingen, Switzerland Dr. M. P. ESCUDIER