Abstract
Mechanically induced air flow is pulsating even when it may seem steady to the physical senses. This pulsation causes errors in velocity measurement that are often as great as twenty per cent when the conventional pitot tube or orifice is used with a liquid manometer, and it causes an increase in the velocity-energy and fluid-friction loss that can easily reach fifty per cent.
A simple portable instrument has been developed by the author which gives the true average air velocity for any amount of pulsation. In this instrument, the air velocity creates a water velocity through a submerged orifice, and this water velocity is always proportional to the air velocity at that instant, and can therefore be used to determine the true average air velocity, regardless of the amount of pulsation.
The instrument was first verified by checking it on velocity-wave forms of known characteristics. Following that a blower was tested for several conditions of operation and the air velocity was determined with the instrument and compared with the values given by the conventional instruments. In a similar manner the intake air velocity was measured for single-, four-, and six-cylinder internal-combustion engines, and the effectiveness of a receiver tank for damping pulsations was studied.
The last section of the paper is devoted to similar tests on the slip-stream velocity of an airplane propeller where the effect of pulsation on the efficiency of the propeller was worked out.
