Electromagnetic Velometry. I. A Method for the Determination of Fluid Velocity Distribution in Space and Time

Abstract

A method is described which permits the determination of the local velocity of flow, as well as of turbulent fluctuations at a given point, or a prescribed series of points in a moving liquid. The method is based on the induction of a potential gradient in the flowing medium as it traverses a magnetic field perpendicularly to its direction. The absence of lag in the process of induction makes the method particularly suitable for the study of rapidly varying velocities and the sharp localization of measurements allows the determination of spatial velocity distributions. In an adaptation of the method for small scale laboratory experiments, the magnetic field is created by a large a.c. magnet in whose gap the conduit is placed, while two minute exploring electrodes pick up the induced e.m.f. at the desired locations. For large scale experiments and field work, a self‐contained unit has been designed which comprises a minute electromagnet with attached pick‐up electrodes. The applicability of this method is demonstrated by velocity distribution curves taken under different conditions and by records of liquid turbulence and transient velocity changes in different forms of flow. This paper is confined to the presentation of the principle involved, to the description of the apparatus, to the discussion of the experimental technique, and to demonstrations of the applicability of the method. A theoretical analysis of the characteristics and limitations of this method will be dealt with in a separate publication.