The Measurement of Low Fluid Velocities with the Aid of a Tethered Sphere

Abstract

Two‐ or three‐dimensional velocity fields in hydraulic models can be measured with the aid of a tethered sphere if the absolute velocity values are very small. The apparatus is relatively simple and quite sensitive. Experimental data can be handled numerically, taking into account all effects, and print‐out is readily available in useful tabular or graphical form. The greatest disadvantage at present seems to be that the position of the tethered sphere is not stable, and it is difficult to define when turbulence is present in the flow. The average position of the sphere will most probably not correspond to the velocity average in this case. This disadvantage is, however, shared by most velocity measuring devices with low frequency response. The second inaccuracy is that the program presently available is based on the assumption that the velocity is uniform over the height of the probe. The development of a new program taking into account continuously varying velocity and temperature profiles is in progress. It is of particular importance for small spheres on long support lines. The instrument may also be inverted by using a sphere slightly heavier than water. Such an instrument would be suitable for velocity measurements near the bottom of a channel. (Key words: Data; density currents; instruments; low velocity)