Using micro-bubbles as acoustic targets for large scale fluid flow experiments

There are many times when it is useful to operate or test acoustic profiling and velocity sensors in laboratory facilities. Unfortunately, the often clean, clear water in such facilities provides little or no backscatter for these instruments to operate. Additional scatterers may be introduced in some cases but this can be unpractical in large facilities or may introduce volumes of particulate matter that are unacceptable. In this note, we describe the use of the Dissolved Air Floatation (DAF) method for creating large quantities of microscopic bubbles to serve as acoustic targets. The advantage of the approach is that it is comparatively inexpensive and does not contaminate the water in any way. A limitation of the approach is that bubbles rise through the water and therefore must be continuously produced. The method is demonstrated in the Institute of Ocean Technology - Ice Tank facility which is 12 m wide, 3 m deep, and 90 m long. In this tank, a large plume of bubbles could be injected at mid-depth and would collectively rise to the surface at a speed of 5 to 10 cm s^-1. The rise speed for individual 100 μm bubbles expected from a DAF system is about 1 cm s^-1 so it is likely that bubble residence time could be increased by dispersing the bubbles through the water column.