Clamp-on transducers for measuring swirl, cross flow and axial flow

It is sometimes said that the ideal flowmeter would be immune to swirl and cross flow. Since no real flowmeter is ideal, it seems that one would like to know if swirl or cross flow are present at a metering site, and if present, in what amount. It turns out that simple modification or reorientation of standard clamp-on transducers for liquid axial flow leads to totally noninvasive solutions for liquid swirl and liquid cross flow. To obtain analogous measurements in gases, however, one to three holes in the pipe or pressure boundary are generally required. Clamp-on laboratory results are presented for water in unperforated 2-inch pipes (1D≈51 mm) and for air in 10-inch (254-mm ID) steel pipe containing one hole for the measurement of swirl or circulation, a second hole for cross flow and a third hole for axial flow. In another air experiment, conducted in a small wind tunnel, circulation about an airfoil is related to lift