Using a 100 gallon sized tank with plexiglass sides a fluid medium test system provides better data than a wind tunnel on scale models. Not to mention extremely lower costs.
Corn oil provides the ability to view the flow patterns better than smoke.
It also provides better data for physical stresses of trans-sonic and super sonic pressures on the airframes. Techniques and types of data acquired are dependent on the tester. But present video cameras and PC stop imagery provides excellent sampling of data.