Stereo Tomography of Turbulent Air Jets*

We experimentally measure the three-dimensional structure and dynamics of turbulent jets of air. Jets at variable speeds (different Reynolds numbers) are made with a standard hair dryer, and variations in air density are observed using background-oriented schlieren (BOS). Two views are obtained using binocular prisms, and 3D reconstruction of air density gradients is made by cross-correlating and triangulating the stereo measurements. Using rectangular and circular apertures, we study both planar and round free turbulent jets.

The entire transition from laminar flow at the outlet to turbulent expansion of the free jet is imaged. Entrainment of the surrounding air is observed, with an envelope expansion and interface intermittency that follow conventional scaling laws. Increasing the Reynolds number does not change the angular spread of expansion but does reduce the spatial and temporal scales of the internal turbulence. The evolution of the axial velocity profile is shown to be self-similar, and the decay of the centerline velocity matches established theory. 

Details of the optical flow and computational reconstruction will be given. Comparisons with conventional stereo BOS and BOS tomography will be discussed.