A LABORATORY INVESTIGATION OF TWO-CELLED VORTEX FLOWS (TORNADOES)

An experimental study of the steady-state kinematics, dynamics, and morphology of two-celled vortex flows has been conducted in the Ward-type tornado vortex chamber (TVC) at Purdue University, with emphasis on exploring the vertical momentum balance in the vortex core and better defining the flow near the external boundaries of the TVC. The TVC was modified for these experiments to more closely compare with numerical models and to allow the implementation of new measurement techniques. Observations of the visualized flow in two-celled vortices and time-averaged static pressure measurements on the axis and at the boundaries of the TVC are reported. Laboratory observations and measurements are compared with results of a numerical model of the TVC flow authored by Rotunno (1984). Laboratory and numerical results are analyzed in terms of the vertical momentum equation. Results show that in the Purdue TVC the flow downstream of the vortex breakdown is everywhere two-celled, with the strongest axial downflow occurring at middle levels. The pressure on the axis in the two-celled vortices increases with height immediately downstream of the breakdown, with the axial pressure gradient tending toward zero farther downstream. The flow-straightening baffle at the downstream terminus of the vortex in the TVC does not critically affect the flow provided the vortex breakdown is well upstream. Analysis of the laboratory findings within the context of the vertical momentum equation shows that the vertical shear stress can play an important role in the axial momentum balance of two-celled vortices by opposing the filling of the vortex core from aloft and so helping to maintain low pressure and high velocities near the surface. The numerical model of Rotunno (1984) is successful in qualitatively replicating several of the flow characteristics in the TVC, including two-celled flow, multiple subsidiary vortices, strongest downflow at middle levels, axial pressure increasing with height, and the role of the vertical shear stress in the axial momentum balance.