A laboratory investigation of the dynamics of tornado-like vortices using a laser Doppler velocimeter
Abstract
Radial and tangential velocity components were measured with a laser Doppler velocimeter for two vortex configurations corresponding to a low swirl ratio (S = 0.226; hereafter C1) and a moderate swirl ratio (S = 0.561; hereafter C2). While vortex breakdown was present in both cases, C1 was single-celled at the surface. C2 had a two-cell circulation throughout the depth of the measurement region. Radial profiles through the cores of both vortices were affected by vortex wander and offset from the line of measurement. A transformation scheme was developed and tested which reduced errors below 5% for r $\sbsp{>}{\sim}$ 1.5 cm. The radial component velocity fields of both vortices show strong inflow very near the surface in response to the frictional dissipation of angular momentum. The tangential velocity fields exhibit the classic features of a Rankine-combined vortex, i.e., a rotational core and irrotational outer flow. Vertical velocities derived through integration of the continuity equation show that w$\sb{\rm max}$ $\sim$ v$\sb{\rm max}$ in C1 and w$\sb{\rm max}$ $\sim$ 0.5v$\sb{\rm max}$ in C2. While the greatest total wind speed is observed in C1, a lower, broader region of total windspeed in excess of 8 m s$\sp{-1}$ occurs in C2. Thus two-cell tornadoes may only seem to have higher wind speeds because this region of relatively high speed flow is brought to bear on surface structures. Fluid parcels that attain the highest rotational speeds derive their vorticity through only modest tilting as they approach the corner region, but principally through subsequent stretching within the corner region. A comparison of the mean velocity fields with the numerical results of Wilson and Rotunno (1986) and Lewellen and Sheng (1980) shows that both the constant eddy viscosity and second-order closure models replicate the salient features of the mean flow field. Angular momentum budgets of C1 and C2 reveal a distinct difference in the eddy divergence pattern surrounding the tangential velocity maxima in the two cases, and it is postulated that the difference is related to a negative viscosity mechanism that exists in the one-cell vortex and is at least partly responsible for the production of extreme wind speeds.
Degree
Ph.D.
Advisors
Snow, Purdue University.
Subject Area
Atmosphere
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.
