Effects of fine-scale debris on different tornado corner flows

The effects of fine-scale debris (e.g., dirt or sand) on tornado corner flows of different types are explored using high-resolution large-eddy simulations. The debris is treated as a second continuous fluid, of variable density and comprised of mono-sized spherical particles, coupled with the airflow through drag forces. Surface fluxes are dependent on the flow structure just above the surface and assumed surface and debris properties. The primary physical variables considered are tornado strength and corner flow swirl ratio, surface conditions, and debris size and density, with some additional tests performed with altered gravity and viscosity. The qualitative structures of the air-flow and debris cloud are found to change significantly depending on tornado strength and swirl ratio and debris properties. For example, a high-swirl tornado corner flow that has multiple strong secondary vortices in the absence of debris will loft debris primarily within the secondary vortices if the velocity scale is modest and/or debris larger or denser, but, given higher velocity scale and/or smaller or lighter debris, will reach sufficient debris loading in the main updraft annulus to severely weaken or even destroy the secondary vortices. Debris mass near the surface can significantly exceed that of the air and remain important to altitudes of several 100 meters. Local air velocities are typically reduced by the presence of debris, but total momentum (air plus debris) sometimes greatly increased, so that the damage potential of the tornado might be either increased or decreased depending on the tornado and debris properties.

Three critical dimensionless parameters affecting the debris dynamics are identified: the corner flow swirl ratio; the ratio of a characteristic core velocity scale to the terminal velocity of the debris in free fall; and the ratio of a characteristic radial acceleration in the core flow to gravity. Quasi-steady state debris cloud properties (mean dimensionless debris-cloud total mass, height, radius and interior cone angle) and effects on flow structure (dimensionless peak mean swirl and vertical velocities and qualitative flow features) are considered as functions of these three parameters.