A Parametric Study of Compact Wind Borne Debris in Tornadoes

Author

Akash YadavFollow

Date of Award

5-2025

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Committee Chair/Advisor

Nigel Kaye

Committee Member

Abdul Khan

Committee Member

Farhang Forghanparast

Abstract

Debris lofted and transported by tornadoes is a significant source of damage during these storms. Several approaches have been taken to model such debris flight, ranging in complexity from fine-scale CFD simulations to coupling the compact debris flight equations with simplified analytic models for the tornado vortex. Both approaches have their merits, though the latter comes with significantly lower computational costs. Despite the simplifications, this second approach has successfully captured details of debris flight events established in post-tornado forensic surveys. Herein, we take advantage of the low computational cost of the latter approach to undertake a detailed parametric study of compact debris flight in tornadoes. The tornado was simulated using the Baker and Sterling vortex model and the compact debris flight equations. The tornado model is parameterized in terms of a tornado aspect ratio (δ = z_m/r_m), the swirl ratio, (S = v_m/u_m), and the tornado Froude number (Ψ = gr_m/u²_m). For the standard compact debris flight model, the debris is characterized by Φ = ρ_air Ar_mC_D /2m.

However, unlike more straight-line windstorms, such as gust fronts of hurricanes, tornadoes have relatively small radii. Therefore, near the center of the tornado, there can be a significant radial pressure gradient. This pressure gradient has the potential to produce an additional force on a piece of debris due to the pressure differential across the surface of the debris. This ‘buoyancy’ force can become significant for less dense debris such as wood. To illustrate this, a simulation was run using the approximate wind field for the July 1, 2023, Didsbury, AB EF4 tornado. The simulation was run for a 20 cm wooden sphere with and without the additional buoyancy force. Their result shows significant differences in the trajectories in the modeled radial location during flight. Including the buoyancy force introduces a second debris parameter Γ = ρ_air /ρ_debris. There are, therefore, three tornado parameters (δ, S, Ψ), two debris parameters (ϕ, Γ) and the initial conditions which require specification of the vertical and radial initial locations and the three components of the initial debris ii velocity for a total of five parameters and five initial conditions.

A large-scale parametric study was done by changing these parameters and initial conditions. Then, a series of simulations are run for a tornado ranging from EF2 to EF5 scale. Debris simulated ranges from tiny raindrops and sediment through larger aggregates like gravels and wood chunks to cars. The result highlights the conditions under which debris can remain airborne for prolonged periods, when debris will be lofted and ejected radially, and when debris will simply fall out of the wind field. It is seen that the visible core of the tornado can be considerably smaller than the actual region of extreme wind speeds, as the visible core only shows small-scale debris that is drawn in and lofted by the tornado. Whereas large-scale debris can remain in the air well outside the visible core.

Recommended Citation

Yadav, Akash, "A Parametric Study of Compact Wind Borne Debris in Tornadoes" (2025). All Theses. 4550.
https://open.clemson.edu/all_theses/4550