Title

Large -eddy simulation of the effects of debris on tornado dynamics

Semester

Fall

Date of Graduation

2006

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

David C. Lewellen.

Abstract

Tornadoes are one of the most intensely violent natural phenomena, which often bring considerable damage. Debris clouds are commonly observed along the tornado path. Studying tornadoes with debris is potentially important for several reasons: particle suspension may dramatically change the structure of the carrier flow; debris clouds represent one of the primary visual signatures of a tornado; large debris loadings can significantly enhance damage potential; differences between air and debris flow can complicate interpretation of Doppler radar measurements of velocity fields.;In earlier work, debris was added to an existing high-resolution LES model using a two-fluid Eulerian-Eulerian approach. The debris cloud was treated as a second continuous fluid of variable density, coupled with the airflow through drag forces in the momentum equations (two-way coupling). A surface model was proposed to implement debris surface boundary conditions where debris surface mass and momentum fluxes are dependent on the flow structure just above the surface and assumed surface and debris properties. An initial LES study was performed using a single tornado type with three debris sizes. In the present study, extensive model validations are performed including Lagrangian particle tracking (Eulerian-Lagrangian approach), multi-species model simulation, quantitative conservation checking and comparisons with existing wind tunnel experiments. Based on extensive parametric analysis, three critical dimensionless parameters affecting the debris dynamics are identified: the corner flow swirl ratio Sc; the ratio of a characteristic core velocity scale to the terminal velocity of the debris in free fall Av ; and the ratio of the characteristic radial acceleration in the core flow to gravity Aa. Then the earlier LES study is extended by performing an extensive set of high-resolution large eddy simulations of debris lofting and transport behaviors in different tornado types using the two-fluid model. Simulation results show that for the same type of tornado, quasi-steady state debris cloud properties (e.g., debris loading, debris-cloud total mass, height, radius and interior cone angle) and effects on airflow structure and dynamics are found to change significantly with variations of Av or Aa. Given the same Av and Aa for different swirl ratio tornadoes, not only can the total debris mass loadings result in large changes, the debris dynamics and structure change significantly. Regardless of how it is achieved, it has been found that an increase in total debris loading generally produces a decrease in peak swirl or vertical velocities in the corner flow region; the location of the peak velocities moves outward relative to that in the no debris case. Peak total momentum is increased dramatically for a tornado with debris, and the motion of debris and airflow are different, especially for large debris.;Further sensitivity study of secondary parameters including variations of particle properties, gravity and surface properties strengthen the conclusion that Av, Aa and Sc are sufficient to parameterize the leading order behavior of a tornado with the presence of debris.

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