An Immersed-Boundary (Ib)-Based 3D Tornado Model And Its Application

A tornado features kinematically combining three major co-existent components, namely, updraft, translation, and rotation, which involve all the three dimensions in space while transient in time. For numerical simulation of a tornado-building interaction scenario, it looks quite challenging to seek a set of physically-rational and meanwhile computationally-practical boundary conditions to accompany traditional CFD approaches; thus, little literature can be found, as of today, in three-dimensional (3D) computational tornado dynamics study. Inspired by the development of immersed boundary (IB) method, this study employed the re-tailored Rankine-combined vortex model (RCVM) that applies the “relative motion” principle to the translational component of tornado, such that the building is viewed as “virtually” translating towards a “pinned” rotational flow that remains time-invariant at the far field region. This revision renders a steady-state kinematic condition applicable to the outer boundary of a large tornado simulation domain, successfully circumventing the boundary condition updating process that the original RCVM would have to suffer, and tremendously accelerating the computation. Here, this re-tailored RCVM was extended to its 3D version with the aid of logarithm law that describes the vertical flow evolution. Eventually, this tornado model was embedded in Incompact3D, an academic high-order finite difference turbulent flow solver, resulting in a practical powerful 3D tornado-building interaction simulation tool. A case study examined the tornadic wind induced loadings on a prismatic building; over all three directions, the vertical force component was found dominant, which effectively suggests the uprooting effect as observed in many reported scenes after a devastating tornado swept over.