APCWE

The Eighth Asia-Pacific Conference on Wind Engineering,December 10–14, 2013, Chennai, India

LES modeling of the effects of ground roughness and translation on tornado-like vortices

Abstract: The numerical simulation by using LES turbulent model for the tornadoes over roughness and those with a translation speed was carried out in a Ward type simulator. The ground roughness was simulated through adding an additional momentum source in the Navier-Stokes equation and the tornado translation was modeled by providing a relative motion on the ground. The effects of ground roughness and the translation of tornado on the flow fields of two typical tornado-like vortices, vortex breakdown and multi-vortex, were investigated and the detailed velocity distributions as well as the ground pressure were examined. It was found that, at the high elevation, Vc and rc shows the same trend versus the external swirl ratio no matter the translation of tornado is introduced or no. However, if the ground is rough, the core radius at high elevation changes dramatically. The ground roughness will expand the size of the core. But for the very small and very high swirl ratio cases, the ground roughness shows the effect of reducing the core size. The similarity of the flow fields after the introduction of the ground roughness and the translation of tornado was also studied.

Keywords: Tornado-like vortex, LES, ground roughness, translation, swirl ratio.

1. Introduction

The occurrence of tornado increases as global warming intensifies around the world. Therefore the researchers pay much attention to study the flow structures, dynamics as well as the similarity of the tornado-like vortex experimentally or numerically in the past decades. Many important findings have been obtained, i.e. the dominant parameters determining the flow structure, the organized swirl motion in tornadoes and the similarity between simulated tornadoes and those in nature. These studies mostly focus on the stationary tornadoes over smooth ground, however, tornadoes are always observed with translation speed, ranging from 10m/s to 30m/s, which will absolutely distort the flow fields near the ground. On the other hand the tornadoes can also happen in urban area, e.g. the tornado occurring in Joplin, Missouri, the US in 2011, killing 158 people (2012). Therefore it will make sense to study the effects of ground roughness to the flow structures close to the ground. However, due to the difficulties along with the investigations of the effects from ground roughness and tornado translation, these effects have not been subjected too much examination.

In this study we would like to study the effects from ground roughness and tornado translation. The details of the model are introduced in section 2 including its dimension, grid distribution, boundary conditions, as well as the method simulating the roughness and translation. In section 3 the flow fields of tornado-like vortices are examined in detail and the effects of roughness and translation are clarified. At last in section 4 we will examine the similarity of the tornadoes with consideration of the effects from ground roughness or tornado translation.

2. Numerical model

2.1 Discretization method

In respect that momentum and mass are mostly transported by large eddies, and considering the current computing capability, large eddy simulation (LES) is adopted to simulate the tornado-like vortex. In LES, large eddies are computed directly, while the influence of eddies

Zhenqing Liu1 and Takeshi Ishihara2

1Doctor of Civil Engineering, Tokyo University, Tokyo, Japan, [email protected] of Civil Engineering, Tokyo University, Tokyo, Japan, [email protected]

Proc. of the 8th Asia-Pacific Conference on Wind Engineering – Nagesh R. Iyer, Prem Krishna, S. Selvi Rajan and P. Harikrishna (eds)Copyright c© 2013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore. ISBN: 978-981-07-8011-1doi:10.3850/978-981-07-8012-8 070 533

Proc. of the 8th Asia-Pacific Conference on Wind Engineering (APCWE-VIII)

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