139 An Eddy Injection Method for Large-eddy Simulations of Tornado-like Vortices

Wednesday, 9 November 2016
Broadway Rooms (Hilton Portland )
George H. Bryan, NCAR, Boulder, CO; and N. A. Dahl, D. S. Nolan, and R. Rotunno

This presentation describes a method for studying the internal structure of tornado-like vortices using large-eddy simulation (LES).  Initial conditions are first generated numerically using a “precursor simulation” with an axisymmetric model that has parameterized turbulence.  A three-dimensional simulation is then integrated simply using these initial conditions plus random initial perturbations.  With this approach the inner core of the simulated vortex becomes turbulent, but the inflowing near-surface flow remains laminar and thus the parameterized turbulence model accounts for most of the vertical momentum flux (in violation of LES assumptions).  To overcome this problem, a second “precursor simulation” is conducted in which turbulence develops within a small-domain LES.  Perturbation flow fields from the small-domain LES are then added (i.e., “injected”) into the large-domain LES at a specified radius.  With this approach, inflowing air near the surface becomes (and remains) turbulent, consistent with LES principles.  Near-surface inflow tends to be deeper and weaker, and inner-core vortex structure changes substantially, when turbulence is injected.
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