Session MM - Chaos and Fractals.
MIXED session, Tuesday afternoon, November 23
Whidbey, Westin Seattle
LES ideas, in particular the dynamic model (Germano et al. 1991), are extended and applied to the problem of SGS drag modeling for flow over trees. The SGS branch drag coefficient is determined as in dynamic LES by enforcing model consistency amongst SGS branches and the smallest resolved branches, and assuming scale invariance of the drag coefficient. The formulation allows for dependence of the drag coefficient on the geometrical configuration of a given branch (e.g. orientation relative to the incoming flow), which results in a linear system for the drag coefficients. Results are obtained by performing LES of high-Re turbulent flow over idealized leafless, self-similar fractal trees as a test case. A posteriori tests show that the dynamic drag coefficients are stable in many cases, although less robust coefficients are associated with trees with a high degree of branching. The total SGS drag force is observed to be a significant fraction of the total drag force on the tree, highlighting the importance of the drag model. Observed nontrivial dependence of the dynamically obtained drag coefficients on branch orientation will also be presented.