Vorticity and helicity of coherent turbulent structures in Taylor-Green and ABC flows

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AGU Fall Meeting

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San Francisco


Coherent structures are thought to play a critical role in the transport of scalar and vector quantities within turbulent flows. Local maxima in the vorticity are one way to define these structures. While helicity is a globally conserved quantity, its importance to the local dynamics of vortical structures is unknown. Vorticity amplification by stretching may lead to alignment of the local vorticity and velocity vectors reducing nonlinearity. We examine the helicity values within vortical structures found in Taylor-Green and ABC forced-turbulence simulations, and find that, although there does not seem to be any correlation between mean structure vorticity and mean structure normalized helicity at a given threshold, there does seem to be a general tendency towards alignment of the pointwise vectors within the structures (Fig. 1). This alignment becomes more pronounced as the vorticity threshold defining the structures is increased. Additionally, while the turbulent structures have a range of helicity values, the helicity of any one structure is quite well defined. The variance around the mean normalized helicity value within a structure decreases as the mean value approaches ±1 (Fig. 2). Scatter plots of mean normalized helicity (x) verse mean vorticity (y) for each structure in the domain, identified by vorticity thresholds 12σ, 15σ, and 18σ, bottom to top.


Abstract only.