(307e) The Role of Helicity in Turbulent Transport of Passive Scalars

Coherent flow structures in turbulent flow are accepted to be quite important in turbulent transport. Graphical representations of three-dimensional flow fields with concentrated vortices have been carried out extensively, with the introduction of laser-Doppler anemometry and the availability of large amount of simulation results. Flow-visualization techniques allow vortices to be experimentally observed, as well as the vortex cores that are characterized by having higher local speeds than the ambient fluid. However, representing such complex flow structures in three-dimensional flow field remains a challenge, especially for high Reynolds number flow. One promising method is to utilize helicity density and normalized helicity, which has been shown to effectively identify and differentiate between primary and secondary vortices, and to trace the vortex-core streamlines [1]. Defined as the cross product of a velocity vector u and vorticity vector ω [2], high values of helicity reflect high speed and high vorticity when the relative angle between them is small. Direction of swirl of the vortex relative to the streamwise velocity could also be derived from the sign of the helicity density. The application of helicity density as a pseudoscalar for characterizing complex three-dimensional flows, however, has been limited to Eulerian schemes only. In this study, simulation of a turbulent channel flow is conducted by direct numerical simulation (DNS), followed by Lagrangian Scalar Tracking (LST) of the motion of passive scalars with different Schmidt numbers (Sc) [3,4]. Mass markers are released from several instantaneous line sources, at different locations away from the channel wall, spanning the viscous wall subregion, the buffer region, the log layer and the outer region of the flow. The channel flow DNS is for friction Reynolds number of 300, and the passive scalars have a Sc ranging from 0.7 to 2,400. By calculating the helicity along the trajectories of these markers in the Lagrangian framework, we can elucidate the correlation between the vortex flow structures with particles dispersion at different regions in the flow field and identify the ones that are most important for scalar turbulent transport.

References:

1. Levy, Y., Degani, D. Seginer, A. Graphical Visualization of Vortical Flows by Means of Helicity. AIAA Journal 28 (1990)
2. H.K. The degree of knottedness of tangled vortex lines. Journal of Fluid Mechanics 35, 117-129 (1969)
3. Nguyen, Q. and D.V. Papavassiliou. A statistical model to predict streamwise turbulent dispersion from the wall at small times. Physics of Fluids, 28(12), Art. 125103 (2016)
4. Nguyen, Q., Feher, S., and D.V. Papavassiliou, “Lagrangian Modeling of Turbulent Dispersion from Instantaneous Point Sources at the Center of a Turbulent Flow Channel,” Fluids, 2(3), Art. 46 (2017)