Helicity in turbulent channel flow with Lagrangian computations

Coherent 3D flow structures are important for turbulent transport. Utilizing helicity and normalized helicity is an effective way to identify and differentiate between primary and secondary vortices, and to trace vortex-core streamlines [1]. Defined as the dot-product of velocity and vorticity [2], high values of helicity reflect high speed and vorticity when the angle between them is small. In this study, flow in a turbulent channel is simulated by DNS, followed by Lagrangian Scalar Tracking of passive scalar markers [3,4]. These are released from instantaneous line sources at different distances from the channel wall. The channel flow DNS is for friction Reynolds number of 300, and the passive scalars have a Schmidt number ranging from 0.7 to 2,400. By calculating the helicity and normalized helicity along the marker trajectories, the correlation between vortex flow structures with particle dispersion can be explored and the flow structures that are important for scalar turbulent transport can be identified.

References

1. Levy Y, Degani D, Seginer A, AIAA J. 28(8), 1347, 1990

2. Moffat HK, JFM, 35(1), 117, 1969

3. Nguyen Q, Papavassiliou DV, Phys. Fluids, 28(12), 125103, 2016

4. Nguyen Q, Feher S, Papavassiliou DV, Fluids 2(3), 46, 2017