(296g) Helicity in Turbulent Transport

Recent experimental measurements of helicity (i.e., the inner product of the velocity and the vorticity vector that indicates the swirling motion of a flow structure) have been reported for laminar flow, allowing the possibility to obtain experimental measurements for helicity in turbulent flow [1]. It is however quite difficult to accomplish such a feat in a laboratory. Numerical results using direct numerical simulations of turbulent flow on the other hand can provide helicity information, while we wait for experimental results. The importance of helicity is mostly related to the structure of the velocity field and the coherent structures that exist in turbulent flows. Theoretical results have shown that high helicity flow regions are associated with low dissipation of kinetic energy [2]. It is, thus, expected that high helicity regions of the flow would be associated with coherent structures that would not dissipate as quickly as others in the flow field. However, how are these structures related to turbulent dispersion of heat or mass? Would such structures of high helicity density contribute to the transport of heat or mass from the wall in the case of wall turbulence? Since high values of helicity indicate flow structures where the velocity and vorticity are high, and the angle between these two vectors is small, can helicity be used to identify flow structures that contribute to turbulent dispersion more than others? We conduct simulations of turbulent channel flow with direct numerical simulation (DNS) techniques at friction Reynolds number of 300, followed by our Lagrangian Scalar Tracking (LST) process of monitoring the motion of passive scalars in the flow filed [3,4]. The Prandtl number (Pr) of the passive scalars varies between 0.7 and 200. [3,4]. The scalar markers are released from several instantaneous line sources, at different locations away from the channel wall, within the viscous wall subregion and the buffer region of the flow. We further calculate the helicity of the flow structures at the location of these Lagrangian markers of scalar transport to obtain a detailed view of the helicity distribution and its changes as the markers disperse in the flow field. The goal is to develop a criterion that can identify coherent structures that contribute mostly to heat or mass transfer from the wall and to use helicity as the main criterion. We will discuss our methodology for obtaining the Lagrangian helicity distribution and will present findings that strongly indicated that flow structures that propagate in the flow field while aligning the velocity and vorticity vectors are the ones that maximize turbulent transport.

References:

1. W. Scheeler, W. M. v. Rees, H. Kedia, D. Kleckner, and W. T. M. Irvine, Science 357, 487 (2017).
2. K. Moffatt, Annual Review of Fluid Mechanics 24, 281 (1992).
3. V. Papavassiliou, International Journal of Heat and Mass Transfer 45, 3571 (2002).
4. 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)
5. Nguyen and D. V. Papavassiliou, Physical Review E 88 (2013).