The relation between the helicity and the rate of dissipation of turbulent kinetic energy in turbulent flows has been a matter of debate. Herein, direct numerical simulations of turbulent Poiseuille and Couette flow were used in combination with the tracking of helicity, helicity density, and dissipation along the trajectories of passive scalar markers to probe the correlation between helicity and dissipation in anisotropic turbulence. The Schmidt number of the scalar markers varied between 0.7, 6, and infinite (i.e., fluid particles), while the friction Reynolds number for both simulations was 300. The probing tools were the autocorrelation coefficients, the cross correlation coefficients between helicity and dissipation, and the joint probability density function calculated in the Lagrangian framework along the positions of the scalar markers. These markers were released at different locations within the flow field, including the viscous wall sublayer, the transition layer, the logarithmic region, and the outer flow. In addition, conditional statistics for scalar markers that dispersed most or least in the flow field were also calculated. It was found that helicity and dissipation changed along the trajectories of scalar markers; however, helicity and dissipation were not correlated in the Lagrangian framework. There was anticorrelation between helicity and dissipation in the near wall region, which was less obvious in the logarithmic region. More importantly, helicity could be used to characterize the alignment of the fluctuating velocity and vorticity vectors along the trajectories of scalar markers that disperse the farthest in the direction normal to the channel wall.
Contrasts between momentum and scalar transport over very rough surfaces
Largeeddy simulations are conducted to contrast momentum and passive scalar transport over large, threedimensional roughness elements in a turbulent channel flow. Special attention is given to the dispersive fluxes, which are shown to be a significant fraction of the total flux within the roughness sublayer. Based on pointwise quadrant analysis, the turbulent components of the transport of momentum and scalars are found to be similar in general, albeit with increasing dissimilarity for roughnesses with low frontal blockage. However, strong dissimilarity is noted between the dispersive momentum and scalar fluxes, especially below the top of the roughness elements. In general, turbulence is found to transport momentum more efficiently than scalars, while the reverse applies to the dispersive contributions. The effects of varying surface geometries, measured by the frontal density, are pronounced on turbulent fluxes and even more so on dispersive fluxes. Increasing frontal density induces a general transition in the flow from a wall bounded type to a mixing layer type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due to reduced contributions from largescale motions in the roughness sublayer. This study highlights the need for distinct parameterizations of the turbulent and dispersive fluxes, as well as the importance of considering the contrasts between momentum and scalar transport for flows over very rough surfaces.
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 Award ID(s):
 1664021
 NSFPAR ID:
 10298395
 Date Published:
 Journal Name:
 Journal of Fluid Mechanics
 Volume:
 880
 ISSN:
 00221120
 Page Range / eLocation ID:
 32 to 58
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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