The shearless turbulence mixing layer

Abstract

The interaction of two energy-containing turbulence scales is studied in the absence of mean shear. The flow, a turbulence mixing layer, is formed in decaying grid turbulence in which there are two distinct scales, one on either side of the stream. This is achieved using a composite grid with a larger mesh spacing on one side of the grid than the other. The solidity of the grid, and thus the mean velocity, is kept constant across the entire flow. Since there is no mean shear there is no turbulence production and thus spreading is caused solely by the fluctuating pressure and velocity fields. Two different types of grids were used: a parallel bar grid and a perforated plate. The mesh spacing ratio was varied from 3.3:1 to 8.9:1 for the bar grid, producing a turbulence lengthscale ratio of 2.4:1 and 4.3:1 for two different experiments. For the perforated plate the mesh ratio was 3:1 producing a turbulence lengthscale ratio of 2.2:1. Cross-stream profiles of the velocity variance and spectra indicate that for the large lengthscale ratio (4.3:1) experiment, a single scale dominates the flow while for the smaller lengthscale ratio experiments, the energetics are controlled by both lengthscales on either side of the flow. In all cases the mixing layer is strongly intermittent and the transverse velocity fluctuations have large skewness. The downstream data of the second, third and fourth moments for all experiments collapse well using a single composite lengthscale. The component turbulent energy budgets show the importance of the triple moment transport and pressure terms within the layer and the dominance of advection and dissipation on the outer edge. It is also shown that the bar grids tend toward self-similarity with downstream distance. The perforated plate could not be measured to the same downstream extent and did not reach self-similarity within its measurement range. In other respects the two types of grids yielded qualitatively similar results. Finally, we emphasize the distinction between intermittent turbulent penetration and turbulent diffusion and show that both play an important role in the spreading of the mixing layer.

Mixing and Diffusion in Regular/Fractal Grid Turbulence

“…In this case, the turbulence intensities are not homogeneous in the y direction, unlike cases (a) and (b), and a shearless mixing layer is generated around y = 0. The result agrees qualitatively with a previous measurement (Veeravalli and Warhaft 1989), although a larger computational domain and Reynolds number will be required to simulate the fully developed shearless mixing layer (Veeravalli and Warhaft 1989). Figure 46 shows the streamwise distribution of the turbulence intensity of the pressure fluctuation p 2 rms for the conventional biplane grid (a).…”

“…47c) are asymmetric, like the turbulence intensity (Fig. 45), as the eddy scale or eddy diffusivity changes in the y direction (Veeravalli and Warhaft 1989). Figure 48 shows the streamwise distributions of the vertical turbulent heat flux < vθ > for the biplane grid (Runs 1 and 2) normalised by of < vθ > follows a power law in the developed region, as reported in previous studies (LaRue and Libby 1981;Huq and Britter 1995).…”

“…The turbulence-generating grid, on which the velocity components are set to zero, is located 20 mm downstream from the entrance. Four types of grid are numerically constructed: (a) a square-mesh biplane grid, (b) a squaremesh single-plane grid, (c) a composite grid consisting of parallel square-bars with different mesh sizes (Veeravalli and Warhaft 1989) and (d) a fractal grid (Seoud and Vassilicos 2007;Hurst and Vassilicos 2007;Mazellier and Vassilicos 2010;Valente and Vassilicos 2011;Gomes-Fernandes et al 2012Nagata et al 2013). The mesh size M and thickness of the square rod d are listed in Table 5 for (d).…”

Interfaces and inhomogeneous turbulence

“…[7], Ishihara et al 2010); and -boundary or internal interfaces associated with discontinuities in density or density gradients (IB), where there are significant body forces.…”

Recent Advances in DNS and LES. Proceedings of the Second AFOSR International Conference on DNS and LES Held at Rutgers - The State University of New Jersey, New Brunswick, New Jersey on June 7-9, 1999

“…The shearless mixing layer is an ideal case for studying the turbulent transportation process without the influence of the instability mechanism introduced by mean shear. Experimental results of transportation of passive scalar in shearless mixing layer have been provided by Veeravalli and Warhaft (1989) [5] and they are good resources for verification of numerical study. Both direct numerical simulation (DNS) and large eddy simulation (LES) are good choices for better understanding of the mechanism and prediction of quantitative properties in the turbulent transportation.…”