The Effect of Initial Conditions on Streamwise Vortices in the Plane Turbulent Mixing Layer

Abstract

In this research we investigate the effect of inflow conditions on the streamwise vortices that exist in the plane turbulent mixing layer. Both initially-laminar and initially-turbulent mixing layers are considered here. The effect of the imposed fluctuations is assessed in the initially-laminar mixing layer simulations through the use of a white noise disturbance environment, and a physically-correlated fluctuation environment produced by an inflow generation method. The initially-turbulent mixing layer has its inflow condition produced by the inflow generation method. Each simulation produces good statistical agreement with the experimental data. The initially-laminar simulation with correlated fluctuations predicts the presence of statistically stationary streamwise vortices, whilst the other two simulations do not.

Resolved Scalar Mixing in Large Eddy Simulations of a Low Reynolds Number Plane Mixing Layer

“…The secondary shear stress contours of Cases WNL and WNH in Figures 15a-15b are very similar, in that there is no particular clustering of u w across the span. This irregular distribution of u w has been reported in other simulations of high Reynolds number turbulent mixing layers, 27 and indicates that any streamwise vorticity present in the mixing layer originating from initially laminar conditions with a white noise disturbance environment is not statistically stationary. In Figure 15c, the u w distribution of Case RRML shows evidence of clustering of the secondary shear stress.…”

“…This type of secondary structure has been observed in other mixing layer simulation originating from initially-laminar conditions with a white noise disturbance environment. 27,34 Further downstream (Figures 6b-6c) a single row of 'mushroom-shaped' eruptions is visible along the span of the braid region, indicating the presence of a row of streamwise vortices. In Case WNH ( Figure 7) a similar cross-plane scalar evolution is observed, although the wavelength of the secondary instability at x = 0.07m is qualitatively much shorter than that of Case WNL.…”

“…It is interesting to note that the peaks and troughs in the entrainment ratio profile are anti-correlated with the peaks and troughs in the mixing layer centreline. Experiments and numerical simulations have noted that the maxima and minima in the centreline distribution can be associated with the presence of statistically stationary streamwise vorticity, 35,36 and this is explored further here.…”

“…It has been shown both experimentally and numerically that the centreline locus can be used to infer the presence of stationary streamwise vorticity in the mixing layer. 35,36 Cases WNL and WNH (Figures 14a and 14b) show no particular periodicity in the variation of the mixing layer centreline across the span, and any variation across the span has a low amplitude. In the simulations where correlated fluctuations provide the inflow disturbances, however, there is evidence for significant spanwise variations in both the mixing layer centreline and the entrainment ratio.…”

“…The character of this streamwise vorticity is quite chaotic however, and the flow has not attained a single row of streamwise vortices that has been observed in both experiments and other numerical simulations. 35,36 At this streamwise measurement station the pairing parameter, x * i = Rx/(30θ i ) has a value of x * i = 5.05. In other experiments, the secondary shear stress contours had a similar distribution for a comparable pairing parameter value.…”