Experimental study of the fine-scale structure of 
conserved scalar mixing in turbulent shear flows. 
Part 2. <i>Sc</i>≈1

Buch, Kenneth A.; Dahm, Werner J. A.

doi:10.1017/s0022112098008726

Cited by 186 publications

(154 citation statements)

References 56 publications

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“…Our data show that the scale l max follows now the same dependence with Reynolds number as the Kolmogorov scale, i.e. l max ∼ R −3/2 λ This result is similar to finding in [10,11] for Sc ∼ 1 and does not change for Sc ≫ 1.…”

Section: Fig 1: (Color Online)supporting

confidence: 89%

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Geometry of Intensive Scalar Dissipation Events in Turbulence

2006

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Maxima of the scalar dissipation rate in turbulence appear in form of sheets and correspond to the potentially most intensive scalar mixing events. Their cross-section extension determines a locally varying diffusion scale of the mixing process and extends the classical Batchelor picture of one mean diffusion scale. The distribution of the local diffusion scales is analysed for different Reynolds and Schmidt numbers with a fast multiscale technique applied to very high-resolution simulation data. The scales take always values across the whole Batchelor range and beyond. Furthermore, their distribution is traced back to the distribution of the contractive short-time Lyapunov exponent of the flow. When a scalar concentration field θ(x, t) is transported in a turbulent flow very large scalar gradients are generated which can be associated with potentially intensive mixing [1]. Frequently, such large-amplitude gradient regions exist across scales that are finer than the smallest turbulent eddies, a case which is known as the Batchelor regime of scalar mixing [2]. Their cross-section is usually estimated by a single mean diffusion scale, the Batchelor scale η B , that equilibrates advection by flow and scalar diffusion. However, scalar gradients are known to fluctuate strongly in turbulent mixing. These fluctuations are caused by the fluctuating scalar amplitudes and by the varying spatial sections across which the scalar differences are built up. Both aspects will cause the strong spatial variability of potentially intensive mixing regions. Thus, a whole range of local diffusion scales l d , which quantifies exactly this variability, will exist around the mean diffusion scale η B . Their distribution is interesting for several reasons. It can enter mixing efficiency measures [3]. The scales prescribe the extension of chemically reactive layers in which the combustion of fuel takes place [4] or the variations of the fluorescence signal as used for the measurement of zooplankton patchiness in the ocean [5].In this Letter, we want to calculate this local diffusion scale distribution p(l d ). It arises from the competition of two dynamic processes. On one hand, the scale distribution will be affected by molecular diffusion that causes a diminishing of existing steep gradients as well as the completion of their formation by reconnection [6,7]. On the other hand, the scales will be determined by the statistics of the local advection flow patterns that pile up scalar differences. While the strongest scalar gradients were related to instantaneous velocity gradients in Ref.[8], we will go one step further in the second part of the Letter and relate the local diffusion scale distribution to the distribution of Lagrangian contraction rates of the flow, as given by the smallest of the three finite-time Lyapunov exponents along Lagrangian trajectories. The Lagrangian approach incorporates the temporal evolution of the persistent flow patterns that eventually generate the strongest scalar gradients or the finest local dissipation...

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Section: Fig 1: (Color Online)supporting

confidence: 89%

“…Experimental studies on the geometry of scalar dissipation fields are very challenging since gradients have to be measured and only a few exist [10,11]. Figure 1 shows a two-dimensional (2D) slice cut through a DNS snapshot of ǫ θ (x, t).…”

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confidence: 99%

Geometry of Intensive Scalar Dissipation Events in Turbulence

2006

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“…Su & Clemens 2003). Scalar dissipation measurements have been used to probe fine-scale mixing in canonical turbulent flows (Buch & Dahm 1996, 1998, constrain combustion models with turbulent jets (Su & Clemens 2003), and understand passive-scalar mixing in isotropic turbulence (Eswaran & Pope 1988;Girimaji 1992). We compute χ from the scalar fields using the numerical approach described in Buch & Dahm (1996).…”

Section: Scalar Dissipation Rate Fields and Mechanisms Of Mixingmentioning

confidence: 99%

An experimental investigation of mixing mechanisms in shock-accelerated flow

et al. 2008

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An experimental investigation of mixing mechanisms in a shock-induced instability flow is described. We obtain quantitative two-dimensional maps of the heavy-gas (SF 6 ) concentration using planar laser-induced fluorescence for the case of a shockaccelerated cylinder of heavy gas in air. The instantaneous scalar dissipation rate, or mixing rate, χ, is estimated experimentally for the first time in this type of flow, and used to identify the regions of most intense post-shock mixing and examine the underlying mechanisms. We observe instability growth in certain regions of the flow beginning at intermediate times. The mixing rate results show that while these unstable regions play a significant role in the mixing process, a large amount of mixing also occurs by mechanisms directly associated with the primary instability, including gradient intensification via the large-scale strain field in a particular non-turbulent region of the flow.

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“…14 In comparison to this parameter, the experimental resolution, L, in liquid mixing layers has been relatively poor (L/ D Ͼ71 13 ) due to the high Schmidt numbers. The data acquired in gaseous mixing layer studies has not resolved the mixing scale, but is much better than the liquid studies ͑e.g., Batt, 12 L/ D Ϸ10͒.…”

Section: Introductionmentioning

confidence: 99%

Passive scalar mixing in a planar shear layer with laminar and turbulent inlet conditions

Pickett

Ghandhi

2002

Physics of Fluids

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The effect of inlet conditions on the mixing of a passive scalar was investigated in a planar shear layer with inlet boundary layers that were laminar, tripped and naturally turbulent-transitional. Planar laser-induced fluorescence measurements of acetone were used to directly evaluate the shear layer structure, and were processed to determine probability density functions ͑PDFs͒ of the mixture fraction. The results agree well with previous studies in aqueous and gaseous systems for laminar inlet conditions. Large-scale structures of a nearly homogeneous composition were found, and the structures spanned the mixing layer width giving rise to a nonmarching style PDF. A high-speed boundary layer that differed from the laminar state ͑produced by tripping a laminar boundary layer, from the naturally turbulent-transitional state at high flow rates, or by tripping the turbulenttransitional condition͒ gave rise to a hybrid-style PDF that was markedly different from either the laminar nonmarching case, or the marching shape that has been found by other investigators. The hybrid PDF shape had a marching character on the high-speed side of the mixing layer, and a distributed nature not favoring any specific composition on the low-speed side of the mixing layer. Tripping the low-speed boundary layer produced no change in the hybrid PDF shape, confirming that the difference observed between the high-and low-speed sides was the result of the shear layer development with turbulent inlet conditions, not an inlet condition effect. In addition, with turbulent and transitional inlet conditions all turbulent passive scalar profiles were found to be self-similar and the velocity power spectra displayed a Ϫ5/3 slope indicating well-developed turbulent conditions prevailed at a relatively low (10 4 ) Reynolds number. Secondary structures were observed in images with turbulent-transitional inlet conditions, but not with tripped inlet conditions.

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Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 2. Sc≈1

Cited by 186 publications

References 56 publications

Geometry of Intensive Scalar Dissipation Events in Turbulence

Geometry of Intensive Scalar Dissipation Events in Turbulence

An experimental investigation of mixing mechanisms in shock-accelerated flow

Passive scalar mixing in a planar shear layer with laminar and turbulent inlet conditions

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