Numerical experiments on turbulent entrainment and mixing of scalars

Cimarelli, Andrea; Boga, G.

doi:10.1017/jfm.2021.779

Cited by 14 publications

(9 citation statements)

References 35 publications

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“…Despite the detailed research available, how entrainment is modified by the turbulent coherent structures is still needed for practical applications, such as large-eddy simulation and the low-order models for flows (Neamtu-Halic et al 2019;Cimarelli & Boga 2021). For small-scale structures, it has been observed that the λ-scaled vortex close to TNTI will modulate its nearby entrainment rate in both numerical and experimental studies (da Silva et al 2014a;Mistry et al 2019;Neamtu-Halic et al 2020), where λ is the Taylor microscale.…”

Section: Introductionmentioning

confidence: 99%

“…More detailed multiscale analyses have been conducted by Philip et al (2014) and Mistry et al (2016): by filtering the flow field with varying filter size Δ, it is found that, although the tortuosity of TNTI and the instantaneous entrainment velocity vary in different Δ, the total entrainment flux is independent of Δ. More recently, the large-scale and small-scale velocity fields in a temporal jet were numerically separated and evolved independently (Cimarelli & Boga 2021). It was found that the entrainment is more violent in the large-scale velocity field, which again suggests the importance of large-scale motions.…”

Section: Introductionmentioning

confidence: 99%

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Universal modulations of large-scale motions on entrainment of turbulent boundary layers

2022

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The modulations of high/low-speed large-scale motions (H/L-SMs) on the turbulent/non-turbulent interface (TNTI) and turbulent entrainment are investigated in turbulent boundary layers via both experimental and numerical studies. The spanwise locations of large-scale motions can be locked by the spanwise heterogeneity, so the boundary layers over such a configuration are investigated first as an instructive case. In the engulfment process, it is found that irrotational ‘bubbles’ near the TNTI are more likely to originate from engulfment, while bubbles far from the TNTI could be produced by the local turbulence itself. Additionally, H-SMs are found to enhance the engulfment by the sweep flow. In the nibbling process, a competition relationship is observed: L-SMs induce stronger instantaneous ‘nibbling’ events by transporting more fluids towards the TNTI, while the H-SMs induced a more distorted TNTI. Consequently, the integral nibbling flux is greater above H-SMs. Furthermore, the explored mechanisms are verified to be insensitive to the wall shapes such as smooth and homogeneous roughness walls, which demonstrates that these modulations are universal for turbulent boundary layers. Finally, a conceptual modulation model is proposed to illustrate the entrainment process above the large-scale motions.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Universal modulations of large-scale motions on entrainment of turbulent boundary layers

2022

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“…However, the entrainment is a multiscale process, which means that the large-scale motions are also involved (Mistry et al 2016). Some recent studies showed that the large-scale motions do modulate the entrainment velocity in the TBL (Long, Wang & Pan 2022) and jet (Cimarelli & Boga 2021). Furthermore, the large-scale motions are shown to dominate the mass and energy transport in the outer region (Adrian, Meinhart & Tomkins 2000) in TBL.…”

Section: Introductionmentioning

confidence: 99%

Reynolds number dependence of the turbulent/non-turbulent interface in temporally developing turbulent boundary layers

2023

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Direct numerical simulations (DNS) of temporally developing turbulent boundary layers are performed with a wide range of Reynolds numbers based on the momentum thickness $Re_{\theta } = 2000$ – $13\,000$ for investigating the Reynolds number dependence of the turbulent/non-turbulent interface (TNTI) layer. The grid spacing in the DNS is determined carefully such that small-scale turbulent motions near the TNTI are well resolved. The outer edge of the TNTI layer, called the irrotational boundary, is detected with vorticity magnitude. The mean thicknesses of the TNTI layer, $\delta _{TNTI}$ , turbulent sublayer, $\delta _{TSL}$ , and viscous superlayer, $\delta _{VSL}$ , are found to be approximately $15\eta _{TI}$ , $10\eta _{TI}$ and $5\eta _{TI}$ , respectively, where $\eta _{TI}$ is the Kolmogorov scale taken in the turbulent region near the TNTI layer. The mean curvature of the irrotational boundary is also characterized by $\eta _{TI}$ . The shear parameter and the shear-to-vorticity ratio show that the mean shear effects near the TNTI layer are not significant for both large and small scales. The anisotropy tensors of Reynolds stress and vorticity suggest that the turbulence under the TNTI layer tends to be isotropic at high $Re_{\theta }$ , for which $\eta _{TI}/\delta \sim Re_{\theta }^{-3/4}$ is valid with the boundary layer thickness $\delta$ . The surface area of the irrotational boundary is consistent with the fractal analysis of the interface, where the fractal dimension $D_f$ is found to be $2.14$ – $2.20$ . The present results suggest that the mean entrainment rate per unit horizontal area normalized by the friction velocity varies slowly as $Re_{\theta }^{(3/4)(D_f-2)}$ for $Re_{\theta } \geq 4000$ .

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“…It has been reported that the entrainment becomes dominated by large-scale engulfment of background fluid under the influence of the coherent motions (e.g. Yule 1978; Bisset, Hunt & Rogers 2002; Cimarelli & Boga 2021; Long, Wang & Pan 2022). For TTIs, Kankanwadi & Buxton (2023) observed that both the turbulence intensity and the integral length scale in the ambient flow correlate to enhanced entrainment in the presence of the large-scale coherent vortices in the near wake of a cylinder – a contrasting result to the far-field study in which background turbulence was observed to suppress entrainment rate (Kankanwadi & Buxton 2020).…”

Section: Introductionmentioning

confidence: 99%

Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

Chen

Buxton

2023

J. Fluid Mech.

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This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are varied independently, and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high Schmidt number ( $Sc$ ) scalar, and a planar laser induced fluorescence experiment was carried out to capture the interface between the wake and the ambient flow from $x/d = 5$ to 40, where $x$ is the streamwise coordinate from the centre of the cylinder, and $d$ is the cylinder's diameter. It is found that the TTI generally spreads faster towards the ambient flow than the TNTI. A transition region of the interfaces’ spreading is found at $x/d \approx 15$ , after which the interfaces propagate at a slower rate than previously (upstream), and the mean interface positions of both the TNTI and TTI scale with the local wake half-width. The locations of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time, we explore the spatial variation of the ‘roughness’ of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity becomes important only for the fractal dimension farther downstream.

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Numerical experiments on turbulent entrainment and mixing of scalars

Cited by 14 publications

References 35 publications

Universal modulations of large-scale motions on entrainment of turbulent boundary layers

Universal modulations of large-scale motions on entrainment of turbulent boundary layers

Reynolds number dependence of the turbulent/non-turbulent interface in temporally developing turbulent boundary layers

Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

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