Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

The perturbation response of small-scale shear layers in turbulence is investigated with direct numerical simulations (DNS). The analysis of shear layers in isotropic turbulence suggests that the typical layer thickness is about four times the Kolmogorov scale $\eta$ . Response for sinusoidal perturbations is investigated for an isolated shear layer, which models a mean flow around the shear layers in turbulence. The vortex formation in the shear layer is optimally promoted by the perturbation whose wavelength divided by the layer thickness is about 7. These results indicate that the small-scale shear instability in turbulence is efficiently promoted by velocity fluctuations with a wavelength of about $30\eta$ . Furthermore, DNS are carried out for decaying turbulence initialised by the artificially modified velocity field of isotropic turbulence. The vortex formation from shear layers is accelerated under the influence of external perturbations with the efficient wavelength to promote the instability. When velocity fluctuations with this wavelength are eliminated by a band-cut filter, the shear layers tend to persist for a long time without producing vortices. These behaviours affect the number of vortices in turbulence, which increases and decreases when velocity perturbations with the unstable wavelength of the instability are artificially amplified and damped, respectively. The increase in the number of vortices results in the enhancement of kinetic energy dissipation, enstrophy production and strain self-amplification. These results indicate that the perturbation response of shear layers is important in the small-scale dynamics of turbulence as well as the modulation of small-scale turbulent motions by external disturbance.

“…2016, 2017 a ; Neamtu-Halic et al. 2021). The increase in the number of vortices by perturbations will be possibly important in the enhancement of entrainment by external disturbance.…”

Section: Discussionmentioning

confidence: 99%

The response of small-scale shear layers to perturbations in turbulence

Watanabe

Nagata

2023

“…2011; Neamtu-Halic et al. 2021). The volume of the structures occupied of the whole volume of the flow field which is also in a good agreement with the value of reported by Jiménez et al.…”

Section: Resultsmentioning

confidence: 99%

“…1993; Jiménez & Wray 1998; Ghira, Elsinga & Da Silva 2022), jets (Ganapathisubramani, Lakshminarasimhan & Clemens 2008; da Silva, dos Reis & Pereira 2011), channel flows (Kang, Tanahashi & Miyauchi 2009) and stratified flows (Neamtu-Halic et al. 2021). Although the alignment of the vorticity vector with the intermediate principal strain holds at small scales, and has been observed in a wide range of flows, at larger length scales the vorticity preferentially aligns instead with the most stretching principal strain (Ishihara, Yamazaki & Kaneda 2001; Leung, Swaminathan & Davidson 2012).…”

Section: Introductionmentioning

confidence: 99%

Entrainment, detrainment and enstrophy transport by small-scale vortex structures

Aligolzadeh,

Holzner,

Dawson

2023

The interaction of small-scale vortical structures with the surrounding fluid are studied using a fully resolved three-dimensional experimental data set of homogeneous turbulence measured at the centre of a von Kármán mixing flow facility and a direct numerical simulation (DNS) data set of forced isotropic turbulence. To identify the small-scale vortices and their boundaries, an objective observer-independent definition was implemented to avoid arbitrariness and is the first implementation applied to experimental measurements of small-scale turbulence. Volume-averaged and conditional statistics are presented to demonstrate consistency between the experimental and DNS data sets. To examine the interaction of the structures with the surrounding flow field, we examine the flow across the boundary of vortex structures by adopting a similar methodological approach to that used to investigate the local entrainment and detrainment across the turbulent–non-turbulent interface. The probability density function (p.d.f.) of entrainment velocity conditioned on the vortex boundary exhibited a non-Gaussian distribution that skewed slightly in favour of entrainment and is remarkably similar to the p.d.f.s of entrainment velocity observed in boundary layers and jets. We analyse the enstrophy transport equation conditioned on radial and axial coordinates of the vortices to quantify the inviscid and viscous components of the entrainment/detrainment process. A comparison with Burgers vortices is made and it is found that the Burgers vortex model captures the vortex structure average size and the mechanisms of enstrophy transport in the radial direction, but is unable to capture local statistics and describe the governing physics along the axes of the vortices.

“…Notably, the TNTI layer forms near small-scale vortices (vortex tubes) and shear layers (da Silva, Dos Reis & Pereira 2011; Elsinga & da Silva 2019; Hayashi, Watanabe & Nagata 2021 b ; Neamtu-Halic et al. 2021). Shear layers, originally termed vortex sheets due to their inherent vorticity, are sheet-like structures exhibiting shearing motion (Vincent & Meneguzzi 1994).…”

Section: Introductionmentioning

confidence: 99%

Efficient enhancement of turbulent entrainment by small-scale shear instability

Watanabe

2024

Turbulent entrainment is a process by which a locally turbulent region draws in an outer irrotational fluid. A large number of small-scale vortices and shear layers exist near the turbulent/non-turbulent interface; these features influence the local entrainment process. Direct numerical simulations of a turbulent front evolving into a quiescent flow without mean shear show that the entrainment rate is amplified by triggering the instability of small-scale shear layers via weak perturbations with a wavelength matching that of the unstable mode of the shear layers. Imposing artificial perturbations with a length scale approximately 30 times the Kolmogorov scale leads to the rapid collapse of small-scale shear layers due to instability, generating vortices near the turbulent/non-turbulent interface. Amplification of the entrainment rate is linked to the enlarged area and increased propagation velocity of the interface. The impact of perturbations on the entrainment rate becomes most pronounced when the flow evolves over approximately 7 times the Kolmogorov time scale, after which their influence diminishes over time. Additionally, the increase in entrainment rate is dictated by the ratio of the perturbation amplitude to the Kolmogorov velocity scale. The entrainment enhancement process is governed by Kolmogorov scales, suggesting that even weak perturbations can amplify the entrainment rate in high Reynolds number flows.