On the self-constraint mechanism of the cross-stream secondary flow in a streamwise-rotating channel

In this paper the multiscale dynamics of streamwise-rotating channel turbulence is studied through direct numerical simulations. Using the generalized Kolmogorov equation, we find that as rotation becomes stronger, the turbulence in the buffer layer is obviously reduced by the intense spatial turbulent convection. On the contrary, in other layers, the turbulence is strengthened mainly by the modified production peak, the intense spatial turbulent convection and the suppressed forward energy cascades. It is also discovered that under a system rotation, small- and large-scale inclined structures have different angles with the streamwise direction, and the difference is strengthened with increasing rotation rates. The multiscale inclined structures are further confirmed quantitatively through a newly defined angle based on the velocity vector. Through the budget balance of Reynolds stresses and the hairpin vortex model, it is discovered that the Coriolis force and the pressure–velocity correlation are responsible for sustaining the inclined structures. The Coriolis force directly decreases the inclination angles but indirectly induces inclined structures in a more predominant way. The pressure–velocity correlation term is related to the strain rate tensor. Finally, the anisotropic generalized Kolmogorov equation is used to validate the above findings and reveals that the multiscale behaviours of the inclined structures are mainly induced by the mean spanwise velocity gradients.

“…However, the tendency is reversed as Ro τ becomes higher. Similar results have also been found by Yang et al (2020a).…”

Section: Basic Turbulence Statisticssupporting

confidence: 91%

Section: Sustaining Mechanisms Of the Inclined Structuresmentioning

confidence: 99%

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Multiscale dynamics in streamwise-rotating channel turbulence

Hu,

Li,

2023

“…DNS datasets were also used to investigate the effect of streamwise system rotation on pressure fluctuation, the self-constraint mechanism and the sustaining mechanism of Taylor–Görtler vortices (Yang et al. 2018, 2020 a , b ). Dai et al.…”

Section: Introductionmentioning

confidence: 99%

“…performed DNS with moderate and high rotation numbers with sufficiently large streamwise computational domains to study the scales of the dynamics of Taylor-Görtler vortices, and Taylor-Görtler vortices can characterize the mean secondary flow according to instability analysis by Wall & Nagata (2006) and Masuda et al (2008). DNS datasets were also used to investigate the effect of streamwise system rotation on pressure fluctuation, the self-constraint mechanism and the sustaining mechanism of Taylor-Görtler vortices (Yang et al , 2020a. Dai et al (2019) studied the coherent structures in turbulent channel flows with streamwise system rotation through DNS and LES, and proposed that rotation promotes cyclones and suppresses anticyclones.…”

mentioning

confidence: 99%

Helicity distributions and transfer in turbulent channel flows with streamwise rotation

Yan

et al. 2022

Helicity is a quadratic inviscid conservative quantity in three-dimensional turbulent flows and is crucial for turbulent system evolution. Helicity effects have mainly been highlighted over the past few decades to explore the intrinsic mechanism of turbulent flows, while the statistical characteristics of helicity itself are nearly absent in general anisotropic turbulent flows. In this paper, we investigate the helicity statistics in turbulent channel flows with streamwise rotation at moderate rotation numbers ( $Ro_{\tau }=7.5,15$ and 30) and Reynolds numbers ( $Re_{\tau }=180$ and 395), including their spatial and scale distributions, anisotropy and cross-scale transfer. The appearance of a mean secondary flow in the spanwise direction corresponds to a mean streamwise vorticity, which indicates the presence of a high-helicity distribution. Numerical results reveal a regular helicity profile along the wall-normal direction, and a new peak is found in the near-wall region around $y^+=6$ of the streamwise or spanwise helicity profiles. The inter-scale helicity transfer is analysed by the filtering method, and the numerical consequences reveal that the second channel of the helicity cascade we proposed previously is dominant in contrast to the first channel. The rotation effects are explored by comparing the numerical results obtained under different rotation numbers. With increasing rotation number, more helical structures in the near-wall regions are present, with peaks of helicity profiles and fluxes coming closer to the wall. With a higher Reynolds number, their amplitudes are larger and scale-space transfer is strengthened. These systematic numerical analyses uncover the helicity distributions and transfer in wall-bounded turbulent flows.

Effects of streamwise rotation on helicity and vortex in channel turbulence

Hu,

Li,

2024

Helicity plays a key role in the evolution of vortex structures and turbulent dynamics. The helicity dynamics and vortex structures in streamwise-rotating channel turbulence are discussed in this paper using the helicity budget equation and the differentiated second-order structure function equation of helicity. Generally, rotation and Reynolds numbers exhibit opposing effects on the interscale helicity dynamics and the vortices. Under the buffer layer, the positions of the helicity peaks are proportional to the ratio between the Reynolds and rotation numbers. The mechanism is related to the opposing effects of convection and rotation. Rotation directly affects the helicity balance through the Coriolis term and corresponding pressure term. In the buffer layer, the scale helicity is negative at small scales but positive at large scales, which is mainly induced by the spatial effects (the production and the spatial turbulent convection) but reduced by interscale cascades. Examination of structures reveals the close association between scale helicity and streaks, with streak lift angles exhibiting an increase with rotation and a decrease with Reynolds numbers. In the log-law layer, the Coriolis terms and corresponding pressure terms are proportional to the rotation numbers but remain independent of the Reynolds numbers. The negative scale helicity is forward cascaded towards small scales. Generally, spanwise vortices in the log-law layer are related to sweep events and forward cascades. Our findings indicate that these spanwise vortices are suppressed by rotation but recover with increasing Reynolds numbers, aligning with the effects observed in the scale helicity balance.