Dynamics of laminar and transitional flows over slip surfaces: effects on the laminar–turbulent separatrix
Dynamics of laminar and transitional flows over slip surfaces: effects on the laminar–turbulent separatrix
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DOI：
10.1017/jfm.2020.282
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发表时间：
2020-04
影响因子：
3.7
通讯作者：
Ethan A. Davis;J. S. Park
中科院分区：
文献类型：
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作者：
Ethan A. Davis;J. S. Park
本文用直接数值模拟的方法研究了滑移面对平面Poillille流层流-湍流分界线的影响。在层流中，包含的滑移面的结果在阻力减少超过10%，这是在良好的协议与以前的研究和理论的层流滑移流。湍流寿命，湍流持续的可能性，研究过渡流与各种滑移长度。我们表明，滑动面减少持续湍流的可能性相比，无滑动的情况下，和可能性进一步降低，滑动长度的增加。通过使用Navier-Stokes方程的非线性行波解（也称为精确相干解），对滑移面对湍流过渡的影响进行了更确定性的分析。使用被称为P3和P4的两个解族，因为它们的下分支解嵌入层流和湍流的吸引盆的边界上（Park & Graham，J. Fluid Mech.，第782卷，2015年，pp. 430-454）。此外，它们表现出不同的流动结构-P3和P4分别表示为核心模式和临界层模式。通过表面摩擦演化、线性增长率和过渡轨迹的相空间投影，观察到滑移面对解的明显影响。滑移面出现修改的过渡动力学的核心模式很少，但相当大的临界层模式。最重要的是，滑移面促进不同的过渡动力学-早期和旁路样过渡的核心模式和延迟和H型或K型过渡的临界层模式。我们解释这些不同的过渡动态的时空和象限分析的基础上。据发现，滑移面促进流行的强壁向运动（扫样事件）靠近通道中心的涡核附近，诱导早期过渡，而长期持续的喷射事件存在于该地区的$\unicode[STIX]{x1 D 6 EC}$形涡核接近临界层，导致延迟过渡。这将促使流动控制策略充分利用这些不同的过渡动力学过渡到湍流。
The effect of slip surfaces on the laminar–turbulent separatrix of plane Poiseuille flow is studied by direct numerical simulation. In laminar flows, the inclusion of the slip surfaces results in a drag reduction of over 10 %, which is in good agreement with previous studies and the theory of laminar slip flows. Turbulence lifetimes, the likelihood that turbulence is sustained, is investigated for transitional flows with various slip lengths. We show that slip surfaces decrease the likelihood of sustained turbulence compared to the no-slip case, and the likelihood is further decreased as slip length is increased. A more deterministic analysis of the effects of slip surfaces on a transition to turbulence is performed by using nonlinear travelling-wave solutions to the Navier–Stokes equations, also known as exact coherent solutions. Two solution families, dubbed P3 and P4, are used since their lower-branch solutions are embedded on the boundary of the basin of attraction of laminar and turbulent flows (Park & Graham, J. Fluid Mech., vol. 782, 2015, pp. 430–454). Additionally, they exhibit distinct flow structures – the P3 and P4 are denoted as core mode and critical-layer mode, respectively. Distinct effects of slip surfaces on the solutions are observed by the skin-friction evolution, linear growth rate and phase-space projection of transitional trajectories. The slip surface appears to modify the transition dynamics very little for the core mode, but quite considerably for the critical-layer mode. Most importantly, the slip surface promotes different transition dynamics – an early and bypass-like transition for the core mode and a delayed and H- or K-type-like transition for the critical-layer mode. We explain these distinct transition dynamics based on spatio-temporal and quadrant analyses. It is found that slip surfaces promote the prevalence of strong wall-toward motions (sweep-like events) near vortex cores close to the channel centre, inducing an early transition, while long sustained ejection events are present in the region of the $\unicode[STIX]{x1D6EC}$-shaped vortex cores close to the critical layer, resulting in a delayed transition. This should motivate flow control strategies to fully exploit these distinct transition dynamics for transition to turbulence.