小尺度湍流磁场中不可能存在反馈

准均匀小尺度磁场被非高斯湍流平流输运的稳态解

Stationary solution for quasi-homogeneous small-scale magnetic field advected by non-Gaussian turbulent flow

本文研究了非高斯各向同性湍流平流输运的小尺度磁场涨落问题，证明了非高斯速度梯度统计与有限泵浦力区域可保证稳态解存在，并计算出均方磁场，发现宽参数范围内强泵浦下无需反馈即可维持稳态。

Passivity-Based Output-Feedback Control of Turbulent Channel Flow

This paper describes a robust linear time-invariant output-feedback control strategy to reduce turbulent fluctuations, and therefore skin-friction drag, in wall-bounded turbulent fluid flows, that nonetheless gives performance guarantees in the nonlinear turbulent regime. The novel strategy is effective in reducing the supply of available energy to feed the turbulent fluctuations, expressed as reducing a bound on the supply rate to a quadratic storage function. The nonlinearity present in the equations that govern the dynamics of the flow is known to be passive and can be considered as a feedb…

The feedback effect caused by bed load on a turbulent liquid flow

Experiments on the effects due solely to a mobile granular layer on a liquid flow are presented (feedback effect). Nonintrusive measurements were performed in a closed conduit channel of rectangular cross section where grains were transported as bed load by a turbulent water flow. The water velocity profiles were measured over fixed and mobile granular beds of same granulometry by Particle Image Velocimetry. The spatial resolution of the measurements allowed the experimental quantification of the feedback effect. The present findings are of importance for predicting the bed-load transport rate…

Feedback Control of Turbulent Shear Flows by Genetic Programming

Turbulent shear flows have triggered fundamental research in nonlinear dynamics, like transition scenarios, pattern formation and dynamical modeling. In particular, the control of nonlinear dynamics is subject of research since decades. In this publication, actuated turbulent shear flows serve as test-bed for a nonlinear feedback control strategy which can optimize an arbitrary cost function in an automatic self-learning manner. This is facilitated by genetic programming providing an analytically treatable control law. Unlike control based on PID laws or neural networks, no structure of the co…

Cluster-based feedback control of turbulent post-stall separated flows

We propose a novel model-free self-learning cluster-based control strategy for general nonlinear feedback flow control technique, benchmarked for high-fidelity simulations of post-stall separated flows over an airfoil. The present approach partitions the flow trajectories (force measurements) into clusters, which correspond to characteristic coarse-grained phases in a low-dimensional feature space. A feedback control law is then sought for each cluster state through iterative evaluation and downhill simplex search to minimize power consumption in flight. Unsupervised clustering of the flow tra…

通过延迟声学自反馈抑制湍流热声系统中的极限环振荡

Mitigation of limit cycle oscillations in a turbulent thermoacoustic system via delayed acoustic self-feedback

本文提出通过延迟声学自反馈抑制钝体稳定湍流燃烧室中的极限环振荡，实验表明耦合管长度为基频声学模态波长3/8时实现振幅死亡，并揭示了同步性破坏与正反馈回路中断的抑制机制。