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On the origin of drag increase in varying-phase opposition control
International Journal of Heat and Fluid Flow ( IF 2.6 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.ijheatfluidflow.2020.108651
Simon Toedtli , Christine Yu , Beverley McKeon

Abstract This study investigates the physics underlying the drag increase in a low Reynolds number turbulent channel flow due to varying-phase opposition control by means of direct numerical simulation and modal analysis. The drag increase occurs for an extended region of the parameter space and we consider a controller with a positive phase shift in Fourier domain between sensor measurement and actuator response as a representative example for this regime. Analyses of instantaneous flow fields as well as spatial power spectra show that the structure of drag-increased flows is remarkably different from that of drag-reduced and canonical flows. In particular, the near-wall region is dominated by structures of short streamwise and large spanwise extent. Isolation of a representative control scale shows that these energetic structures can be characterized as spanwise rollers, which induce strong ejection and sweep motions and lead to drag increase. The presence of rollers, and therefore drag increase, in the full nonlinear system correlates well with the presence of an amplified eigenvalue in the eigenspectrum of the linearized Navier–Stokes operator. It is further shown that the scales responsible for drag increase at positive phase shifts are inactive at negative phase shifts and do not contribute to drag reduction. These scales can therefore be excluded from a controller aimed at drag reduction, which relaxes the spatial resolution requirements on the control hardware. The eigenspectrum may be used as a computationally cheap tool to identify such detrimental scales during an early design stage.

中文翻译:

变相反向控制中阻力增加的起源

摘要 本研究通过直接数值模拟和模态分析研究了变相对立控制导致低雷诺数湍流通道流阻力增加的物理原理。阻力增加发生在参数空间的扩展区域,我们将传感器测量和执行器响应之间在傅立叶域中具有正相移的控制器视为该机制的代表性示例。对瞬时流场和空间功率谱的分析表明,增阻流的结构与减阻流和典型流的结构有显着差异。特别是近壁区以短流向和大展向结构为主。具有代表性的控制尺度的隔离表明,这些能量结构可以被表征为展向滚子,其引起强烈的抛射和扫掠运动并导致阻力增加。在完全非线性系统中,滚子的存在以及阻力的增加与线性化纳维-斯托克斯算子的特征谱中放大特征值的存在密切相关。进一步表明,在正相移时导致阻力增加的尺度在负相移时不起作用,并且对减阻没有贡献。因此,这些比例可以从旨在减少阻力的控制器中排除,这放宽了对控制硬件的空间分辨率要求。本征谱可用作计算成本低廉的工具,以在早期设计阶段识别此类有害尺度。
更新日期:2020-10-01
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