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Advances in active control and optimization in turbulence.

机译：主动控制和湍流优化方面的进展。

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The main objective of this research is to explore the effectiveness of pulsed plasma actuators for turbulence control. In particular, a pulsed plasma actuator is used in this research to implement active control, in the form of a localized body force, over turbulent separated shear layers. Applications of tins research include controlling the formation and distribution of large scale turbulent structures and optimizing turbulence-aberrated laser propagation. This research is primarily experimental, with the motivation for the work derived from theoretical analysis of a turbulent shear layer. The experimental work is considered within two primary flow regimes, compressible and incompressible. For both cases, a turbulent shear layer is generated and then forced with plasma which is introduced periodically at frequencies ranging between 1.0 kHz and 25.0 kHz. The Reynolds numbers, based on visual thickness, of the compressible and incompressible flows investigated in this research are 6.0 106 and 8.0 104 respectively. Experimental results for the compressible case, based on Shack-Hartmann profiling of turbulence-aberrated laser wavefronts, for laser propagation through forced and unforced shear flows show reductions in the laser aberrations of up to 27.5% with a pulsing frequency of 5.0 kHz as well as increases of up to 16.9% with a pulsing frequency of 1.0 kHz. Other pulsing frequencies within the specified range were experimental analyzed and found to exhibit little or no significant change in the laser aberrations compared to the unforced case. The direct results from the Shack-Hartmann wavefront sensor are used to calculate the power spectra of the recorded Optical Path Difference profiles to verify the correlation between large aero-optical aberrations and propagation through large turbulent structures. Shadowgraph imaging of the compressible flow field was conducted to visually demonstrate the same. The experimental procedure for the incompressible shear layer involves imaging the flow field using fog-Mie scattering. The analysis for the resulting incompressible shear layer images include investigations of the distribution of large scale structures and the associated effects that periodic forcing has on the shear layer relating to mixing enhancement and scalar geometry. The effects of periodic forcing on mixing will be determined based on the scalar probability density function and the scalar power spectrum. In addition, the geometry of the scalar interfaces will be examined in terms of the generalized fractal dimension to determine the effects that periodic forcing has on the scale dependency of self-similarity within the flow field. Results from the experiments for the incompressible shear layer show that mixing can be increased by up to 8.4% as determined based on increases within the intermediate scalar probability density function and decreased by as much as 30.8% at forcing frequencies of 25.0 kHz and 1.0 kHz respectively. Additionally, this research shows that the extent of the range of scales of geometrical self-similarity of iso-concentration interfaces extracted from the flow images can be increased by up to 75.0% or reduced by as much as 75.0% depending on the forcing frequency applied. These results show that aero-optical interactions in a compressible shear layer as well as both mixing and the interfacial geometry in incompressible shear layers can be substantially modified by the periodic forcing.
机译：这项研究的主要目的是探索脉冲等离子体致动器对湍流控制的有效性。特别地，在该研究中使用脉冲等离子体致动器来以局部体力的形式对湍流分离的剪切层实施主动控制。罐头研究的应用包括控制大型湍流结构的形成和分布以及优化湍流畸变的激光传播。这项研究主要是实验性的，其工作动机来自湍流剪切层的理论分析。实验工作被认为是在两个主要的流动状态，可压缩和不可压缩。对于这两种情况，都将产生湍流的剪切层，然后施加等离子，该等离子以1.0 kHz至25.0 kHz的频率周期性地引入。基于视觉厚度，本研究中研究的可压缩和不可压缩流的雷诺数分别为6.0 106和8.0 104。基于湍流畸变的激光波阵面的Shack-Hartmann剖析的可压缩情况的实验结果，表明激光在强制和非强制剪切流中的传播表明，在5.0 kHz的脉冲频率下，激光像差最多可降低27.5％，并且脉冲频率为1.0 kHz时，最高可增加16.9％。对特定范围内的其他脉冲频率进行了实验分析，发现与非强迫情况相比，激光像差几乎没有或没有显着变化。 Shack-Hartmann波前传感器的直接结果用于计算记录的光程差剖面的功率谱，以验证大航空像差与通过大湍流结构传播之间的相关性。进行了可压缩流场的阴影图成像，以目视证明这一点。不可压缩剪切层的实验程序涉及使用雾-米氏散射对流场进行成像。对所得不可压缩剪切层图像的分析包括调查大型结构的分布，以及周期性强迫对剪切层产生的与混合增强和标量几何形状相关的影响。周期性强迫对混合的影响将基于标量概率密度函数和标量功率谱来确定。另外，将根据广义分形维数检查标量界面的几何形状，以确定周期性强迫对流场内自相似性的比例依赖性的影响。不可压缩剪切层的实验结果表明，根据中间标量概率密度函数内的增加确定，混合可以最多增加8.4％，而在25.0 kHz和1.0 kHz的强迫频率下，混合最多可以减少30.8％。 。此外，这项研究表明，从流图像中提取的等浓度界面的几何自相似性的尺度范围范围可以增加高达75.0％或减少多达75.0％，具体取决于施加的强制频率。这些结果表明，可压缩剪力层中的气动相互作用以及不可压缩剪力层中的混合和界面几何形状都可以通过周期性强迫而得到显着改变。

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