Active Control of Laminar Separation Bubbles using a Synergistic Combination of Wind Tunnel Experiments and Direct Numerical Simulations

层流分离气泡广泛存在于飞机机翼、旋翼机叶片和燃气轮机叶片等实际系统中。这一相对较小的流动区域在支配物理过程中起着主导作用，但人们对它仍然知之甚少。这一研究项目将通过彻底评估相关流动不稳定性之间的相互作用，极大地促进当前对层流分离气泡的理解。通过这样做，这将促进控制层流分离气泡的新策略，从而提高许多空气动力学平台的性能和效率。该项目将部分支持一名博士后研究员、一名博士生和一名本科生。现有的外联方案将扩大到包括针对代表性不足的少数群体的试验性和数值性流动可视化。图森、亚利桑那州及周边地区丰富的文化多样性，以及航空航天对州经济的重要性，都支持了这一推广计划。这项研究的主要目的是研究剪切层不稳定性和3D(Klebanoff)模式之间的竞争，因为它与层流分离气泡的主动流动控制有关。以前在亚利桑那大学进行的直接数值模拟表明，通过以与时间平均流场的剪切层不稳定性相关的频率进行2D周期强迫，可以消除层流分离气泡(即使它已经转变为分离下游的湍流)。这种方法的有效性取决于模拟中的自由流湍流水平，数值结果表明，即使是很小的水平也会使这种方法的效果变得不那么有效。最近在亚利桑那大学的成功演示表明，这项技术是可行的，但有效性受到压力梯度和局部雷诺数的影响，这进一步使基本物理变得复杂。这项工作将使用最新的仪器和最先进的低速风洞，能够适应不同级别的自由流湍流，以研究与主动流动控制有关的流动不稳定性之间的竞争。采用高保真的直接数值模拟方法，利用实验数据对自由流湍流进行数值模拟，从而量化压力梯度和雷诺数的影响。实验中将采用介质阻挡放电等离子体形式的主动流动控制，并在模拟中对其进行建模。这种协同研究方法将加强已建立的外展和国际交流计划，同时在这一丰富而重要的领域取得突破。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Laminar separation bubbles are prevalent in many practical systems including aircraft wings, rotorcraft blades and gas turbine blades among others. This relatively small region of the flow plays a dominant role in the governing physical processes, yet it remains poorly understood. This research project will drastically advance the current understanding of laminar separation bubbles by thoroughly evaluating the interplay between relevant flow instabilities. In doing so, this will foster new strategies for the control of laminar separation bubbles leading to improvements in performance and efficiency across many aerodynamic platforms. The project will partially support one post-doctoral researcher, one PhD student and one undergraduate student. Existing outreach programs will be expanded to include experimental and numerical flow visualizations targeting underrepresented minority groups. The outreach program is buoyed by the rich cultural diversity of Tucson, AZ and surrounding areas as well as the importance of aerospace to the state economy. The primary objective of the proposed research is to investigate competition between the shear layer instability and the 3D (Klebanoff) mode as it relates to active flow control of laminar separation bubbles. Previous direct numerical simulations performed at the University of Arizona have shown that it is possible to eliminate a laminar separation bubble (even if it has transitioned to turbulence downstream of separation) by 2D periodic forcing at a frequency related to the shear layer instability of the time-averaged flow field. The efficacy of this approach is dependent on the level of freestream turbulence in the simulation, and it has been shown numerically that even small levels render this technique less effective. Recent successful demonstrations at the University of Arizona suggest the technique is feasible, but effectiveness is influenced by pressure gradient and local Reynolds number, which further complicate the fundamental physics. This work will employ the latest instrumentation and a state-of-the-art low-speed wind tunnel capable of various levels of freestream turbulence to investigate the competition between flow instabilities as they pertain to active flow control. Freestream turbulence will be modeled in high-fidelity direct numerical simulations using experimental data thus quantifying the influence of pressure gradient and Reynolds number. Active flow control in the form of dielectric barrier discharge plasma will be employed in the experiment and modeled in the simulation. This synergistic research approach will strengthen established outreach and international exchange programs all while producing a breakthrough in this rich and important field.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.