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

层流分离气泡在许多实际系统中普遍存在，包括飞行器机翼、旋翼机叶片和燃气涡轮机叶片等。这个相对较小的流动区域在控制物理过程中起着主导作用，但人们对它的了解仍然很少。这项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，大大推进目前对层流分离气泡的理解。在这样做的过程中，这将促进层流分离气泡控制的新策略，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。现有的外展计划将扩大到包括针对代表性不足的少数群体的实验和数字流可视化。该推广计划是由图森，亚利桑那州和周边地区丰富的文化多样性，以及航空航天对国家经济的重要性。所提出的研究的主要目标是调查之间的竞争的剪切层不稳定性和3D（Klebanoff）模式，因为它涉及到层流分离气泡的主动流动控制。先前在亚利桑那大学进行的直接数值模拟表明，通过与时间平均流场的剪切层不稳定性相关的频率下的2D周期性强迫，可以消除层流分离气泡（即使它已在分离下游过渡为湍流）。这种方法的功效取决于模拟中自由流湍流的水平，并且数值计算表明，即使水平很小，这种技术的效果也会降低。最近在亚利桑那大学的成功演示表明该技术是可行的，但有效性受到压力梯度和局部雷诺数的影响，这进一步使基础物理复杂化。这项工作将采用最新的仪器和一个国家的最先进的低速风洞能够不同程度的自由湍流，以调查流动不稳定性之间的竞争，因为它们属于主动流动控制。将使用实验数据在高保真直接数值模拟中对自由湍流进行建模，从而量化压力梯度和雷诺数的影响。介质阻挡放电等离子体形式的主动流动控制将在实验中采用，并在模拟中建模。这种协同研究方法将加强既定的推广和国际交流计划，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，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

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.