The Role of Inlet Perturbations on Superstructures of Turbulent Boundary Layers- Toward Global Flow Control

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This proposal is about the investigation of a very fundamental issue in turbulent flows, the issue of the effects of small flow changes close to a solid wall on the development of very long eddies farther downstream. The practical outcome of such understanding would be the possibility to control turbulence downstream by small modifications of the flow upstream, leading to improvements in industrial applications, such as film cooling technology, commercial and military flight, design of wind energy turbine blades and saving energy in cases where fluid pumped over a solid surface is turbulent. Although turbulent boundary layers have been studied for over 50 years, the understanding of them remains the cause of ontroversy in the turbulence community. Such understanding is critical, however, if we are to develop practical control schemes that can help mitigate the energy penalty of turbulent boundary layers. Recent studies have shown that there are Large Scale Motions (LSMs) and Very Large Scale Motions (VLSMs) that carry more than 50% of the Reynolds stresses and kinetic energy. The largest of these motions have scales of the order of O(1δ) ≤ L ≤ O(10δ) and are often also referred to as superstructures. The hypothesis in the proposed study is to demonstrate the possibility to achieve Global Flow Control of the downstream flow by manipulating the inlet conditions in such a way to control the "superstructures". Our team from the USA and Australia seeks to explain the mechanisms by which such complex interactions are possible, and also to explore how these superstructures can modulate the inner region and vice versa. The proposed research consists of high Reynolds number wind tunnel experiments and high fidelity Direct Numerical Simulations (DNS) to study spatially-developing turbulent boundary layers under the influence of different inlet perturbations (e.g., including steady and unsteady). The low Reynolds number simulations will uncover the impact of inlet perturbations on superstructures and will provide optimum configurations to consider in the moderate and high Reynolds number studies. Recruitment of students and outreach will be facilitated by synergies with the ongoing Summer Research Institute established by the PI in 2013. In addition, there will be links to Puerto Rico institutions, where the co-PIs have a strong track record of student recruitment. The PIs also have a plan bringing K-12 teachers to Workshop on Wind Energy Turbulence each summer for the duration of the grant.