Periodic Reporting for period 4 - GLOWING (Spatio-temporal measurement and plasma-based control of crossflow instabilities for drag reduction)
Période du rapport: 2023-08-01 au 2024-01-31
Delay of laminar-turbulent flow transition on aircraft wings can potentially reduce aerodynamic drag by up to 15%, reducing emissions and fuel consumption considerably. The main cause of laminar-turbulent transition on commonly used swept wings is the development of crossflow (CF) instabilities. Despite their importance, our fundamental understanding of CF instabilities is limited due to inability of current measurement techniques to capture their complex and multi-scale spatio-temporal features. This severely limits our ability to delay CF transition, which is further impeded by the lack of simple, robust and efficient control concepts. In this project we will achieve unprecedented spatio-temporal measurements of CF instabilities and develop a novel active flow control system that can successfully delay transition on swept wings. To achieve these goals, we bring forth a unique combination of cutting-edge technologies, such as tomographic particle image velocimetry, advanced plasma-based actuators and linear/non-linear stability and control theory.
Spatio-temporal volumetric velocity measurements of CF instabilities will be achieved at three important stages of their life, namely inception, growth and breakdown, providing breakthrough insights into the underlying physics of swept wing transition and turbulence production. The results will be used to postulate and validate linear and non-linear stability and control theory models and provide top benchmarks for high-fidelity CFD. The unprecedented wealth of information, enabled through these advances, will be used to design and demonstrate the first synergetic plasma-based laminar flow control system. This system will feature minimum-thickness plasma actuators, able to suppress the growth of CF instabilities and achieve and sustain considerable transition delay at high Reynolds numbers. These advances will finally enable robust and efficient laminar flow on future air transport.
Spatio-temporal volumetric velocity measurements of CF instabilities will be achieved at three important stages of their life, namely inception, growth and breakdown, providing breakthrough insights into the underlying physics of swept wing transition and turbulence production. The results will be used to postulate and validate linear and non-linear stability and control theory models and provide top benchmarks for high-fidelity CFD. The unprecedented wealth of information, enabled through these advances, will be used to design and demonstrate the first synergetic plasma-based laminar flow control system. This system will feature minimum-thickness plasma actuators, able to suppress the growth of CF instabilities and achieve and sustain considerable transition delay at high Reynolds numbers. These advances will finally enable robust and efficient laminar flow on future air transport.
The project embarked on a twofold objective plan, namely to accurately measure the spatio-temporal development of key transitional flows on swept wings and control them using plasma-based methods. A diverse set of methods and tools were developed and applied towards these objectives. Experimentally, two state-of art experimental facilities have been created, which have helped us to isolate important features in these flows. The two facilities were further furbished with a state of art Particle Tracking Velocimetry system which provided us with the first fully three-dimensional and volumetric description of the receptivity process in the vicinity of roughness elements. Similarly, advanced numerical methodologies able to model these processes have also been developed. The most important tool developed here was a novel non-linear Harmonic Navier Stoke solver which is now published and available open-access. This framework was instrumental for the confirmation of transient growth mechanisms. The outcomes form the numerical tools have also helped us reconcile our experimental observations. Based on both the experimental and numerical tools we have developed, we have gained the first description of how external disturbances in the form of roughness enter the boundary layer flow and trigger instabilities. Towards the second objective, we have applied novel plasma actuators on the same flows. To achieve a seamless integration of these systems, we developed a novel conductive material deposition method based on inkjet printing which allowed us to produce extremely thin electrodes which have a minimal negative effect on flow. The application of plasma actuators led to the first experimentally confirmed transition delay in swept wings and revealed a wealth of previously unobserved phenomena, especially relevant in cases of flow control. Finally, numerical simulations have advanced our understanding on how these instabilities interact with critical roughness features such as steps (passive) pr plasma actuators (active). New growth mechanisms are found as well as a novel transition delay mechanism which can be further exploited.
A list of key results and following exploitation/dissemination activities is given here:
- Key insights into the physics of swept wing transition through state-of-art measurements and simulations. Dissemination through leading peer-reviewed journal publications and outreach through invited talks at interantional conferences.
- Elucidation of roughness effects on transition and discovery of a novel flow control method. Dissemination through publications. Exploitation through a patent application for the Delft Laminar Hump. Further valorisation through an ERC PoC grant (DeLaH) and research funding from the Dutch Research Council.
- Development of an open-source numerical simulation framework for complex transition problems, published and available online.
- Development of a holistic system for the production of ultra-thin plasma actuators for flow control problems
A list of key results and following exploitation/dissemination activities is given here:
- Key insights into the physics of swept wing transition through state-of-art measurements and simulations. Dissemination through leading peer-reviewed journal publications and outreach through invited talks at interantional conferences.
- Elucidation of roughness effects on transition and discovery of a novel flow control method. Dissemination through publications. Exploitation through a patent application for the Delft Laminar Hump. Further valorisation through an ERC PoC grant (DeLaH) and research funding from the Dutch Research Council.
- Development of an open-source numerical simulation framework for complex transition problems, published and available online.
- Development of a holistic system for the production of ultra-thin plasma actuators for flow control problems
New insights are gained here, particularly involving a newly discovered mechanisms governing the breakdown of crossflow vortices under supercritical roughness. By the end of the project the mechanisms have been generalised and scaled as well as conceptualised into a predictive model. In addition, the discovery of new growth mechanisms in the vicinity of roughness elements was an unexpected development emerging from the spatio-temporal measurements. The implications are profound, as this explains many discrepancies in past predictions. Initial validation and confirmation of this new discovery was done by using the newly developed numerical methodologies. Finally, the basic machanism behind base flow modification has been experimentally measured and conceptualised into a generic "hump" device called Delft Laminar Hump. Steps are currently taken to upscale it to higher Reynolds numbers within the framework of an ERC PoC grant.