- ・Koji Fukagata (Keio University)
- ・Ekaterinaris, John A. (Embry-Riddle Aeronautical University)
・Huihe Qiu (Building Energy Research Center, FYTGS, The Hong Kong University of Science & Technology)
・Jinjun Wang (Beijing University of Aeronautics and Astronautics)
・Henrik Alfredsson (KTH)
・Ephraim (Effie) Gutmark (University of Cincinnati)
Presentation Title Application of machine learning to fluid mechanics problems Abstract Application of machine learning is currently one of the hottest topics in the fluid mechanics field. While machine learning seems to have a great possibility, its limitations should also be clarified. In our group, we have started a research project to construct a nonlinear feature extraction method by applying machine learning technology to “turbulence big data,” extracting the nonlinear modes essential to the regeneration mechanism of turbulence, and deriving the time evolution equation of those nonlinear modes. In this presentation, we will introduce some examples on learning and regeneration of temporal evolution of cross-sectional velocity field in a turbulent channel flow using convolutional neural network (CNN). We will also introduce the application of CNN for super-resolution analysis and extraction of low-dimensional nonlinear modes for flow around a bluff body accompanying vortex shedding. We also introduce our attempts to interpret the nonlinear modes extracted by CNN autoencoder and to use them for an advanced design of flow control, as well as an attempt for uncertainty quantification and applications to experimental data.
Presentation Title Simulations requirements ensuring accurate predictions of vortex dominated fields and vortices generated from wings and rotor blades Abstract Accurate resolution of vortex dominated fields generated by wing and rotor blades is important for the aerospace industry and concentrated the efforts of researchers and engineers from the early stages of computational fluid dynamics and a new emerging technology. To date accurate prediction of vortex dominated flow fields still is a subject of interest. For industrial scale applications such as aircraft and rotorcraft vortical flows only simulations performed with the Reynolds averaged Navier-Stokes (RANS) equations are feasible while large eddy simulations (LES) can hardly reach the Reynolds numbers of practical applications in aerodynamics and remains a research tool. It will be demonstrated that despite the tremendous increase of resources, computational speed, and the progress in turbulence modeling achieved in the last years, the high resolution requirements and turbulence modeling deficiencies are still an issue for industrial scale applications. In this presentations some remedies to these issues are suggested and simulation examples for leading edge vortices, dynamic stall, and wing tip vortices generated by fixed and rotating blades are shown. The improvements achieved are highlighted and the remaining deficiencies are discussed.
Presentation Title Synergetic Flow Control and Wing Flexibility Effect in a Tandem Wing Insect Flight Abstract The behavior of insect flight is of significant interest for designing human-engineered flapping-wing-based micro air vehicles. Tandem wing insects, such as dragonflies reveal quick, demanding maneuvers or takeoff, steady flight and hovering utilizing different phases between the fore- and hindwings. The forewing and the hindwing of a dragonfly have different geometry that could be an evolutionary specialization for better aerodynamic performance via sophisticated wing pitch control. Under different extent of wing pitching by the wing root musculature, the fore- and hindwings could exhibit different shape deformation and aerodynamic characteristics as a result of passive shape deformation. It is debatable whether the pitching motion of an insect wing is only induced passively by aerodynamic and inertial forces or some of the pitching are consciously actuated. Furthermore, the vortex-wing interaction between the forewing and hindwing is also very complex as it involves forewing’s leading edge vortex and trailing edge vortex shedding and hindwing vortex capturing which is affected by wing kinematics, wing flexibility, wing spacing and wing shape.
In this talk, the vortex-wing interactions and active/passive natures of wing pitching in several observed wing kinematics, including the wing motions of tethered and free-fly dragonflies are presented. We measured the flow around the flapping wings using time-resolved particle image velocimetry (TR-PIV) to investigate the consequences of shape and the pitching mechanisms of the wings on the aerodynamics of dragonflies. The dynamic deformation of the wings was optically probed. The flow fields and pitching angle variations of the naturally actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. To reveal the extent of active pitching, the flow fields and pitching angle of the actively actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. We found that the trailing edge vortex dynamics and the wake were affected by the wing shape only for the in-vivo experiment with muscle induced pitching. These results suggest the pitching motion of the wings can be active and passive depending on flight models of insect flight. These results provided quantified evidence to the extent and importance of the pitching motion of the wings in dragonfly flight The spanwise variation in vortex-wing interactions can be exploited in the design of artificial wings to achieve greater agility and higher efficiency. The results of this work can be useful for the design of wings, their actuation mechanism, and the in-flight kinematics control of flapping wing micro air vehicles (MAVs).
Presentation Title Synthetic Jet Impinging onto Porous Walls Abstract Since the concept of synthetic jets was proposed in the 1950s, it has attracted great attention. By blowing and suction of fluid across the exit periodically, a synthetic jet is created. During an actuation cycle, the amount of fluid ejected from the exit is equal to that drawn in, therefore, synthetic jets are also named “zero-net-mass-flux” (ZNMF) jets. When a synthetic jet is actuated, a train of vortices is ejected into the flow field, which promotes the flow mixing and entrainment. On the other hand, the secondary flow structure induced by the vortex impinging onto a wall could improve the mass and momentum transfer of the near-wall flow. Thus, impinging synthetic jets are promising to develop as an efficient cooling method in the future.
In the presentation, we will report the results from an experimental study on impinging synthetic jets by using laser induced fluorescence (LIF) and particle image velocimetry (PIV) techniques. Influences of stroke length and Reynolds number on the flow behavior of an impinging synthetic jet are analyzed. Special attention has been paid to the near-wall flow in order to build the links between the vortical structures and the characteristics of the wall jet, which is important for understanding of vortex/wall interaction mechanism as well as for the related heat transfer applications. Finally, vortex rings of a synthetic jet impinging onto porous walls with a constant surface porosity has been investigated to provide a better understanding of the mechanism of the interaction between vortex rings and complex boundaries.
Presentation Title Rotating disks and cones – a centennial of von Kármán's 1921 paper Abstract In 1921 Theodor von Kármán presented a paper in the first issue of Zeitschrift für Angewandte Mathematik und Mechanik (ZAMM) with the title "Über laminare und turbulente Reibung". That paper is only 20 pages and deals with various aspects of boundary layers in 9 different sections, and certainly enough topics to result in several papers today. One of the sections develops the similarity theory of the laminar boundary layer on a rotating disk and another section deals with experimental results of the frictional resistance of rotating disks in the turbulent region. This talk will review results over the last 100 years since the work of von Kármán and the difference between rotating broad cones (including the disk) and rotating slender cones (including the cylinder) will be highlighted. The relevant parameters and mechanisms determining the instability will be discussed in the light of old and new results, with a focus on work over the last 10+ years at the Fluid Physics laboratory, KTH (see Kato et al, 2021 and references therein).
Kato, K., Segalini, A., Alfredsson, P.H. & Lingwood, R.J. 2021 Instability and transition in the boundary layer driven by a rotating slender cone. J. Fluid. Mech. vol. 915, R4.
Presentation Title Flow and Acoustics of supersonic twin jets Abstract Some modern supersonic airplanes have two or more engines near each other. The supersonic exhaust jets can interact with each other and impact the flow structure of each other and the acoustic emissions from them. This presentation describes the interaction between two jets emanating from circular, low aspect ratio rectangular, and square nozzles that are in proximity to each other. The impact of the interaction between the jets on the flow field and the near and far field acoustics and compares them to single nozzles of the same geometry is shown. The mode of interaction for different nozzle geometry and nozzle pressure ratio (NPR) will be discussed. Pressure waves that propagate between the jets change their mutual shock structure and shear layer evolution, modifying the acoustic field normal and in-line with the common plane of the two jets.