Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J03: Jets: Control (8:00am - 8:45am CST)Interactive On Demand
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J03.00001: Control of ferrofluid jets with magnetic fields Romain Canu, Marie-Charlotte Renoult A linear stability analysis of a Newtonian ferrofluid cylinder under a steady magnetic field with a general shape is performed. Surrounding fluid and gravity effects are ignored and isothermal axisymmetric conditions are considered. The admissible magnetic field shapes are found with a radial dependence for the radial and azimuthal components and a constant axial component. A dispersion relation is obtained and an equation for the cut-off wavenumber is developed. They are solved for different magnetic field shapes and the applicability to jet experiments is studied by linking the spatial and temporal analysis of the jet. The stabilizing effect of the axial and azimuthal shapes, already reported in the literature for the inviscid case, is retrieved. The influence of a solenoid or a wire, that can be used to create experimentally these fields, is also quantified. Above all, the wire is employed to prevent the singularity at the cylinder center due to the radial dependence. In addition to these two cases, a magnetic field with a destabilizing effect is sought. We show that a radial shape is one solution. Such ability to control ferrofluid jets with magnetic fields could be of interest for applications in the printing and medical fields. [Preview Abstract] |
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J03.00002: Extending the planar mixing layer analogy for jets in cross flow by including viscous, curvature and asymmetry effects Davi Souza, Romulo Freitas, Leo Alves A simplified model is developed in the present study to predict the jet in cross flow transition to absolute instability. It is based on a linear stability analysis of the local mean flow extracted from the upstream shear layer of the jet in cross flow, where the wave maker is located. In order to do so, the planar mixing layer analogy [Iyer and Mahesh, JFM, vol. 790, pp. 275-307, 2016] is extended to include curvature, viscous and asymmetry effects. The addition of the former two effects significantly improves the predictive capabilities of the analogy, which is confirmed through validation against experimental data for a jet in cross flow issued from a flush mounted convergent nozzle [Shoji et al, JFM, vol. 890, A7-1, 2020]. Furthermore, azimuthal asymmetry is also added to the analogy. Preliminary results from this latter extension indicate that helical modes could also contribute to the transition to absolute/global instability in jets in cross flow. [Preview Abstract] |
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J03.00003: Dynamics of Coaxial Transverse Jets Elijah Harris, David D. W. Ren, Ann Karagozian The present experimental study investigates shear layer instabilities, structural dynamics, and mixing characteristics of a coaxial gaseous jet injected normally into crossflow via acetone PLIF and stereo PIV. The coaxial jet is tested in a suction configuration, where the core jet, in the absence of the outer suction, has a naturally convectively unstable (CU) upstream shear layer (USL) at a jet-to-crossflow momentum flux ratio of J=41, with an asymmetric mean cross-section. For isolated suction in the upstream or downstream edge of the jet, systematically increasing the suction can enhance the jet cross-sectional symmetry and improve molecular mixing. Velocity field based POD modes demonstrate enhancement of traveling wave structures and periodicity along the USL with suction upstream, and upright wake dynamics when suction is applied downstream. With strong enough suction upstream, the USL instabilities undergo a transition from CU to absolutely unstable flow, while the USL instabilities are largely unaffected by suction downstream. These induced alterations to the instabilities along the coaxial transverse jet's USL have qualitative consistency with a counter-current shear layer analogy\footnote{Shoji, et al., {\bf JFM}, 890, A7, 2020} from linear stability theory. [Preview Abstract] |
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J03.00004: Towards optimal forcing analysis of transverse jets Samantha Harel, Krishnan Mahesh A linear input-output analysis capability is developed in an unstructured grid and parallel code with the objective of identifying the optimal location and frequency of external periodic forcing applied to a transverse jet. Past work has performed global linear stability and adjoint analyses to characterize the least stable and most sensitive perturbations for transverse jets. The time-stepper approach is used along with the Arnoldi algorithm. The lid-driven cavity and Blasius boundary problems are used as validation. Optimal and suboptimal forcing results and the respective flow responses will be discussed for multiple frequencies. [Preview Abstract] |
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J03.00005: The formation of side jets by streamwise vortices and a vortex ring in a controlled round jet Akinori Muramatsu, Kohei Tanaka When the density of jet fluid is sufficiently lower than that of the ambient fluid, branched flows are formed at the end of potential core in a jet. The branched flows are referred to as \textit{side jets}. The formation of the side jets in a round jet is concerned with both the wavy deformation of vortex rings and a sinusoidal velocity fluctuation with relatively high level in the potential core. A round jet with the branched flows was artificially formed using synthetic jets, as shown in 2019 APS DFD. The controlled jet using three synthetic jets is used to investigate the mechanism for side-jets formation experimentally. The starting positions of the branched flow are fixed in the controlled jet, although the starting positions of the side jets vary unsteadily in the natural jet. The branched flows become a periodic phenomenon with synchronizing the roll-ups in the controlled jet. The time-resolved 3D imaging using a laser sheet, an oscillating mirror, and a high-speed camera and the measurement of a dynamic PIV were carried out in the controlled jet. It is found that the branched flow is formed by an induced flow at the top of wavy vortex ring from the experimental results. The induced flow is generated by a pair of streamwise vortices and a vortex ring. [Preview Abstract] |
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J03.00006: Closed-loop control of a globally unstable jet using genetic programming Zhijian Yang, Bo Yin, Yu Guan, Stephane Redonnet, Yuanhang Zhu, Vikrant Gupta, Larry K.B. Li When the density of a jet is sufficiently below that of its surroundings, it can become globally unstable, transitioning from a steady state to a self-excited state characterized by axisymmetric limit-cycle oscillations. We present experiments on the closed-loop control of such oscillations using an unsupervised data-driven model-free framework based on genetic programming (GP). Our implementation of this GP-based control framework relies on a hot-wire probe for sensing and a loudspeaker for actuation. We first initialize a generation of candidate control laws and evaluate their individual performance on the basis of a cost function that accounts for the amplitude of the global mode in a low-density jet and the actuation effort. We then breed further generations of control laws by enrolling them in a tournament and by executing genetic operations such as mutation, crossover, replication and elitism. By benchmarking the best GP-based control law against the best periodic forcing strategy found via conventional open-loop mapping, we show that GP-based control can provide a more efficient means of global mode suppression, offering new insight into the physics of hydrodynamically self-excited jets. [Preview Abstract] |
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