Nature Of Large-scale Structure Research Articles

Excitation of Free Shear-Layer Instabilities for High-Speed Flow Control

Free shear layers are building blocks of many flows of interest in applications, including jets, cavity flows, and separated flows. It was found several decades ago that free shear layers are unstable to small perturbations over a wide range of frequencies, that they are dominated by coherent large-scale structures (even at high Reynolds numbers), and that the dynamics of these structures dominate important processes: entrainment, mixing, momentum transport, and noise generation. These findings motivated extensive research activities to actively control their development using excitation of instabilities, but early experimental research focused primarily on the control of low-speed, low-Reynolds-number free shear layers. This extensive body of the literature is briefly reviewed. The authors’ recent work using localized arc filament plasma actuators in jets shows that free shear layers respond to the excitation over a large range of conditions that have been explored: jet Mach number (up to 1.65), convective Mach number (up to 1), and Reynolds number (up to ). However, the nature of large-scale structures, shear-layer growth rate, and generation of Mach waves all depend on the jet Mach number and compressibility level. The results clearly demonstrate the similarity of instability processes and development of large-scale structures in free shear layers, regardless of the Mach or Reynolds numbers.

The evolution and nature of large-scale structures in the turbulent jet

The structure and evolution of natural, circularly and axially forced turbulent round jets in the far field are investigated. A three-dimensional insovalue surface, which compactly shows the global evolution of the large-scale vortical structures, is extracted from x-y-t laser-induced fluorescence data. Correlation techniques are used to determine whether these structures are helical or axisymmetric. The present results suggest that both the helical and axisymmetric instability modes are present in the natural jet, while the helical mode is dominant in both types of forced jets. Circular forcing increases the spreading rate of the jet.

Three dimensional structures of coronal streamers, holes and CME plasmoids from multifrequency imaging observations

Throughout the quiet Sun years 1982–1987, the Clark Lake Radioheliograph routinely mapped the solar corona on a daily basis at frequencies from 30 to 100 MHz. The Clark Lake maps show a variety of features which we have analyzed quantitatively, providing information about the three dimensional nature of large scale structures of the solar corona.