On the evolution of laminar vortex rings

Using Laser Doppler Anemometry (LDA) and Digital Particle Image Velocimetry (DPIV), the physical properties of laminar vortex rings are investigated in the Reynolds-number range 830 ≤ Re ≤ 1650. The measured initial circulations of the vortex rings are found to agree well with corrected versions of the vorticity-flux (slug-flow) model proposed by Didden and Pullin. The DPIV and LDA data show excellent agreement regarding local velocities and vortex-ring circulations. The DPIV data depict the distribution of the vorticity and circulation in the core regions, where the resulting vorticity distributions are found to be self-similar Gaussian profiles. The propagation velocity of the vortex rings is well approximated by an analytical model of Saffman for large core sizes. In the asymptotic limit t → ∞, the trajectories are in excellent agreement with the exact Stokes-dipole solution of Cantwell and Rott.     

Simultaneous mapping of the velocity and deformation field at a free surface

The oblique interaction of a turbulent vortex ring with a clean water surface is experimentally investigated during the transition stage using Digital Particle Image Velocimetry (DPIV) and the shadowgraph technique to map the surface velocity and deformation field simultaneously. The transitional vortex-ring/free-surface interaction leads to the formation of a trifurcation pattern at the free surface. Similar to the laminar flow case, the vortex ring initially bifurcates into two symmetric and separately connected vortex loops. The turbulent break-up of those vortex loops results in the formation of longitudinal wake vortices that symmetrically connect to the surface and eventually lead to a trifurcation pattern. In the absence of large-deformation surface waves, the simultaneous DPIV and shadowgraph measurements reveal good agreement between the surface vorticity and deformation field for small- and large-scale vortical structures. The simultaneous measurement technique is not restricted to the qualitative shadowgraph visualization, but can be easily extended to quantitative methods such as grating-imaging techniques, Color Schlieren, or Color Surface Mapping (CSM) techniques.This work has been supported by the Office of Naval Research, ONR-URI grant N00014-92-J-1610.     

Laser Diagnostic Study of the Mechanism of a Periodic Combustion Instability in a Gas Turbine Model Combustor

To investigate the mechanisms leading to sustained thermoacoustic oscillations in swirl flames, a gas turbine model combustor was equipped with an optically accessible combustion chamber allowing the application of various laser techniques. The flame investigated was a swirled CH₄/air diffusion flame (thermal power 10 kW, global equivalence ratio φ = 0.75) at atmospheric pressure which exhibited self-excited thermoacoustic oscillations at a frequency of 290 Hz. In separate experiments, the flow velocities were measured by laser Doppler velocimetry, the flame structures and heat release rates by planar laser-induced fluorescence of CH and by OH chemiluminescence, and the joint probability density functions of the major species concentrations, mixture fraction, and temperature by laser Raman scattering. All measurements were performed in a phase-locked mode, i.e., triggered with respect to the oscillating pressure level measured by a microphone. The results revealed large periodic variations of all measured quantities and showed that the heat release rate was correlated with the degree of mixing of hot products with unburned fuel/air mixtures before ignition. The thermal expansion of the reacting gases had, in turn, a strong influence on the flow field and induced a periodic motion of the inner and outer recirculation zones. The combination of all results yielded a deeper understanding of the events sustaining the oscillations in the flame under investigation. The results also represent a data base that can be used for the validation and improvement of CFD codes.     

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