Periodic Flow Structures Research Articles

Instability of buoyant diffusion flames

Buoyant jet diffusion flames are known to exhibit large scale vortical flow structures strongly interacting with flame structures. In the present work, the formation and evolution of coherent flow structures is studied in a methane/ air coflow arrangement. This is accomplished by utilizing visualization techniques (planar laser induced hydroxyl fluorescence and Mie-scattering) and Laser Doppler Velocimetry. A striking repeatability and correlation of evolving coherent structures of the air co-flow and the reaction zone is observed. In the transitional region, flow and flame structures oscillate at very pure frequencies ranging from 10–15 Hz. A local absolutely unstable velocity profile close to the burner rim seems to be responsible. Self-excited axisymmetric wavelike structures propagate up- and downstream of this location. We study the influence of the exit velocities and the type of coflowing oxidizer (air or oxygen) on the location of transition to periodic flow structures and related frequencies. Conditional averages of image and velocity data are employed to describe the evolution of coherent flow structures and their interaction with flame structures.

Corotating disk flow in an axisymmetric enclosure with and without a bluff body

An experimental study was performed for the airflow between a pair of coaxial disks corotating in an axisymmetric enclosure with and without an obstruction in the form of a rigid flat rectangular bluff body (arm). Long-term averages of the pressure coefficient across the arm and of the circumferential velocity component in the space between the disks were obtained as a function of position and disk angular velocity in a system of fixed dimensions. The unobstructed configuration exhibits an inner flow region approaching solid body rotation and an outer strongly sheard region with high turbulence levels. For this case, the assumption of steady axisymmetric flow allowed numerical calculations to be perfomed using the procedures of Chang et. al. (1989). The calculations yield mean velocity distributions in good agreement with the unobstructed flow measurements but fail to reproduce some features of the velocity fluctuations that are believed to be due to circumferentially periodic mean flow structures. The measurements show that the presence of an arm has a profound effect on the flow.

Laser sheet technique for visualizing a periodic rotor wake

Introduction L sheets have been used to illuminate cross sections of steady flowfields for several years. Intense sheets of light only a few millimeters in thickness can be produced by either expanding a single laser beam with a cylindrical lens or by rapidly sweeping the beam by means of a vibrating mirror or other optical device.' Generally, smoke or other seed particles which scatter light as they pass through the laser sheet are introduced upstream of the region of interest. Streamlines and vortical structures are identified respectively as streaks and swirl patterns in the illuminated region.' Recent experiments conducted in the John J. Harper 2.31 m x 2.74 m (7 x 9 ft) low-speed wind tunnel at the Georgia Institute of Technology employed a strobed laser sheet to visualize the periodic flowfield between a model rotor and an airframe in forward flight. A nonintrusive method was developed to determine accurately tip vortex trajectories in a uniformly seeded flow. Unlike Schlieren, shadowgraphic, or interferometric methods, the laser sheet technique is well-suited for visualizing the incompressible (low Mach number) flows common to rotorcraft. Quantitative data valuable for use in the interpretation of vortex/airframe interaction measurements were gathered using this technique, as reported in Ref. 5. Used in conjunction with laser velocimeter measurements, the laser sheet flow visualization approach provides a powerful means for documenting vortex jitter or other flow instabilities that can affect measurements. The methods described are adaptable to many situations requiring visualization of a periodic flow structure.

The intrinsic structure of turbulent jets

The object of this paper is to present some experimental results relating to the part of the jet where a potential core exists. The measurements have been made, in the main, in the nonturbulent regions of the jet ( viz. the potential core and the entrainment region), and they suggest that underlying the normally-accepted picture of a random process in the mixing region of the jet there exists a relatively periodic and deterministic flow structure. A simple model of jet structure, purporting to explain the particular phase relationships between the fluctuating pressure and the components of the fluctuating velocity obtained here, is postulated. This model appears to be well substantiated and found to be applicable even to conditions in the mixing region, provided allowance is made for non-linear effects to take place in the mixing region. A special study was made of the use of standard microphones for measuring pressure fluctuations in regions of significant mean flow, and it was found that, to a first-order approximation, this method was justified.