Large-scale Vortex Rings Research Articles

Active control of lifted diffusion flames with arrayed micro actuators

Active control of a lifted flame issued from a coaxial nozzle is investigated. Arrayed micro flap actuators are employed to introduce disturbances locally into the initial shear layer. Shedding of large-scale vortex rings is modified with the flap motion, and the flame characteristics such as liftoff height, blowoff limit, and emission trend, are successfully manipulated. Spatio-temporal evolution of large-scale vortical structures and fuel concentration is examined with the aid of PIV and PLIF in order to elucidate the control mechanisms. It is found that, depending on the driving signal of the flaps, the near-field vortical structures are significantly modified and two types of lifted flames having different stabilization mechanisms are realized.

DFW microburst model based on AA-539 data

Analysis of the August 2, 1985 crash for an L-1011 jumbo jet (DL-191) on approach to the Dallas-Ft. Worth International Airport (DFW) in a thunderstorm indicates that the severe windshear microburst that caused the crash was composed not only of a strong downflow and outflow but also included several large-scale vortex rings entrained in the flowfield. This paper presents a detailed two-dimensional model of the DFW microburst based on data from the MD-80 (AA-539) that followed behind DL-191 and flew through the microburst about two minutes after the crash of DL-191. The model was developed using wind-vector and flight-path data reconstructed by NASA Ames Research Center and a combination of interactive graphics and least-squares error best fit between the modeled and measured wind vectors along the AA-539 flight path. The model indicates that the flowfield contains some significant elements and vortices not previously reported. The alternating direction of rotation of the vortices in the model suggests a microburst structure based on a von Karman vortex street rather than on a Kelvin-Helmholtz instability. The model also indicates that the reconstructed wind-vector data contain a time lag of at least one second in the horizontal winds.

Flow visualization and numerical analysis of a coflowing jet: a three-dimensional approach

The instabilities of an acoustically excited coflowing jet are investigated by using a time series of cross-sections of large-scale structures present in the flow and by a fully three-dimensional numerical simulation via the vortex-element method. These structures are first revealed by a flow-visualization technique based on smoke seeding the inner jet flow and observing the flow cross-sections as the structures are convected past a stationary sheet of laser light. Usage of image-processing techniques along with computer graphics allows the full three-dimensional reconstruction of the structure. The general morphology of the flow is determined from the reconstructed views and shows that the vorticity is mainly concentrated into a single large-scale vortex ring. Hypotheses regarding the nature and origin of the longitudinal instabilities observed in the flow can be drawn from the reconstructed views, and are supported by the numerical simulations. Quantitative values such as the surface-to-volume ratio and the entrainment level are also obtained from the experimental data. The onset of streamwise vorticity and its distribution are derived from the numerical calculations as well. Three-dimensional views and a holographic display of a characteristic eddy are also presented.

Forced convective heat transfer from premixed flames: —Part 1: Flame structure

In this, the first part of a two-part study of convective heat transfer from impinging flames, the aerodynamic structure of four flames was studied. The flames examined were of stoichiometric mixtures of methane and air with a Reynolds number range extending from the laminar to fully turbulent flow regimes. Instantaneous Schlieren photographs revealed that with increasing Reynolds number the flame reaction zone extended further downstream and became thicker and more diffuse. Associated with this, measurements of mean and rms velocities and mean temperatures showed that the properties of the flames became drawn out in the downstream direction as Reynolds number increased. Schlierenstroboscopic techniques also revealed the existence of large scale vortex rings which originated in the shear layer of the flames, and which were found to cause low frequency oscillations in measured instantaneous velocities. These oscillations lead to misleadingly high levels of rms velocities downstream of the flame reaction zone which should not be interpreted as representing turbulence within the flame.

Flow Visualization Study on Large-Scale Vortex Ring in He II

The behavior of large-scale vortex rings in He II is studied by the application of the method of flow visualization. A vortex ring is formed by pushing a piston and the resulting flow field is visualized with neutrally buoyant hydrogen-deuterium particles as tracers. The development and the translational motion of rings are examined from sequential still pictures.

Experiments on the circulation and propagation of large-scale vortex rings in He II

The normal-fluid and superfluid vorticity, produced by forcing He II out of a tube 0.8 cm in inner diameter at velocities of about 10 cm/sec, is investigated in the region in front of the tube orifice. The method of circulation measurement uses the flow-induced variations of the traveling times of second-sound shock waves. The evolution of large-scale vortex rings with a nearly homogeneous vorticity distribution is observed and measured. Their translational velocity and circulation are measured at temperatures 1.28 K≤T≤2.02 K in the region 1–7 orifice diameters downstream from the orifice. Strong evidence for a marked dependence of the normal-fluid circulation in the vortex ring on temperature and time is found. It points out the importance of the mutual friction force for both macroscopic velocity fields.

Experimental investigation of the circulation of large scale vortex rings in HE II

The vorticity produced by forcing He II out of a tube 8 mm in diameter was investigated in the region in front of the tube orifice. By a non-disturbing method of measurement based on the flow-induced differences in running times of second-sound shock waves, the formation of large scale vortex rings was observed and measured. The circulation of their normalfluid velocity field was found to be not conserved.

Visualization of the acoustic excitation of a subsonic jet

The response of a subsonic round jet to internal excitation by an acoustic wave has been examined using flow visualization and noise measurements. The jet was subjected to pulse excitation and to harmonic excitation. In both cases large-scale vortex ring structures were observed in the jet mixing layer. With pulse excitation, interactions between the vortex rings were observed, but no associated noise emission could be detected.

A numerical model of instability in axisymmetric jets

The detailed flow fields associated with instability in axisymmetric jets are realized by numerical integration of the time-dependent Navier-Stokes equations.The mechanism of amplification of small disturbances is shown to be two-dimensional for a thin boundary layer profile and conclusions regarding three-dimensionality which have been inferred from recent models of axisymmetrical systems are clarified.The computed flow field is shown to be dominated by the large-scale vortex ring structure which has been observed experimentally. Although the wavelength of vortex shedding is found to be slightly variable owing to the randomness of the initial perturbation, the results are shown to agree closely with experiment.