Vortex Rings In Air Research Articles

Determination of the drag coefficient of a toroidal plasma vortex in air

Forces acting on toroidal vortices in an unbounded medium (plasma vortices in air and vortex rings in air and water) are investigated. A solution to the equations describing such votrices is obtained. It is shown that this solution satisfactorily agrees with experiment. Based on the experimental results and the solution obtained, the drag coefficient C x of such vortices is found. For the same Reynolds numbers, the value of C x may be much less than the drag coefficient of a drop-shaped axisymmetric body (0.045), which is known to be the best streamlined object.

Influence of initial and boundary conditions on vortex ring development

THIS paper presents the results of an experimental investigation comparing the mass and energy content of fully formed laminar vortex rings in air with that of the original pulse that generated them for a variety of initial and boundary conditions. In particular, the fractional entrainment of mass and the partition of initial energy between kinetic energy of translation and kinetic energy of rotation are studied. It is found that these characteristics depend strongly on the boundary conditions. A technique is presented which enables calculation of kinetic energy of rotation from motion picture sequences. The ratio of characteristi c translational speed to characteristic rotational speed is shown to be a useful parameter for correlation of data. Data on vortex size and speed are presented using this correlation, and it is seen that all data, regardless of initial and boundary conditions, fall on a single curve. A theoretical curve is derived, and it is seen that the data compare well with it. Contents In many applications utilizing nonsteady flow, it is of importance to understand the mechanisms of mass, momentum, and energy exchange between a primary flow and a secondary flow. The present work attempts to shed light on such mechanisms by ultilizing a simple example: the formation of vortex rings. A sketch of the system under study is shown in Fig. 1. We consider that, at time £ = 0, a uniform pulse is initiated with velocity Uj(t) for a duration Tp through a chimney of height H and diameter D0. Note that, when //=0, we have a simple orifice. After a certain period of time, the pulse becomes a fully formed vortex ring of diameter D which propagates at a speed U. The nature of the vortex ring will depend on the boundary conditions (H, DQ) and on the initial conditions luj(t), T p }. Since all of the vorticity essential to the existence of a vortex is generated in boundary layers prior to formation, different geometries will produce different amounts of vorticity. Furthermore, in the case of the simple orifice, vorticity of opposite sense is generated on the outside of the orifice by the boundary layer formed from the entrained flow, passing over the solid surface, thereby impeding the influx of entrained flow and diminishing the net available vorticity. These effects will result in the generation of vortices with different sizes and different propagation speeds, as well as differences in internal flowfields. In the experimental investigation, vortex rings were generated in still air for various sizes of simple orifices and chimneys by means of a large loudspeaker energized by a step in voltage, producing a controllable jet-pulse intensity. The pulse intensity and duration were monitored by means of a hot-film anemometer. Oscillographic records of the

Travelling air vortex rings as potential communication signals in a cricket

1. Crickets of the non-stridulating speciesPhaeophilacris spectrum (Phalangopsidae) generate travelling air vortex rings, which might serve as a hitherto unknown method of non-acoustic communication involving air particle movements. 2. The males produce a series of forward wing flicks during courtship and single wing flicks during aggressive encounters (Figs. 1, 2). In simulation experiments with moving wing pairs (Fig. 3) travelling vortex rings were visualized by using TiCl4 smoke or tracer particles in the air flow field (Fig. 4). 3. The vortex rings have an initial diameter of about 2 cm, increasing to 5 cm, and travel a maximum distance of 15 cm, depending on wing size and wing flick duration. Both wing size and flick duration determine the Reynolds number (Fig. 6). 4. Generally the velocity of propagation of vortex rings (from an initial velocity of about 40 cm/s) decays exponentially with distance (Fig. 5B). 5. The internal structure of the vortex rings, as visualized with tracer particles (Fig. 8), reveals a typical rotational velocity field. Particle velocities approach 15 cm/s (Fig. 9). 6. The physical basis of vortex ring generation by wing flicks and their possible communication role are discussed.

An empirical model of the motion of turbulent vortex rings

D IMENSIONAL arguments indicate that the motion of both axial turbulent puffs (strongly turbulent masses of fluid moving through surroundings with which they readily mix)1 and turbulent vortex rings2 will remain similar at all distances from their virtual sources if their spreading rates are linear and their translational velocities decay as the inverse cube of the distance from their respective virtual sources. This hypothesis appears to be well substantiated for the case of the rapidly spreading turbulent puffs.1-3 However, the validity of the model is much more difficult to determine for the case of the vortex ring, where the extremely slow ring growth leads to the prediction of very large virtual origins. In fact, recent experiments4 indicate that measurements would be required at least 1000 diam from the discharging orifice, in order to satisfactoril y test the similarity model. Since such measurements have not been made and do not appear to be in the offing, the following empirical model is proposed for use in engineering calculations. Quantitative agreement with experimental results is good to distances on the order of 70 diam from the discharging orifice. The model is based on measurements of air vortex rings, seeded with a mist of dioctyl phthalate droplets, traveling through quiescent air. The time of flight was recorded by using the signal from a hot-wire anemometer to stop an electronic event timer as the vortex passed the anemometer. The hot-wire circuitry was also used to flash a stroboscope, thereby illuminating the vortex to an open-lensed camera, which recorded its geometry. All measurements were repeated at least thirty times and the mean values were used in further computations. It was apparent from the data that the behavior of the rings is very accurately described by (t - tv)/to = C[(x - x (D