Vortex Rings In Air Research Articles

Blow-off of diffusion flame by moving air vortex ring

The blow-off is one of the typical flame extinction mechanisms, and such a principle has been widely used in firefighting when the water-based extinguisher is limited. This work explores the blow-off extinctions of different diffusion flames by air vortex ring. The vortex ring harnesses its kinetic energy within the fast-rotating vortex core, enabling the transmission of power over several meters to blow off a remote flame. The power required for vortex ring blow-off is found to be two to three orders of magnitude smaller than the power of the flame itself, demonstrating exceptional energy efficiency. It is observed that the poloidal flow (circulation) surrounding the vortex core can stretch the flame base to the critical state and then cause instantaneous extinction. To explain the vortex-induced blow-off limit, a critical Damköhler number that accounts for the competition between fuel gas flow and flame stretch was formulated. This work provides a fundamental understanding of the extinction mechanism by vortex ring, and it offers technical guidelines for using air as a flame extinguisher for remote firefighting within minimum energy input.

Parametric study on flow characteristics of air vortex ring supply using large-eddy simulation

Air vortex-ring-based supply mode was proposed to deliver fresh air. The influence of air supply parameters on the entrainment and mixing of air vortex rings was investigated numerically. The air supply parameters, including the supply air velocity, formation time, and supply air temperature difference, were specified by theoretical analysis based on both the simple cylindrical-slug model and the application scenario. A large eddy simulation with a dynamic Smagorinsky–Lilly model was used to generate the data required for analysis. Sensitivity tests of the computational parameters and experimental validations were conducted to ensure the accuracy and applicability of the numerical method. The results showed that when the formation time was less than the formation number, increasing the formation time reduced the entrance gap width and consequently restricted the entrainment of the air vortex ring. When the formation time exceeded the formation number, the additional air appeared as a trailing wake and enhanced the diffusive entrainment. If the supply air velocity is too high, the entrainment and mixing process of the air diffused from the boundary of the vortex ring is accelerated. However, if the supply air velocity is too low, the air vortex ring collapses downstream, increasing the amount of pulsed air jet deposited behind the vortex ring. The non-isothermal pulsed jet caused more air from the pulsed jet to be deposited behind the vortex ring and accelerated the attenuation rate of the air vortex ring, indicating that the entrainment was enhanced.

Investigation into fresh air delivery performance with vortex ring

An air supply mode based on vortex ring structures was proposed to deliver fresh air for good indoor air quality. This study focused on the influence of air supply parameters, including formation time (T*), supply air velocity (U0), and supply air temperature difference (ΔT), on the fresh air delivery performance of an air vortex ring using numerical simulations. The cross-sectional average mass fraction of fresh air (Ca) was proposed to evaluate the fresh air delivery performance of the air vortex ring supply. The results showed that the convective entrainment of the vortex ring resulted from the combined effect of the induced velocity generated by the rotational motion of the vortex core and the negative pressure zone. When the formation time T* < 3.5, Ca increases with an increase in the formation time but decreases with an increase in the formation time for T* > 3.5. The variation in Ca is insignificant for U0 < 3 m/s or U0 > 3 m/s but decreases with an increase in the supply air temperature difference (ΔT). Thus, the optimal air supply parameters for air vortex ring supply are identified as T* = 3.5, U0 = 3 m/s, ΔT = 0 °C. Compared with the traditional circular and square free air jets, within 25Ds (Ds: the diameter of air supply inlet) downstream from the air inlet, the fresh air delivery performance of the air vortex ring supply is increased by a maximum of nearly 200 %, with an average improvement of approximately 120 %.

Flow characteristics and formation optimization of vortex ring air supply.

The vortex rings ventilation (VRV) is a new type of air supply system comprising vortex rings. Compared with an air supply jet, a vortex ring reduces the loss of fresh air during transportation because of its stable structure. However, during the formation of the vortex ring, it entrains the ambient air and reduces the fresh air in the vortex ring. In this study, a vortex ring generator with a fresh air cavity is proposed to form confined vortex rings. This improved the fresh air ratio of the VRV. Based on previous experiments, a piston-orifice axisymmetric model with a dynamic grid was developed to form an air vortex ring. The flow characteristics of free and confined vortex rings during the generation stage were studied. First, the evolution of free vortex rings with different stroke lengths was studied, and the optimal piston stroke length and radial constraint size were determined. Subsequently, the mixing ratios of the free and confined vortex rings were compared and analyzed. The results showed that the mixing ratio of the confined vortex ring formed by the fresh air cavity in the formation stage was zero. Moving to the location of 4.9 times the orifice diameter ( ), reduced the mixing ratio of the confined vortex ring by 77.78% compared with that of the free vortex ring. In addition, the influence of three inner diameters and four outlet diameters of fresh air cavities on the vortex rings was studied to optimize the size of the vortex ring generator. The results showed that the inner diameter of the fresh air cavity was greater than and that an outlet diameter greater than had little influence on the vortex rings.

Effect of thermal buoyancy on vortex ring air supply mode

Owing to the air buoyancy effect in the non-isothermal environment, the air supply efficiency will be greatly reduced due to the severe interference of the air distribution and air supply performance. Although the vortex ring air supply mode shows better air supply efficiency than the symmetrical round jet under the isothermal conditions, it is still unclear under non-isothermal conditions. Therefore, the dynamic mesh method is applied to the formation of the air vortex ring. Then the influence of non-isothermal conditions on the transport characteristics of the air vortex ring is studied, and the temperature attenuation trend of the vortex ring air supply mode are evaluated. The results show that the attenuation of the translational velocity of the vortex ring under isothermal conditions can be used to approximate the translational velocity of the thermal air vortex ring, with a deviation of no more than 3%; second, the vortex ring air supply mode has better directionality, and the deviation can be reduced by 85% compared with the symmetrical round jet; third, an improvement method of the vortex ring air supply mode is proposed, the vortex core temperature increased by 4.5 °C; finally, the temperature attenuation of the thermal vortex ring is 25.8% lower than that of the thermal jet in the range of Lpath/D < 20.

The Evolution of Acoustic Radiation by an Ensemble of Vortex Rings in Air

The evolution of acoustic radiation emitted by an ensemble of vortex rings in air is studied on the basis of nonstationary Navier–Stokes equations. We use the expansions of required functions into a power series of the initial vorticity which is a small value. The Navier–Stokes equation system reduces to a parabolic system with constant coefficients for the higher derivatives. The problem is posed as follows. The vorticity is defined inside the toroid at t = 0. The other parameters of the gas are assumed to be constant throughout the space at the initial instant of time. The solution is expressed in terms of multiple integrals, which are calculated using Korobov grids. The density oscillations were investigated. The results show that the frequency spectrum depends on time; high-frequency oscillations are observed at small times and low-frequency oscillations then occur. At the same time, the amplitude of high-frequency oscillations decreases in comparison with low-frequency oscillations. Thus, a transition of energy from the high-frequency spectrum to the lowfrequency spectrum occurs. These results can be useful for modeling decaying grid turbulence.

Symmetrical collision of multiple vortex rings

In this work, we investigate the motion, interaction, and simultaneous collision between many initially stable vortex rings arranged symmetrically in two initial configurations, three and six rings making an angle of 60 and 120° between their straight path lines, respectively. We report results for laminar vortex rings in air obtained through numerical simulations of the ring velocity, pressure, and vorticity fields, both in free flight and during the entire collision. Each collision was studied for small Reynolds numbers Re&amp;lt;103 based on both the self-induced velocity and diameter of the ring. The case of three rings produces secondary vortical structures formed by laterally expanding dipolar arms with top and bottom secondary vortex rings. The case of six colliding rings produces, as secondary structures, two big rings moving in opposite directions, a process that reminds us of the head-on collision of two rings [T. T. Lim and T. B. Nickels, “Instability and reconnection in the head-on collision of two vortex rings,” Nature 357, 225–227 (1992)] under a hypothetical time reversal transformation. Both collisions display a characteristic kinetic energy evolution where mean collision stages can be identified within the range of Reynolds numbers investigated here.

Acoustic Radiation by 3D Vortex Rings in Air

Acoustic radiation emitted by three-dimensional (3D) vortex rings in air has been investigated on the basis of the unsteady Navier–Stokes equations. Power series expansions of the unknown functions with respect to the initial vorticity which is supposed to be small are used. In such a manner the system of the Navier–Stokes equations is reduced to a parabolic system with constant coefficients at high derivatives. The initial value problem is as follows. The vorticity is defined inside a toroid at t = 0. Other gas parameters are assumed to be constant throughout the whole space at t = 0. The solution is expressed by multiple integrals which are evaluated with the aid of the Korobov grids. Density oscillations are analyzed. The results show that the frequency band depends on the initial size of the vortex ring and its helicity. The presented data may be applied to the study of a flow in a wake region behind an aerodynamic body.

Experimental observation of the collision of three vortex rings

We investigate for the first time the motion, interaction and simultaneous collision between three initially stable vortex rings arranged symmetrically, making an angle of 120 degrees between their straight path lines. We report results with laminar vortex rings in air and water obtained through measurements of the ring velocity field with a hot-wire anemometer, both in free flight and during the entire collision. In the air experiment, our flow visualizations allowed us to identify two main collision stages. A first ring-dominated stage where the rings slowdown progressively, increasing their diameter rapidly, followed by secondary vortex structures resulting after the rings make contact. Local portions of the vortex tubes of opposite circulation are coupled together thus creating local arm-like vortex structures moving radially in outward directions, rapidly dissipating kinetic energy. From a similar water experiment, we provide detailed shadowgraph visualizations of both the ring bubble and the full size collision, showing clearly the final expanding vortex structure. It is accurately resolved that the physical contact between vortex ring tubes gives rise to three symmetric expanding vortex arms but also the vortex reconnection of the top and lower vortex tubes. The central collision zone was found to have the lowest kinetic energy during the entire collision and therefore it can be identified as a safe zone. The preserved collision symmetries leading to the weak kinematic activity in the safe zone is the first step into the development of an intermittent hydrodynamic trap for small and lightweight particles.

Water Drops Dancing on Ice: How Sublimation Leads to Drop Rebound

Drop rebound is a spectacular event that appears after impact on hydrophobic or superhydrophobic surfaces but can also be induced through the so-called Leidenfrost effect. Here we demonstrate that drop rebound can also originate from another physical phenomenon, the solid substrate sublimation. Through drop impact experiments on a superhydrophobic surface, a hot plate, and solid carbon dioxide (commonly known as dry ice), we compare drop rebound based on three different physical mechanisms, which apparently share nothing in common (superhydrophobicity, evaporation, and sublimation), but lead to the same rebound phenomenon in an extremely wide temperature range, from 300 °C down to even below -79 °C. The formation and unprecedented visualization of an air vortex ring around an impacting drop are also reported.

Vortex rings and plasma toroidal vortices in homogeneous infinite media. III. Diffusion anisotropy effect in vortices

The effect of diffusion anisotropy of material particles in toroidal vortices (TVs) is experimentally studied in plasma TVs in air, in vortex rings in air and water. The essence of this effect consists in the fact that diffusion coefficients of material particles in TVs in the direction perpendicular to the rotation axis D⊥ are much lower than in the direction parallel to this axis D‖. The coefficient D⊥ is determined for propane molecules in air TVs, and the dependence of the coefficient D⊥ of ink particles in water TVs on the rotation angular velocity ω is studied. It is shown that the coefficient D⊥ can decrease with increasing ω by a factor of 10–25 in comparison with the coefficient D‖.

Vortex rings and plasma toroidal vortices in homogeneous unbounded media. II. The study of vortex formation process

The conditions of the formation of toroidal vortices (TVs), i.e., the plasma TV and air vortex ring at atmospheric pressure, are experimentally determined. The TV formation process is numerically simulated. Local characteristics of the pulsed jet, induced flow of ambient medium (air), and forming TV at various time points are obtained. It is shown that the results of the preliminary numerical calculation are in agreement with experimental data.

Application of air‐jet nozzle in short staple Siro spinning system

With the objective of reducing the hairiness of Siro spun yarns, two types of air‐jet nozzle differing in the angle of sub‐holes and suitable for an air vortex ring spinning system were designed and fabricated. The performance of the JetSiro spinning system on short staple fiber materials and the effects of the different parameters on the hairiness of JetSiro spun yarns, such as nozzle pressure of compressed air, distance between front roller nip and inlet of nozzle, and nozzle structure, were investigated using the Taguchi method. The physical properties of JetSiro spun yarns with those of conventional Siro spun yarns were compared. The results show that the parameters, air pressure and distance between front roller nip and inlet of nozzle have the strongest and weakest effect on yarn hairiness. In addition, the optimum spinning conditions were determined. The application of the air‐jet nozzle exhibits significant reductions in yarn hairiness of 40%. The results revealed no significant effect of the air‐jet nozzle on the yarn tensile properties and evenness.

Air vortex ring communication between mobile robots

This project investigates the biomimetic implementation of a form of communication observed in cave dwelling crickets. The cricket Phaeophilacris spectrum uses air vortices as a form of short-range communication. This project aims to mimic this communication technique for use in robotic systems and to assess its capabilities in terms of technical requirements, range and the data it can provide. The design of an air vortex generator and receiver are described. Results of practical experiments to code information into sequences of vortices as well as to determine range of the source and direction of arrival are also presented.

Propagation of orifice- and nozzle-generated vortex rings in air

The effect of the exit geometry of a vortex ring generator was studied experimentally. Two types of exit geometries were chosen: an orifice and a nozzle. Vortex rings were generated by pushing a solenoid-valve-controlled, pressurized-air jet through the circular opening of the orifice or nozzle. Experiments were performed over a wide range of initial Reynolds number (450 l Re l 4580) and length-to-diameter ratio (0.7 l L / D l 7.0) of the air jet. The exit geometry was found to significantly influence the entire course of propagation of the vortex ring. The orifice-generated vortex ring had superior characteristics to that produced by the nozzle under the same conditions. The vorticity generated along the wall in the orifice exit plane had a negligible effect on the circulation of the vortex ring within the specified range of Reynolds number. Compared to the nozzle-generated vortex ring, the orifice-generated ring showed reduced initial vorticity losses and less diffusive entrainment of ambient fluid. The vortex rings produced by the orifice attained more circulation, less entrainment of ambient fluid and hence rapidly propagated through longer distances in comparison to the nozzle-generated rings.

Robot Pheromone Communication Using Vortex Ring Transmission

Unpredictable air movements have proved to be a problem in previous studies investigating robot communication by means of airborne pheromone chemicals. The project described in this paper investigates the use of air vortex rings as a means of carrying pheromone chemicals between transmitting and receiving robots. Sensitivity to chemicals including pheromones released by conspecifics is essential for many aspects of an insect's life. They assist in finding food, locating a mate, avoiding danger and help coordinate the activities of social insects. In the future, autonomous robots will be challenged by many situations similar to those that face insects and other simple creatures. Chemical communication may prove useful for these robots as well. This paper describes the equipment developed for generating and detecting vortex rings. Results of experiments involving location and tracking of a sequence of pheromone vortex rings are also presented.

G0601-6-3 渦輪を利用した噴流拡散火炎の消火特性(熱工学(6)燃焼(1))

A vortex ring is easily produced by pushing fluid through a circular orifice. If the vortex ring interacts with a diffusion flame, a local extinction occurs. In order to use a vortex ring for fire fighting, we have investigated the characteristics of the air vortex ring extinguishment of a methane-air jet diffusion flame. From the blowout experiments, we calculated probability of the vortex ring blowout and measured displacement speed of the vortex ring. As a result, when the air vortex ring impacts with the flame base zone of the jet diffusion flame at the displacement speed enough to generate a local extinction, the whole flame can be extinguished.

Effect of an Exit-Wedge Angle on Pinch-off and Mass Entrainment of Vortex Rings in Air

The effects of exit-wedge angle on evolution, formation, pinch-off, propagation and diffusive mass entrainment of vortex rings in air were studied using digital particle image velocimetry. Vortex rings were generated by passing a solenoid-valve-controlled air jet through a cylindrical nozzle. Experiments were performed over a wide range of exit-wedge angles (10° ≤ α ≤ 90°) of the cylindrical nozzle, initial Reynolds numbers (450 ≤ Re ≤ 4,580) and length-to-diameter ratios (0.9 ≤ L/D ≤ 11) of the air jet. For sharp edges (α ≤ 10°), a secondary ring may emerge at high Reynolds numbers, which tended to distort the vortex ring if ingested into it. For blunt edges (α ≥ 45°), by contrast, stable vortex rings were produced. The formation phase of a vortex ring was found to be closely related to its evolution pattern. An exit-wedge angle of 45° was found to be optimal for rapid pinch-off and faster propagation and better stability of a vortex ring. Diffusive mass entrainment was found to be between 35% and 40% in the early stages of a vortex ring propagation and it gradually increased throughout the course of vortex ring propagation. Entrainment fraction was found to be sensitive to the L/D ratio of the initial jet and decreases when the L/D ratio is increased.

755 CFDによる空気砲の形状検討 : 第1報 : 渦輪のシミュレーション(流体パワーによる駆動と制御IV,オーガナイズドセッション)

755 CFDによる空気砲の形状検討 : 第1報 : 渦輪のシミュレーション(流体パワーによる駆動と制御IV,オーガナイズドセッション)

空気砲による渦輪放出の数値シミュレーション

空気砲による渦輪放出の数値シミュレーション