Vortex Ring Formation Process Research Articles

Vortex ring formation process in starting jets with uniform background co- and counter-flow

The formation process of the leading vortex ring in starting jets with uniform background co- and counter-flow has been studied numerically for $-0.5\leq R_v\leq 0.5$ , where $R_v$ is the ratio of background velocity to jet velocity. For the cases with background counter-flow, the normal formation process of the leading vortex ring would be destroyed when $R_v<-0.4$ , i.e. the trailing jet would overtake the leading vortex ring through the centre, a phenomenon reminiscent of vortex leapfrogging. As the velocity ratio $R_v$ increases, the formation number $F_{t^*}$ decreases from $9.6$ at $R_v=-0.4$ to $1.92$ at $R_v=0.5$ . An analytical model based on the kinematic criterion has been developed so as to describe the relationship between the formation number $F_{t^*}$ and velocity ratio $R_v$ . A linear relationship between the vortex core parameter and stroke ratio of starting jet ( $\varepsilon \sim k_1L/D$ ) for the Norbury vortex ring has been established and used effectively to close the model. For co-flow with $0< R_v\leq 0.5$ , the results from this model are consistent with the present numerical simulation and the experiments by Krueger et al. (J. Fluid Mech., vol. 556, 2006, pp. 147–166). For counter-flow, two different equations are proposed for $-0.4\leq R_v\leq -0.2$ and $-0.2< R_v<0$ , respectively.

곡률 변화에 따른 코안다 와류 고리 추진기의 유동 해석

The vortex ring thruster (VRT) is a propulsion system mimicking the propulsive technique of Mollusca, such as squids and jellyfish. VRT’s acknowledged advantages include low noise generation, delicate attitude control, and precise maneuverability. However, due to the low propulsion efficiency, its application is limited to low-speed unmanned underwater vehicle (UUV) only. In the present study, the propulsion performance evaluation of Coanda vortex ring thruster (CoVoRT) with different curvature was performed to examine the application of CoVoRT. The flows around the typical VRT and CoVoRT were analyzed using SNUFOAM, which was open-source toolkit CFD code based on the OpenFOAM. For computational validation, the CFD results for the flow around VRT were compared with the previous simulations and experimental results. Then propulsion performance of VRT and CoVoRT was compared in terms of the PPEI, and the vortex ring formation process. The results showed that CoVoRT

Fluid Flow Characterization in IPMC Actuated Contractile Water Jet Thruster

Contractile water jet thruster is an alternative method for Autonomous Underwater Vehicle (AUV) locomotion as well as for the AUV’s manoeuvrability control. Currently, studies on contractile water jet thruster focus more on the thrust force performance and its efficiency, such as the characterization of vortex ring formation process, the slug model L/D ratio and the influence of the thrust performance on the AUV. The aim of this paper is to discuss the fluid flow behaviour in the thruster during contraction process under different actuation frequency. This research utilized Ionic Polymer Metal Composite (IPMC) as the actuators instead of electric motor for the contractile function. The result shows that the Reynolds number of the intake flow has negative linear relation with the actuation frequency.

Numerical investigation on mechanism of multiple vortex rings formation in late boundary-layer transition

Mechanism of multiple vortex ring formation is one of key issues of late flow transition in a boundary layer. In this paper, a new mechanism is presented that the multiple ring formation is a result of shear layer instability. According to our DNS observation, a momentum deficit zone is formed above the original hairpin vortex tube due to the ejection of the streamwise hairpin vortex tube, which brings the low speed flow from the lower level to the higher level of the boundary layer through vortex rotation. Then the momentum deficit zone generates a sharp olive-shaped shear layer surrounded by the high speed flow. As the shear layer moves up, inviscid instability is developed due to the existence of inflection points (inflection surface in 3-D). Consequently, the new vortex rings are formed one by one. During the new spanwise vortex ring formation process, the spanwise vorticity is increased; however, the original streamwise hairpin vortex tube is weakened with decreasing of the streamwise vorticity. This process follows the Helmoholtz vorticity conservation law.

Formation Process of Vortex Ring Generated by a Pulsating Jet Flow

The formation process of a vortex ring generated by a pulsating jet flow, which is called the cyclic vortex ring, is investigated experimentally with PIV measurement. The waveform of the jet flow is conducted using sine curves by changing the amplitude of Reynolds' numbers Re0 of 1200-5040 and Womersley numbers α of 12.5-24.4. The results indicate that the formation process of the cyclic vortex ring is different from the laminar vortex ring. Another vortex ring which is formed from separated wall boundary layers in the nozzle during the suction period, which is called the separation vortex ring, is transported toward the nozzle exit during the discharge period. This separation vortex ring interferes with the formation process of the cyclic vortex ring at the nozzle exit plane. The circulation of the separation vortex ring depends on the pulsating conditions of the jet flow. Distribution of the vorticity around the core center of the cyclic vortex ring is in good agreement with the Oseen vortex model. The formation process of the cyclic vortex ring is classified into three categories, depending on Strouhal number.

Experimental Study of a Starting Vortex Ring Generated by a Thin Circular Disk

The formation process of vortex ring generated by a thin circular disk was studied experimentally in this paper. A thin circular disk installed a linear motion stage was used to generate the vortex rings. Digital Particle Image Velocimetry (DPIV) was used to measure the velocity and vorticity fields. The finite-time Lyapunov exponent field corresponding to the vortex flow was computed to identify Lagrangian coherent structures of the starting vortex. The results reveal the existence of a flux window between repelling Lagrangian Coherent Structures (rLCS) and attracting Lagrangian Coherent Structures (aLCS), through which the shear flow is entrained into the vortex. The flux window is closed gradually during the starting process. Once the flux window shut down, the formation process of the vortex ring finishes, as the shear flow with vorticity cannot be entrained in the vortex ring.

Pulsatile vortex generators for low-speed maneuvering of small underwater vehicles

Compact zero-mass pulsatile jet actuators are proposed for low-speed maneuvering and station keeping of small underwater vehicles. 1 1 Our research in this area was reported as the cover story of the New Scientist on October 23, 2004 ( Frost Gorder, 2004, science writer). The flow field of such jets are initially dominated by vortex ring formation. Pinched-off vortices characterize the extremum impulse accumulated by the leading vortex ring in a vortex ring formation process. Relevant parameters in this process are identified in order to design simple and low cost zero-mass pulsatile jet actuators. Thrust optimization of synthetic jets for maximal thrust generation is achieved by enforcing the jet formation number to be around 4. Prototypes of such actuators are built and tested for underwater maneuvering and propulsion. The actuators could be used in two ways: (i) to improve the low-speed maneuvering and station keeping capabilities of traditional propeller driven underwater vehicles, and (ii) as a synthetic jet for flow control and drag reduction at higher cruising speeds. A model for calculating the rotation rate of the underwater vehicle is also proposed and verified.

The pinch-off process in a starting buoyant plume

The vortex ring formation process of a starting buoyant plume was studied experimentally using digital particle image thermometry and velocimetry (DPITV). The vortex ring was observed to pinch-off, or become disconnected, from the trailing plume. Pinch-off occurred at non-dimensional times, or formation numbers, between 4.4 and 4.9. The observed pinch-off process is consistent with an explanation based upon the Kelvin–Benjamin variational principle. This is analogous to the pinch-off of a vortex ring generated using a piston–cylinder apparatus, suggesting that pinch-off is a general component of the vortex ring formation process for various generation mechanisms.