Free Vortex Ring Research Articles

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.

Development of an Unsteady Vortex Lattice Method to Model Propellers at Incidence

The complex aerodynamics of a propeller at incidence involve multiple asymmetric flow conditions around the disk, with each originating from different physical phenomena. These asymmetries give rise to in-plane forces and out-of-plane moments that are difficult to model with traditional low-order computational aerodynamics methods. This paper presents a three-dimensional unsteady vortex lattice method with a free vortex ring wake formulation that has been developed to model these effects for propellers at incidence. Convergence quality is investigated with respect to simulation time, grid resolution, and time-step resolution. Solution accuracy is evaluated by comparing to computational fluid dynamics (CFD) analyses. Good agreement was found with CFD results. The paper concludes with a discussion on the challenges of modeling propellers at incidence with vortex lattice methods.

Hydrodynamic impulse enhancement of a vortex ring interacting with an axisymmetric co-axial aperture

Abstract

Pinning of scroll waves to flat and highly branched unexcitable heterogeneities.

System heterogeneities such as organelles, cells, and anatomical features strongly affect nonlinear wave patterns in biological systems. These effects are more readily studied in otherwise homogeneous chemical reactions that allow the introduction of tailored structures. Following this approach, we investigate the dynamics of three-dimensional excitation vortices pinned to inert sheets with circular holes arranged on a hexagonal lattice. Experiments with the Belousov-Zhabotinsky reaction and numerical simulations of an excitable reaction-diffusion model reveal vortex pinning that circumvents the rapid collapse of free vortex rings. The pinned scroll waves are affected by the topological mismatch between their looplike rotation backbone and the branched pinning structure. Depending on the initial condition, a multitude of stable vortex states exist, all of which obey topological constraints, suggesting spinlike states for the involved obstacle holes.

Generation and characteristics of vortex rings free of piston vortex and stopping vortex effects

This paper presents a novel method for generating vortex rings that circumvents some of the drawbacks associated with existing methods in producing them. The predominant effects that occur in previously used methods are due to the presence of some of the other vortices such as the stopping vortex, piston vortex, image vortex and orifice lip generated vortices in the early stage of development. These disturbances influence the geometric, kinematic and dynamic characteristics of a vortex ring and lead to mismatches with classical theoretical predictions. It is shown in the present study that the disturbance free vortex rings produced follow the classical theory. Flow visualization and particle image velocimetry experiments are carried out in the Reynolds number (defined as the ratio of circulation ($\unicode[STIX]{x1D6E4}$) and kinematic viscosity ($\unicode[STIX]{x1D708}$)) range, $2270<Re_{\unicode[STIX]{x1D6E4}}<6790$, to find the translational velocity, total and core circulation, core diameter, ring diameter and bubble diameter. In reference to the earlier studies, significant differences are noted in the variations of the vortex ring diameter and core diameter. A model for the core diameter during the formation stage is proposed. The translational velocity variation with time shows that the second-order accurate formula derived using Hamilton’s equation by Fraenkel (J. Fluid Mech., vol. 51, 1972, pp. 119–135) predicts it best.

Mussels blow rings: Jet behavior affects local mixing

AbstractBenthic suspension feeders such as dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis) are often found in remarkably dense aggregations (i.e. > 105 mussels m− 2), which is surprising, given their high clearance rates and limited mixing within the benthic boundary layer. Results from flow visualization in flow chamber experiments indicate that there is indeed limited mixing around mussel aggregations at low flows and that siphonal jets can increase mixing around and above these aggregations. Using particle image velocimetry (PIV) to further investigate the underlying hydrodynamics of these jets, we characterized differences in velocity and vorticity among four siphonal behaviors (e.g., slow flux, streaming, exhalant jets, and inhalant jets), including both continuous and pulsatile jets, the latter of which generate free vortex rings. Incorporating these hydrodynamic characteristics into a computational fluid dynamic (CFD) model revealed that siphonal jets increased mixing, expressed as vertical diffusivity in the benthic boundary layer. These differences were most pronounced at slow vs. fast cross‐stream velocities, but those differences diminished several body lengths (i.e. 10−1 m to 10−2 m) downstream. The results from PIV measurements and CFD modeling suggest that benthic suspension feeders can influence patterns of local mixing, which would affect mass transport and biogeochemical processes in the near‐bed region. This underscores the need for physical‐biologically linked models to incorporate the behavior of benthic suspension feeding invertebrates.

Interaction of a laminar vortex ring with a thin permeable screen

AbstractThe canonical case of a vortex ring interacting with a solid surface orthogonal to its symmetry axis exhibits a variety of intricate behaviours, including stretching of the primary vortex ring and generation of secondary vorticity, which illustrate key features of vortex interactions with boundaries. Replacing the solid boundary with a permeable screen allows for new behaviour by relaxing the no-through-flow condition, and can provide a useful analogue for the interaction of large-scale vortices with permeable structures or closely spaced obstructions. The present investigation considers the interaction of experimentally generated vortex rings with a thin permeable screen. The vortex rings were generated using a piston-in-cylinder mechanism using piston stroke-to-diameter ratios ($L/ D$) of 1.0 and 3.0 (nominal) with jet Reynolds numbers ($R{e}_{j} $) of 3000 and 6000 (nominal). Planar laser-induced fluorescence and digital particle image velocimetry (DPIV) were used to study the interaction with wire-mesh screens having surface open-area ratios ($\phi $) in the range 0.44–0.79. Solid surfaces ($\phi = 0$) and free vortex rings ($\phi = 1$) were also included as special cases. Measurement of the vortex trajectories showed expansion of the vortex ring diameter as it approached the boundary and generation of secondary vorticity similar to the case of a solid boundary, but the primary vortex diameter then began to contract towards the symmetry axis as the flow permeated the screen and reorganized into a transmitted vortex downstream. The trajectories were highly dependent on $\phi $, with little change in the incident ring trajectory for $\phi = 0. 79$. Measurement of the hydrodynamic impulse and kinetic energy using DPIV showed that the change between the average upstream and downstream values of these quantities also depended primarily on $\phi $, with a slight decrease in the relative change as $L/ D$ and/or ${\mathit{Re}}_{j} $ were increased. The kinetic energy dissipation ($ \mrm{\Delta} E$) was much more sensitive to $\phi $, with a strongly nonlinear dependence, while the decrease in impulse ($ \mrm{\Delta} I$) was nearly linear in $\phi $. A simple model is proposed to relate $ \mrm{\Delta} E$ and $ \mrm{\Delta} I$ in terms of bulk flow parameters. The model incorporates the decrease in flow velocity during the interaction due to the drag force exerted by the screen on the flow.

Turbulence in Boundary Flow of SuperfluidHe4Triggered by Free Vortex Rings

The transition to turbulence in the boundary flow of superfluid 4He is investigated using a vortex-free vibrating wire. At high wire vibration velocities, we found that stable alternating flow around the wire enters a turbulent phase triggered by free vortex rings. Numerical simulations of vortex dynamics demonstrate that vortex rings can attach to the surface of an oscillating obstacle and expand unstably due to the boundary flow of the superfluid, forming turbulence. Experimental investigations indicate that the turbulent phase continues even after stopping the injection of vortex rings, which is also confirmed by the simulations.

A Finite-Difference Scheme for Three-Dimensional Incompressible Flows in Cylindrical Coordinates

A finite-difference scheme for the direct simulation of the incompressible time-dependent three-dimensional Navier-Stokes equations in cylindrical coordinates is presented. The equations in primitive variables (vr,vθ,vzandp) are solved by a fractional-step method together with an approximate-factorization technique. Cylindrical coordinates are singular at the axis; the introduction of the radial fluxqr=r·vron a staggered grid simplifies the treatment of the region atr= 0. The method is tested by comparing the evolution of a free vortex ring and its collision with a wall with the theory, experiments, and other numerical results. The formation of a tripolar vortex, where the highest vorticity is atr= 0, is also considered. Finally to emphasize the accurate treatment near the axis, the motion of a Lamb dipole crossing the origin is simulated.

Free Vortex Ring Formation in the Left Atrium Originating in the Left Auricle

An 80-year-old woman was evaluated by transesophageal echocardiography after coronary artery bypass surgery and subsequent cardioembolic stroke. In addition to spontaneous echo contrast demonstrating a counterclockwise rotating blood flow pattern, we observed free vortex ring formation in the left atrium, originating in the left auricle. To our knowledge, this is the first reported case of abnormal free vortex ring type flow pattern in the left atrium.

Some Properties of Quantized Vortex Rings near a Wall in Superfluid Helium

)' We show that, because of frictional forces due to quasi-particle collisions, the free vortex rings move slower and their radii are diminished. The influence on rings near a wall is such as to stop them at a closer distance to the wall. We also show that the ring stops in front of a wall in a quite stable way. We also investigate how the rings behave in an a. c. field.