Compressible vortex ring propagating faster than the speed of sound simulated for the first time

Scilight ◽  
2021 ◽  
Vol 2021 (38) ◽  
pp. 381106
Author(s):  
Jodi Ackerman Frank
Keyword(s):  
Vortex Ring ◽  
Speed Of Sound ◽  
First Time ◽  
Compressible Vortex Ring

Laminar vortex rings impinging onto porous walls with a constant porosity

2018 ◽  
Vol 837 ◽  
pp. 729-764 ◽  
Author(s):  
Yang Xu ◽  
Jin-Jun Wang ◽  
Li-Hao Feng ◽  
Guo-Sheng He ◽  
Zhong-Yi Wang
Keyword(s):  
Particle Image Velocimetry ◽  
Vortex Ring ◽  
Particle Image ◽  
Porous Wall ◽  
Hole Diameter ◽  
Vortex Rings ◽  
Flow Structures ◽  
Image Velocimetry ◽  
Porous Walls ◽  
First Time

For the first time, an experiment has been conducted to investigate synthetic jet laminar vortex rings impinging onto porous walls with different geometries by time-resolved particle image velocimetry. The geometry of the porous wall is changed by varying the hole diameter on the wall (from 1.0 mm to 3.0 mm) when surface porosity is kept constant ($\unicode[STIX]{x1D719}=75\,\%$). The finite-time Lyapunov exponent and phase-averaged vorticity field derived from particle image velocimetry data are presented to reveal the evolution of the vortical structures. A mechanism associated with vorticity cancellation is proposed to explain the formation of downstream transmitted vortex rings; and both the vortex ring trajectory and the time-mean flow feature are compared between different cases. It is found that the hole diameter significantly influences the evolution of the flow structures on both the upstream and downstream sides of the porous wall. In particular, for a porous wall with a small hole diameter ($d_{h}^{\ast }=0.067$, 0.10 and 0.133), the transmitted finger-type jets will reorganize into a well-formed transmitted vortex ring in the downstream flow. However, for the case of a large hole diameter of $d_{h}^{\ast }=0.20$, the transmitted vortex ring is not well formed because of insufficient vorticity cancellation. Additionally, the residual vorticity gradually evolves into discrete jet-like structures downstream, which further weaken the intensity of the transmitted vortex ring. Consequently, the transmitted flow structures for the $d_{h}^{\ast }=0.20$ case would lose coherence more easily (or probably even transition to turbulence), resulting in a faster decay of the axial velocity and stronger entrainment of the transmitted jet. For all porous wall cases, the velocity profile of the transmitted jet exhibits self-similar behaviour in the far field ($z/D_{0}\geqslant 6.03$), which agrees well with the velocity distribution of free synthetic jets. With the help of the control-volume approach, the time-mean drag of the porous wall is evaluated experimentally for the first time. It is shown that the porous wall drag increases with the decrease in the hole diameter. Moreover, for a porous wall with a small hole diameter ($d_{h}^{\ast }=0.067$, 0.10 and 0.133), it appears that the porous wall drag mainly derives from the viscous effect. However, as $d_{h}^{\ast }$ increases to 0.20, the form drag associated with the porous wall geometry becomes significant.

Numerical Modeling of Spores Dispersal of Sphagnum Moss Using ANSYS FLUENT

2017 ◽  
Author(s):  
Dwight L. Whitaker ◽  
Robert Simsiman ◽  
Emily S. Chang ◽  
Samuel Whitehead ◽  
Hesam Sarvghad-Moghaddam
Keyword(s):  
Vortex Ring ◽  
High Speed ◽  
Cost Effective ◽  
Video Data ◽  
Vortex Rings ◽  
Peat Moss ◽  
Ansys Fluent ◽  
Formation Number ◽  
Piston Stroke ◽  
First Time

The common peat moss, Sphagnum, is able to explosively disperse its spores by producing a vortex ring from a pressurized sporophyte to carry a cloud of spores to heights over 15 cm where the turbulent boundary layer can lift and carry them indefinitely. While vortex ring production is fairly common in the animal kingdom (e.g. squid, jellyfish, and the human heart), this is the first report of vortex rings generated by a plant. In other cases of biologically created vortex rings, it has been observed that vortices are produced with a maximum formation number of L/D = 4, where L is the length of the piston stroke and D is the diameter of the outlet. At this optimal formation number, the circulation and thus impulse of the vortex ring is maximized just as the ring is pinched off. In the current study, we modeled this dispersal phenomenon for the first time using ANSYS FLUENT 17.2. The spore capsule at the time of burst was approximated as a cylinder with a thin cylindrical cap attached to it. They were then placed inside a very large domain representing the air in which the expulsion was modeled. Due to the symmetry of our model about the central axis, we performed a 2D axisymmetric simulation. Also, due the complexity of the fluid domain as a result of the capsule-cap interface, as well as the need for a dynamic mesh for simulating the motion of the cap, first a mesh study was performed to generate an efficient mesh in order to make simulations computationally cost-effective. The domain was discretized using triangular elements and the mesh was refined at the capsule-cap interface to accurately capture the ring vortices formed by the expulsed cap. The dispersal was modeled using a transient simulation by setting a pressure difference between inside of the capsule and the surrounding atmospheric air. Pressure and vorticity contours were recorded at different time instances. Our simulation results were interpreted and compared to high-speed video data of sporophyte expulsions to deduce the pressure within the capsule upon dispersal, as well as the formation number of resulting vortex rings. Vorticity contours predicted by our model were in agreement with the experimental results. We hypothesized that the vortex rings from Sphagnum are sub-optimal since a slower vortex bubble would carry spores more effectively than a faster one.

Numerical simulation and PIV study of compressible vortex ring evolution

Shock Waves ◽  
2011 ◽  
Vol 22 (1) ◽  
pp. 69-83 ◽  
Author(s):  
T. Murugan ◽  
S. De ◽  
C. L. Dora ◽  
D. Das
Keyword(s):  
Numerical Simulation ◽  
Vortex Ring ◽  
Compressible Vortex Ring
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