Numerical investigation into collision of vortex-rings with round cylinders

• Main Author: Lim, Yi Long
• Other Authors: New Tze How, Daniel
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/145299
• Institution: Nanyang Technological University

Large Eddy Simulations (LES) involving the head-on impingement of vortex rings upon round cylinders were carried out in ANSYS Fluent. Two cylinder-to-vortex-ring diameter ratios D/ d=0.5 and D/ d=0.25 were examined at Re=2000 for the purpose of studying ring-boundary collision interactions and comparing with larger-than-one diameter-ratio experimental results. A User-Defined Function (UDF) file, incorporating the kinematics properties of a vortex ring, was used for the numerical generation of the ring within the computational domain. The UDF’s capability to model the vortex ring’s translation and development was also validated by simulating the unimpeded translation of the ring. Results obtained from simulated cases show good qualitative matches when compared with experimental results. Both simulated and experimental cases show similar observations in the initial structural distortion of the primary ring, the formation of the secondary ring, and the movement of the secondary ring towards the collision axis. Unlike the experimental cases, however, the D/ d=0.5 simulated case also reveal that the primary ring undergoes azimuthal instability during the formation of the secondary ring. This flow behaviour was not observed when diameter-ratio was decreased to 0.25. Vortex cut-and-reconnection processes were observed in both simulated and experimental results but ring structural differences were noted during and after bifurcation. For instance, the simulation results show that the secondary ring bifurcates into two smaller rings but experimental results indicated the ejection of vortex-ringlets and further development of vortex-dipoles due to the collision between ringlets. Moreover, experimental results show the formation of tertiary rings but no tertiary rings were observed in the simulated cases. It is important to note, however, that these structural differences could be verified with longer simulation runs. Structural differences, apart from the primary ring’s instability, also exist between the two simulated cases. For D/ d=0.25, the vortex core of the primary ring is noticeably larger than that of the secondary ring. When diameter-ratio is increased to 0.5, however, there is little observable difference in core sizes between the rings. The lower diameter-ratio case also shows the reconstruction of the primary ring via vortex reconnection but this observation was not verified in the higher diameter-ratio case due to limited simulation time.