DIRECT NUMERICAL-SIMULATION OF A LAMINAR VORTEX RING

Results are presented of direct numerical simulations (DNS) of a visco us, laminar ring. The effects of different generator configurations an d velocity programs on the formation and post-formation characteristic s of the ring are studied. It is shown that during the formation phase of the ring, total circulation and impulse in the flowfield are appro ximately the same for the ''nozzle'' and ''orifice'' generators. It is also found that throughout this period the slug flow model under-pred icts the total circulation in the flow. During the formation phase, th e simulation results for the time evolution of total circulation and l ocation of the vortex spiral center are in agreement with the experime ntal findings of Didden [J. Appl. Mech. Phys. (ZAMP) 30, 101 (1979)]. The results of the flow visualization studies show that during the pos t-formation phase a vortex bubble is formed. As the bubble propels its elf forward a wake is formed in the rear of the bubble. The impulse an d vorticity from the bubble are continuously shed into this wake. It i s found that the total value of the circulation in the flow varies as (t(1)())-0.33 which is consistent with Maxworthy's [J. Fluid Mech. 81 , 1 465 (1977)] prediction of the decay of circulation for a vortex ri ng. The transport of a passive Shvab-Zeldovich scalar variable is used to study the mixing and to obtain the maximum product formation in a chemical reaction of the type A A + B Products in a vortex ring. It is found that as the bubble containing the fuel propels itself forward, the outside oxidizer flow is entrained into it and reacts to form a pr oduct. Some of this product then is de-entrained into the wake of the bubble. (C) 1996 American Institute of Physics.