最大循環をもつ渦輪の生成

抄録

A numerical simulation by a three dimensional vortex blob method has shown that an axisymmetric sinusoidal forcing generates a vortex ring with circulation which is greater than that for the unforced flow, under the condition that the same volume of fluid issues from a circular nozzle during a fixed time interval. The time interval is chosen as 4.0(2R/U) based on the experiment of Gharib et al. (1998), where R is the radius of the nozzle and U is the average velocity of the issuing fluid. At a fixed amplitude of forcing, this maximum circulation is realized at a forcing frequency of 0.125(U/2R). This is because most of the circulation shed from the nozzle is absorbed into the vortex ring at this optimum frequency. The maximum circulation is greater than that of the unforced flow by an amount proportional to the amplitudes squared. This is discussed in terms of the circulation shed from the nozzle during the issuing time.