The Experimental Study Of A Starting Vortex Ring Generated By A Thin Circular Disk

Vortex ring is one of the most of fundamental and fascinating fluid mechanical phenomena. This doctor dissertation presents an experimental study on the formation process of an axisymmetric vortex ring generated by a thin circular disk in detail.The article mainly studies the following aspects:a software used for digital particle image velocimetry (DPIV) was developed based on Matlab graphical interface; distribution of velocity and vortisity around the vortex ring generated by a disk was measured by DPIV; The finite-time Lyapunov exponent (FTLE) fields and the Lagrangian coherent structures (LCS) of the vortex flow were computed; boundary of the vortex ring was identified and transport process of flow fluid of the vortex ring was analyzed; a vorticity-flux window was found. Based on the boundary of the disk vortex ring, the volume, circulation and energy of the vortex ring were studied.The formation process of the vortex ring consists of three phases:the rapid growth phase, the stable growth phase and the non-axisymmetric phase. In the rapid growth phase (the dimensionless time0<Tn<0.2), the circulation of the vortex ring rapidly grows and the velocity of the vortex core decreases, where the Taylor’s inviscid estimation works well; in the stable growth phase (0.2<Tn<4), the growth rate of the circulation decreases gradually; and in the non-axisymmetric phase (Tn>4), results of the repeatability test, the flow visualization and the numerical simulation show that the disk vortex ring cannot maintain axisymmetry due to instability.Based on the DPIV results, a new non-invasive approach measuring force of the moving disk in the fluid are proposed, that is named’the direct momentum conservation method’(DMCM) and the temporal impulse exerted by the moving disk on the fluid is estimated. Results show the momentum of the control volume enclosing LCS is64%-68%of the entire impulse exerted by the disk on the fluid at last, which value is very close to2/3, results of Saffman’s calculation(1992). Compared with vorticity-moment method (VMM), the results of DMCM and VMM agree with each other very well at high experimental data accuracy, but DMCM requires lower experimental data accuracy than VMM.