Experimental Study On The Instability Of Vortex Rings Generated By An Oscillating Disk

In this thesis,a new mechanism of vortex ring instabilities generated by a controlled oscillat-ing disk is studied experimentally by flow visualization method.The major diameter of the disk is c.We define two key control parameters:the frequency which is scaled as the Stokes number β=f0c2/v,and the non-dimensional amplitude or Keulegan-Carpenter number KC = 2πA/c,where v is the kinematic viscosity of the fluid and A is the amplitude of oscillation.To determine the parametric points for experiments,we firstly perform the Floquet stability analysis to identify the unstable modes arising from the interacting vortex rings.Within the(KC,β)parametric space,we find two different flow regimes.First,for low β,due to the low frequency of the oscillation,the interaction between the shedding vortex rings is weak,the flow breaks its symmetry with a single wavenumber mode(m=1)at sufficiently large values of KC.Second,for high β,the interaction between the vortex rings is pronounced,resulting in high-order unstable modes.The high-mode number instabilities can be further divided into two branches,characterized by the phase proper-ties of the three-dimensional vortex structures above and below the disk.We reproduce the flow structures for these two different branches by using three-dimensional direct simulations(DNS),in which the in-phase and out-of-phase properties are clearly identified.For the most important part of this thesis,we carry out experimental studies in the high mode number region,i.e.0.3<KC<15 and 100<β<1200.By releasing fluorescent tracers from the edge of the disk,we observe the flow structure with asymptotically stable,organized modes with countable wave numbers arising from the vortex ring instabilities.These modes cover a large range of wave numbers from m = 1 to m = 9,which is in good agreement with the previous linear stability analysis.Furthermore,the secondary symmetry breaking with respect to the disk plane is observed,according well with our DNS results.It is shown that for a fixed β,as increasing KC,the vortex bubbles shedding to the upper side of the disk are located between those shedding to the lower side of the disk,or in the sense of out-of-phase property.The two-dimensional base flow has also been compared to our PIV experiments,showing a good agreement between them.