| dc.description.abstract | In the present work, we study experimentally the effect of flexible exit on vortex?pair generation. Vortex rings are frequently observed flow phenomena and have often been associated with unsteady starting jets issuing out of an orifice or nozzle. Such flows occur in various biological systems such as propulsive action of some aquatic creatures, the discharge of blood in the human heart valve amongst others. While vortex rings have been studied extensively, the effect of exit flexibility on such flows has not been explored. This could be important for biological systems that themselves are composed of flexible membranes.
A vortex?pair can be looked at as the two?dimensional analogue of a vortex ring. We study the generation, formation, and evolution of a vortex?pair using a piston?cylinder mechanism in a water tank. Fluid (water) is pushed out of a rectangular high aspect?ratio channel in a water tank using a servo?motor controlled piston. The issuing fluid separates at the channel exit and the separating boundary layer rolls up to form a vortex?pair. The forming vortex?pair is visualized using fluorescein dye and the flow field is also measured quantitatively at different instants of time using Particle Image Velocimetry (PIV). In addition, for the flexible exit cases, the dynamics of the flexible exit flap are measured optically, simultaneously with the flow field.
The main parameters in the present problem are the distance (L) moved by the fluid at the exit if the fluid moved as a slug, and the channel width (d) perpendicular to the direction of piston motion. A normalized fluid slug length (L/d) can be defined to study the time?dependent evolution of the vortex?pair.
For the rigid exit case, experiments show that at small times after the piston motion initiation, the flow resembles potential flow with no observable shed vorticity. This phenomenon was observed for fluid slug lengths (L/d) less than about 1, after which shedding of vorticity and formation of vortices could be seen. For larger fluid slug lengths, systematic experiments were carried out to study the formation of vortex?pair using different piston velocity programs. These experiments show the growth and convection downstream of the vortex?pair without the presence of any trailing?jet for fluid slug lengths (L/d) even up to 9. This is in contrast to prior axisymmetric studies that show vortex pinch?off and the formation of trailing?jet for L/d ? 4. For a given fluid slug length (L/d), it was found that variations in the piston velocity program affect the streamwise location of the vortex?pair. Experiments also showed that a change in exit configuration from nozzle type to orifice type resulted in significant changes in the vortex?pair. Surprisingly, it was observed that the orifice configuration resulted in a vortex?pair that convects at nearly the same rate as observed for the vortex?ring in prior axisymmetric studies using nozzles. However, further work is required to ascertain if this result is generic.
For the flexible exit studies, we use rigid flaps that are flexibly mounted (hinged) at the channel exit. For this case, systematic experiments have been done for different piston velocity programs and flap lengths to understand their effects on both the flap dynamics and the formation of the vortex?pair. In each case, it was observed that the flaps moved outwards during starting times after which they returned to their initial position. The initial opening can be attributed to the development of an over?pressure in the channel because of added mass effects. In typical experiments, this outward and subsequent inward motion of the flaps causes formation of additional vortices at the flap tip that then merge with the vortices formed due to flow issuing out of the channel. Circulation measurements with PIV indicate a reduction in circulation of the vortex?pair when compared to the rigid exit case. This may be attributed to the changes in exit velocity caused by the flap opening. Measurements with varying flap lengths show that larger flap lengths result in lower circulation values and delayed onset of the trailing?jet.
The present experiments suggest that the presence of a flexible exit can cause significant changes in the properties of a vortex?pair. This can help towards a better understanding of some biological flows. | |
