Riho Hiramoto

Manipulation of vortex rings for flow control

This paper reviews the dynamics of vortex rings and the control of flow by the manipulation of vortex rings. Vortex rings play key roles in many flows; hence, the understanding of the dynamics of vortex rings is crucial for scientists and engineers dealing with flow phenomena. We describe the structures and motions of vortex rings in circular and noncircular jets, which are typical examples of flows evolving into vortex rings. For circular jets the mechanism of evolving, merging and breakdown of vortex rings is described, and for noncircular jets the dynamics of three-dimensional deformation and interaction of noncircular vortex rings under the effect of self- and mutual induction is discussed. The application of vortex-ring manipulation to the control of various flows is reviewed with successful examples, based on the relationship between the vortex ring dynamics and the flow properties.

Dynamics of the Streamwise Vortices in an Axisymmetric Mixing Layer

The interaction of axisymmetric and streamwise vortices in the axisymmetric mixing layer developing along the jet boundary was investigated by a flow visualization technique. The jet was excited by axial and azimuthal perturbations to stabilize and enhance axisymmetric and streamwise vortices. A laser fluorescent dye and a laser light sheet were used to visualize the mixing layer. The three-dimensional views of the boundary surface of mixing layer were constructed by applying the Taylor hypothesis to the cross-sectional images. By the three-dimensional views, the interaction of axisymmetric and streamwise vortices was revealed, and the entrainment rate of the jet was also discussed in relation to the vortical structure.Copyright

Simultaneous Measurements of Velocity and Fluctuating Static-Pressure in a Circular Jet

Velocity and fluctuating static-pressure were measured simultaneously in a circular jet in order to discuss the pressure transport of turbulent energy and the generation mechanism of aerodynamic sound in relation to vortical structure. A combined sensor, which consists of X-type hot-wire and pressure probe, was used for the simultaneous measurements. The air jet was issued from a sharp-edged circular orifice under the excitation at the interaction mode so that velocity and fluctuating static-pressure can be determined in space and time domains by using phase-average technique. The results suggest that the simultaneous measurements with the phase-average technique give us useful information to reveal the turbulent shear flow phenomena. In particular, the pressure transport is closely related to vortical structure, and intense sound sources are caused by the vortex merging and by the acceleration of vortices during vortex pairing.Copyright

Visualization of Three-Dimensional Vortical Structure in a Rectangular Jet by Large Eddy Simulation

The vortex rings generated in non-circular jets are deformed and interacted three-dimensionally owing to self-induced velocity, and the vortex motion enhances mixing. To understand the mechanism of the mixing enhancement, it is crucial to make clear the three-dimensional vortical structure caused by the interaction of non-circular vortices. In the present study, the details of three-dimensional vortical structure of a rectangular jet of aspect ratio 4 is clarified using Large Eddy Simulation (LES), and the interaction mechanism of rectangular vortices is discussed, focusing the deformation of vortices. The simulation results reveal the details of the vortex interaction in the rectangular jet: the vortices deform owing to the non-uniform curvature effect, and the interaction of vortices generates hairpin vortices via vortex stretching.

Eduction of Three-Dimensional Vortical Structure in Jet by Measurements of Phase-Average Fluctuating Static-Pressure

Measurements of fluctuating static-pressure in a rectangular jet were conducted to educe three-dimensional vortical structure with phase-averaging technique. The experiments were performed for the air issuing from a sharp-edged rectangular orifice. The jet was excited at the interaction mode, where the stable interaction of vortices was generated. For phase-averaging processing, the reference signals were detected by a single normal hot-wire probe fixed at a downstream station whose distance from the jet exit corresponded to an equivalent diameter of the exit, and fluctuating static-pressure were measured over the flow field by a static pressure probe. Phase-average incoherent fluctuating static-pressure, which is related to jittering motions of vortices and is defined in phase-averaging, and adequacy of Taylor hypothesis to obtain three-dimensional structure were discussed. The results suggested that interaction and stretching of vortices contributes to generation of the incoherent vortex motions, and the use of a constant convection velocity in the hypothesis is acceptable for qualitative consideration.

Relation of the Vortical Structure and the Entrainment in the Initial Region of a Plane Jet

The relation of the vortical structure and the entrainment in the initial region of a plane jet was investigated under the four conditions of vortex generators by vorticity measurements. The vortices were enhanced and stabilized by streamwise and spanwise disturbances. The three-dimensional views of the vortices were constructed by applying the Taylor hypothesis to the phase-average vorticities, and the entrainment was discussed in relation to the vortical structures. The result reveals the enhancement of entrainment by the interaction of spanwise and streamwise vortices