Takashi Naitoh

Scaling and instability of a junction vortex

This paper details experiments in the region where an impulsively started moving wall slides under a stationary wall. The experiments were conducted over a Reynolds number range of Re Γ =5×10 2 –5×10 5 . The length scale for the Reynolds number is defined as the distance the wall has moved from rest and increases during an experiment. Experiments show that for Re Γ >10 3 a vortex forms close to the junction where the moving wall meets the stationary one. The data shows that while the vortical structure is small, in relation to the fixed-apparatus length scale, the size of the vortex normalized with respect to the wall speed and viscosity scales in a universal fashion with respect to Re Γ . The scaling rate is proportional to t 5/6 when the Reynolds number is large. The kinematic behaviour of the vortex is related to the impulse that the moving wall applies to the fluid and results in a prediction that the transient structure should grow as t 5/6 and the velocity field should scale as t −1/6 . The spatial-growth prediction is in good agreement with the experimental results and the velocity scaling is moderately successful in collapsing the experimental data. For Re Γ >2×10 4 three-dimensional instabilities appear on the perimeter of the vortical structure and the flow transitions from an unsteady two-dimensional flow to a strongly three-dimensional vortical structure at Re Γ ≃ 4 × 10 4 . The instability mechanism is centrifugal. The formation and growth of these instability structures and their ingestion into the primary vortex core causes the three-dimensional breakdown of the primary vortex. Two movies are available with the online version of the paper.

Experimental study of axial flow in a vortex ring

For a vortex ring at Re=1600, it was found that two types of azimuthal flow (axial flow) exist inside the vortex tube. The axial flow appears when the amplitude of a wavy deformation of the vortex ring becomes appreciable. The axial flow starts to grow before the wave-breaking and its speed increases gradually. In order to investigate the relation between the deformation of the vortex core and the axial flow, the flow field was examined experimentally using smoke visualization and the smoke wire technique simultaneously. Axial flow velocities are estimated from flow visualization images.

Longitudinal vortex structure in the flow field produced by a vortex ring impinging on a flat plate

The flow field produced by a vortex ring at Re = 2600 impinging on a flat plate was examined in detail using the smoke visualization method. The longitudinal vortex structure with a finger-like configuration was found which plays a crucial role in forming azimuthal coherent structures with a cross section that resembles that of an orange; the structures promote the flow field into turbulent state. The generating and developing process of the longitudinal vortex are discussed, and the topological aspects are inferred by integrating the information obtained through various local visualizations.

Experimental study of the production of vortex rings using a variable diameter orifice

A method for producing a vortex ring with an apparatus that utilizes a time varying aperture that closes during ring production is shown to have the capability for the production of rings of low nondimensional energy similar to a Hill’s spherical vortex. This represents a difference of more than 50%, in terms of the nondimensional energy, of those which have been produced with fixed diameter generators. Experiments show that modulation of the rate at which the vortex ring generator produces kinetic energy, circulation, and impulse is the critical factor in determining what type of ring is formed.

On the evolution of vortex rings with swirl

A laminar vortex ring with swirl, which has the meridional velocity component inside the vortex core, was experimentally generated by the brief fluid ejection from a rotating outlet. The evolution of the vortex ring was investigated with flow visualizations and particle image velocimetry measurements in order to find the influence of swirling flow in particular upon the transition to turbulence. Immediately after the formation of a vortex ring with swirl, a columnar strong vortex along the symmetric axis is observed in all cases of the present experiment. Then the characteristic fluid discharging from a vortex ring with swirl referred to as “peeling off” appears. The amount of discharging fluid due to the “peeling off” increases with the angular velocity of the rotating outlet. We conjectured that the mechanism generating the “peeling off” is related to the columnar strong vortex by close observations of the spatio-temporal development of the vorticity distribution and the cutting 3D images constructed fr...

A vortex ring travelling across a thin circular cylinder

Some specific features, primarily observed by smoke visualization, are described for a vortex ring interaction with a thin bluff body in the shape of a circular cylinder. Secondary vortices were found to be induced by boundary layer separation from the cylinder surface when the vortex ring travelled across it, and played a crucial role in the subsequent flowfield. The main vortex ring itself underwent distortion and moved like an elliptical ring. The ratio of the cylinder diameter to the core diameter of the vortex ring was found to be an essential parameter which governs the vortex motion after crossing the circular cylinder. In particular, the velocity of the vortex ring decreased as the ratio increased from 0.0063 to 0.25, but then increased as the ratio increased from 0.25 to 0.38. Increasing the ratio above 0.38 resulted in the velocity again decreasing after crossing the cylinder

An experimental study on wall bounded vortices

Smoke visualization studies have been made on the flow fields of wall bounded vortices generated by puffing air through an orifice. The characteristics of wall bounded vortices produced by three orifices constructed by placing a dividing plate perpendicular to a circular orifice plate have been investigated. The three orifices were a full circular opening, a three‐quarter circular opening, and a semicircular opening. It was found that the vorticity concentrated in the wall shear layer of the three‐quarter and semicircular orifices was due to the separation of the boundary layer on the dividing plate and its interaction with the arch shaped vortex generated at the perimeter of the orifice. An interesting observation is that the wall bounded vortex from the semicircular orifice moved in a stable manner, whereas, the vortex ring produced by the circular orifice with the dividing plate at the bottom of the orifice was extremely unstable.

Penetration by a Negatively Buoyant Vortex Ring

In order to investigate the vortex motion affected by buoyancy force, we experimentally formed a negatively buoyant vortex ring by pushing less dense fluid vertically downward out of a nozzle and measured the penetration depth of it in the wide Reynolds number range, 3,000 2. The formation conditions of buoyant vortex rings to be turbulent or laminar are also examined, since the characteristic features of buoyant turbulent vortex rings are markedly different from those of buoyant laminar rings.

Experiment on Puff Consisting of Leading Vortex and Trailing Jet

This paper describes spatiotemporal features of a pulsating jet. We restrict our attention, in particular, to a single puff, which is specified as a body of fluid, ejected during one period of pulsation, and consisting of a fully developed vortex ring (called a leading vortex ring) with a trailing jet following behind the ring. The flow fields and the motions of the puff were examined using both smoke visualization and velocity measurement with a hot wire anemometer. We dealt with laminar and turbulent puffs, judged by the initial flow states. The velocity variations in both cases are discussed at first with using the results of smoke visualization. Then, they are analyzed using the results from discrete wavelet transformation. The transformation proved to be adequate to express the specific features of the puffs, which were characterized by highly unsteady and irregular variation, in a time frequency domain.

Flow field around airfoil subjected to ground effect

This paper describes both the flow field around an airfoil and the aerodynamical properties of the airfoil subjected to the ground effect. The Reynolds number was 1.2×104. Our attention was primarily focussed on pressure measurement on a ground plate. Some specific features of the pressure distribution were clarified. We attempted to evaluate lift acting on the airfoil by performing surface integral of the pressure on the ground. Two types of end plates were tested as a preliminary experiment. The results obtained were anticipated: the pressure tends to become higher as an end plate has such shape as to decrease the spanwise velocity component over the airfoil. The results of the measured pressure distribution were found, in general, to reflect strongly enough the features of the flow field visualized by the smoke wire method.