Yuko Oshima

Cross-linking of two vortex rings

The mechanism for cross‐linking two vortex rings has been investigated experimentally. Phase‐locked velocity measurements using x‐type hot wires were carried out point by point over the entire flow field, and time‐dependent vorticity fields were educed. It is found that a new pair of vortex tubes is created, which connects the interacting vortex rings and grows gradually, while the vorticity intensity of the main rings decreases. This is considered to be a type of bridging phenomenon of the vortex tubes, as proposed by Kida and Takaoka [Phys. Fluids 30, 2911 (1987)] in their numerical simulation.

Bifurcation of an elliptic vortex ring

Time-dependent vorticity fields of elliptic vortex rings of aspect ratios 2, 3 and 4 were measured by means of hot-wire anemometry. The time evolution of their vorticity fields was analyzed and the processes of vortex ring formation, advection, interaction and decay, and the mechanism of vortex bifurcation are studied. The following crosslinking model is proposed: A thick vortical region composed of many equivalent vortex filaments with distributed cores is initially formed at the orifice and they behave as inviscid filaments. The elliptic ring deforms and the end parts of its major axis get closer. Then, the vortex filaments interact at the touching point and the ring partially bifurcates. Almost simultaneously, turbulent spot appears at this point, and propagates around the ring cross section, thus preventing further bifurcation. And it becomes a turbulent blob. This model is also supported by numerical simulation by a high-order vortex method and the Navier-Stokes solution.

Numerical study of interaction of two vortex rings

Three-dimensional interaction of vortices with finite core is investigated numerically using the Rosenhead-Moore approximation. As the computational scheme in this approximation cannot deal with the structure of the vortex core, a bundle of vortex elements is employed to represent a single physical vortex tube. After the validity of this method was confirmed by comparing the numerical result for a single vortex ring composed of various number of the elements with the analytical solution, two cases of interaction of two vortex rings were studied. The first case is two vortex rings traveling along a common axis, and the second is two vortex rings moving side by side along parallel axes. Comparison with the experiments showed good agreement.

Merging of Two Dimensional Vortices by the Discrete Vortex Method

Interaction of two dimensional vortex blobs with finite core, particularly their convective merging, is investigated numerically by the use of the discrete vortex method, in which each vortex blob is replaced by a group of a number of point vortices. The numerical experiment is performed for the two cases. One is the interaction of two identical vortex blobs, and the other is the interaction of three vortex blobs with the same strength, where two of them have a like-signed circulation and the rest has the opposite. The results clarify the process of convective merging of two like-signed vortices and the effect of the third opposite-signed vortex to the merging.

Thermal Convection Caused by Ring-Type Heat Source

Flow field due to thermal convection upon a ring type heat source is studied numerically and experimentally, which simulates hopefully the flow due to the devastating combustion just after the Great Kanto Earthquake. New numerical scheme was developed, which enables us to treat accurately singular or nonuniform grid system in finite-difference method. In experimental measurements, extremely low velocity and its fluctuation were detected by laser doppler velocimeter. Both results show the main flow due to the convection is strongly concentrated within the central portion above the ring where no heat is supplied at the bottom. Conspicuous unsteadiness was observed, which is considered to be the main cause of the tornado at the calamity of the Great Kanto Earthquake.

Finite element analysis of viscous incompressible flow around an oscillating airfoil

The Navier-Stokes equations of viscous incompressible flow around an elliptic airfoil under heaving and pitching motion was numerically analysed using the finite element method. The time relaxation procedure was adopted for several oscillating period, starting with the steady viscous flow condition. It was found that the rear stagnation point moves upstream considerably, though it does exist and no separation takes place.

Flow simulation by discrete vortex method

Applicability of discrete vortex approximation was tested experimentally for four types of flow conditions; an oscillating airfoil, roll-up of wake vortex layer originated from an oscillating plate, an impulsively started flat plate with an angle-of-attack and a two-dimensional rotating elliptic airfoil. Detailed flow visualization reveals the mechanism of creation, growth and migration of vortices and the comparison with those predicted by discrete vortex method has been done. It is concluded that this numerical simulation method is most usefull to predict global feature of the flow fields and care must be taken not to excessively increase the spacial and time resolution.

Flow Field of Ultrasonic Flow Meter

Basic principle of ultrasonic flow meters is to measure the traveling time difference of ultrasonic wave between the go and back signals traveling through gas flows. The flow fields are one of the most important factors of the meter quality together with the sensors and the electronic systems. That is, the signal to the noise ratio of the sensor depends on the flow condition of the passage of ultrasonic wave. Typical ultrasonic flow meters are analyzed from the viewpoint of fluid dynamics, using hot-wire anemometry, visualization with PIV method and numerical simulation. The flow fields with low turbulence level are required for meter.

Visualization of Internal Flow of Water Meter

Rotating vane type flow meter has been widely used to measure the water quantity to each household through water supply network. Depending on the water quantity, three types of meter, single case type for small, double case type for medium, and vertical type for large amount, are used. All type devices consist of vane rotor and it is set across the water piping. The rotor rotation is supposed to be proportional to the total flow quantity. Since this is a measuring device, it must be reliably accurate for defined usage condition, which is usually very strict and wide. In order to design such device, knowledge on the flow condition in them is essential. Here we used Hydrogen Bubble Flow Visualization technique, and obtained complete understanding of the flow character for the double case flow meter.

Visualizations Study of the Flow Field ia Rotating Vane Flow Meter

Rotating vane type flow meter has been widely used to measure the tap water quantity to each household through water supply network. This device consists of vane rotor and is set across the water piping. The rotor rotation is supposed to be proportional to the total flow quantity. Since this is a measuring device, it must be reliably accurate for defined usage condition, which is usually very strict and wide. In order to design such device, knowledge on the flow condition in them is essential, and since the flow conditions are very complicated, theoretical or computational fluid dynamic analyses are hardly competent. Here we used Hydrogen Bubble Flow Visualization technique, and obtained complete understanding of the flow character in such device.